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This manual describes how to use `gcj', the GNU compiler for the Java
programming language.  `gcj' can generate both `.class' files and
object files, and it can read both Java source code and `.class' files.

* Menu:

* Copying::             The GNU General Public License
* GNU Free Documentation License::
                        How you can share and copy this manual
* Invoking gcj::        Compiler options supported by `gcj'
* Compatibility::       Compatibility between gcj and other tools for Java
* Invoking jcf-dump::   Print information about class files
* Invoking gij::        Interpreting Java bytecodes
* Invoking gcj-dbtool:: Tool for manipulating class file databases.
* Invoking jv-convert:: Converting from one encoding to another
* Invoking grmic::      Generate stubs for Remote Method Invocation.
* Invoking gc-analyze:: Analyze Garbage Collector (GC) memory dumps.
* Invoking aot-compile:: Compile bytecode to native and generate databases.
* Invoking rebuild-gcj-db:: Merge the per-solib databases made by aot-compile
                            into one system-wide database.
* About CNI::           Description of the Compiled Native Interface
* System properties::   Modifying runtime behavior of the libgcj library
* Resources::           Where to look for more information
* Index::               Index.

File:,  Node: Copying,  Next: GNU Free Documentation License,  Prev: Top,  Up: Top

GNU General Public License

                        Version 3, 29 June 2007

     Copyright (C) 2007 Free Software Foundation, Inc. `'

     Everyone is permitted to copy and distribute verbatim copies of this
     license document, but changing it is not allowed.


The GNU General Public License is a free, copyleft license for software
and other kinds of works.

   The licenses for most software and other practical works are designed
to take away your freedom to share and change the works.  By contrast,
the GNU General Public License is intended to guarantee your freedom to
share and change all versions of a program-to make sure it remains free
software for all its users.  We, the Free Software Foundation, use the
GNU General Public License for most of our software; it applies also to
any other work released this way by its authors.  You can apply it to
your programs, too.

   When we speak of free software, we are referring to freedom, not
price.  Our General Public Licenses are designed to make sure that you
have the freedom to distribute copies of free software (and charge for
them if you wish), that you receive source code or can get it if you
want it, that you can change the software or use pieces of it in new
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   To protect your rights, we need to prevent others from denying you
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have certain responsibilities if you distribute copies of the software,
or if you modify it: responsibilities to respect the freedom of others.

   For example, if you distribute copies of such a program, whether
gratis or for a fee, you must pass on to the recipients the same
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know their rights.

   Developers that use the GNU GPL protect your rights with two steps:
(1) assert copyright on the software, and (2) offer you this License
giving you legal permission to copy, distribute and/or modify it.

   For the developers' and authors' protection, the GPL clearly explains
that there is no warranty for this free software.  For both users' and
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changed, so that their problems will not be attributed erroneously to
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   Some devices are designed to deny users access to install or run
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   The precise terms and conditions for copying, distribution and
modification follow.


  0. Definitions.

     "This License" refers to version 3 of the GNU General Public

     "Copyright" also means copyright-like laws that apply to other
     kinds of works, such as semiconductor masks.

     "The Program" refers to any copyrightable work licensed under this
     License.  Each licensee is addressed as "you".  "Licensees" and
     "recipients" may be individuals or organizations.

     To "modify" a work means to copy from or adapt all or part of the
     work in a fashion requiring copyright permission, other than the
     making of an exact copy.  The resulting work is called a "modified
     version" of the earlier work or a work "based on" the earlier work.

     A "covered work" means either the unmodified Program or a work
     based on the Program.

     To "propagate" a work means to do anything with it that, without
     permission, would make you directly or secondarily liable for
     infringement under applicable copyright law, except executing it
     on a computer or modifying a private copy.  Propagation includes
     copying, distribution (with or without modification), making
     available to the public, and in some countries other activities as

     To "convey" a work means any kind of propagation that enables other
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     An interactive user interface displays "Appropriate Legal Notices"
     to the extent that it includes a convenient and prominently visible
     feature that (1) displays an appropriate copyright notice, and (2)
     tells the user that there is no warranty for the work (except to
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     convey the work under this License, and how to view a copy of this
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  1. Source Code.

     The "source code" for a work means the preferred form of the work
     for making modifications to it.  "Object code" means any
     non-source form of a work.

     A "Standard Interface" means an interface that either is an
     official standard defined by a recognized standards body, or, in
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     The "System Libraries" of an executable work include anything,
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     specific operating system (if any) on which the executable work
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     The "Corresponding Source" for a work in object code form means all
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     The Corresponding Source need not include anything that users can
     regenerate automatically from other parts of the Corresponding

     The Corresponding Source for a work in source code form is that
     same work.

  2. Basic Permissions.

     All rights granted under this License are granted for the term of
     copyright on the Program, and are irrevocable provided the stated
     conditions are met.  This License explicitly affirms your unlimited
     permission to run the unmodified Program.  The output from running
     a covered work is covered by this License only if the output,
     given its content, constitutes a covered work.  This License
     acknowledges your rights of fair use or other equivalent, as
     provided by copyright law.

     You may make, run and propagate covered works that you do not
     convey, without conditions so long as your license otherwise
     remains in force.  You may convey covered works to others for the
     sole purpose of having them make modifications exclusively for
     you, or provide you with facilities for running those works,
     provided that you comply with the terms of this License in
     conveying all material for which you do not control copyright.
     Those thus making or running the covered works for you must do so
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     terms that prohibit them from making any copies of your
     copyrighted material outside their relationship with you.

     Conveying under any other circumstances is permitted solely under
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     10 makes it unnecessary.

  3. Protecting Users' Legal Rights From Anti-Circumvention Law.

     No covered work shall be deemed part of an effective technological
     measure under any applicable law fulfilling obligations under
     article 11 of the WIPO copyright treaty adopted on 20 December
     1996, or similar laws prohibiting or restricting circumvention of
     such measures.

     When you convey a covered work, you waive any legal power to forbid
     circumvention of technological measures to the extent such
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     to limit operation or modification of the work as a means of
     enforcing, against the work's users, your or third parties' legal
     rights to forbid circumvention of technological measures.

  4. Conveying Verbatim Copies.

     You may convey verbatim copies of the Program's source code as you
     receive it, in any medium, provided that you conspicuously and
     appropriately publish on each copy an appropriate copyright notice;
     keep intact all notices stating that this License and any
     non-permissive terms added in accord with section 7 apply to the
     code; keep intact all notices of the absence of any warranty; and
     give all recipients a copy of this License along with the Program.

     You may charge any price or no price for each copy that you convey,
     and you may offer support or warranty protection for a fee.

  5. Conveying Modified Source Versions.

     You may convey a work based on the Program, or the modifications to
     produce it from the Program, in the form of source code under the
     terms of section 4, provided that you also meet all of these

       a. The work must carry prominent notices stating that you
          modified it, and giving a relevant date.

       b. The work must carry prominent notices stating that it is
          released under this License and any conditions added under
          section 7.  This requirement modifies the requirement in
          section 4 to "keep intact all notices".

       c. You must license the entire work, as a whole, under this
          License to anyone who comes into possession of a copy.  This
          License will therefore apply, along with any applicable
          section 7 additional terms, to the whole of the work, and all
          its parts, regardless of how they are packaged.  This License
          gives no permission to license the work in any other way, but
          it does not invalidate such permission if you have separately
          received it.

       d. If the work has interactive user interfaces, each must display
          Appropriate Legal Notices; however, if the Program has
          interactive interfaces that do not display Appropriate Legal
          Notices, your work need not make them do so.

     A compilation of a covered work with other separate and independent
     works, which are not by their nature extensions of the covered
     work, and which are not combined with it such as to form a larger
     program, in or on a volume of a storage or distribution medium, is
     called an "aggregate" if the compilation and its resulting
     copyright are not used to limit the access or legal rights of the
     compilation's users beyond what the individual works permit.
     Inclusion of a covered work in an aggregate does not cause this
     License to apply to the other parts of the aggregate.

  6. Conveying Non-Source Forms.

     You may convey a covered work in object code form under the terms
     of sections 4 and 5, provided that you also convey the
     machine-readable Corresponding Source under the terms of this
     License, in one of these ways:

       a. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by the
          Corresponding Source fixed on a durable physical medium
          customarily used for software interchange.

       b. Convey the object code in, or embodied in, a physical product
          (including a physical distribution medium), accompanied by a
          written offer, valid for at least three years and valid for
          as long as you offer spare parts or customer support for that
          product model, to give anyone who possesses the object code
          either (1) a copy of the Corresponding Source for all the
          software in the product that is covered by this License, on a
          durable physical medium customarily used for software
          interchange, for a price no more than your reasonable cost of
          physically performing this conveying of source, or (2) access
          to copy the Corresponding Source from a network server at no

       c. Convey individual copies of the object code with a copy of
          the written offer to provide the Corresponding Source.  This
          alternative is allowed only occasionally and noncommercially,
          and only if you received the object code with such an offer,
          in accord with subsection 6b.

       d. Convey the object code by offering access from a designated
          place (gratis or for a charge), and offer equivalent access
          to the Corresponding Source in the same way through the same
          place at no further charge.  You need not require recipients
          to copy the Corresponding Source along with the object code.
          If the place to copy the object code is a network server, the
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          by you or a third party) that supports equivalent copying
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          Regardless of what server hosts the Corresponding Source, you
          remain obligated to ensure that it is available for as long
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       e. Convey the object code using peer-to-peer transmission,
          provided you inform other peers where the object code and
          Corresponding Source of the work are being offered to the
          general public at no charge under subsection 6d.

     A separable portion of the object code, whose source code is
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     not be included in conveying the object code work.

     A "User Product" is either (1) a "consumer product", which means
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     industrial or non-consumer uses, unless such uses represent the
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     "Installation Information" for a User Product means any methods,
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     The information must suffice to ensure that the continued
     functioning of the modified object code is in no case prevented or
     interfered with solely because modification has been made.

     If you convey an object code work under this section in, or with,
     or specifically for use in, a User Product, and the conveying
     occurs as part of a transaction in which the right of possession
     and use of the User Product is transferred to the recipient in
     perpetuity or for a fixed term (regardless of how the transaction
     is characterized), the Corresponding Source conveyed under this
     section must be accompanied by the Installation Information.  But
     this requirement does not apply if neither you nor any third party
     retains the ability to install modified object code on the User
     Product (for example, the work has been installed in ROM).

     The requirement to provide Installation Information does not
     include a requirement to continue to provide support service,
     warranty, or updates for a work that has been modified or
     installed by the recipient, or for the User Product in which it
     has been modified or installed.  Access to a network may be denied
     when the modification itself materially and adversely affects the
     operation of the network or violates the rules and protocols for
     communication across the network.

     Corresponding Source conveyed, and Installation Information
     provided, in accord with this section must be in a format that is
     publicly documented (and with an implementation available to the
     public in source code form), and must require no special password
     or key for unpacking, reading or copying.

  7. Additional Terms.

     "Additional permissions" are terms that supplement the terms of
     this License by making exceptions from one or more of its
     conditions.  Additional permissions that are applicable to the
     entire Program shall be treated as though they were included in
     this License, to the extent that they are valid under applicable
     law.  If additional permissions apply only to part of the Program,
     that part may be used separately under those permissions, but the
     entire Program remains governed by this License without regard to
     the additional permissions.

     When you convey a copy of a covered work, you may at your option
     remove any additional permissions from that copy, or from any part
     of it.  (Additional permissions may be written to require their own
     removal in certain cases when you modify the work.)  You may place
     additional permissions on material, added by you to a covered work,
     for which you have or can give appropriate copyright permission.

     Notwithstanding any other provision of this License, for material
     you add to a covered work, you may (if authorized by the copyright
     holders of that material) supplement the terms of this License
     with terms:

       a. Disclaiming warranty or limiting liability differently from
          the terms of sections 15 and 16 of this License; or

       b. Requiring preservation of specified reasonable legal notices
          or author attributions in that material or in the Appropriate
          Legal Notices displayed by works containing it; or

       c. Prohibiting misrepresentation of the origin of that material,
          or requiring that modified versions of such material be
          marked in reasonable ways as different from the original
          version; or

       d. Limiting the use for publicity purposes of names of licensors
          or authors of the material; or

       e. Declining to grant rights under trademark law for use of some
          trade names, trademarks, or service marks; or

       f. Requiring indemnification of licensors and authors of that
          material by anyone who conveys the material (or modified
          versions of it) with contractual assumptions of liability to
          the recipient, for any liability that these contractual
          assumptions directly impose on those licensors and authors.

     All other non-permissive additional terms are considered "further
     restrictions" within the meaning of section 10.  If the Program as
     you received it, or any part of it, contains a notice stating that
     it is governed by this License along with a term that is a further
     restriction, you may remove that term.  If a license document
     contains a further restriction but permits relicensing or
     conveying under this License, you may add to a covered work
     material governed by the terms of that license document, provided
     that the further restriction does not survive such relicensing or

     If you add terms to a covered work in accord with this section, you
     must place, in the relevant source files, a statement of the
     additional terms that apply to those files, or a notice indicating
     where to find the applicable terms.

     Additional terms, permissive or non-permissive, may be stated in
     the form of a separately written license, or stated as exceptions;
     the above requirements apply either way.

  8. Termination.

     You may not propagate or modify a covered work except as expressly
     provided under this License.  Any attempt otherwise to propagate or
     modify it is void, and will automatically terminate your rights
     under this License (including any patent licenses granted under
     the third paragraph of section 11).

     However, if you cease all violation of this License, then your
     license from a particular copyright holder is reinstated (a)
     provisionally, unless and until the copyright holder explicitly
     and finally terminates your license, and (b) permanently, if the
     copyright holder fails to notify you of the violation by some
     reasonable means prior to 60 days after the cessation.

     Moreover, your license from a particular copyright holder is
     reinstated permanently if the copyright holder notifies you of the
     violation by some reasonable means, this is the first time you have
     received notice of violation of this License (for any work) from
     that copyright holder, and you cure the violation prior to 30 days
     after your receipt of the notice.

     Termination of your rights under this section does not terminate
     the licenses of parties who have received copies or rights from
     you under this License.  If your rights have been terminated and
     not permanently reinstated, you do not qualify to receive new
     licenses for the same material under section 10.

  9. Acceptance Not Required for Having Copies.

     You are not required to accept this License in order to receive or
     run a copy of the Program.  Ancillary propagation of a covered work
     occurring solely as a consequence of using peer-to-peer
     transmission to receive a copy likewise does not require
     acceptance.  However, nothing other than this License grants you
     permission to propagate or modify any covered work.  These actions
     infringe copyright if you do not accept this License.  Therefore,
     by modifying or propagating a covered work, you indicate your
     acceptance of this License to do so.

 10. Automatic Licensing of Downstream Recipients.

     Each time you convey a covered work, the recipient automatically
     receives a license from the original licensors, to run, modify and
     propagate that work, subject to this License.  You are not
     responsible for enforcing compliance by third parties with this

     An "entity transaction" is a transaction transferring control of an
     organization, or substantially all assets of one, or subdividing an
     organization, or merging organizations.  If propagation of a
     covered work results from an entity transaction, each party to that
     transaction who receives a copy of the work also receives whatever
     licenses to the work the party's predecessor in interest had or
     could give under the previous paragraph, plus a right to
     possession of the Corresponding Source of the work from the
     predecessor in interest, if the predecessor has it or can get it
     with reasonable efforts.

     You may not impose any further restrictions on the exercise of the
     rights granted or affirmed under this License.  For example, you
     may not impose a license fee, royalty, or other charge for
     exercise of rights granted under this License, and you may not
     initiate litigation (including a cross-claim or counterclaim in a
     lawsuit) alleging that any patent claim is infringed by making,
     using, selling, offering for sale, or importing the Program or any
     portion of it.

 11. Patents.

     A "contributor" is a copyright holder who authorizes use under this
     License of the Program or a work on which the Program is based.
     The work thus licensed is called the contributor's "contributor

     A contributor's "essential patent claims" are all patent claims
     owned or controlled by the contributor, whether already acquired or
     hereafter acquired, that would be infringed by some manner,
     permitted by this License, of making, using, or selling its
     contributor version, but do not include claims that would be
     infringed only as a consequence of further modification of the
     contributor version.  For purposes of this definition, "control"
     includes the right to grant patent sublicenses in a manner
     consistent with the requirements of this License.

     Each contributor grants you a non-exclusive, worldwide,
     royalty-free patent license under the contributor's essential
     patent claims, to make, use, sell, offer for sale, import and
     otherwise run, modify and propagate the contents of its
     contributor version.

     In the following three paragraphs, a "patent license" is any
     express agreement or commitment, however denominated, not to
     enforce a patent (such as an express permission to practice a
     patent or covenant not to sue for patent infringement).  To
     "grant" such a patent license to a party means to make such an
     agreement or commitment not to enforce a patent against the party.

     If you convey a covered work, knowingly relying on a patent
     license, and the Corresponding Source of the work is not available
     for anyone to copy, free of charge and under the terms of this
     License, through a publicly available network server or other
     readily accessible means, then you must either (1) cause the
     Corresponding Source to be so available, or (2) arrange to deprive
     yourself of the benefit of the patent license for this particular
     work, or (3) arrange, in a manner consistent with the requirements
     of this License, to extend the patent license to downstream
     recipients.  "Knowingly relying" means you have actual knowledge
     that, but for the patent license, your conveying the covered work
     in a country, or your recipient's use of the covered work in a
     country, would infringe one or more identifiable patents in that
     country that you have reason to believe are valid.

     If, pursuant to or in connection with a single transaction or
     arrangement, you convey, or propagate by procuring conveyance of, a
     covered work, and grant a patent license to some of the parties
     receiving the covered work authorizing them to use, propagate,
     modify or convey a specific copy of the covered work, then the
     patent license you grant is automatically extended to all
     recipients of the covered work and works based on it.

     A patent license is "discriminatory" if it does not include within
     the scope of its coverage, prohibits the exercise of, or is
     conditioned on the non-exercise of one or more of the rights that
     are specifically granted under this License.  You may not convey a
     covered work if you are a party to an arrangement with a third
     party that is in the business of distributing software, under
     which you make payment to the third party based on the extent of
     your activity of conveying the work, and under which the third
     party grants, to any of the parties who would receive the covered
     work from you, a discriminatory patent license (a) in connection
     with copies of the covered work conveyed by you (or copies made
     from those copies), or (b) primarily for and in connection with
     specific products or compilations that contain the covered work,
     unless you entered into that arrangement, or that patent license
     was granted, prior to 28 March 2007.

     Nothing in this License shall be construed as excluding or limiting
     any implied license or other defenses to infringement that may
     otherwise be available to you under applicable patent law.

 12. No Surrender of Others' Freedom.

     If conditions are imposed on you (whether by court order,
     agreement or otherwise) that contradict the conditions of this
     License, they do not excuse you from the conditions of this
     License.  If you cannot convey a covered work so as to satisfy
     simultaneously your obligations under this License and any other
     pertinent obligations, then as a consequence you may not convey it
     at all.  For example, if you agree to terms that obligate you to
     collect a royalty for further conveying from those to whom you
     convey the Program, the only way you could satisfy both those
     terms and this License would be to refrain entirely from conveying
     the Program.

 13. Use with the GNU Affero General Public License.

     Notwithstanding any other provision of this License, you have
     permission to link or combine any covered work with a work licensed
     under version 3 of the GNU Affero General Public License into a
     single combined work, and to convey the resulting work.  The terms
     of this License will continue to apply to the part which is the
     covered work, but the special requirements of the GNU Affero
     General Public License, section 13, concerning interaction through
     a network will apply to the combination as such.

 14. Revised Versions of this License.

     The Free Software Foundation may publish revised and/or new
     versions of the GNU General Public License from time to time.
     Such new versions will be similar in spirit to the present
     version, but may differ in detail to address new problems or

     Each version is given a distinguishing version number.  If the
     Program specifies that a certain numbered version of the GNU
     General Public License "or any later version" applies to it, you
     have the option of following the terms and conditions either of
     that numbered version or of any later version published by the
     Free Software Foundation.  If the Program does not specify a
     version number of the GNU General Public License, you may choose
     any version ever published by the Free Software Foundation.

     If the Program specifies that a proxy can decide which future
     versions of the GNU General Public License can be used, that
     proxy's public statement of acceptance of a version permanently
     authorizes you to choose that version for the Program.

     Later license versions may give you additional or different
     permissions.  However, no additional obligations are imposed on any
     author or copyright holder as a result of your choosing to follow a
     later version.

 15. Disclaimer of Warranty.


 16. Limitation of Liability.


 17. Interpretation of Sections 15 and 16.

     If the disclaimer of warranty and limitation of liability provided
     above cannot be given local legal effect according to their terms,
     reviewing courts shall apply local law that most closely
     approximates an absolute waiver of all civil liability in
     connection with the Program, unless a warranty or assumption of
     liability accompanies a copy of the Program in return for a fee.


How to Apply These Terms to Your New Programs

If you develop a new program, and you want it to be of the greatest
possible use to the public, the best way to achieve this is to make it
free software which everyone can redistribute and change under these

   To do so, attach the following notices to the program.  It is safest
to attach them to the start of each source file to most effectively
state the exclusion of warranty; and each file should have at least the
"copyright" line and a pointer to where the full notice is found.

     Copyright (C) YEAR NAME OF AUTHOR

     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
     the Free Software Foundation, either version 3 of the License, or (at
     your option) any later version.

     This program is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     General Public License for more details.

     You should have received a copy of the GNU General Public License
     along with this program.  If not, see `'.

   Also add information on how to contact you by electronic and paper

   If the program does terminal interaction, make it output a short
notice like this when it starts in an interactive mode:

     This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
     This is free software, and you are welcome to redistribute it
     under certain conditions; type `show c' for details.

   The hypothetical commands `show w' and `show c' should show the
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   You should also get your employer (if you work as a programmer) or
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   The GNU General Public License does not permit incorporating your
program into proprietary programs.  If your program is a subroutine
library, you may consider it more useful to permit linking proprietary
applications with the library.  If this is what you want to do, use the
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File:,  Node: GNU Free Documentation License,  Next: Invoking gcj,  Prev: Copying,  Up: Top

GNU Free Documentation License

                      Version 1.2, November 2002

     Copyright (C) 2000,2001,2002 Free Software Foundation, Inc.
     51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA

     Everyone is permitted to copy and distribute verbatim copies
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     You may not copy, modify, sublicense, or distribute the Document
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ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of
the License in the document and put the following copyright and license
notices just after the title page:

       Copyright (C)  YEAR  YOUR NAME.
       Permission is granted to copy, distribute and/or modify this document
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       or any later version published by the Free Software Foundation;
       with no Invariant Sections, no Front-Cover Texts, and no Back-Cover
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   If you have Invariant Sections, Front-Cover Texts and Back-Cover
Texts, replace the "with...Texts." line with this:

         with the Invariant Sections being LIST THEIR TITLES, with
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   If you have Invariant Sections without Cover Texts, or some other
combination of the three, merge those two alternatives to suit the

   If your document contains nontrivial examples of program code, we
recommend releasing these examples in parallel under your choice of
free software license, such as the GNU General Public License, to
permit their use in free software.

File:,  Node: Invoking gcj,  Next: Compatibility,  Prev: GNU Free Documentation License,  Up: Top

1 Invoking gcj

As `gcj' is just another front end to `gcc', it supports many of the
same options as gcc.  *Note Option Summary: (gcc)Option Summary.  This
manual only documents the options specific to `gcj'.

* Menu:

* Input and output files::
* Input Options::               How gcj finds files
* Encodings::                   Options controlling source file encoding
* Warnings::                    Options controlling warnings specific to gcj
* Linking::                     Options for making an executable
* Code Generation::             Options controlling the output of gcj
* Configure-time Options::      Options you won't use

File:,  Node: Input and output files,  Next: Input Options,  Up: Invoking gcj

1.1 Input and output files

A `gcj' command is like a `gcc' command, in that it consists of a
number of options and file names.  The following kinds of input file
names are supported:

     Java source files.

     Java bytecode files.

     An archive containing one or more `.class' files, all of which are
     compiled.  The archive may be compressed.  Files in an archive
     which don't end with `.class' are treated as resource files; they
     are compiled into the resulting object file as `core:' URLs.

     A file containing a whitespace-separated list of input file names.
     (Currently, these must all be `.java' source files, but that may
     change.)  Each named file is compiled, just as if it had been on
     the command line.

     Libraries to use when linking.  See the `gcc' manual.

   You can specify more than one input file on the `gcj' command line,
in which case they will all be compiled.  If you specify a `-o FILENAME'
option, all the input files will be compiled together, producing a
single output file, named FILENAME.  This is allowed even when using
`-S' or `-c', but not when using `-C' or `--resource'.  (This is an
extension beyond the what plain `gcc' allows.)  (If more than one input
file is specified, all must currently be `.java' files, though we hope
to fix this.)

File:,  Node: Input Options,  Next: Encodings,  Prev: Input and output files,  Up: Invoking gcj

1.2 Input Options

`gcj' has options to control where it looks to find files it needs.
For instance, `gcj' might need to load a class that is referenced by
the file it has been asked to compile.  Like other compilers for the
Java language, `gcj' has a notion of a "class path".  There are several
options and environment variables which can be used to manipulate the
class path.  When `gcj' looks for a given class, it searches the class
path looking for matching `.class' or `.java' file.  `gcj' comes with a
built-in class path which points at the installed `libgcj.jar', a file
which contains all the standard classes.

   In the text below, a directory or path component can refer either to
an actual directory on the filesystem, or to a `.zip' or `.jar' file,
which `gcj' will search as if it is a directory.

     All directories specified by `-I' are kept in order and prepended
     to the class path constructed from all the other options.  Unless
     compatibility with tools like `javac' is important, we recommend
     always using `-I' instead of the other options for manipulating the
     class path.

     This sets the class path to PATH, a colon-separated list of paths
     (on Windows-based systems, a semicolon-separate list of paths).
     This does not override the builtin ("boot") search path.

     Deprecated synonym for `--classpath'.

     Where to find the standard builtin classes, such as

     For each directory in the PATH, place the contents of that
     directory at the end of the class path.

     This is an environment variable which holds a list of paths.

   The final class path is constructed like so:

   * First come all directories specified via `-I'.

   * If `--classpath' is specified, its value is appended.  Otherwise,
     if the `CLASSPATH' environment variable is specified, then its
     value is appended.  Otherwise, the current directory (`"."') is

   * If `--bootclasspath' was specified, append its value.  Otherwise,
     append the built-in system directory, `libgcj.jar'.

   * Finally, if `--extdirs' was specified, append the contents of the
     specified directories at the end of the class path.  Otherwise,
     append the contents of the built-in extdirs at

   The classfile built by `gcj' for the class `java.lang.Object' (and
placed in `libgcj.jar') contains a special zero length attribute
`gnu.gcj.gcj-compiled'. The compiler looks for this attribute when
loading `java.lang.Object' and will report an error if it isn't found,
unless it compiles to bytecode (the option
`-fforce-classes-archive-check' can be used to override this behavior
in this particular case.)

     This forces the compiler to always check for the special zero
     length attribute `gnu.gcj.gcj-compiled' in `java.lang.Object' and
     issue an error if it isn't found.

     This option is used to choose the source version accepted by
     `gcj'.  The default is `1.5'.

File:,  Node: Encodings,  Next: Warnings,  Prev: Input Options,  Up: Invoking gcj

1.3 Encodings

The Java programming language uses Unicode throughout.  In an effort to
integrate well with other locales, `gcj' allows `.java' files to be
written using almost any encoding.  `gcj' knows how to convert these
encodings into its internal encoding at compile time.

   You can use the `--encoding=NAME' option to specify an encoding (of
a particular character set) to use for source files.  If this is not
specified, the default encoding comes from your current locale.  If
your host system has insufficient locale support, then `gcj' assumes
the default encoding to be the `UTF-8' encoding of Unicode.

   To implement `--encoding', `gcj' simply uses the host platform's
`iconv' conversion routine.  This means that in practice `gcj' is
limited by the capabilities of the host platform.

   The names allowed for the argument `--encoding' vary from platform
to platform (since they are not standardized anywhere).  However, `gcj'
implements the encoding named `UTF-8' internally, so if you choose to
use this for your source files you can be assured that it will work on
every host.

File:,  Node: Warnings,  Next: Linking,  Prev: Encodings,  Up: Invoking gcj

1.4 Warnings

`gcj' implements several warnings.  As with other generic `gcc'
warnings, if an option of the form `-Wfoo' enables a warning, then
`-Wno-foo' will disable it.  Here we've chosen to document the form of
the warning which will have an effect - the default being the opposite
of what is listed.

     With this flag, `gcj' will warn about redundant modifiers.  For
     instance, it will warn if an interface method is declared `public'.

     This causes `gcj' to warn about empty statements.  Empty statements
     have been deprecated.

     This option will cause `gcj' not to warn when a source file is
     newer than its matching class file.  By default `gcj' will warn
     about this.

     Warn if a deprecated class, method, or field is referred to.

     This is the same as `gcc''s `-Wunused'.

     This is the same as `-Wredundant-modifiers -Wextraneous-semicolon

File:,  Node: Linking,  Next: Code Generation,  Prev: Warnings,  Up: Invoking gcj

1.5 Linking

To turn a Java application into an executable program, you need to link
it with the needed libraries, just as for C or C++.  The linker by
default looks for a global function named `main'.  Since Java does not
have global functions, and a collection of Java classes may have more
than one class with a `main' method, you need to let the linker know
which of those `main' methods it should invoke when starting the
application.  You can do that in any of these ways:

   * Specify the class containing the desired `main' method when you
     link the application, using the `--main' flag, described below.

   * Link the Java package(s) into a shared library (dll) rather than an
     executable.  Then invoke the application using the `gij' program,
     making sure that `gij' can find the libraries it needs.

   * Link the Java packages(s) with the flag `-lgij', which links in
     the `main' routine from the `gij' command.  This allows you to
     select the class whose `main' method you want to run when you run
     the application.  You can also use other `gij' flags, such as `-D'
     flags to set properties.  Using the `-lgij' library (rather than
     the `gij' program of the previous mechanism) has some advantages:
     it is compatible with static linking, and does not require
     configuring or installing libraries.

   These `gij' options relate to linking an executable:

     This option is used when linking to specify the name of the class
     whose `main' method should be invoked when the resulting
     executable is run.

     This option can only be used with `--main'.  It defines a system
     property named NAME with value VALUE.  If VALUE is not specified
     then it defaults to the empty string.  These system properties are
     initialized at the program's startup and can be retrieved at
     runtime using the `java.lang.System.getProperty' method.

     Create an application whose command-line processing is that of the
     `gij' command.

     This option is an alternative to using `--main'; you cannot use

     This option causes linking to be done against a static version of
     the libgcj runtime library.  This option is only available if
     corresponding linker support exists.

     *Caution:* Static linking of libgcj may cause essential parts of
     libgcj to be omitted.  Some parts of libgcj use reflection to load
     classes at runtime.  Since the linker does not see these
     references at link time, it can omit the referred to classes.  The
     result is usually (but not always) a `ClassNotFoundException'
     being thrown at runtime. Caution must be used when using this
     option.  For more details see:

File:,  Node: Code Generation,  Next: Configure-time Options,  Prev: Linking,  Up: Invoking gcj

1.6 Code Generation

In addition to the many `gcc' options controlling code generation,
`gcj' has several options specific to itself.

     This option is used to tell `gcj' to generate bytecode (`.class'
     files) rather than object code.

`--resource RESOURCE-NAME'
     This option is used to tell `gcj' to compile the contents of a
     given file to object code so it may be accessed at runtime with
     the core protocol handler as `core:/RESOURCE-NAME'.  Note that
     RESOURCE-NAME is the name of the resource as found at runtime; for
     instance, it could be used in a call to `ResourceBundle.getBundle'.
     The actual file name to be compiled this way must be specified

     This can be used with `-C' to choose the version of bytecode
     emitted by `gcj'.  The default is `1.5'.  When not generating
     bytecode, this option has no effect.

     When used with `-C', this causes all generated `.class' files to
     be put in the appropriate subdirectory of DIRECTORY.  By default
     they will be put in subdirectories of the current working

     By default, `gcj' generates code which checks the bounds of all
     array indexing operations.  With this option, these checks are
     omitted, which can improve performance for code that uses arrays
     extensively.  Note that this can result in unpredictable behavior
     if the code in question actually does violate array bounds
     constraints.  It is safe to use this option if you are sure that
     your code will never throw an `ArrayIndexOutOfBoundsException'.

     Don't generate array store checks.  When storing objects into
     arrays, a runtime check is normally generated in order to ensure
     that the object is assignment compatible with the component type
     of the array (which may not be known at compile-time).  With this
     option, these checks are omitted.  This can improve performance
     for code which stores objects into arrays frequently.  It is safe
     to use this option if you are sure your code will never throw an

     With `gcj' there are two options for writing native methods: CNI
     and JNI.  By default `gcj' assumes you are using CNI.  If you are
     compiling a class with native methods, and these methods are
     implemented using JNI, then you must use `-fjni'.  This option
     causes `gcj' to generate stubs which will invoke the underlying JNI

     Don't recognize the `assert' keyword.  This is for compatibility
     with older versions of the language specification.

     When the optimization level is greater or equal to `-O2', `gcj'
     will try to optimize the way calls into the runtime are made to
     initialize static classes upon their first use (this optimization
     isn't carried out if `-C' was specified.) When compiling to native
     code, `-fno-optimize-static-class-initialization' will turn this
     optimization off, regardless of the optimization level in use.

     Don't include code for checking assertions in the compiled code.
     If `=CLASS-OR-PACKAGE' is missing disables assertion code
     generation for all classes, unless overridden by a more specific
     `--enable-assertions' flag.  If CLASS-OR-PACKAGE is a class name,
     only disables generating assertion checks within the named class
     or its inner classes.  If CLASS-OR-PACKAGE is a package name,
     disables generating assertion checks within the named package or a

     By default, assertions are enabled when generating class files or
     when not optimizing, and disabled when generating optimized

     Generates code to check assertions.  The option is perhaps
     misnamed, as you still need to turn on assertion checking at
     run-time, and we don't support any easy way to do that.  So this
     flag isn't very useful yet, except to partially override

     `gcj' has a special binary compatibility ABI, which is enabled by
     the `-findirect-dispatch' option.  In this mode, the code
     generated by `gcj' honors the binary compatibility guarantees in
     the Java Language Specification, and the resulting object files do
     not need to be directly linked against their dependencies.
     Instead, all dependencies are looked up at runtime.  This allows
     free mixing of interpreted and compiled code.

     Note that, at present, `-findirect-dispatch' can only be used when
     compiling `.class' files.  It will not work when compiling from
     source.  CNI also does not yet work with the binary compatibility
     ABI.  These restrictions will be lifted in some future release.

     However, if you compile CNI code with the standard ABI, you can
     call it from code built with the binary compatibility ABI.

     This option can be use to tell `libgcj' that the compiled classes
     should be loaded by the bootstrap loader, not the system class
     loader.  By default, if you compile a class and link it into an
     executable, it will be treated as if it was loaded using the
     system class loader.  This is convenient, as it means that things
     like `Class.forName()' will search `CLASSPATH' to find the desired

     This option causes the code generated by `gcj' to contain a
     reduced amount of the class meta-data used to support runtime
     reflection. The cost of this savings is the loss of the ability to
     use certain reflection capabilities of the standard Java runtime
     environment. When set all meta-data except for that which is
     needed to obtain correct runtime semantics is eliminated.

     For code that does not use reflection (i.e. serialization, RMI,
     CORBA or call methods in the `java.lang.reflect' package),
     `-freduced-reflection' will result in proper operation with a
     savings in executable code size.

     JNI (`-fjni') and the binary compatibility ABI
     (`-findirect-dispatch') do not work properly without full
     reflection meta-data.  Because of this, it is an error to use
     these options with `-freduced-reflection'.

     *Caution:* If there is no reflection meta-data, code that uses a
     `SecurityManager' may not work properly.  Also calling
     `Class.forName()' may fail if the calling method has no reflection

File:,  Node: Configure-time Options,  Prev: Code Generation,  Up: Invoking gcj

1.7 Configure-time Options

Some `gcj' code generations options affect the resulting ABI, and so
can only be meaningfully given when `libgcj', the runtime package, is
configured.  `libgcj' puts the appropriate options from this group into
a `spec' file which is read by `gcj'.  These options are listed here
for completeness; if you are using `libgcj' then you won't want to
touch these options.

     This enables the use of the Boehm GC bitmap marking code.  In
     particular this causes `gcj' to put an object marking descriptor
     into each vtable.

     By default, synchronization data (the data used for `synchronize',
     `wait', and `notify') is pointed to by a word in each object.
     With this option `gcj' assumes that this information is stored in a
     hash table and not in the object itself.

     On some systems, a library routine is called to perform integer
     division.  This is required to get exception handling correct when
     dividing by zero.

     On some systems it's necessary to insert inline checks whenever
     accessing an object via a reference.  On other systems you won't
     need this because null pointer accesses are caught automatically
     by the processor.

File:,  Node: Compatibility,  Next: Invoking jcf-dump,  Prev: Invoking gcj,  Up: Top

2 Compatibility with the Java Platform

As we believe it is important that the Java platform not be fragmented,
`gcj' and `libgcj' try to conform to the relevant Java specifications.
However, limited manpower and incomplete and unclear documentation work
against us.  So, there are caveats to using `gcj'.

* Menu:

* Limitations::
* Extensions::

File:,  Node: Limitations,  Next: Extensions,  Up: Compatibility

2.1 Standard features not yet supported

This list of compatibility issues is by no means complete.

   * `gcj' implements the JDK 1.2 language.  It supports inner classes
     and the new 1.4 `assert' keyword.  It does not yet support the
     Java 2 `strictfp' keyword (it recognizes the keyword but ignores

   * `libgcj' is largely compatible with the JDK 1.2 libraries.
     However, `libgcj' is missing many packages, most notably
     `java.awt'.  There are also individual missing classes and methods.
     We currently do not have a list showing differences between
     `libgcj' and the Java 2 platform.

   * Sometimes the `libgcj' implementation of a method or class differs
     from the JDK implementation.  This is not always a bug.  Still, if
     it affects you, it probably makes sense to report it so that we
     can discuss the appropriate response.

   * `gcj' does not currently allow for piecemeal replacement of
     components within `libgcj'. Unfortunately, programmers often want
     to use newer versions of certain packages, such as those provided
     by the Apache Software Foundation's Jakarta project.  This has
     forced us to place the `org.w3c.dom' and `org.xml.sax' packages
     into their own libraries, separate from `libgcj'.  If you intend to
     use these classes, you must link them explicitly with
     `-l-org-w3c-dom' and `-l-org-xml-sax'.  Future versions of `gcj'
     may not have this restriction.

File:,  Node: Extensions,  Prev: Limitations,  Up: Compatibility

2.2 Extra features unique to gcj

The main feature of `gcj' is that it can compile programs written in
the Java programming language to native code.  Most extensions that
have been added are to facilitate this functionality.

   * `gcj' makes it easy and efficient to mix code written in Java and
     C++.  *Note About CNI::, for more info on how to use this in your

   * When you compile your classes into a shared library using
     `-findirect-dispatch' then add them to the system-wide classmap.db
     file using `gcj-dbtool', they will be automatically loaded by the
     `libgcj' system classloader.  This is the new, preferred
     classname-to-library resolution mechanism.  *Note Invoking
     gcj-dbtool::, for more information on using the classmap database.

   * The old classname-to-library lookup mechanism is still supported
     through the `gnu.gcj.runtime.VMClassLoader.library_control'
     property, but it is deprecated and will likely be removed in some
     future release.  When trying to load a class `gnu.pkg.SomeClass'
     the system classloader will first try to load the shared library
     `', if that fails to load the class then
     it will try to load `' and finally when the class is
     still not loaded it will try to load `'.  Note that all
     `.'s will be transformed into `-'s and that searching for inner
     classes starts with their outermost outer class.  If the class
     cannot be found this way the system classloader tries to use the
     `libgcj' bytecode interpreter to load the class from the standard
     classpath.  This process can be controlled to some degree via the
     `gnu.gcj.runtime.VMClassLoader.library_control' property; *Note
     libgcj Runtime Properties::.

   * `libgcj' includes a special `gcjlib' URL type.  A URL of this form
     is like a `jar' URL, and looks like
     `gcjlib:/path/to/shared/!/path/to/resource'.  An access
     to one of these URLs causes the shared library to be `dlopen()'d,
     and then the resource is looked for in that library.  These URLs
     are most useful when used in conjunction with
     `'.  Note that, due to implementation
     limitations, currently any such URL can be accessed by only one
     class loader, and libraries are never unloaded.  This means some
     care must be exercised to make sure that a `gcjlib' URL is not
     accessed by more than one class loader at once.  In a future
     release this limitation will be lifted, and such libraries will be
     mapped privately.

   * A program compiled by `gcj' will examine the `GCJ_PROPERTIES'
     environment variable and change its behavior in some ways.  In
     particular `GCJ_PROPERTIES' holds a list of assignments to global
     properties, such as would be set with the `-D' option to `java'.
     For instance, `java.compiler=gcj' is a valid (but currently
     meaningless) setting.

File:,  Node: Invoking jcf-dump,  Next: Invoking gij,  Prev: Compatibility,  Up: Top

3 Invoking jcf-dump

This is a class file examiner, similar to `javap'.  It will print
information about a number of classes, which are specified by class name
or file name.

     Disassemble method bodies.  By default method bodies are not

     Print the constant pool.  When printing a reference to a constant
     also print its index in the constant pool.

     Generate output in `javap' format.  The implementation of this
     feature is very incomplete.

`-o FILE'
     These options as the same as the corresponding `gcj' options.

     Print help, then exit.

     Print version number, then exit.

`-v, --verbose'
     Print extra information while running.  Implies

File:,  Node: Invoking gij,  Next: Invoking gcj-dbtool,  Prev: Invoking jcf-dump,  Up: Top

4 Invoking gij

`gij' is a Java bytecode interpreter included with `libgcj'.  `gij' is
not available on every platform; porting it requires a small amount of
assembly programming which has not been done for all the targets
supported by `gcj'.

   The primary argument to `gij' is the name of a class or, with
`-jar', a jar file.  Options before this argument are interpreted by
`gij'; remaining options are passed to the interpreted program.

   If a class name is specified and this class does not have a `main'
method with the appropriate signature (a `static void' method with a
`String[]' as its sole argument), then `gij' will print an error and

   If a jar file is specified then `gij' will use information in it to
determine which class' `main' method will be invoked.

   `gij' will invoke the `main' method with all the remaining
command-line options.

   Note that `gij' is not limited to interpreting code.  Because
`libgcj' includes a class loader which can dynamically load shared
objects, it is possible to give `gij' the name of a class which has
been compiled and put into a shared library on the class path.

`-cp PATH'
`-classpath PATH'
     Set the initial class path.  The class path is used for finding
     class and resource files.  If specified, this option overrides the
     `CLASSPATH' environment variable.  Note that this option is
     ignored if `-jar' is used.

     This defines a system property named NAME with value VALUE.  If
     VALUE is not specified then it defaults to the empty string.
     These system properties are initialized at the program's startup
     and can be retrieved at runtime using the
     `java.lang.System.getProperty' method.

     Equivalent to `-Xms'.

     Equivalent to `-Xmx'.

     Do not verify compliance of bytecode with the VM specification. In
     addition, this option disables type verification which is
     otherwise performed on BC-ABI compiled code.

     Supplying `-X' by itself will cause `gij' to list all the
     supported `-X' options.  Currently these options are supported:

          Set the initial heap size.

          Set the maximum heap size.

          Set the thread stack size.

     Unrecognized `-X' options are ignored, for compatibility with
     other runtimes.

     This indicates that the name passed to `gij' should be interpreted
     as the name of a jar file, not a class.

     Print help, then exit.

     Print version number and continue.

     Print detailed version information, then exit.

     Print version number, then exit.

     Each time a class is initialized, print a short message on
     standard error.

   `gij' also recognizes and ignores the following options, for
compatibility with existing application launch scripts: `-client',
`-server', `-hotspot', `-jrockit', `-agentlib', `-agentpath', `-debug',
`-d32', `-d64', `-javaagent', `-noclassgc', `-verify', and

File:,  Node: Invoking gcj-dbtool,  Next: Invoking jv-convert,  Prev: Invoking gij,  Up: Top

5 Invoking gcj-dbtool.

`gcj-dbtool' is a tool for creating and manipulating class file mapping
databases.  `libgcj' can use these databases to find a shared library
corresponding to the bytecode representation of a class.  This
functionality is useful for ahead-of-time compilation of a program that
has no knowledge of `gcj'.

   `gcj-dbtool' works best if all the jar files added to it are
compiled using `-findirect-dispatch'.

   Note that `gcj-dbtool' is currently available as "preview
technology".  We believe it is a reasonable way to allow
application-transparent ahead-of-time compilation, but this is an
unexplored area.  We welcome your comments.

     This creates a new database.  Currently, databases cannot be
     resized; you can choose a larger initial size if desired.  The
     default size is 32,749.

     This adds a jar file to the database.  For each class file in the
     jar, a cryptographic signature of the bytecode representation of
     the class is recorded in the database.  At runtime, a class is
     looked up by its signature and the compiled form of the class is
     looked for in the corresponding shared library.  The `-a' option
     will verify that LIB exists before adding it to the database; `-f'
     skips this check.

`[`-'][`-0'] -m DBFILE DBFILE,[DBFILE]'
     Merge a number of databases.  The output database overwrites any
     existing database.  To add databases into an existing database,
     include the destination in the list of sources.

     If `-' or `-0' are used, the list of files to read is taken from
     standard input instead of the command line.  For `-0', Input
     filenames are terminated by a null character instead of by
     whitespace.  Useful when arguments might contain white space.  The
     GNU find -print0 option produces input suitable for this mode.

     Test a database.

     List the contents of a database.

     Print the name of the default database.  If there is no default
     database, this prints a blank line.  If LIBDIR is specified, use
     it instead of the default library directory component of the
     database name.

     Print a help message, then exit.

     Print version information, then exit.

File:,  Node: Invoking jv-convert,  Next: Invoking grmic,  Prev: Invoking gcj-dbtool,  Up: Top

6 Invoking jv-convert

`jv-convert' [`OPTION'] ... [INPUTFILE [OUTPUTFILE]]

   `jv-convert' is a utility included with `libgcj' which converts a
file from one encoding to another.  It is similar to the Unix `iconv'

   The encodings supported by `jv-convert' are platform-dependent.
Currently there is no way to get a list of all supported encodings.

`--encoding NAME'
`--from NAME'
     Use NAME as the input encoding.  The default is the current
     locale's encoding.

`--to NAME'
     Use NAME as the output encoding.  The default is the `JavaSrc'
     encoding; this is ASCII with `\u' escapes for non-ASCII characters.

`-i FILE'
     Read from FILE.  The default is to read from standard input.

`-o FILE'
     Write to FILE.  The default is to write to standard output.

     Swap the input and output encodings.

     Print a help message, then exit.

     Print version information, then exit.

File:,  Node: Invoking grmic,  Next: Invoking gc-analyze,  Prev: Invoking jv-convert,  Up: Top

7 Invoking grmic

`grmic' [`OPTION'] ... CLASS ...

   `grmic' is a utility included with `libgcj' which generates stubs
for remote objects.

   Note that this program isn't yet fully compatible with the JDK
`grmic'.  Some options, such as `-classpath', are recognized but
currently ignored.  We have left these options undocumented for now.

   Long options can also be given with a GNU-style leading `--'.  For
instance, `--help' is accepted.

     By default, `grmic' deletes intermediate files.  Either of these
     options causes it not to delete such files.

     Cause `grmic' to create stubs and skeletons for the 1.1 protocol

     Cause `grmic' to create stubs and skeletons compatible with both
     the 1.1 and 1.2 protocol versions.  This is the default.

     Cause `grmic' to create stubs and skeletons for the 1.2 protocol

     Don't compile the generated files.

     Print information about what `grmic' is doing.

     Put output files in DIRECTORY.  By default the files are put in
     the current working directory.

     Print a help message, then exit.

     Print version information, then exit.

File:,  Node: Invoking gc-analyze,  Next: Invoking aot-compile,  Prev: Invoking grmic,  Up: Top

8 Invoking gc-analyze

`gc-analyze' [`OPTION'] ... [FILE]

   `gc-analyze' prints an analysis of a GC memory dump to standard out.

   The memory dumps may be created by calling
`gnu.gcj.util.GCInfo.enumerate(String namePrefix)' from java code.  A
memory dump will be created on an out of memory condition if
`gnu.gcj.util.GCInfo.setOOMDump(String namePrefix)' is called before
the out of memory occurs.

   Running this program will create two files: `TestDump001' and

     import gnu.gcj.util.*;
     import java.util.*;

     public class GCDumpTest
         static public void main(String args[])
             ArrayList<String> l = new ArrayList<String>(1000);

             for (int i = 1; i < 1500; i++) {
                 l.add("This is string #" + i);

   The memory dump may then be displayed by running:

     gc-analyze -v TestDump001

     Verbose output.

     Prefix added to the names of the `nm' and `readelf' commands.

     Directory that contains the executable and shared libraries used
     when the dump was generated.

     Print a help message, then exit.

     Print version information, then exit.

File:,  Node: Invoking aot-compile,  Next: Invoking rebuild-gcj-db,  Prev: Invoking gc-analyze,  Up: Top

9 Invoking aot-compile

`aot-compile' is a script that searches a directory for Java bytecode
(as class files, or in jars) and uses `gcj' to compile it to native
code and generate the databases from it.

`-M, --make=PATH'
     Specify the path to the `make' executable to use.

`-C, --gcj=PATH'
     Specify the path to the `gcj' executable to use.

`-D, --dbtool=PATH'
     Specify the path to the `gcj-dbtool' executable to use.

`-m, --makeflags=FLAGS'
     Specify flags to pass to `make' during the build.

`-c, --gcjflags=FLAGS'
     Specify flags to pass to `gcj' during compilation, in addition to
     '-fPIC -findirect-dispatch -fjni'.

`-l, --ldflags=FLAGS'
     Specify flags to pass to `gcj' during linking, in addition to

`-e, --exclude=PATH'
     Do not compile PATH.

File:,  Node: Invoking rebuild-gcj-db,  Next: About CNI,  Prev: Invoking aot-compile,  Up: Top

10 Invoking rebuild-gcj-db

`rebuild-gcj-db' is a script that merges the per-solib databases made by
`aot-compile' into one system-wide database so `gij' can find the

File:,  Node: About CNI,  Next: System properties,  Prev: Invoking rebuild-gcj-db,  Up: Top

11 About CNI

This documents CNI, the Compiled Native Interface, which is is a
convenient way to write Java native methods using C++.  This is a more
efficient, more convenient, but less portable alternative to the
standard JNI (Java Native Interface).

* Menu:

* Basic concepts::              Introduction to using CNI.
* Packages::                    How packages are mapped to C++.
* Primitive types::             Handling primitive Java types in C++.
* Reference types::             Handling Java reference types in C++.
* Interfaces::                  How Java interfaces map to C++.
* Objects and Classes::         C++ and Java classes.
* Class Initialization::        How objects are initialized.
* Object allocation::           How to create Java objects in C++.
* Memory allocation::           How to allocate and free memory.
* Arrays::                      Dealing with Java arrays in C++.
* Methods::                     Java methods in C++.
* Strings::                     Information about Java Strings.
* Mixing with C++::             How CNI can interoperate with C++.
* Exception Handling::          How exceptions are handled.
* Synchronization::             Synchronizing between Java and C++.
* Invocation::                  Starting the Java runtime from C++.
* Reflection::                  Using reflection from C++.

File:,  Node: Basic concepts,  Next: Packages,  Up: About CNI

11.1 Basic concepts

In terms of languages features, Java is mostly a subset of C++.  Java
has a few important extensions, plus a powerful standard class library,
but on the whole that does not change the basic similarity.  Java is a
hybrid object-oriented language, with a few native types, in addition
to class types.  It is class-based, where a class may have static as
well as per-object fields, and static as well as instance methods.
Non-static methods may be virtual, and may be overloaded.  Overloading
is resolved at compile time by matching the actual argument types
against the parameter types.  Virtual methods are implemented using
indirect calls through a dispatch table (virtual function table).
Objects are allocated on the heap, and initialized using a constructor
method.  Classes are organized in a package hierarchy.

   All of the listed attributes are also true of C++, though C++ has
extra features (for example in C++ objects may be allocated not just on
the heap, but also statically or in a local stack frame).  Because
`gcj' uses the same compiler technology as G++ (the GNU C++ compiler),
it is possible to make the intersection of the two languages use the
same ABI (object representation and calling conventions).  The key idea
in CNI is that Java objects are C++ objects, and all Java classes are
C++ classes (but not the other way around).  So the most important task
in integrating Java and C++ is to remove gratuitous incompatibilities.

   You write CNI code as a regular C++ source file.  (You do have to use
a Java/CNI-aware C++ compiler, specifically a recent version of G++.)

A CNI C++ source file must have:

     #include <gcj/cni.h>

and then must include one header file for each Java class it uses, e.g.:

     #include <java/lang/Character.h>
     #include <java/util/Date.h>
     #include <java/lang/IndexOutOfBoundsException.h>

These header files are automatically generated by `gcjh'.

   CNI provides some functions and macros to make using Java objects and
primitive types from C++ easier.  In general, these CNI functions and
macros start with the `Jv' prefix, for example the function
`JvNewObjectArray'.  This convention is used to avoid conflicts with
other libraries.  Internal functions in CNI start with the prefix
`_Jv_'.  You should not call these; if you find a need to, let us know
and we will try to come up with an alternate solution.

11.1.1 Limitations

Whilst a Java class is just a C++ class that doesn't mean that you are
freed from the shackles of Java, a CNI C++ class must adhere to the
rules of the Java programming language.

   For example: it is not possible to declare a method in a CNI class
that will take a C string (`char*') as an argument, or to declare a
member variable of some non-Java datatype.

File:,  Node: Packages,  Next: Primitive types,  Prev: Basic concepts,  Up: About CNI

11.2 Packages

The only global names in Java are class names, and packages.  A
"package" can contain zero or more classes, and also zero or more
sub-packages.  Every class belongs to either an unnamed package or a
package that has a hierarchical and globally unique name.

   A Java package is mapped to a C++ "namespace".  The Java class
`java.lang.String' is in the package `java.lang', which is a
sub-package of `java'.  The C++ equivalent is the class
`java::lang::String', which is in the namespace `java::lang' which is
in the namespace `java'.

Here is how you could express this:

     (// Declare the class(es), possibly in a header file:
     namespace java {
       namespace lang {
         class Object;
         class String;

     class java::lang::String : public java::lang::Object

The `gcjh' tool automatically generates the necessary namespace

11.2.1 Leaving out package names

Always using the fully-qualified name of a java class can be tiresomely
verbose.  Using the full qualified name also ties the code to a single
package making code changes necessary should the class move from one
package to another.  The Java `package' declaration specifies that the
following class declarations are in the named package, without having
to explicitly name the full package qualifiers.  The `package'
declaration can be followed by zero or more `import' declarations, which
allows either a single class or all the classes in a package to be
named by a simple identifier.  C++ provides something similar with the
`using' declaration and directive.

In Java:


allows the program text to refer to CLASS-NAME as a shorthand for the
fully qualified name: `PACKAGE-NAME.CLASS-NAME'.

To achieve the same effect C++, you have to do this:


Java can also cause imports on demand, like this:

     import PACKAGE-NAME.*;

Doing this allows any class from the package PACKAGE-NAME to be
referred to only by its class-name within the program text.

The same effect can be achieved in C++ like this:

     using namespace PACKAGE-NAME;

File:,  Node: Primitive types,  Next: Reference types,  Prev: Packages,  Up: About CNI

11.3 Primitive types

Java provides 8 "primitives" types which represent integers, floats,
characters and booleans (and also the void type).  C++ has its own very
similar concrete types.  Such types in C++ however are not always
implemented in the same way (an int might be 16, 32 or 64 bits for
example) so CNI provides a special C++ type for each primitive Java

*Java type*    *C/C++ typename*   *Description*
`char'         `jchar'            16 bit Unicode character
`boolean'      `jboolean'         logical (true or false) values
`byte'         `jbyte'            8-bit signed integer
`short'        `jshort'           16 bit signed integer
`int'          `jint'             32 bit signed integer
`long'         `jlong'            64 bit signed integer
`float'        `jfloat'           32 bit IEEE floating point number
`double'       `jdouble'          64 bit IEEE floating point number
`void'         `void'             no value

   When referring to a Java type You should always use these C++
typenames (e.g.: `jint') to avoid disappointment.

11.3.1 Reference types associated with primitive types

In Java each primitive type has an associated reference type, e.g.:
`boolean' has an associated `java.lang.Boolean.TYPE' class.  In order
to make working with such classes easier GCJ provides the macro

 -- macro: JvPrimClass type
     Return a pointer to the `Class' object corresponding to the type

          JvPrimClass(void) => java.lang.Void.TYPE

File:,  Node: Reference types,  Next: Interfaces,  Prev: Primitive types,  Up: About CNI

11.4 Reference types

A Java reference type is treated as a class in C++.  Classes and
interfaces are handled this way.  A Java reference is translated to a
C++ pointer, so for instance a Java `java.lang.String' becomes, in C++,
`java::lang::String *'.

   CNI provides a few built-in typedefs for the most common classes:
*Java type*            *C++ typename*     *Description*
`java.lang.Object'     `jobject'          Object type
`java.lang.String'     `jstring'          String type
`java.lang.Class'      `jclass'           Class type

   Every Java class or interface has a corresponding `Class' instance.
These can be accessed in CNI via the static `class$' field of a class.
The `class$' field is of type `Class' (and not `Class *'), so you will
typically take the address of it.

   Here is how you can refer to the class of `String', which in Java
would be written `String.class':

     using namespace java::lang;
     doSomething (&String::class$);

File:,  Node: Interfaces,  Next: Objects and Classes,  Prev: Reference types,  Up: About CNI

11.5 Interfaces

A Java class can "implement" zero or more "interfaces", in addition to
inheriting from a single base class.

   CNI allows CNI code to implement methods of interfaces.  You can
also call methods through interface references, with some limitations.

   CNI doesn't understand interface inheritance at all yet.  So, you
can only call an interface method when the declared type of the field
being called matches the interface which declares that method.  The
workaround is to cast the interface reference to the right

   For example if you have:

     interface A
       void a();

     interface B extends A
       void b();

   and declare a variable of type `B' in C++, you can't call `a()'
unless you cast it to an `A' first.

File:,  Node: Objects and Classes,  Next: Class Initialization,  Prev: Interfaces,  Up: About CNI

11.6 Objects and Classes

11.6.1 Classes

All Java classes are derived from `java.lang.Object'.  C++ does not
have a unique root class, but we use the C++ class `java::lang::Object'
as the C++ version of the `java.lang.Object' Java class.  All other
Java classes are mapped into corresponding C++ classes derived from

   Interface inheritance (the `implements' keyword) is currently not
reflected in the C++ mapping.

11.6.2 Object fields

Each object contains an object header, followed by the instance fields
of the class, in order.  The object header consists of a single pointer
to a dispatch or virtual function table.  (There may be extra fields
_in front of_ the object, for example for memory management, but this
is invisible to the application, and the reference to the object points
to the dispatch table pointer.)

   The fields are laid out in the same order, alignment, and size as in
C++.  Specifically, 8-bit and 16-bit native types (`byte', `short',
`char', and `boolean') are _not_ widened to 32 bits.  Note that the
Java VM does extend 8-bit and 16-bit types to 32 bits when on the VM
stack or temporary registers.

   If you include the `gcjh'-generated header for a class, you can
access fields of Java classes in the _natural_ way.  For example, given
the following Java class:

     public class Int
       public int i;
       public Int (int i) { this.i = i; }
       public static Int zero = new Int(0);

   you can write:

     #include <gcj/cni.h>;
     #include <Int>;

     mult (Int *p, jint k)
       if (k == 0)
         return Int::zero;  // Static member access.
       return new Int(p->i * k);

11.6.3 Access specifiers

CNI does not strictly enforce the Java access specifiers, because Java
permissions cannot be directly mapped into C++ permission.  Private
Java fields and methods are mapped to private C++ fields and methods,
but other fields and methods are mapped to public fields and methods.

File:,  Node: Class Initialization,  Next: Object allocation,  Prev: Objects and Classes,  Up: About CNI

11.7 Class Initialization

Java requires that each class be automatically initialized at the time
of the first active use.  Initializing a class involves initializing
the static fields, running code in class initializer methods, and
initializing base classes.  There may also be some implementation
specific actions, such as allocating `String' objects corresponding to
string literals in the code.

   The GCJ compiler inserts calls to `JvInitClass' at appropriate
places to ensure that a class is initialized when required.  The C++
compiler does not insert these calls automatically--it is the
programmer's responsibility to make sure classes are initialized.
However, this is fairly painless because of the conventions assumed by
the Java system.

   First, `libgcj' will make sure a class is initialized before an
instance of that object is created.  This is one of the
responsibilities of the `new' operation.  This is taken care of both in
Java code, and in C++ code.  When G++ sees a `new' of a Java class, it
will call a routine in `libgcj' to allocate the object, and that
routine will take care of initializing the class.  Note however that
this does not happen for Java arrays; you must allocate those using the
appropriate CNI function.  It follows that you can access an instance
field, or call an instance (non-static) method and be safe in the
knowledge that the class and all of its base classes have been

   Invoking a static method is also safe.  This is because the Java
compiler adds code to the start of a static method to make sure the
class is initialized.  However, the C++ compiler does not add this
extra code.  Hence, if you write a native static method using CNI, you
are responsible for calling `JvInitClass' before doing anything else in
the method (unless you are sure it is safe to leave it out).

   Accessing a static field also requires the class of the field to be
initialized.  The Java compiler will generate code to call
`JvInitClass' before getting or setting the field.  However, the C++
compiler will not generate this extra code, so it is your
responsibility to make sure the class is initialized before you access
a static field from C++.

File:,  Node: Object allocation,  Next: Memory allocation,  Prev: Class Initialization,  Up: About CNI

11.8 Object allocation

New Java objects are allocated using a "class instance creation
expression", e.g.:

     new TYPE ( ... )

   The same syntax is used in C++.  The main difference is that C++
objects have to be explicitly deleted; in Java they are automatically
deleted by the garbage collector.  Using CNI, you can allocate a new
Java object using standard C++ syntax and the C++ compiler will allocate
memory from the garbage collector.  If you have overloaded
constructors, the compiler will choose the correct one using standard
C++ overload resolution rules.

For example:

     java::util::Hashtable *ht = new java::util::Hashtable(120);

File:,  Node: Memory allocation,  Next: Arrays,  Prev: Object allocation,  Up: About CNI

11.9 Memory allocation

When allocating memory in CNI methods it is best to handle
out-of-memory conditions by throwing a Java exception.  These functions
are provided for that purpose:

 -- Function: void* JvMalloc (jsize SIZE)
     Calls malloc.  Throws `java.lang.OutOfMemoryError' if allocation

 -- Function: void* JvRealloc (void* PTR, jsize SIZE)
     Calls realloc.  Throws `java.lang.OutOfMemoryError' if
     reallocation fails.

 -- Function: void JvFree (void* PTR)
     Calls free.

File:,  Node: Arrays,  Next: Methods,  Prev: Memory allocation,  Up: About CNI

11.10 Arrays

While in many ways Java is similar to C and C++, it is quite different
in its treatment of arrays.  C arrays are based on the idea of pointer
arithmetic, which would be incompatible with Java's security
requirements.  Java arrays are true objects (array types inherit from
`java.lang.Object').  An array-valued variable is one that contains a
reference (pointer) to an array object.

   Referencing a Java array in C++ code is done using the `JArray'
template, which as defined as follows:

     class __JArray : public java::lang::Object
       int length;

     template<class T>
     class JArray : public __JArray
       T data[0];
       T& operator[](jint i) { return data[i]; }

   There are a number of `typedef's which correspond to `typedef's from
the JNI.  Each is the type of an array holding objects of the relevant

     typedef __JArray *jarray;
     typedef JArray<jobject> *jobjectArray;
     typedef JArray<jboolean> *jbooleanArray;
     typedef JArray<jbyte> *jbyteArray;
     typedef JArray<jchar> *jcharArray;
     typedef JArray<jshort> *jshortArray;
     typedef JArray<jint> *jintArray;
     typedef JArray<jlong> *jlongArray;
     typedef JArray<jfloat> *jfloatArray;
     typedef JArray<jdouble> *jdoubleArray;

 -- Method on template<class T>: T* elements (JArray<T> ARRAY)
     This template function can be used to get a pointer to the
     elements of the `array'.  For instance, you can fetch a pointer to
     the integers that make up an `int[]' like so:

          extern jintArray foo;
          jint *intp = elements (foo);

     The name of this function may change in the future.

 -- Function: jobjectArray JvNewObjectArray (jsize LENGTH, jclass
          KLASS, jobject INIT)
     This creates a new array whose elements have reference type.
     `klass' is the type of elements of the array and `init' is the
     initial value put into every slot in the array.

     using namespace java::lang;
     JArray<String *> *array
       = (JArray<String *> *) JvNewObjectArray(length, &String::class$, NULL);

11.10.1 Creating arrays

For each primitive type there is a function which can be used to create
a new array of that type.  The name of the function is of the form:


For example:


can be used to create an array of Java primitive boolean types.

The following function definition is the template for all such

 -- Function: jbooleanArray JvNewBooleanArray (jint LENGTH)
     Creates an array LENGTH indices long.

 -- Function: jsize JvGetArrayLength (jarray ARRAY)
     Returns the length of the ARRAY.

File:,  Node: Methods,  Next: Strings,  Prev: Arrays,  Up: About CNI

11.11 Methods

Java methods are mapped directly into C++ methods.  The header files
generated by `gcjh' include the appropriate method definitions.
Basically, the generated methods have the same names and
_corresponding_ types as the Java methods, and are called in the
natural manner.

11.11.1 Overloading

Both Java and C++ provide method overloading, where multiple methods in
a class have the same name, and the correct one is chosen (at compile
time) depending on the argument types.  The rules for choosing the
correct method are (as expected) more complicated in C++ than in Java,
but given a set of overloaded methods generated by `gcjh' the C++
compiler will choose the expected one.

   Common assemblers and linkers are not aware of C++ overloading, so
the standard implementation strategy is to encode the parameter types
of a method into its assembly-level name.  This encoding is called
"mangling", and the encoded name is the "mangled name".  The same
mechanism is used to implement Java overloading.  For C++/Java
interoperability, it is important that both the Java and C++ compilers
use the _same_ encoding scheme.

11.11.2 Static methods

Static Java methods are invoked in CNI using the standard C++ syntax,
using the `::' operator rather than the `.' operator.

For example:

     jint i = java::lang::Math::round((jfloat) 2.3);

C++ method definition syntax is used to define a static native method.
For example:

     #include <java/lang/Integer>
     java::lang::Integer::getInteger(jstring str)

11.11.3 Object Constructors

Constructors are called implicitly as part of object allocation using
the `new' operator.

For example:

     java::lang::Integer *x = new java::lang::Integer(234);

   Java does not allow a constructor to be a native method.  This
limitation can be coded round however because a constructor can _call_
a native method.

11.11.4 Instance methods

Calling a Java instance method from a C++ CNI method is done using the
standard C++ syntax, e.g.:

     // First create the Java object.
     java::lang::Integer *x = new java::lang::Integer(234);
     // Now call a method.
     jint prim_value = x->intValue();
     if (x->longValue == 0)

Defining a Java native instance method is also done the natural way:

     #include <java/lang/Integer.h>

       return (jdouble) value;

11.11.5 Interface methods

In Java you can call a method using an interface reference.  This is
supported, but not completely.  *Note Interfaces::.

File:,  Node: Strings,  Next: Mixing with C++,  Prev: Methods,  Up: About CNI

11.12 Strings

CNI provides a number of utility functions for working with Java Java
`String' objects.  The names and interfaces are analogous to those of

 -- Function: jstring JvNewString (const jchar* CHARS, jsize LEN)
     Returns a Java `String' object with characters from the array of
     Unicode characters CHARS up to the index LEN in that array.

 -- Function: jstring JvNewStringLatin1 (const char* BYTES, jsize LEN)
     Returns a Java `String' made up of LEN bytes from BYTES.

 -- Function: jstring JvNewStringLatin1 (const char* BYTES)
     As above but the length of the `String' is `strlen(BYTES)'.

 -- Function: jstring JvNewStringUTF (const char* BYTES)
     Returns a `String' which is made up of the UTF encoded characters
     present in the C string BYTES.

 -- Function: jchar* JvGetStringChars (jstring STR)
     Returns a pointer to an array of characters making up the `String'

 -- Function: int JvGetStringUTFLength (jstring STR)
     Returns the number of bytes required to encode the contents of the
     `String' STR in UTF-8.

 -- Function: jsize JvGetStringUTFRegion (jstring STR, jsize START,
          jsize LEN, char* BUF)
     Puts the UTF-8 encoding of a region of the `String' STR into the
     buffer `buf'.  The region to fetch is marked by START and LEN.

     Note that BUF is a buffer, not a C string.  It is _not_ null

File:,  Node: Mixing with C++,  Next: Exception Handling,  Prev: Strings,  Up: About CNI

11.13 Interoperating with C/C++

Because CNI is designed to represent Java classes and methods it cannot
be mixed readily with C/C++ types.

   One important restriction is that Java classes cannot have non-Java
type instance or static variables and cannot have methods which take
non-Java types as arguments or return non-Java types.

None of the following is possible with CNI:

     class ::MyClass : public java::lang::Object
        char* variable;  // char* is not a valid Java type.

     ::SomeClass::someMethod (char *arg)
     }   // `uint' is not a valid Java type, neither is `char*'

Of course, it is ok to use C/C++ types within the scope of a method:

     ::SomeClass::otherMethod (jstring str)
        char *arg = ...

11.13.1 RawData

The above restriction can be problematic, so CNI includes the
`gnu.gcj.RawData' class.  The `RawData' class is a "non-scanned
reference" type.  In other words variables declared of type `RawData'
can contain any data and are not checked by the compiler or memory
manager in any way.

   This means that you can put C/C++ data structures (including classes)
in your CNI classes, as long as you use the appropriate cast.

Here are some examples:

     class ::MyClass : public java::lang::Object
        gnu.gcj.RawData string;

        MyClass ();
        gnu.gcj.RawData getText ();
        void printText ();

     ::MyClass::MyClass ()
        char* text = ...
        string = text;

     ::MyClass::getText ()
        return string;

     ::MyClass::printText ()
       printf("%s\n", (char*) string);

11.13.2 RawDataManaged

`gnu.gcj.RawDataManaged' is another type used to indicate special data
used by native code. Unlike the `RawData' type, fields declared as
`RawDataManaged' will be "marked" by the memory manager and considered
for garbage collection.

   Native data which is allocated using CNI's `JvAllocBytes()' function
and stored in a `RawDataManaged' will be automatically freed when the
Java object it is associated with becomes unreachable.

11.13.3 Native memory allocation

 -- Function: void* JvAllocBytes (jsize SIZE)
     Allocates SIZE bytes from the heap.  The memory returned is zeroed.
     This memory is not scanned for pointers by the garbage collector,
     but will be freed if no references to it are discovered.

     This function can be useful if you need to associate some native
     data with a Java object. Using a CNI's special `RawDataManaged'
     type, native data allocated with `JvAllocBytes' will be
     automatically freed when the Java object itself becomes

11.13.4 Posix signals

On Posix based systems the `libgcj' library uses several signals
internally.  CNI code should not attempt to use the same signals as
doing so may cause `libgcj' and/or the CNI code to fail.

   SIGSEGV is used on many systems to generate `NullPointerExceptions'.
SIGCHLD is used internally by `Runtime.exec()'.  Several other signals
(that vary from platform to platform) can be used by the memory manager
and by `Thread.interrupt()'.

File:,  Node: Exception Handling,  Next: Synchronization,  Prev: Mixing with C++,  Up: About CNI

11.14 Exception Handling

While C++ and Java share a common exception handling framework, things
are not yet perfectly integrated.  The main issue is that the run-time
type information facilities of the two languages are not integrated.

   Still, things work fairly well.  You can throw a Java exception from
C++ using the ordinary `throw' construct, and this exception can be
caught by Java code.  Similarly, you can catch an exception thrown from
Java using the C++ `catch' construct.

Here is an example:

     if (i >= count)
        throw new java::lang::IndexOutOfBoundsException();

   Normally, G++ will automatically detect when you are writing C++
code that uses Java exceptions, and handle them appropriately.
However, if C++ code only needs to execute destructors when Java
exceptions are thrown through it, GCC will guess incorrectly.  Sample
problematic code:

     struct S { ~S(); };

     extern void bar();    // Is implemented in Java and may throw exceptions.

     void foo()
       S s;

   The usual effect of an incorrect guess is a link failure,
complaining of a missing routine called `__gxx_personality_v0'.

   You can inform the compiler that Java exceptions are to be used in a
translation unit, irrespective of what it might think, by writing
`#pragma GCC java_exceptions' at the head of the file.  This `#pragma'
must appear before any functions that throw or catch exceptions, or run
destructors when exceptions are thrown through them.

File:,  Node: Synchronization,  Next: Invocation,  Prev: Exception Handling,  Up: About CNI

11.15 Synchronization

Each Java object has an implicit monitor.  The Java VM uses the
instruction `monitorenter' to acquire and lock a monitor, and
`monitorexit' to release it.

   The corresponding CNI macros are `JvMonitorEnter' and
`JvMonitorExit' (JNI has similar  methods `MonitorEnter' and

   The Java source language does not provide direct access to these
primitives.  Instead, there is a `synchronized' statement that does an
implicit `monitorenter' before entry to the block, and does a
`monitorexit' on exit from the block.  Note that the lock has to be
released even when the block is abnormally terminated by an exception,
which means there is an implicit `try finally' surrounding
synchronization locks.

   From C++, it makes sense to use a destructor to release a lock.  CNI
defines the following utility class:

     class JvSynchronize() {
       jobject obj;
       JvSynchronize(jobject o) { obj = o; JvMonitorEnter(o); }
       ~JvSynchronize() { JvMonitorExit(obj); }

   So this Java code:

     synchronized (OBJ)

might become this C++ code:

        JvSynchronize dummy (OBJ);

   Java also has methods with the `synchronized' attribute.  This is
equivalent to wrapping the entire method body in a `synchronized'
statement.  (Alternatively, an implementation could require the caller
to do the synchronization.  This is not practical for a compiler,
because each virtual method call would have to test at run-time if
synchronization is needed.)  Since in `gcj' the `synchronized'
attribute is handled by the method implementation, it is up to the
programmer of a synchronized native method to handle the synchronization
(in the C++ implementation of the method).  In other words, you need to
manually add `JvSynchronize' in a `native synchronized' method.

File:,  Node: Invocation,  Next: Reflection,  Prev: Synchronization,  Up: About CNI

11.16 Invocation

CNI permits C++ applications to make calls into Java classes, in
addition to allowing Java code to call into C++. Several functions,
known as the "invocation API", are provided to support this.

 -- Function: jint JvCreateJavaVM (JvVMInitArgs* VM_ARGS)
     Initializes the Java runtime. This function performs essential
     initialization of the threads interface, garbage collector,
     exception handling and other key aspects of the runtime. It must
     be called once by an application with a non-Java `main()'
     function, before any other Java or CNI calls are made.  It is
     safe, but not recommended, to call `JvCreateJavaVM()' more than
     once provided it is only called from a single thread.  The VMARGS
     parameter can be used to specify initialization parameters for the
     Java runtime. It may be `NULL'.

     JvVMInitArgs represents a list of virtual machine initialization
     arguments. `JvCreateJavaVM()' ignores the version field.

          typedef struct JvVMOption
            // a VM initialization option
            char* optionString;
            // extra information associated with this option
            void* extraInfo;
          } JvVMOption;

          typedef struct JvVMInitArgs
            // for compatibility with JavaVMInitArgs
            jint version;

            // number of VM initialization options
            jint nOptions;

            // an array of VM initialization options
            JvVMOption* options;

            // true if the option parser should ignore unrecognized options
            jboolean ignoreUnrecognized;
          } JvVMInitArgs;

     `JvCreateJavaVM()' returns `0' upon success, or `-1' if the
     runtime is already initialized.

     _Note:_ In GCJ 3.1, the `vm_args' parameter is ignored. It is
     recognized and used as of release 4.0.

 -- Function: java::lang::Thread* JvAttachCurrentThread (jstring NAME,
          java::lang::ThreadGroup* GROUP)
     Registers an existing thread with the Java runtime.  This must be
     called once from each thread, before that thread makes any other
     Java or CNI calls. It must be called after `JvCreateJavaVM'.  NAME
     specifies a name for the thread. It may be `NULL', in which case a
     name will be generated.  GROUP is the ThreadGroup in which this
     thread will be a member. If it is `NULL', the thread will be a
     member of the main thread group.  The return value is the Java
     `Thread' object that represents the thread.  It is safe to call
     `JvAttachCurrentThread()' more than once from the same thread. If
     the thread is already attached, the call is ignored and the current
     thread object is returned.

 -- Function: jint JvDetachCurrentThread ()
     Unregisters a thread from the Java runtime. This should be called
     by threads that were attached using `JvAttachCurrentThread()',
     after they have finished making calls to Java code. This ensures
     that any resources associated with the thread become eligible for
     garbage collection.  This function returns `0' upon success, or
     `-1' if the current thread is not attached.

11.16.1 Handling uncaught exceptions

If an exception is thrown from Java code called using the invocation
API, and no handler for the exception can be found, the runtime will
abort the application. In order to make the application more robust, it
is recommended that code which uses the invocation API be wrapped by a
top-level try/catch block that catches all Java exceptions.

11.16.2 Example

The following code demonstrates the use of the invocation API. In this
example, the C++ application initializes the Java runtime and attaches
itself. The `java.lang.System' class is initialized in order to access
its `out' field, and a Java string is printed. Finally, the thread is
detached from the runtime once it has finished making Java calls.
Everything is wrapped with a try/catch block to provide a default
handler for any uncaught exceptions.

   The example can be compiled with `c++ -c; gcj test.o'.

     #include <gcj/cni.h>
     #include <java/lang/System.h>
     #include <java/io/PrintStream.h>
     #include <java/lang/Throwable.h>

     int main(int argc, char *argv[])
       using namespace java::lang;

         JvAttachCurrentThread(NULL, NULL);

         String *message = JvNewStringLatin1("Hello from C++");

       catch (Throwable *t)
         System::err->println(JvNewStringLatin1("Unhandled Java exception:"));

File:,  Node: Reflection,  Prev: Invocation,  Up: About CNI

11.17 Reflection

Reflection is possible with CNI code, it functions similarly to how it
functions with JNI.

   The types `jfieldID' and `jmethodID' are as in JNI.

The functions:

   * `JvFromReflectedField',

   * `JvFromReflectedMethod',

   * `JvToReflectedField'

   * `JvToFromReflectedMethod'

will be added shortly, as will other functions corresponding to JNI.

File:,  Node: System properties,  Next: Resources,  Prev: About CNI,  Up: Top

12 System properties

The runtime behavior of the `libgcj' library can be modified by setting
certain system properties.  These properties can be compiled into the
program using the `-DNAME[=VALUE]' option to `gcj' or by setting them
explicitly in the program by calling the
`java.lang.System.setProperty()' method.  Some system properties are
only used for informational purposes (like giving a version number or a
user name).  A program can inspect the current value of a property by
calling the `java.lang.System.getProperty()' method.

* Menu:

* Standard Properties::         Standard properties supported by `libgcj'
* GNU Classpath Properties::    Properties found in Classpath based libraries
* libgcj Runtime Properties::   Properties specific to `libgcj'

File:,  Node: Standard Properties,  Next: GNU Classpath Properties,  Up: System properties

12.1 Standard Properties

The following properties are normally found in all implementations of
the core libraries for the Java language.

     The `libgcj' version number.

     Set to `The Free Software Foundation, Inc.'

     Set to `'.

     The directory where `gcj' was installed.  Taken from the `--prefix'
     option given to `configure'.

     The class format version number supported by the libgcj byte code
     interpreter.  (Currently `46.0')

     The Virtual Machine Specification version implemented by `libgcj'.
     (Currently `1.0')

     The name of the Virtual Machine specification designer.

     The name of the Virtual Machine specification (Set to `Java
     Virtual Machine Specification').

     The `gcj' version number.

     Set to `The Free Software Foundation, Inc.'

     Set to `GNU libgcj'.

     The Runtime Environment specification version implemented by
     `libgcj'.  (Currently set to `1.3')

     The Runtime Environment specification designer.

     The name of the Runtime Environment specification (Set to `Java
     Platform API Specification').

     The paths (jar files, zip files and directories) used for finding
     class files.

     Directory path used for finding native libraries.

     The directory used to put temporary files in.

     Name of the Just In Time compiler to use by the byte code
     interpreter.  Currently not used in `libgcj'.

     Directories containing jar files with extra libraries.  Will be
     used when resolving classes.

     A `|' separated list of package names that is used to find classes
     that implement handlers for `'.

     A list of URLs that is used by the `java.rmi.server.RMIClassLoader'
     to load classes from.

     A list of class names that will be loaded by the
     `java.sql.DriverManager' when it starts up.

     The separator used in when directories are included in a filename
     (normally `/' or `\' ).

     The default character encoding used when converting platform
     native files to Unicode (usually set to `8859_1').

     The standard separator used when a string contains multiple paths
     (normally `:' or `;'), the string is usually not a valid character
     to use in normal directory names.)

     The default line separator used on the platform (normally `\n',
     `\r' or a combination of those two characters).

     The class name used for the default policy provider returned by

     The name of the user running the program.  Can be the full name,
     the login name or empty if unknown.

     The default directory to put user specific files in.

     The current working directory from which the program was started.

     The default language as used by the `java.util.Locale' class.

     The default region as used by the `java.util.Local' class.

     The default variant of the language and region local used.

     The default timezone as used by the `java.util.TimeZone' class.

     The operating system/kernel name that the program runs on.

     The hardware that we are running on.

     The version number of the operating system/kernel.

     The string to display when an untrusted applet is displayed.
     Returned by `java.awt.Window.getWarningString()' when the window is

     The class name used for initializing the default
     `java.awt.Toolkit'.  Defaults to `gnu.awt.gtk.GtkToolkit'.

     Name of proxy host for http connections.

     Port number to use when a proxy host is in use.

File:,  Node: GNU Classpath Properties,  Next: libgcj Runtime Properties,  Prev: Standard Properties,  Up: System properties

12.2 GNU Classpath Properties

`libgcj' is based on the GNU Classpath (Essential Libraries for Java) a
GNU project to create free core class libraries for use with virtual
machines and compilers for the Java language.  The following properties
are common to libraries based on GNU Classpath.

     Enables printing serialization debugging by the
     `' and `' classes when set
     to something else then the empty string.  Only used when running a
     debug build of the library.

     This is a succinct name of the virtual machine.  For `libgcj',
     this will always be `libgcj'.

     A base URL used for finding system property files (e.g.,
     `').  By default this is a `file:' URL pointing
     to the `lib' directory under `java.home'.

File:,  Node: libgcj Runtime Properties,  Prev: GNU Classpath Properties,  Up: System properties

12.3 libgcj Runtime Properties

The following properties are specific to the `libgcj' runtime and will
normally not be found in other core libraries for the java language.

     The combination of `' and `java.vm.version'.

     Same as `java.fullversion'.

     Used by the `' class when set to something
     else then the empty string.  When set all newly created
     `DatagramSocket's will try to load a class
     `[impl.prefix]DatagramSocketImpl' instead of the normal

     The class or binary name that was used to invoke the program. This
     will be the name of the "main" class in the case where the `gij'
     front end is used, or the program binary name in the case where an
     application is compiled to a native binary.

     The real name of the user, as taken from the password file.  This
     may not always hold only the user's name (as some sites put extra
     information in this field).  Also, this property is not available
     on all platforms.

     Whether an external process, `addr2line', should be used to
     determine line number information when tracing the stack. Setting
     this to `false' may suppress line numbers when printing stack
     traces and when using the java.util.logging infrastructure.
     However, performance may improve significantly for applications
     that print stack traces or make logging calls frequently.

     Whether the address of a stack frame should be printed when the
     line number is unavailable. Setting this to `true' will cause the
     name of the object and the offset within that object to be printed
     when no line number is available.  This allows for off-line
     decoding of stack traces if necessary debug information is
     available.  The default is `false', no raw addresses are printed.

     Whether stack frames for non-java code should be included in a
     stack trace.  The default value is `true', stack frames for
     non-java code are suppressed.  Setting this to `false' will cause
     any non-java stack frames to be printed in addition to frames for
     the java code.

     This controls how shared libraries are automatically loaded by the
     built-in class loader.  If this property is set to `full', a full
     search is done for each requested class.  If this property is set
     to `cache', then any failed lookups are cached and not tried again.
     If this property is set to `never' (the default), then lookups are
     never done.  For more information, *Note Extensions::.

     This is like the standard `java.endorsed.dirs', property, but
     specifies some extra directories which are searched after the
     standard endorsed directories.  This is primarily useful for
     telling `libgcj' about additional libraries which are ordinarily
     incorporated into the JDK, and which should be loaded by the
     bootstrap class loader, but which are not yet part of `libgcj'
     itself for some reason.

     This is the full path to `gcj' executable which should be used to
     compile classes just-in-time when `ClassLoader.defineClass' is
     called.  If not set, `gcj' will not be invoked by the runtime;
     this can also be controlled via `Compiler.disable'.

     This is a space-separated string of options which should be passed
     to `gcj' when in JIT mode.  If not set, a sensible default is

     This is the directory where cached shared library files are
     stored.  If not set, JIT compilation is disabled.  This should
     never be set to a directory that is writable by any other user.

     This is a sequence of file names, each referring to a file created
     by `gcj-dbtool'.  These files will be used by `libgcj' to find
     shared libraries corresponding to classes that are loaded from
     bytecode.  `libgcj' often has a built-in default database; it can
     be queried using `gcj-dbtool -p'.

File:,  Node: Resources,  Next: Index,  Prev: System properties,  Up: Top

13 Resources

While writing `gcj' and `libgcj' we have, of course, relied heavily on
documentation from Sun Microsystems.  In particular we have used The
Java Language Specification (both first and second editions), the Java
Class Libraries (volumes one and two), and the Java Virtual Machine
Specification.  In addition we've used the online documentation at

   The current `gcj' home page is `'.

   For more information on gcc, see `'.

   Some `libgcj' testing is done using the Mauve test suite.  This is a
free software Java class library test suite which is being written
because the JCK is not free.  See `'
for more information.

File:,  Node: Index,  Prev: Resources,  Up: Top


* Menu:

* class path:                            Input Options.        (line  6)
* class$:                                Reference types.      (line 20)
* elements on template<class T>:         Arrays.               (line 46)
* FDL, GNU Free Documentation License:   GNU Free Documentation License.
                                                               (line  6)
* GCJ_PROPERTIES:                        Extensions.           (line 56)
* jclass:                                Reference types.      (line 16)
* jobject:                               Reference types.      (line 16)
* jstring:                               Reference types.      (line 16)
* JvAllocBytes:                          Mixing with C++.      (line 99)
* JvAttachCurrentThread:                 Invocation.           (line 55)
* JvCreateJavaVM:                        Invocation.           (line 11)
* JvDetachCurrentThread:                 Invocation.           (line 68)
* JvFree:                                Memory allocation.    (line 19)
* JvGetArrayLength:                      Arrays.               (line 86)
* JvGetStringChars:                      Strings.              (line 25)
* JvGetStringUTFLength:                  Strings.              (line 29)
* JvGetStringUTFRegion:                  Strings.              (line 34)
* JvMalloc:                              Memory allocation.    (line 11)
* JvNewBooleanArray:                     Arrays.               (line 83)
* JvNewObjectArray:                      Arrays.               (line 57)
* JvNewString:                           Strings.              (line 11)
* JvNewStringLatin1:                     Strings.              (line 15)
* JvNewStringUTF:                        Strings.              (line 21)
* JvPrimClass:                           Primitive types.      (line 36)
* JvRealloc:                             Memory allocation.    (line 15)