1 \input texinfo @c -*-texinfo-*-
3 @settitle Guide to GNU gcj
5 @include gcc-common.texi
7 @c Note: When reading this manual you'll find lots of strange
8 @c circumlocutions like ``compiler for the Java language''.
9 @c This is necessary due to Sun's restrictions on the use of
12 @c When this manual is copyrighted.
13 @set copyrights-gcj 2001, 2002
16 @set which-gcj GCC-@value{version-GCC}
19 @c man begin COPYRIGHT
20 Copyright @copyright{} @value{copyrights-gcj} Free Software Foundation, Inc.
22 Permission is granted to copy, distribute and/or modify this document
23 under the terms of the GNU Free Documentation License, Version 1.2 or
24 any later version published by the Free Software Foundation; with the
25 Invariant Sections being ``GNU General Public License'', the Front-Cover
26 texts being (a) (see below), and with the Back-Cover Texts being (b)
27 (see below). A copy of the license is included in the
30 ``GNU Free Documentation License''.
32 @c man begin COPYRIGHT
37 @c man begin COPYRIGHT
39 (a) The FSF's Front-Cover Text is:
43 (b) The FSF's Back-Cover Text is:
45 You have freedom to copy and modify this GNU Manual, like GNU
46 software. Copies published by the Free Software Foundation raise
47 funds for GNU development.
53 @dircategory Programming
55 * Gcj: (gcj). Ahead-of-time compiler for the Java language
58 @dircategory Individual utilities
60 * gcjh: (gcj)Invoking gcjh.
61 Generate header files from Java class files
62 * jv-scan: (gcj)Invoking jv-scan.
63 Print information about Java source files
64 * jcf-dump: (gcj)Invoking jcf-dump.
65 Print information about Java class files
66 * gij: (gcj)Invoking gij. GNU interpreter for Java bytecode
67 * jv-convert: (gcj)Invoking jv-convert.
68 Convert file from one encoding to another
69 * grmic: (gcj)Invoking grmic.
70 Generate stubs for Remote Method Invocation.
71 * grmiregistry: (gcj)Invoking grmiregistry.
72 The remote object registry.
84 @vskip 0pt plus 1filll
85 For the @value{which-gcj} Version*
87 Published by the Free Software Foundation @*
88 59 Temple Place - Suite 330@*
89 Boston, MA 02111-1307, USA@*
100 This manual describes how to use @command{gcj}, the GNU compiler for the
101 Java programming language. @command{gcj} can generate both @file{.class}
102 files and object files, and it can read both Java source code and
106 * Copying:: The GNU General Public License
107 * GNU Free Documentation License::
108 How you can share and copy this manual
109 * Invoking gcj:: Compiler options supported by @command{gcj}
110 * Compatibility:: Compatibility between gcj and other tools for Java
111 * Invoking gcjh:: Generate header files from class files
112 * Invoking jv-scan:: Print information about source files
113 * Invoking jcf-dump:: Print information about class files
114 * Invoking gij:: Interpreting Java bytecodes
115 * Invoking jv-convert:: Converting from one encoding to another
116 * Invoking grmic:: Generate stubs for Remote Method Invocation.
117 * Invoking grmiregistry:: The remote object registry.
118 * About CNI:: Description of the Compiled Native Interface
119 * System properties:: Modifying runtime behavior of the libgcj library
120 * Resources:: Where to look for more information
130 @chapter Invoking gcj
132 @c man title gcj Ahead-of-time compiler for the Java language
135 @c man begin SYNOPSIS gcj
136 gcj [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
137 [@option{--CLASSPATH}=@var{path}] [@option{--classpath}=@var{path}]
138 [@option{-f}@var{option}@dots{}] [@option{--encoding}=@var{name}]
139 [@option{--main}=@var{classname}] [@option{-D}@var{name}[=@var{value}]@dots{}]
140 [@option{-C}] [@option{--resource} @var{resource-name}] [@option{-d} @var{directory}]
141 [@option{-W}@var{warn}@dots{}]
142 @var{sourcefile}@dots{}
144 @c man begin SEEALSO gcj
145 gcc(1), gcjh(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
146 and the Info entries for @file{gcj} and @file{gcc}.
150 @c man begin DESCRIPTION gcj
152 As @command{gcj} is just another front end to @command{gcc}, it supports many
153 of the same options as gcc. @xref{Option Summary, , Option Summary,
154 gcc, Using the GNU Compiler Collection (GCC)}. This manual only documents the
155 options specific to @command{gcj}.
160 * Input and output files::
161 * Input Options:: How gcj finds files
162 * Encodings:: Options controlling source file encoding
163 * Warnings:: Options controlling warnings specific to gcj
164 * Linking:: Options for making an executable
165 * Code Generation:: Options controlling the output of gcj
166 * Configure-time Options:: Options you won't use
169 @c man begin OPTIONS gcj
171 @node Input and output files
172 @section Input and output files
174 A @command{gcj} command is like a @command{gcc} command, in that it
175 consists of a number of options and file names. The following kinds
176 of input file names are supported:
179 @item @var{file}.java
181 @item @var{file}.class
184 @itemx @var{file}.jar
185 An archive containing one or more @code{.class} files, all of
186 which are compiled. The archive may be compressed. Files in
187 an archive which don't end with @samp{.class} are treated as
188 resource files; they are compiled into the resulting object file
189 as @samp{core:} URLs.
191 A file containing a whitespace-separated list of input file names.
192 (Currently, these must all be @code{.java} source files, but that
194 Each named file is compiled, just as if it had been on the command line.
195 @item @var{library}.a
196 @itemx @var{library}.so
197 @itemx -l@var{libname}
198 Libraries to use when linking. See the @command{gcc} manual.
201 You can specify more than one input file on the @command{gcj} command line,
202 in which case they will all be compiled. If you specify a
203 @code{-o @var{FILENAME}}
204 option, all the input files will be compiled together, producing a
205 single output file, named @var{FILENAME}.
206 This is allowed even when using @code{-S} or @code{-c},
207 but not when using @code{-C} or @code{--resource}.
208 (This is an extension beyond the what plain @command{gcc} allows.)
209 (If more than one input file is specified, all must currently
210 be @code{.java} files, though we hope to fix this.)
213 @section Input Options
217 @command{gcj} has options to control where it looks to find files it needs.
218 For instance, @command{gcj} might need to load a class that is referenced
219 by the file it has been asked to compile. Like other compilers for the
220 Java language, @command{gcj} has a notion of a @dfn{class path}. There are
221 several options and environment variables which can be used to
222 manipulate the class path. When @command{gcj} looks for a given class, it
223 searches the class path looking for matching @file{.class} or
224 @file{.java} file. @command{gcj} comes with a built-in class path which
225 points at the installed @file{libgcj.jar}, a file which contains all the
228 In the below, a directory or path component can refer either to an
229 actual directory on the filesystem, or to a @file{.zip} or @file{.jar}
230 file, which @command{gcj} will search as if it is a directory.
234 All directories specified by @code{-I} are kept in order and prepended
235 to the class path constructed from all the other options. Unless
236 compatibility with tools like @code{javac} is important, we recommend
237 always using @code{-I} instead of the other options for manipulating the
240 @item --classpath=@var{path}
241 This sets the class path to @var{path}, a colon-separated list of paths
242 (on Windows-based systems, a semicolon-separate list of paths).
243 This does not override the builtin (``boot'') search path.
245 @item --CLASSPATH=@var{path}
246 Deprecated synonym for @code{--classpath}.
248 @item --bootclasspath=@var{path}
249 Where to find the standard builtin classes, such as @code{java.lang.String}.
251 @item --extdirs=@var{path}
252 For each directory in the @var{path}, place the contents of that
253 directory at the end of the class path.
256 This is an environment variable which holds a list of paths.
259 The final class path is constructed like so:
263 First come all directories specified via @code{-I}.
266 If @option{--classpath} is specified, its value is appended.
267 Otherwise, if the @code{CLASSPATH} environment variable is specified,
268 then its value is appended.
269 Otherwise, the current directory (@code{"."}) is appended.
272 If @code{--bootclasspath} was specified, append its value.
273 Otherwise, append the built-in system directory, @file{libgcj.jar}.
276 Finally, if @code{--extdirs} was specified, append the contents of the
277 specified directories at the end of the class path. Otherwise, append
278 the contents of the built-in extdirs at @code{$(prefix)/share/java/ext}.
281 The classfile built by @command{gcj} for the class @code{java.lang.Object}
282 (and placed in @code{libgcj.jar}) contains a special zero length
283 attribute @code{gnu.gcj.gcj-compiled}. The compiler looks for this
284 attribute when loading @code{java.lang.Object} and will report an error
285 if it isn't found, unless it compiles to bytecode (the option
286 @code{-fforce-classes-archive-check} can be used to override this
287 behavior in this particular case.)
290 @item -fforce-classes-archive-check
291 This forces the compiler to always check for the special zero length
292 attribute @code{gnu.gcj.gcj-compiled} in @code{java.lang.Object} and
293 issue an error if it isn't found.
299 The Java programming language uses Unicode throughout. In an effort to
300 integrate well with other locales, @command{gcj} allows @file{.java} files
301 to be written using almost any encoding. @command{gcj} knows how to
302 convert these encodings into its internal encoding at compile time.
304 You can use the @code{--encoding=@var{NAME}} option to specify an
305 encoding (of a particular character set) to use for source files. If
306 this is not specified, the default encoding comes from your current
307 locale. If your host system has insufficient locale support, then
308 @command{gcj} assumes the default encoding to be the @samp{UTF-8} encoding
311 To implement @code{--encoding}, @command{gcj} simply uses the host
312 platform's @code{iconv} conversion routine. This means that in practice
313 @command{gcj} is limited by the capabilities of the host platform.
315 The names allowed for the argument @code{--encoding} vary from platform
316 to platform (since they are not standardized anywhere). However,
317 @command{gcj} implements the encoding named @samp{UTF-8} internally, so if
318 you choose to use this for your source files you can be assured that it
319 will work on every host.
325 @command{gcj} implements several warnings. As with other generic
326 @command{gcc} warnings, if an option of the form @code{-Wfoo} enables a
327 warning, then @code{-Wno-foo} will disable it. Here we've chosen to
328 document the form of the warning which will have an effect -- the
329 default being the opposite of what is listed.
332 @item -Wredundant-modifiers
333 With this flag, @command{gcj} will warn about redundant modifiers. For
334 instance, it will warn if an interface method is declared @code{public}.
336 @item -Wextraneous-semicolon
337 This causes @command{gcj} to warn about empty statements. Empty statements
338 have been deprecated.
340 @item -Wno-out-of-date
341 This option will cause @command{gcj} not to warn when a source file is
342 newer than its matching class file. By default @command{gcj} will warn
345 @item -Wno-deprecated
346 Warn if a deprecated class, method, or field is referred to.
349 This is the same as @command{gcc}'s @code{-Wunused}.
352 This is the same as @code{-Wredundant-modifiers -Wextraneous-semicolon
360 To turn a Java application into an executable program,
361 you need to link it with the needed libraries, just as for C or C++.
362 The linker by default looks for a global function named @code{main}.
363 Since Java does not have global functions, and a
364 collection of Java classes may have more than one class with a
365 @code{main} method, you need to let the linker know which of those
366 @code{main} methods it should invoke when starting the application.
367 You can do that in any of these ways:
371 Specify the class containing the desired @code{main} method
372 when you link the application, using the @code{--main} flag,
375 Link the Java package(s) into a shared library (dll) rather than an
376 executable. Then invoke the application using the @code{gij} program,
377 making sure that @code{gij} can find the libraries it needs.
379 Link the Java packages(s) with the flag @code{-lgij}, which links
380 in the @code{main} routine from the @code{gij} command.
381 This allows you to select the class whose @code{main} method you
382 want to run when you run the application. You can also use
383 other @code{gij} flags, such as @code{-D} flags to set properties.
384 Using the @code{-lgij} library (rather than the @code{gij} program
385 of the previous mechanism) has some advantages: it is compatible with
386 static linking, and does not require configuring or installing libraries.
389 These @code{gij} options relate to linking an executable:
392 @item --main=@var{CLASSNAME}
393 This option is used when linking to specify the name of the class whose
394 @code{main} method should be invoked when the resulting executable is
397 @item -D@var{name}[=@var{value}]
398 This option can only be used with @code{--main}. It defines a system
399 property named @var{name} with value @var{value}. If @var{value} is not
400 specified then it defaults to the empty string. These system properties
401 are initialized at the program's startup and can be retrieved at runtime
402 using the @code{java.lang.System.getProperty} method.
405 Create an application whose command-line processing is that
406 of the @code{gij} command.
408 This option is an alternative to using @code{--main}; you cannot use both.
411 @node Code Generation
412 @section Code Generation
414 In addition to the many @command{gcc} options controlling code generation,
415 @command{gcj} has several options specific to itself.
420 This option is used to tell @command{gcj} to generate bytecode
421 (@file{.class} files) rather than object code.
423 @item --resource @var{resource-name}
424 This option is used to tell @command{gcj} to compile the contents of a
425 given file to object code so it may be accessed at runtime with the core
426 protocol handler as @samp{core:/@var{resource-name}}. Note that
427 @var{resource-name} is the name of the resource as found at runtime; for
428 instance, it could be used in a call to @code{ResourceBundle.getBundle}.
429 The actual file name to be compiled this way must be specified
432 @item -d @var{directory}
433 When used with @code{-C}, this causes all generated @file{.class} files
434 to be put in the appropriate subdirectory of @var{directory}. By
435 default they will be put in subdirectories of the current working
438 @item -fno-bounds-check
439 By default, @command{gcj} generates code which checks the bounds of all
440 array indexing operations. With this option, these checks are omitted, which
441 can improve performance for code that uses arrays extensively. Note that this
442 can result in unpredictable behavior if the code in question actually does
443 violate array bounds constraints. It is safe to use this option if you are
444 sure that your code will never throw an @code{ArrayIndexOutOfBoundsException}.
446 @item -fno-store-check
447 Don't generate array store checks. When storing objects into arrays, a runtime
448 check is normally generated in order to ensure that the object is assignment
449 compatible with the component type of the array (which may not be known
450 at compile-time). With this option, these checks are omitted. This can
451 improve performance for code which stores objects into arrays frequently.
452 It is safe to use this option if you are sure your code will never throw an
453 @code{ArrayStoreException}.
456 With @command{gcj} there are two options for writing native methods: CNI
457 and JNI@. By default @command{gcj} assumes you are using CNI@. If you are
458 compiling a class with native methods, and these methods are implemented
459 using JNI, then you must use @code{-fjni}. This option causes
460 @command{gcj} to generate stubs which will invoke the underlying JNI
464 Don't recognize the @code{assert} keyword. This is for compatibility
465 with older versions of the language specification.
467 @item -fno-optimize-static-class-initialization
468 When the optimization level is greater or equal to @code{-O2},
469 @command{gcj} will try to optimize the way calls into the runtime are made
470 to initialize static classes upon their first use (this optimization
471 isn't carried out if @code{-C} was specified.) When compiling to native
472 code, @code{-fno-optimize-static-class-initialization} will turn this
473 optimization off, regardless of the optimization level in use.
475 @item --disable-assertions[=@var{class-or-package}]
476 Don't include code for checking assertions in the compiled code.
477 If @code{=@var{class-or-package}} is missing disables assertion code
478 generation for all classes, unless overridden by a more
479 specific @code{--enable-assertions} flag.
480 If @var{class-or-package} is a class name, only disables generating
481 assertion checks within the named class or its inner classes.
482 If @var{class-or-package} is a package name, disables generating
483 assertion checks within the named package or a subpackage.
485 By default, assertions are enabled when generating class files
486 or when not optimizing, and disabled when generating optimized binaries.
488 @item --enable-assertions[=@var{class-or-package}]
489 Generates code to check assertions. The option is perhaps misnamed,
490 as you still need to turn on assertion checking at run-time,
491 and we don't support any easy way to do that.
492 So this flag isn't very useful yet, except to partially override
493 @code{--disable-assertions}.
498 @node Configure-time Options
499 @section Configure-time Options
501 Some @command{gcj} code generations options affect the resulting ABI, and
502 so can only be meaningfully given when @code{libgcj}, the runtime
503 package, is configured. @code{libgcj} puts the appropriate options from
504 this group into a @samp{spec} file which is read by @command{gcj}. These
505 options are listed here for completeness; if you are using @code{libgcj}
506 then you won't want to touch these options.
510 This enables the use of the Boehm GC bitmap marking code. In particular
511 this causes @command{gcj} to put an object marking descriptor into each
514 @item -fhash-synchronization
515 By default, synchronization data (the data used for @code{synchronize},
516 @code{wait}, and @code{notify}) is pointed to by a word in each object.
517 With this option @command{gcj} assumes that this information is stored in a
518 hash table and not in the object itself.
520 @item -fuse-divide-subroutine
521 On some systems, a library routine is called to perform integer
522 division. This is required to get exception handling correct when
525 @item -fcheck-references
526 On some systems it's necessary to insert inline checks whenever
527 accessing an object via a reference. On other systems you won't need
528 this because null pointer accesses are caught automatically by the
535 @chapter Compatibility with the Java Platform
537 As we believe it is important that the Java platform not be fragmented,
538 @command{gcj} and @code{libgcj} try to conform to the relevant Java
539 specifications. However, limited manpower and incomplete and unclear
540 documentation work against us. So, there are caveats to using
549 @section Standard features not yet supported
551 This list of compatibility issues is by no means complete.
555 @command{gcj} implements the JDK 1.2 language. It supports inner classes
556 and the new 1.4 @code{assert} keyword. It does not yet support the Java 2
557 @code{strictfp} keyword (it recognizes the keyword but ignores it).
560 @code{libgcj} is largely compatible with the JDK 1.2 libraries.
561 However, @code{libgcj} is missing many packages, most notably
562 @code{java.awt}. There are also individual missing classes and methods.
563 We currently do not have a list showing differences between
564 @code{libgcj} and the Java 2 platform.
567 Sometimes the @code{libgcj} implementation of a method or class differs
568 from the JDK implementation. This is not always a bug. Still, if it
569 affects you, it probably makes sense to report it so that we can discuss
570 the appropriate response.
573 @command{gcj} does not currently allow for piecemeal replacement of
574 components within @code{libgcj}. Unfortunately, programmers often want
575 to use newer versions of certain packages, such as those provided by
576 the Apache Software Foundation's Jakarta project. This has forced us
577 to place the @code{org.w3c.dom} and @code{org.xml.sax} packages into
578 their own libraries, separate from @code{libgcj}. If you intend to
579 use these classes, you must link them explicitly with
580 @code{-l-org-w3c-dom} and @code{-l-org-xml-sax}. Future versions of
581 @command{gcj} may not have this restriction.
585 @section Extra features unique to gcj
587 The main feature of @command{gcj} is that it can compile programs written in
588 the Java programming language to native code. Most extensions that have been
589 added are to facilitate this functionality.
593 @command{gcj} makes it easy and efficient to mix code written in Java and C++.
594 @xref{About CNI}, for more info on how to use this in your programs.
597 When you compile your classes into a shared library they can be automatically
598 loaded by the @code{libgcj} system classloader. When trying to load a class
599 @code{gnu.pkg.SomeClass} the system classloader will first try to load the
600 shared library @file{lib-gnu-pkg-SomeClass.so}, if that fails to load the
601 class then it will try to load @file{lib-gnu-pkg.so} and finally when the
602 class is still not loaded it will try to load @file{lib-gnu.so}. Note that
603 all @samp{.}s will be transformed into @samp{-}s and that searching
604 for inner classes starts with their outermost outer class. If the class
605 cannot be found this way the system classloader tries to use
606 the @code{libgcj} bytecode interpreter to load the class from the standard
607 classpath. This process can be controlled to some degree via the
608 @code{gnu.gcj.runtime.VMClassLoader.library_control} property;
609 @xref{libgcj Runtime Properties}.
612 @code{libgcj} includes a special @samp{gcjlib} URL type. A URL of
613 this form is like a @code{jar} URL, and looks like
614 @samp{gcjlib:/path/to/shared/library.so!/path/to/resource}. An access
615 to one of these URLs causes the shared library to be @code{dlopen()}d,
616 and then the resource is looked for in that library. These URLs are
617 most useful when used in conjunction with @code{java.net.URLClassLoader}.
618 Note that, due to implementation limitations, currently any such URL
619 can be accessed by only one class loader, and libraries are never
620 unloaded. This means some care must be exercised to make sure that
621 a @code{gcjlib} URL is not accessed by more than one class loader at once.
622 In a future release this limitation will be lifted, and such
623 libraries will be mapped privately.
626 A program compiled by @command{gcj} will examine the
627 @env{GCJ_PROPERTIES} environment variable and change its behavior in
628 some ways. In particular @env{GCJ_PROPERTIES} holds a list of
629 assignments to global properties, such as would be set with the
630 @option{-D} option to @command{java}. For instance,
631 @samp{java.compiler=gcj} is a valid (but currently meaningless)
633 @cindex GCJ_PROPERTIES
634 @vindex GCJ_PROPERTIES
640 @chapter Invoking gcjh
642 @c man title gcjh generate header files from Java class files
644 @c man begin DESCRIPTION gcjh
646 The @code{gcjh} program is used to generate header files from class
647 files. It can generate both CNI and JNI header files, as well as stub
648 implementation files which can be used as a basis for implementing the
649 required native methods.
654 @c man begin SYNOPSIS gcjh
655 gcjh [@option{-stubs}] [@option{-jni}]
656 [@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
657 [@option{-preprend} @var{text}]
658 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
659 [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
660 [@option{-o} @var{file}] [@option{-td} @var{dir}]
661 [@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
662 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
663 @var{classname}@dots{}
665 @c man begin SEEALSO gcjh
666 gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
667 and the Info entries for @file{gcj} and @file{gcc}.
671 @c man begin OPTIONS gcjh
675 This causes @code{gcjh} to generate stub files instead of header files.
676 By default the stub file will be named after the class, with a suffix of
677 @samp{.cc}. In JNI mode, the default output file will have the suffix
681 This tells @code{gcjh} to generate a JNI header or stub. By default,
682 CNI headers are generated.
684 @item -add @var{text}
685 Inserts @var{text} into the class body. This is ignored in JNI mode.
687 @item -append @var{text}
688 Inserts @var{text} into the header file after the class declaration.
689 This is ignored in JNI mode.
691 @item -friend @var{text}
692 Inserts @var{text} into the class as a @code{friend} declaration.
693 This is ignored in JNI mode.
695 @item -prepend @var{text}
696 Inserts @var{text} into the header file before the class declaration.
697 This is ignored in JNI mode.
699 @item --classpath=@var{path}
700 @itemx --CLASSPATH=@var{path}
701 @itemx -I@var{directory}
702 @itemx -d @var{directory}
704 These options are all identical to the corresponding @command{gcj} options.
707 Sets the output file name. This cannot be used if there is more than
708 one class on the command line.
710 @item -td @var{directory}
711 Sets the name of the directory to use for temporary files.
714 Print all dependencies to stdout; suppress ordinary output.
717 Print non-system dependencies to stdout; suppress ordinary output.
720 Print all dependencies to stdout.
723 Print non-system dependencies to stdout.
726 Print help about @code{gcjh} and exit. No further processing is done.
729 Print version information for @code{gcjh} and exit. No further
733 Print extra information while running.
736 All remaining options are considered to be names of classes.
740 @node Invoking jv-scan
741 @chapter Invoking jv-scan
743 @c man title jv-scan print information about Java source file
745 @c man begin DESCRIPTION jv-scan
747 The @code{jv-scan} program can be used to print information about a Java
748 source file (@file{.java} file).
753 @c man begin SYNOPSIS jv-scan
754 jv-scan [@option{--no-assert}] [@option{--complexity}]
755 [@option{--encoding}=@var{name}] [@option{--print-main}]
756 [@option{--list-class}] [@option{--list-filename}]
757 [@option{--version}] [@option{--help}]
758 [@option{-o} @var{file}] @var{inputfile}@dots{}
760 @c man begin SEEALSO jv-scan
761 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
762 and the Info entries for @file{gcj} and @file{gcc}.
766 @c man begin OPTIONS jv-scan
770 Don't recognize the @code{assert} keyword, for backwards compatibility
771 with older versions of the language specification.
774 This prints a complexity measure, related to cyclomatic complexity, for
777 @item --encoding=@var{name}
778 This works like the corresponding @command{gcj} option.
781 This prints the name of the class in this file containing a @code{main}
785 This lists the names of all classes defined in the input files.
787 @item --list-filename
788 If @code{--list-class} is given, this option causes @code{jv-scan} to
789 also print the name of the file in which each class was found.
792 Print output to the named file.
795 Print help, then exit.
798 Print version number, then exit.
803 @node Invoking jcf-dump
804 @chapter Invoking jcf-dump
806 @c man title jcf-dump print information about Java class files
809 @c man begin SYNOPSIS jcf-dump
810 jcf-dump [@option{-c}] [@option{--javap}]
811 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
812 [@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
813 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
814 @var{classname}@dots{}
816 @c man begin SEEALSO jcf-dump
817 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
818 and the Info entries for @file{gcj} and @file{gcc}.
822 @c man begin DESCRIPTION jcf-dump
824 This is a class file examiner, similar to @code{javap}. It will print
825 information about a number of classes, which are specified by class name
830 @c man begin OPTIONS jcf-dump
834 Disassemble method bodies. By default method bodies are not printed.
836 @item --print-constants
837 Print the constant pool. When printing a reference to a constant
838 also print its index in the constant pool.
841 Generate output in @code{javap} format. The implementation of this
842 feature is very incomplete.
844 @item --classpath=@var{path}
845 @itemx --CLASSPATH=@var{path}
846 @itemx -I@var{directory}
848 These options as the same as the corresponding @command{gcj} options.
851 Print help, then exit.
854 Print version number, then exit.
857 Print extra information while running.
858 Implies @code{--print-constants}.
864 @chapter Invoking gij
866 @c man title gij GNU interpreter for Java bytecode
869 @c man begin SYNOPSIS gij
870 gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
872 gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
873 [@option{-cp} @var{path}] [@option{-classpath} @var{path}]
874 [@option{-D}@var{name}[=@var{value}]@dots{}]
875 [@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
876 [@option{-X@var{argument}}] [@option{-verbose}] [@option{-verbose:class}]
877 [@option{--showversion}] [@option{--version}] [@option{--help}][@option{-?}]
879 @c man begin SEEALSO gij
880 gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
881 and the Info entries for @file{gcj} and @file{gcc}.
885 @c man begin DESCRIPTION gij
887 @code{gij} is a Java bytecode interpreter included with @code{libgcj}.
888 @code{gij} is not available on every platform; porting it requires a
889 small amount of assembly programming which has not been done for all the
890 targets supported by @command{gcj}.
892 The primary argument to @code{gij} is the name of a class or, with
893 @code{-jar}, a jar file. Options before this argument are interpreted
894 by @code{gij}; remaining options are passed to the interpreted program.
896 If a class name is specified and this class does not have a @code{main}
897 method with the appropriate signature (a @code{static void} method with
898 a @code{String[]} as its sole argument), then @code{gij} will print an
901 If a jar file is specified then @code{gij} will use information in it to
902 determine which class' @code{main} method will be invoked.
904 @code{gij} will invoke the @code{main} method with all the remaining
905 command-line options.
907 Note that @code{gij} is not limited to interpreting code. Because
908 @code{libgcj} includes a class loader which can dynamically load shared
909 objects, it is possible to give @code{gij} the name of a class which has
910 been compiled and put into a shared library on the class path.
914 @c man begin OPTIONS gij
918 @itemx -classpath @var{path}
919 Set the initial class path. The class path is used for finding
920 class and resource files. If specified, this option overrides the
921 @code{CLASSPATH} environment variable. Note that this option is
922 ignored if @code{-jar} is used.
924 @item -D@var{name}[=@var{value}]
925 This defines a system property named @var{name} with value @var{value}.
926 If @var{value} is not specified then it defaults to the empty string.
927 These system properties are initialized at the program's startup and can
928 be retrieved at runtime using the @code{java.lang.System.getProperty}
931 @item -ms=@var{number}
932 This sets the initial heap size.
934 @item -mx=@var{number}
935 This sets the maximum heap size.
938 @itemx -X@var{argument}
939 Supplying @code{-X} by itself will cause @code{gij} to list all the
940 supported @code{-X} options. Currently there are none. Unrecognized
941 @code{-X} options are ignored, for compatibility with other runtimes.
944 This indicates that the name passed to @code{gij} should be interpreted
945 as the name of a jar file, not a class.
949 Print help, then exit.
952 Print version number and continue.
955 Print version number, then exit.
958 @itemx -verbose:class
959 Each time a class is initialized, print a short message on standard error.
964 @node Invoking jv-convert
965 @chapter Invoking jv-convert
967 @c man title jv-convert Convert file from one encoding to another
969 @c man begin SYNOPSIS jv-convert
970 @command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
973 [@option{--encoding} @var{name}]
974 [@option{--from} @var{name}]
975 [@option{--to} @var{name}]
976 [@option{-i} @var{file}] [@option{-o} @var{file}]
977 [@option{--reverse}] [@option{--help}] [@option{--version}]
981 @c man begin DESCRIPTION jv-convert
983 @command{jv-convert} is a utility included with @code{libgcj} which
984 converts a file from one encoding to another. It is similar to the Unix
985 @command{iconv} utility.
987 The encodings supported by @command{jv-convert} are platform-dependent.
988 Currently there is no way to get a list of all supported encodings.
992 @c man begin OPTIONS jv-convert
995 @item --encoding @var{name}
996 @itemx --from @var{name}
997 Use @var{name} as the input encoding. The default is the current
1000 @item --to @var{name}
1001 Use @var{name} as the output encoding. The default is the
1002 @code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
1003 non-ASCII characters.
1006 Read from @var{file}. The default is to read from standard input.
1009 Write to @var{file}. The default is to write to standard output.
1012 Swap the input and output encodings.
1015 Print a help message, then exit.
1018 Print version information, then exit.
1023 @node Invoking grmic
1024 @chapter Invoking grmic
1026 @c man title grmic Generate stubs for Remote Method Invocation
1028 @c man begin SYNOPSIS grmic
1029 @command{grmic} [@option{OPTION}] @dots{} @var{class} @dots{}
1032 [@option{-keepgenerated}]
1036 [@option{-nocompile}]
1038 [@option{-d} @var{directory}]
1044 @c man begin DESCRIPTION grmic
1046 @command{grmic} is a utility included with @code{libgcj} which generates
1047 stubs for remote objects.
1049 @c FIXME: Add real information here.
1050 @c This really isn't much more than the --help output.
1052 Note that this program isn't yet fully compatible with the JDK
1053 @command{grmic}. Some options, such as @option{-classpath}, are
1054 recognized but currently ignored. We have left these options
1055 undocumented for now.
1057 Long options can also be given with a GNU-style leading @samp{--}. For
1058 instance, @option{--help} is accepted.
1062 @c man begin OPTIONS grmic
1066 @itemx -keepgenerated
1067 By default, @command{grmic} deletes intermediate files. Either of these
1068 options causes it not to delete such files.
1071 Cause @command{grmic} to create stubs and skeletons for the 1.1
1075 Cause @command{grmic} to create stubs and skeletons compatible with both
1076 the 1.1 and 1.2 protocol versions. This is the default.
1079 Cause @command{grmic} to create stubs and skeletons for the 1.2
1083 Don't compile the generated files.
1086 Print information about what @command{grmic} is doing.
1088 @item -d @var{directory}
1089 Put output files in @var{directory}. By default the files are put in
1090 the current working directory.
1093 Print a help message, then exit.
1096 Print version information, then exit.
1102 @node Invoking grmiregistry
1103 @chapter Invoking grmiregistry
1105 @c man title grmiregistry Remote object registry
1107 @c man begin SYNOPSIS grmiregistry
1108 @command{grmic} [@option{OPTION}] @dots{} [@var{port}]
1111 [@option{--version}]
1115 @c man begin DESCRIPTION grmiregistry
1117 @command{grmiregistry} starts a remote object registry on the current
1118 host. If no port number is specified, then port 1099 is used.
1120 @c FIXME: Add real information here.
1121 @c This really isn't much more than the --help output.
1125 @c man begin OPTIONS grmiregistry
1129 Print a help message, then exit.
1132 Print version information, then exit.
1141 This documents CNI, the Compiled Native Interface,
1142 which is is a convenient way to write Java native methods using C++.
1143 This is a more efficient, more convenient, but less portable
1144 alternative to the standard JNI (Java Native Interface).
1147 * Basic concepts:: Introduction to using CNI@.
1148 * Packages:: How packages are mapped to C++.
1149 * Primitive types:: Handling Java types in C++.
1150 * Interfaces:: How Java interfaces map to C++.
1151 * Objects and Classes:: C++ and Java classes.
1152 * Class Initialization:: How objects are initialized.
1153 * Object allocation:: How to create Java objects in C++.
1154 * Arrays:: Dealing with Java arrays in C++.
1155 * Methods:: Java methods in C++.
1156 * Strings:: Information about Java Strings.
1157 * Mixing with C++:: How CNI can interoperate with C++.
1158 * Exception Handling:: How exceptions are handled.
1159 * Synchronization:: Synchronizing between Java and C++.
1160 * Invocation:: Starting the Java runtime from C++.
1161 * Reflection:: Using reflection from C++.
1165 @node Basic concepts
1166 @section Basic concepts
1168 In terms of languages features, Java is mostly a subset
1169 of C++. Java has a few important extensions, plus a powerful standard
1170 class library, but on the whole that does not change the basic similarity.
1171 Java is a hybrid object-oriented language, with a few native types,
1172 in addition to class types. It is class-based, where a class may have
1173 static as well as per-object fields, and static as well as instance methods.
1174 Non-static methods may be virtual, and may be overloaded. Overloading is
1175 resolved at compile time by matching the actual argument types against
1176 the parameter types. Virtual methods are implemented using indirect calls
1177 through a dispatch table (virtual function table). Objects are
1178 allocated on the heap, and initialized using a constructor method.
1179 Classes are organized in a package hierarchy.
1181 All of the listed attributes are also true of C++, though C++ has
1182 extra features (for example in C++ objects may be allocated not just
1183 on the heap, but also statically or in a local stack frame). Because
1184 @command{gcj} uses the same compiler technology as G++ (the GNU
1185 C++ compiler), it is possible to make the intersection of the two
1186 languages use the same ABI (object representation and calling
1187 conventions). The key idea in CNI is that Java objects are C++
1188 objects, and all Java classes are C++ classes (but not the other way
1189 around). So the most important task in integrating Java and C++ is to
1190 remove gratuitous incompatibilities.
1192 You write CNI code as a regular C++ source file. (You do have to use
1193 a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
1195 @noindent A CNI C++ source file must have:
1198 #include <gcj/cni.h>
1201 @noindent and then must include one header file for each Java class it uses, e.g.:
1204 #include <java/lang/Character.h>
1205 #include <java/util/Date.h>
1206 #include <java/lang/IndexOutOfBoundsException.h>
1209 @noindent These header files are automatically generated by @code{gcjh}.
1212 CNI provides some functions and macros to make using Java objects and
1213 primitive types from C++ easier. In general, these CNI functions and
1214 macros start with the @code{Jv} prefix, for example the function
1215 @code{JvNewObjectArray}. This convention is used to avoid conflicts
1216 with other libraries. Internal functions in CNI start with the prefix
1217 @code{_Jv_}. You should not call these; if you find a need to, let us
1218 know and we will try to come up with an alternate solution.
1221 @subsection Limitations
1223 Whilst a Java class is just a C++ class that doesn't mean that you are
1224 freed from the shackles of Java, a @acronym{CNI} C++ class must adhere to the
1225 rules of the Java programming language.
1227 For example: it is not possible to declare a method in a CNI class
1228 that will take a C string (@code{char*}) as an argument, or to declare a
1229 member variable of some non-Java datatype.
1235 The only global names in Java are class names, and packages. A
1236 @dfn{package} can contain zero or more classes, and also zero or more
1237 sub-packages. Every class belongs to either an unnamed package or a
1238 package that has a hierarchical and globally unique name.
1240 A Java package is mapped to a C++ @dfn{namespace}. The Java class
1241 @code{java.lang.String} is in the package @code{java.lang}, which is a
1242 sub-package of @code{java}. The C++ equivalent is the class
1243 @code{java::lang::String}, which is in the namespace @code{java::lang}
1244 which is in the namespace @code{java}.
1246 @noindent Here is how you could express this:
1249 (// @r{Declare the class(es), possibly in a header file:}
1258 class java::lang::String : public java::lang::Object
1264 @noindent The @code{gcjh} tool automatically generates the necessary namespace
1268 @subsection Leaving out package names
1270 Always using the fully-qualified name of a java class can be
1271 tiresomely verbose. Using the full qualified name also ties the code
1272 to a single package making code changes necessary should the class
1273 move from one package to another. The Java @code{package} declaration
1274 specifies that the following class declarations are in the named
1275 package, without having to explicitly name the full package
1276 qualifiers. The @code{package} declaration can be
1277 followed by zero or more @code{import} declarations, which
1278 allows either a single class or all the classes in a package to be
1279 named by a simple identifier. C++ provides something similar with the
1280 @code{using} declaration and directive.
1285 import @var{package-name}.@var{class-name};
1288 @noindent allows the program text to refer to @var{class-name} as a shorthand for
1289 the fully qualified name: @code{@var{package-name}.@var{class-name}}.
1292 @noindent To achieve the same effect C++, you have to do this:
1295 using @var{package-name}::@var{class-name};
1299 @noindent Java can also cause imports on demand, like this:
1302 import @var{package-name}.*;
1305 @noindent Doing this allows any class from the package @var{package-name} to be
1306 referred to only by its class-name within the program text.
1309 @noindent The same effect can be achieved in C++ like this:
1312 using namespace @var{package-name};
1316 @node Primitive types
1317 @section Primitive types
1319 Java provides 8 @dfn{primitives} types which represent integers, floats,
1320 characters and booleans (and also the void type). C++ has its own
1321 very similar concrete types. Such types in C++ however are not always
1322 implemented in the same way (an int might be 16, 32 or 64 bits for example)
1323 so CNI provides a special C++ type for each primitive Java type:
1325 @multitable @columnfractions .20 .25 .60
1326 @item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
1327 @item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
1328 @item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
1329 @item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
1330 @item @code{short} @tab @code{jshort} @tab 16 bit signed integer
1331 @item @code{int} @tab @code{jint} @tab 32 bit signed integer
1332 @item @code{long} @tab @code{jlong} @tab 64 bit signed integer
1333 @item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
1334 @item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
1335 @item @code{void} @tab @code{void} @tab no value
1338 When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
1339 to avoid disappointment.
1342 @subsection Reference types associated with primitive types
1344 In Java each primitive type has an associated reference type,
1345 e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
1346 In order to make working with such classes easier GCJ provides the macro
1349 @deffn macro JvPrimClass type
1350 Return a pointer to the @code{Class} object corresponding to the type supplied.
1353 JvPrimClass(void) @result{} java.lang.Void.TYPE
1362 A Java class can @dfn{implement} zero or more
1363 @dfn{interfaces}, in addition to inheriting from
1364 a single base class.
1366 @acronym{CNI} allows CNI code to implement methods of interfaces.
1367 You can also call methods through interface references, with some
1370 @acronym{CNI} doesn't understand interface inheritance at all yet. So,
1371 you can only call an interface method when the declared type of the
1372 field being called matches the interface which declares that
1373 method. The workaround is to cast the interface reference to the right
1376 For example if you have:
1384 interface B extends A
1390 and declare a variable of type @code{B} in C++, you can't call
1391 @code{a()} unless you cast it to an @code{A} first.
1393 @node Objects and Classes
1394 @section Objects and Classes
1398 All Java classes are derived from @code{java.lang.Object}. C++ does
1399 not have a unique root class, but we use the C++ class
1400 @code{java::lang::Object} as the C++ version of the
1401 @code{java.lang.Object} Java class. All other Java classes are mapped
1402 into corresponding C++ classes derived from @code{java::lang::Object}.
1404 Interface inheritance (the @code{implements} keyword) is currently not
1405 reflected in the C++ mapping.
1408 @subsection Object fields
1410 Each object contains an object header, followed by the instance fields
1411 of the class, in order. The object header consists of a single
1412 pointer to a dispatch or virtual function table. (There may be extra
1413 fields @emph{in front of} the object, for example for memory
1414 management, but this is invisible to the application, and the
1415 reference to the object points to the dispatch table pointer.)
1417 The fields are laid out in the same order, alignment, and size as in
1418 C++. Specifically, 8-bite and 16-bit native types (@code{byte},
1419 @code{short}, @code{char}, and @code{boolean}) are @emph{not} widened
1420 to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
1421 to 32 bits when on the VM stack or temporary registers.
1423 If you include the @code{gcjh}-generated header for a
1424 class, you can access fields of Java classes in the @emph{natural}
1425 way. For example, given the following Java class:
1431 public Integer (int i) @{ this.i = i; @}
1432 public static zero = new Integer(0);
1439 #include <gcj/cni.h>;
1443 mult (Int *p, jint k)
1446 return Int::zero; // @r{Static member access.}
1447 return new Int(p->i * k);
1452 @subsection Access specifiers
1454 CNI does not strictly enforce the Java access
1455 specifiers, because Java permissions cannot be directly mapped
1456 into C++ permission. Private Java fields and methods are mapped
1457 to private C++ fields and methods, but other fields and methods
1458 are mapped to public fields and methods.
1462 @node Class Initialization
1463 @section Class Initialization
1465 Java requires that each class be automatically initialized at the time
1466 of the first active use. Initializing a class involves
1467 initializing the static fields, running code in class initializer
1468 methods, and initializing base classes. There may also be
1469 some implementation specific actions, such as allocating
1470 @code{String} objects corresponding to string literals in
1473 The GCJ compiler inserts calls to @code{JvInitClass} at appropriate
1474 places to ensure that a class is initialized when required. The C++
1475 compiler does not insert these calls automatically---it is the
1476 programmer's responsibility to make sure classes are initialized.
1477 However, this is fairly painless because of the conventions assumed by
1480 First, @code{libgcj} will make sure a class is initialized before an
1481 instance of that object is created. This is one of the
1482 responsibilities of the @code{new} operation. This is taken care of
1483 both in Java code, and in C++ code. When G++ sees a @code{new} of a
1484 Java class, it will call a routine in @code{libgcj} to allocate the
1485 object, and that routine will take care of initializing the class.
1486 Note however that this does not happen for Java arrays; you must
1487 allocate those using the appropriate CNI function. It follows that
1488 you can access an instance field, or call an instance (non-static)
1489 method and be safe in the knowledge that the class and all of its base
1490 classes have been initialized.
1492 Invoking a static method is also safe. This is because the
1493 Java compiler adds code to the start of a static method to make sure
1494 the class is initialized. However, the C++ compiler does not
1495 add this extra code. Hence, if you write a native static method
1496 using CNI, you are responsible for calling @code{JvInitClass}
1497 before doing anything else in the method (unless you are sure
1498 it is safe to leave it out).
1500 Accessing a static field also requires the class of the
1501 field to be initialized. The Java compiler will generate code
1502 to call @code{Jv_InitClass} before getting or setting the field.
1503 However, the C++ compiler will not generate this extra code,
1504 so it is your responsibility to make sure the class is
1505 initialized before you access a static field from C++.
1508 @node Object allocation
1509 @section Object allocation
1511 New Java objects are allocated using a
1512 @dfn{class instance creation expression}, e.g.:
1515 new @var{Type} ( ... )
1518 The same syntax is used in C++. The main difference is that
1519 C++ objects have to be explicitly deleted; in Java they are
1520 automatically deleted by the garbage collector.
1521 Using @acronym{CNI}, you can allocate a new Java object
1522 using standard C++ syntax and the C++ compiler will allocate
1523 memory from the garbage collector. If you have overloaded
1524 constructors, the compiler will choose the correct one
1525 using standard C++ overload resolution rules.
1527 @noindent For example:
1530 java::util::Hashtable *ht = new java::util::Hashtable(120);
1537 While in many ways Java is similar to C and C++, it is quite different
1538 in its treatment of arrays. C arrays are based on the idea of pointer
1539 arithmetic, which would be incompatible with Java's security
1540 requirements. Java arrays are true objects (array types inherit from
1541 @code{java.lang.Object}). An array-valued variable is one that
1542 contains a reference (pointer) to an array object.
1544 Referencing a Java array in C++ code is done using the
1545 @code{JArray} template, which as defined as follows:
1548 class __JArray : public java::lang::Object
1555 class JArray : public __JArray
1559 T& operator[](jint i) @{ return data[i]; @}
1564 There are a number of @code{typedef}s which correspond to @code{typedef}s
1565 from the @acronym{JNI}. Each is the type of an array holding objects
1566 of the relevant type:
1569 typedef __JArray *jarray;
1570 typedef JArray<jobject> *jobjectArray;
1571 typedef JArray<jboolean> *jbooleanArray;
1572 typedef JArray<jbyte> *jbyteArray;
1573 typedef JArray<jchar> *jcharArray;
1574 typedef JArray<jshort> *jshortArray;
1575 typedef JArray<jint> *jintArray;
1576 typedef JArray<jlong> *jlongArray;
1577 typedef JArray<jfloat> *jfloatArray;
1578 typedef JArray<jdouble> *jdoubleArray;
1582 @deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
1583 This template function can be used to get a pointer to the elements of
1584 the @code{array}. For instance, you can fetch a pointer to the
1585 integers that make up an @code{int[]} like so:
1588 extern jintArray foo;
1589 jint *intp = elements (foo);
1592 The name of this function may change in the future.
1596 @deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
1597 Here @code{klass} is the type of elements of the array and
1598 @code{init} is the initial value put into every slot in the array.
1602 @subsection Creating arrays
1604 For each primitive type there is a function which can be used to
1605 create a new array of that type. The name of the function is of the
1609 JvNew@var{Type}Array
1612 @noindent For example:
1618 @noindent can be used to create an array of Java primitive boolean types.
1620 @noindent The following function definition is the template for all such functions:
1622 @deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
1623 Create's an array @var{length} indices long.
1626 @deftypefun jsize JvGetArrayLength (jarray @var{array})
1627 Returns the length of the @var{array}.
1634 Java methods are mapped directly into C++ methods.
1635 The header files generated by @code{gcjh}
1636 include the appropriate method definitions.
1637 Basically, the generated methods have the same names and
1638 @emph{corresponding} types as the Java methods,
1639 and are called in the natural manner.
1641 @subsection Overloading
1643 Both Java and C++ provide method overloading, where multiple
1644 methods in a class have the same name, and the correct one is chosen
1645 (at compile time) depending on the argument types.
1646 The rules for choosing the correct method are (as expected) more complicated
1647 in C++ than in Java, but given a set of overloaded methods
1648 generated by @code{gcjh} the C++ compiler will choose
1651 Common assemblers and linkers are not aware of C++ overloading,
1652 so the standard implementation strategy is to encode the
1653 parameter types of a method into its assembly-level name.
1654 This encoding is called @dfn{mangling},
1655 and the encoded name is the @dfn{mangled name}.
1656 The same mechanism is used to implement Java overloading.
1657 For C++/Java interoperability, it is important that both the Java
1658 and C++ compilers use the @emph{same} encoding scheme.
1660 @subsection Static methods
1662 Static Java methods are invoked in @acronym{CNI} using the standard
1663 C++ syntax, using the @code{::} operator rather
1664 than the @code{.} operator.
1666 @noindent For example:
1669 jint i = java::lang::Math::round((jfloat) 2.3);
1672 @noindent C++ method definition syntax is used to define a static native method.
1676 #include <java/lang/Integer>
1677 java::lang::Integer*
1678 java::lang::Integer::getInteger(jstring str)
1685 @subsection Object Constructors
1687 Constructors are called implicitly as part of object allocation
1688 using the @code{new} operator.
1690 @noindent For example:
1693 java::lang::Integer *x = new java::lang::Integer(234);
1696 Java does not allow a constructor to be a native method.
1697 This limitation can be coded round however because a constructor
1698 can @emph{call} a native method.
1701 @subsection Instance methods
1703 Calling a Java instance method from a C++ @acronym{CNI} method is done
1704 using the standard C++ syntax, e.g.:
1707 // @r{First create the Java object.}
1708 java::lang::Integer *x = new java::lang::Integer(234);
1709 // @r{Now call a method.}
1710 jint prim_value = x->intValue();
1711 if (x->longValue == 0)
1715 @noindent Defining a Java native instance method is also done the natural way:
1718 #include <java/lang/Integer.h>
1721 java::lang:Integer::doubleValue()
1723 return (jdouble) value;
1728 @subsection Interface methods
1730 In Java you can call a method using an interface reference. This is
1731 supported, but not completely. @xref{Interfaces}.
1739 @acronym{CNI} provides a number of utility functions for
1740 working with Java Java @code{String} objects.
1741 The names and interfaces are analogous to those of @acronym{JNI}.
1744 @deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
1745 Returns a Java @code{String} object with characters from the C string
1746 @var{chars} up to the index @var{len} in that array.
1749 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
1750 Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
1754 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
1755 As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
1758 @deftypefun jstring JvNewStringUTF (const char* @var{bytes})
1759 Returns a @code{String} which is made up of the UTF encoded characters
1760 present in the C string @var{bytes}.
1763 @deftypefun jchar* JvGetStringChars (jstring @var{str})
1764 Returns a pointer to an array of characters making up the @code{String} @var{str}.
1767 @deftypefun int JvGetStringUTFLength (jstring @var{str})
1768 Returns the number of bytes required to encode the contents of the
1769 @code{String} @var{str} in UTF-8.
1772 @deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
1773 Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
1774 the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
1776 Note that @var{buf} is a buffer, not a C string. It is @emph{not}
1781 @node Mixing with C++
1782 @section Interoperating with C/C++
1784 Because @acronym{CNI} is designed to represent Java classes and methods it
1785 cannot be mixed readily with C/C++ types.
1787 One important restriction is that Java classes cannot have non-Java
1788 type instance or static variables and cannot have methods which take
1789 non-Java types as arguments or return non-Java types.
1791 @noindent None of the following is possible with CNI:
1795 class ::MyClass : public java::lang::Object
1797 char* variable; // @r{char* is not a valid Java type.}
1802 ::SomeClass::someMethod (char *arg)
1807 @} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
1810 @noindent Of course, it is ok to use C/C++ types within the scope of a method:
1815 ::SomeClass::otherMethod (jstring str)
1826 The above restriction can be problematic, so @acronym{CNI} includes the
1827 @code{gnu.gcj.RawData} class. The @code{RawData} class is a
1828 @dfn{non-scanned reference} type. In other words variables declared
1829 of type @code{RawData} can contain any data and are not checked by the
1830 compiler or memory manager in any way.
1832 This means that you can put C/C++ data structures (including classes)
1833 in your @acronym{CNI} classes, as long as you use the appropriate cast.
1835 @noindent Here are some examples:
1839 class ::MyClass : public java::lang::Object
1841 gnu.gcj.RawData string;
1844 gnu.gcj.RawData getText ();
1848 ::MyClass::MyClass ()
1855 ::MyClass::getText ()
1861 ::MyClass::printText ()
1863 printf("%s\n", (char*) string);
1868 @subsection RawDataManaged
1870 @code{gnu.gcj.RawDataManaged} is another type used to indicate special data used
1871 by native code. Unlike the @code{RawData} type, fields declared as
1872 @code{RawDataManaged} will be "marked" by the memory manager and
1873 considered for garbage collection.
1875 Native data which is allocated using CNI's @code{JvAllocBytes()}
1876 function and stored in a @code{RawDataManaged} will be automatically
1877 freed when the Java object it is associated with becomes unreachable.
1879 @subsection Native memory allocation
1881 @deftypefun void* JvAllocBytes (jsize @var{size})
1882 Allocates @var{size} bytes from the heap. The memory returned is zeroed.
1883 This memory is not scanned for pointers by the garbage collector, but will
1884 be freed if no references to it are discovered.
1886 This function can be useful if you need to associate some native data with a
1887 Java object. Using a CNI's special @code{RawDataManaged} type, native data
1888 allocated with @code{JvAllocBytes} will be automatically freed when the Java
1889 object itself becomes unreachable.
1892 @subsection Posix signals
1894 On Posix based systems the @code{libgcj} library uses several signals
1895 internally. @acronym{CNI} code should not attempt to use the same
1896 signals as doing so may cause @code{libgcj} and/or the @acronym{CNI}
1899 SIGSEGV is used on many systems to generate
1900 @code{NullPointerExceptions}. SIGCHLD is used internally by
1901 @code{Runtime.exec()}. Several other signals (that vary from platform to
1902 platform) can be used by the memory manager and by
1903 @code{Thread.interrupt()}.
1905 @node Exception Handling
1906 @section Exception Handling
1908 While C++ and Java share a common exception handling framework,
1909 things are not yet perfectly integrated. The main issue is that the
1910 run-time type information facilities of the two
1911 languages are not integrated.
1913 Still, things work fairly well. You can throw a Java exception from
1914 C++ using the ordinary @code{throw} construct, and this
1915 exception can be caught by Java code. Similarly, you can catch an
1916 exception thrown from Java using the C++ @code{catch}
1919 @noindent Here is an example:
1923 throw new java::lang::IndexOutOfBoundsException();
1926 Normally, G++ will automatically detect when you are writing C++
1927 code that uses Java exceptions, and handle them appropriately.
1928 However, if C++ code only needs to execute destructors when Java
1929 exceptions are thrown through it, GCC will guess incorrectly. Sample
1933 struct S @{ ~S(); @};
1935 extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
1944 The usual effect of an incorrect guess is a link failure, complaining of
1945 a missing routine called @code{__gxx_personality_v0}.
1947 You can inform the compiler that Java exceptions are to be used in a
1948 translation unit, irrespective of what it might think, by writing
1949 @code{#pragma GCC java_exceptions} at the head of the
1950 file. This @code{#pragma} must appear before any
1951 functions that throw or catch exceptions, or run destructors when
1952 exceptions are thrown through them.
1954 @node Synchronization
1955 @section Synchronization
1957 Each Java object has an implicit monitor.
1958 The Java VM uses the instruction @code{monitorenter} to acquire
1959 and lock a monitor, and @code{monitorexit} to release it.
1961 The corresponding CNI macros are @code{JvMonitorEnter} and
1962 @code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
1963 and @code{MonitorExit}).
1966 The Java source language does not provide direct access to these primitives.
1967 Instead, there is a @code{synchronized} statement that does an
1968 implicit @code{monitorenter} before entry to the block,
1969 and does a @code{monitorexit} on exit from the block.
1970 Note that the lock has to be released even when the block is abnormally
1971 terminated by an exception, which means there is an implicit
1972 @code{try finally} surrounding synchronization locks.
1974 From C++, it makes sense to use a destructor to release a lock.
1975 @acronym{CNI} defines the following utility class:
1978 class JvSynchronize() @{
1980 JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
1981 ~JvSynchronize() @{ JvMonitorExit(obj); @}
1994 @noindent might become this C++ code:
1998 JvSynchronize dummy (OBJ);
2003 Java also has methods with the @code{synchronized} attribute.
2004 This is equivalent to wrapping the entire method body in a
2005 @code{synchronized} statement.
2006 (Alternatively, an implementation could require the caller to do
2007 the synchronization. This is not practical for a compiler, because
2008 each virtual method call would have to test at run-time if
2009 synchronization is needed.) Since in @command{gcj}
2010 the @code{synchronized} attribute is handled by the
2011 method implementation, it is up to the programmer
2012 of a synchronized native method to handle the synchronization
2013 (in the C++ implementation of the method).
2014 In other words, you need to manually add @code{JvSynchronize}
2015 in a @code{native synchronized} method.
2020 CNI permits C++ applications to make calls into Java classes, in addition to
2021 allowing Java code to call into C++. Several functions, known as the
2022 @dfn{invocation API}, are provided to support this.
2024 @deftypefun jint JvCreateJavaVM (void* @var{vm_args})
2025 Initializes the Java runtime. This function performs essential initialization
2026 of the threads interface, garbage collector, exception handling and other key
2027 aspects of the runtime. It must be called once by an application with
2028 a non-Java @code{main()} function, before any other Java or CNI calls are made.
2029 It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
2030 once provided it is only called from a single thread.
2031 The @var{vmargs} parameter can be used to specify initialization parameters
2032 for the Java runtime. It may be @code{NULL}.
2033 This function returns @code{0} upon success, or @code{-1} if the runtime is
2034 already initialized.
2036 @emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
2037 used in a future release.
2040 @deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{group})
2041 Registers an existing thread with the Java runtime. This must be called once
2042 from each thread, before that thread makes any other Java or CNI calls. It
2043 must be called after @code{JvCreateJavaVM}.
2044 @var{name} specifies a name for the thread. It may be @code{NULL}, in which
2045 case a name will be generated.
2046 @var{group} is the ThreadGroup in which this thread will be a member. If it
2047 is @code{NULL}, the thread will be a member of the main thread group.
2048 The return value is the Java @code{Thread} object that represents the thread.
2049 It is safe to call @code{JvAttachCurrentThread()} more than once from the same
2050 thread. If the thread is already attached, the call is ignored and the current
2051 thread object is returned.
2054 @deftypefun jint JvDetachCurrentThread ()
2055 Unregisters a thread from the Java runtime. This should be called by threads
2056 that were attached using @code{JvAttachCurrentThread()}, after they have
2057 finished making calls to Java code. This ensures that any resources associated
2058 with the thread become eligible for garbage collection.
2059 This function returns @code{0} upon success, or @code{-1} if the current thread
2063 @subsection Handling uncaught exceptions
2065 If an exception is thrown from Java code called using the invocation API, and
2066 no handler for the exception can be found, the runtime will abort the
2067 application. In order to make the application more robust, it is recommended
2068 that code which uses the invocation API be wrapped by a top-level try/catch
2069 block that catches all Java exceptions.
2073 The following code demonstrates the use of the invocation API. In this
2074 example, the C++ application initializes the Java runtime and attaches
2075 itself. The @code{java.lang.System} class is initialized in order to
2076 access its @code{out} field, and a Java string is printed. Finally, the thread
2077 is detached from the runtime once it has finished making Java calls. Everything
2078 is wrapped with a try/catch block to provide a default handler for any uncaught
2081 The example can be compiled with @command{c++ test.cc -lgcj}.
2085 #include <gcj/cni.h>
2086 #include <java/lang/System.h>
2087 #include <java/io/PrintStream.h>
2088 #include <java/lang/Throwable.h>
2090 int main(int argc, char *argv)
2092 using namespace java::lang;
2096 JvCreateJavaVM(NULL);
2097 JvAttachCurrentThread(NULL, NULL);
2099 String *message = JvNewStringLatin1("Hello from C++");
2100 JvInitClass(&System::class$);
2101 System::out->println(message);
2103 JvDetachCurrentThread();
2105 catch (Throwable *t)
2107 System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
2108 t->printStackTrace();
2116 Reflection is possible with CNI code, it functions similarly to how it
2117 functions with JNI@.
2119 @c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
2120 The types @code{jfieldID} and @code{jmethodID}
2123 @noindent The functions:
2126 @item @code{JvFromReflectedField},
2127 @item @code{JvFromReflectedMethod},
2128 @item @code{JvToReflectedField}
2129 @item @code{JvToFromReflectedMethod}
2132 @noindent will be added shortly, as will other functions corresponding to JNI@.
2135 @node System properties
2136 @chapter System properties
2138 The runtime behavior of the @code{libgcj} library can be modified by setting
2139 certain system properties. These properties can be compiled into the program
2140 using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
2141 setting them explicitly in the program by calling the
2142 @code{java.lang.System.setProperty()} method. Some system properties are only
2143 used for informational purposes (like giving a version number or a user name).
2144 A program can inspect the current value of a property by calling the
2145 @code{java.lang.System.getProperty()} method.
2148 * Standard Properties:: Standard properties supported by @code{libgcj}
2149 * GNU Classpath Properties:: Properties found in Classpath based libraries
2150 * libgcj Runtime Properties:: Properties specific to @code{libgcj}
2153 @node Standard Properties
2154 @section Standard Properties
2156 The following properties are normally found in all implementations of the core
2157 libraries for the Java language.
2162 The @code{libgcj} version number.
2165 Set to @samp{The Free Software Foundation, Inc.}
2167 @item java.vendor.url
2168 Set to @uref{http://gcc.gnu.org/java/}.
2171 The directory where @code{gcj} was installed. Taken from the @code{--prefix}
2172 option given to @command{configure}.
2174 @item java.class.version
2175 The class format version number supported by the libgcj byte code interpreter.
2176 (Currently @samp{46.0})
2178 @item java.vm.specification.version
2179 The Virtual Machine Specification version implemented by @code{libgcj}.
2180 (Currently @samp{1.0})
2182 @item java.vm.specification.vendor
2183 The name of the Virtual Machine specification designer.
2185 @item java.vm.specification.name
2186 The name of the Virtual Machine specification
2187 (Set to @samp{Java Virtual Machine Specification}).
2189 @item java.vm.version
2190 The @command{gcj} version number.
2192 @item java.vm.vendor
2193 Set to @samp{The Free Software Foundation, Inc.}
2196 Set to @samp{GNU libgcj}.
2198 @item java.specification.version
2199 The Runtime Environment specification version implemented by @code{libgcj}.
2200 (Currently set to @samp{1.3})
2202 @item java.specification.vendor
2203 The Runtime Environment specification designer.
2205 @item java.specification.name
2206 The name of the Runtime Environment specification
2207 (Set to @samp{Java Platform API Specification}).
2209 @item java.class.path
2210 The paths (jar files, zip files and directories) used for finding class files.
2212 @item java.library.path
2213 Directory path used for finding native libraries.
2215 @item java.io.tmpdir
2216 The directory used to put temporary files in.
2219 Name of the Just In Time compiler to use by the byte code interpreter.
2220 Currently not used in @code{libgcj}.
2223 Directories containing jar files with extra libraries. Will be used when
2224 resolving classes. Currently not used in @code{libgcj}.
2226 @item java.protocol.handler.pkgs
2227 A @samp{|} separated list of package names that is used to find classes that
2228 implement handlers for @code{java.net.URL}.
2230 @item java.rmi.server.codebase
2231 A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
2232 to load classes from.
2235 A list of class names that will be loaded by the @code{java.sql.DriverManager}
2238 @item file.separator
2239 The separator used in when directories are included in a filename
2240 (normally @samp{/} or @samp{\} ).
2243 The default character encoding used when converting platform native files to
2244 Unicode (usually set to @samp{8859_1}).
2246 @item path.separator
2247 The standard separator used when a string contains multiple paths
2248 (normally @samp{:} or @samp{;}), the string is usually not a valid character
2249 to use in normal directory names.)
2251 @item line.separator
2252 The default line separator used on the platform (normally @samp{\n}, @samp{\r}
2253 or a combination of those two characters).
2255 @item policy.provider
2256 The class name used for the default policy provider returned by
2257 @code{java.security.Policy.getPolicy}.
2260 The name of the user running the program. Can be the full name, the login name
2261 or empty if unknown.
2264 The default directory to put user specific files in.
2267 The current working directory from which the program was started.
2270 The default language as used by the @code{java.util.Locale} class.
2273 The default region as used by the @code{java.util.Local} class.
2276 The default variant of the language and region local used.
2279 The default timezone as used by the @code{java.util.TimeZone} class.
2282 The operating system/kernel name that the program runs on.
2285 The hardware that we are running on.
2288 The version number of the operating system/kernel.
2290 @item awt.appletWarning
2291 The string to display when an untrusted applet is displayed.
2292 Returned by @code{java.awt.Window.getWarningString()} when the window is
2296 The class name used for initializing the default @code{java.awt.Toolkit}.
2297 Defaults to @code{gnu.awt.gtk.GtkToolkit}.
2299 @item http.proxyHost
2300 Name of proxy host for http connections.
2302 @item http.proxyPort
2303 Port number to use when a proxy host is in use.
2307 @node GNU Classpath Properties
2308 @section GNU Classpath Properties
2310 @code{libgcj} is based on the GNU Classpath (Essential Libraries for Java) a
2311 GNU project to create free core class libraries for use with virtual machines
2312 and compilers for the Java language. The following properties are common to
2313 libraries based on GNU Classpath.
2317 @item gcj.dumpobject
2318 Enables printing serialization debugging by the @code{java.io.ObjectInput} and
2319 @code{java.io.ObjectOutput} classes when set to something else then the empty
2320 string. Only used when running a debug build of the library.
2322 @item gnu.classpath.vm.shortname
2323 This is a succint name of the virtual machine. For @code{libgcj},
2324 this will always be @samp{libgcj}.
2326 @item gnu.classpath.home.url
2327 A base URL used for finding system property files (e.g.,
2328 @file{classpath.security}). By default this is a @samp{file:} URL
2329 pointing to the @file{lib} directory under @samp{java.home}.
2333 @node libgcj Runtime Properties
2334 @section libgcj Runtime Properties
2336 The following properties are specific to the @code{libgcj} runtime and will
2337 normally not be found in other core libraries for the java language.
2341 @item java.fullversion
2342 The combination of @code{java.vm.name} and @code{java.vm.version}.
2345 Same as @code{java.fullversion}.
2348 Used by the @code{java.net.DatagramSocket} class when set to something else
2349 then the empty string. When set all newly created @code{DatagramSocket}s will
2350 try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
2351 the normal @code{java.net.PlainDatagramSocketImpl}.
2353 @item gnu.gcj.progname
2354 The name that was used to invoked the program.
2356 @item gnu.gcj.runtime.NameFinder.demangle
2357 Whether names in a stack trace should be demangled. Defaults to @code{true}.
2359 @item gnu.gcj.runtime.NameFinder.sanitize
2360 Whether calls to initialize exceptions and starting the runtime system
2361 should be removed from the stack trace. Only done when names are
2362 demangled. Defaults to @code{true}.
2364 @item gnu.gcj.runtime.NameFinder.remove_unknown
2365 Whether calls to unknown functions (class and method names are unknown)
2366 should be removed from the stack trace. Only done when the stack is
2367 sanitized. Ignored if this means no stack trace information would be
2368 available anymore. Defaults to @code{true}.
2370 @item gnu.gcj.runtime.NameFinder.remove_interpreter
2371 Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
2372 and functions starting with @samp{ffi_}) should be removed from the stack
2373 trace. Only done when the stack is sanitized. Defaults to @code{true}.
2376 @item gnu.gcj.runtime.NameFinder.use_addr2line
2377 Whether an external process (@command{addr2line} or @command{addr2name.awk})
2378 should be used as fallback to convert the addresses to function names when
2379 the runtime is unable to do it through @code{dladdr}.
2381 @item gnu.gcj.runtime.VMClassLoader.library_control
2382 This controls how shared libraries are automatically loaded by the
2383 built-in class loader. By default, or if this property is set to
2384 @samp{full}, a full search is done for each requested class. If this
2385 property is set to @samp{cache}, then any failed lookups are cached
2386 and not tried again. If this property is set to @samp{never}, then
2387 lookups are never done. For more information, @xref{Extensions}.
2389 @item gnu.gcj.jit.compiler
2390 @c FIXME we should probably have a whole node on this...
2391 This is the full path to @command{gcj} executable which should be
2392 used to compile classes just-in-time when
2393 @code{ClassLoader.defineClass} is called. If not set, @command{gcj}
2394 will not be invoked by the runtime; this can also be controlled via
2395 @code{Compiler.disable}.
2397 @item gnu.gcj.jit.options
2398 This is a space-separated string of options which should be passed to
2399 @command{gcj} when in JIT mode. If not set, a sensible default is
2402 @item gnu.gcj.jit.cachedir
2403 This is the directory where cached shared library files are
2404 stored. If not set, JIT compilation is disabled. This should never
2405 be set to a directory that is writable by any other user.
2413 While writing @command{gcj} and @code{libgcj} we have, of course, relied
2414 heavily on documentation from Sun Microsystems. In particular we have
2415 used The Java Language Specification (both first and second editions),
2416 the Java Class Libraries (volumes one and two), and the Java Virtual
2417 Machine Specification. In addition we've used the online documentation
2418 at @uref{http://java.sun.com/}.
2420 The current @command{gcj} home page is
2421 @uref{http://gcc.gnu.org/java/}.
2423 For more information on gcc, see @uref{http://gcc.gnu.org/}.
2425 Some @code{libgcj} testing is done using the Mauve test suite. This is
2426 a free software Java class library test suite which is being written
2427 because the JCK is not free. See
2428 @uref{http://sources.redhat.com/mauve/} for more information.