1 @\input texinfo @c -*-texinfo-*-
3 @settitle Guide to GNU gcj
5 @c Note: When reading this manual you'll find lots of strange
6 @c circumlocutions like ``compiler for the Java language''.
7 @c This is necessary due to Sun's restrictions on the use of
10 @c When this manual is copyrighted.
11 @set copyrights-gcj 2001, 2002
15 @set which-gcj GCC-@value{version-gcc}
17 @macro gcctabopt{body}
23 @dircategory Programming
25 * Gcj: (gcj). Ahead-of-time compiler for the Java language
28 @dircategory Individual utilities
30 * gcjh: (gcj)Invoking gcjh.
31 Generate header files from Java class files
32 * jv-scan: (gcj)Invoking jv-scan.
33 Print information about Java source files
34 * jcf-dump: (gcj)Invoking jcf-dump.
35 Print information about Java class files
36 * gij: (gcj)Invoking gij. GNU interpreter for Java bytecode
37 * jv-convert: (gcj)Invoking jv-convert.
38 Convert file from one encoding to another
39 * rmic: (gcj)Invoking rmic.
40 Generate stubs for Remote Method Invocation.
41 * rmiregistry: (gcj)Invoking rmiregistry.
42 The remote object registry.
46 @c man begin COPYRIGHT
47 Copyright (C) @value{copyrights-gcj} Free Software Foundation, Inc.
49 Permission is granted to copy, distribute and/or modify this document
50 under the terms of the GNU Free Documentation License, Version 1.1 or
51 any later version published by the Free Software Foundation; with the
52 Invariant Sections being ``GNU General Public License'', the Front-Cover
53 texts being (a) (see below), and with the Back-Cover Texts being (b)
54 (see below). A copy of the license is included in the
57 ``GNU Free Documentation License''.
59 @c man begin COPYRIGHT
64 (a) The FSF's Front-Cover Text is:
68 (b) The FSF's Back-Cover Text is:
70 You have freedom to copy and modify this GNU Manual, like GNU
71 software. Copies published by the Free Software Foundation raise
72 funds for GNU development.
80 @vskip 0pt plus 1filll
81 Copyright @copyright{} @value{copyrights-gcj} Free Software Foundation, Inc.
83 For the @value{which-gcj} Version*
85 Published by the Free Software Foundation @*
86 59 Temple Place - Suite 330@*
87 Boston, MA 02111-1307, USA@*
89 Permission is granted to copy, distribute and/or modify this document
90 under the terms of the GNU Free Documentation License, Version 1.1 or
91 any later version published by the Free Software Foundation; with the
92 Invariant Sections being ``GNU General Public License'', the Front-Cover
93 texts being (a) (see below), and with the Back-Cover Texts being (b)
94 (see below). A copy of the license is included in the section entitled
95 ``GNU Free Documentation License''.
97 (a) The FSF's Front-Cover Text is:
101 (b) The FSF's Back-Cover Text is:
103 You have freedom to copy and modify this GNU Manual, like GNU
104 software. Copies published by the Free Software Foundation raise
105 funds for GNU development.
114 This manual describes how to use @command{gcj}, the GNU compiler for the
115 Java programming language. @command{gcj} can generate both @file{.class}
116 files and object files, and it can read both Java source code and
120 * Copying:: The GNU General Public License
121 * GNU Free Documentation License::
122 How you can share and copy this manual
123 * Invoking gcj:: Compiler options supported by @command{gcj}
124 * Compatibility:: Compatibility between gcj and other tools for Java
125 * Invoking gcjh:: Generate header files from class files
126 * Invoking jv-scan:: Print information about source files
127 * Invoking jcf-dump:: Print information about class files
128 * Invoking gij:: Interpreting Java bytecodes
129 * Invoking jv-convert:: Converting from one encoding to another
130 * Invoking rmic:: Generate stubs for Remote Method Invocation.
131 * Invoking rmiregistry:: The remote object registry.
132 * About CNI:: Description of the Cygnus Native Interface
133 * System properties:: Modifying runtime behavior of the libgcj library
134 * Resources:: Where to look for more information
144 @chapter Invoking gcj
146 @c man title gcj Ahead-of-time compiler for the Java language
149 @c man begin SYNOPSIS gcj
150 gcj [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
151 [@option{--CLASSPATH}=@var{path}] [@option{--classpath}=@var{path}]
152 [@option{-f}@var{option}@dots{}] [@option{--encoding}=@var{name}]
153 [@option{--main}=@var{classname}] [@option{-D}@var{name}[=@var{value}]@dots{}]
154 [@option{-C}] [@option{--resource} @var{resource-name}] [@option{-d} @var{directory}]
155 [@option{-W}@var{warn}@dots{}]
156 @var{sourcefile}@dots{}
158 @c man begin SEEALSO gcj
159 gcc(1), gcjh(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
160 and the Info entries for @file{gcj} and @file{gcc}.
164 @c man begin DESCRIPTION gcj
166 As @command{gcj} is just another front end to @command{gcc}, it supports many
167 of the same options as gcc. @xref{Option Summary, , Option Summary,
168 gcc, Using the GNU Compiler Collection (GCC)}. This manual only documents the
169 options specific to @command{gcj}.
174 * Input and output files::
175 * Input Options:: How gcj finds files
176 * Encodings:: Options controlling source file encoding
177 * Warnings:: Options controlling warnings specific to gcj
178 * Code Generation:: Options controlling the output of gcj
179 * Configure-time Options:: Options you won't use
182 @c man begin OPTIONS gcj
184 @node Input and output files
185 @section Input and output files
187 A @command{gcj} command is like a @command{gcc} command, in that it
188 consists of a number of options and file names. The following kinds
189 of input file names are supported:
192 @item @var{file}.java
194 @item @var{file}.class
197 @itemx @var{file}.jar
198 An archive containing one or more @code{.class} files, all of
199 which are compiled. The archive may be compressed.
201 A file containing a whitespace-separated list of input file names.
202 (Currently, these must all be @code{.java} source files, but that
204 Each named file is compiled, just as if it had been on the command line.
205 @item @var{library}.a
206 @itemx @var{library}.so
207 @itemx -l@var{libname}
208 Libraries to use when linking. See the @command{gcc} manual.
211 You can specify more than one input file on the @command{gcj} command line,
212 in which case they will all be compiled. If you specify a
213 @code{-o @var{FILENAME}}
214 option, all the input files will be compiled together, producing a
215 single output file, named @var{FILENAME}.
216 This is allowed even when using @code{-S} or @code{-c},
217 but not when using @code{-C} or @code{--resource}.
218 (This is an extension beyond the what plain @command{gcc} allows.)
219 (If more than one input file is specified, all must currently
220 be @code{.java} files, though we hope to fix this.)
223 @section Input Options
227 @command{gcj} has options to control where it looks to find files it needs.
228 For instance, @command{gcj} might need to load a class that is referenced
229 by the file it has been asked to compile. Like other compilers for the
230 Java language, @command{gcj} has a notion of a @dfn{class path}. There are
231 several options and environment variables which can be used to
232 manipulate the class path. When @command{gcj} looks for a given class, it
233 searches the class path looking for matching @file{.class} or
234 @file{.java} file. @command{gcj} comes with a built-in class path which
235 points at the installed @file{libgcj.jar}, a file which contains all the
238 In the below, a directory or path component can refer either to an
239 actual directory on the filesystem, or to a @file{.zip} or @file{.jar}
240 file, which @command{gcj} will search as if it is a directory.
244 All directories specified by @code{-I} are kept in order and prepended
245 to the class path constructed from all the other options. Unless
246 compatibility with tools like @code{javac} is important, we recommend
247 always using @code{-I} instead of the other options for manipulating the
250 @item --classpath=@var{path}
251 This sets the class path to @var{path}, a colon-separated list of paths
252 (on Windows-based systems, a semicolon-separate list of paths).
253 This does not override the builtin (``boot'') search path.
255 @item --CLASSPATH=@var{path}
256 Deprecated synonym for @code{--classpath}.
258 @item --bootclasspath=@var{path}
259 Where to find the standard builtin classes, such as @code{java.lang.String}.
261 @item --extdirs=@var{path}
262 For each directory in the @var{path}, place the contents of that
263 directory at the end of the class path.
266 This is an environment variable which holds a list of paths.
269 The final class path is constructed like so:
273 First come all directories specified via @code{-I}.
276 If @option{--classpath} is specified, its value is appended.
277 Otherwise, if the @code{CLASSPATH} environment variable is specified,
278 then its value is appended.
279 Otherwise, the current directory (@code{"."}) is appended.
282 If @code{--bootclasspath} was specified, append its value.
283 Otherwise, append the built-in system directory, @file{libgcj.jar}.
286 Finally, if @code{--extdirs} was specified, append the contents of the
287 specified directories at the end of the class path. Otherwise, append
288 the contents of the built-in extdirs at @code{$(prefix)/share/java/ext}.
291 The classfile built by @command{gcj} for the class @code{java.lang.Object}
292 (and placed in @code{libgcj.jar}) contains a special zero length
293 attribute @code{gnu.gcj.gcj-compiled}. The compiler looks for this
294 attribute when loading @code{java.lang.Object} and will report an error
295 if it isn't found, unless it compiles to bytecode (the option
296 @code{-fforce-classes-archive-check} can be used to override this
297 behavior in this particular case.)
300 @item -fforce-classes-archive-check
301 This forces the compiler to always check for the special zero length
302 attribute @code{gnu.gcj.gcj-compiled} in @code{java.lang.Object} and
303 issue an error if it isn't found.
309 The Java programming language uses Unicode throughout. In an effort to
310 integrate well with other locales, @command{gcj} allows @file{.java} files
311 to be written using almost any encoding. @command{gcj} knows how to
312 convert these encodings into its internal encoding at compile time.
314 You can use the @code{--encoding=@var{NAME}} option to specify an
315 encoding (of a particular character set) to use for source files. If
316 this is not specified, the default encoding comes from your current
317 locale. If your host system has insufficient locale support, then
318 @command{gcj} assumes the default encoding to be the @samp{UTF-8} encoding
321 To implement @code{--encoding}, @command{gcj} simply uses the host
322 platform's @code{iconv} conversion routine. This means that in practice
323 @command{gcj} is limited by the capabilities of the host platform.
325 The names allowed for the argument @code{--encoding} vary from platform
326 to platform (since they are not standardized anywhere). However,
327 @command{gcj} implements the encoding named @samp{UTF-8} internally, so if
328 you choose to use this for your source files you can be assured that it
329 will work on every host.
335 @command{gcj} implements several warnings. As with other generic
336 @command{gcc} warnings, if an option of the form @code{-Wfoo} enables a
337 warning, then @code{-Wno-foo} will disable it. Here we've chosen to
338 document the form of the warning which will have an effect -- the
339 default being the opposite of what is listed.
342 @item -Wredundant-modifiers
343 With this flag, @command{gcj} will warn about redundant modifiers. For
344 instance, it will warn if an interface method is declared @code{public}.
346 @item -Wextraneous-semicolon
347 This causes @command{gcj} to warn about empty statements. Empty statements
348 have been deprecated.
350 @item -Wno-out-of-date
351 This option will cause @command{gcj} not to warn when a source file is
352 newer than its matching class file. By default @command{gcj} will warn
356 This is the same as @command{gcc}'s @code{-Wunused}.
359 This is the same as @code{-Wredundant-modifiers -Wextraneous-semicolon
364 @node Code Generation
365 @section Code Generation
367 In addition to the many @command{gcc} options controlling code generation,
368 @command{gcj} has several options specific to itself.
371 @item --main=@var{CLASSNAME}
372 This option is used when linking to specify the name of the class whose
373 @code{main} method should be invoked when the resulting executable is
374 run. @footnote{The linker by default looks for a global function named
375 @code{main}. Since Java does not have global functions, and a
376 collection of Java classes may have more than one class with a
377 @code{main} method, you need to let the linker know which of those
378 @code{main} methods it should invoke when starting the application.}
380 @item -D@var{name}[=@var{value}]
381 This option can only be used with @code{--main}. It defines a system
382 property named @var{name} with value @var{value}. If @var{value} is not
383 specified then it defaults to the empty string. These system properties
384 are initialized at the program's startup and can be retrieved at runtime
385 using the @code{java.lang.System.getProperty} method.
388 This option is used to tell @command{gcj} to generate bytecode
389 (@file{.class} files) rather than object code.
391 @item --resource @var{resource-name}
392 This option is used to tell @command{gcj} to compile the contents of a
393 given file to object code so it may be accessed at runtime with the core
394 protocol handler as @samp{core:/@var{resource-name}}. Note that
395 @var{resource-name} is the name of the resource as found at runtime; for
396 instance, it could be used in a call to @code{ResourceBundle.getBundle}.
397 The actual file name to be compiled this way must be specified
400 @item -d @var{directory}
401 When used with @code{-C}, this causes all generated @file{.class} files
402 to be put in the appropriate subdirectory of @var{directory}. By
403 default they will be put in subdirectories of the current working
406 @item -fno-bounds-check
407 By default, @command{gcj} generates code which checks the bounds of all
408 array indexing operations. With this option, these checks are omitted, which
409 can improve performance for code that uses arrays extensively. Note that this
410 can result in unpredictable behavior if the code in question actually does
411 violate array bounds constraints. It is safe to use this option if you are
412 sure that your code will never throw an @code{ArrayIndexOutOfBoundsException}.
414 @item -fno-store-check
415 Don't generate array store checks. When storing objects into arrays, a runtime
416 check is normally generated in order to ensure that the object is assignment
417 compatible with the component type of the array (which may not be known
418 at compile-time). With this option, these checks are omitted. This can
419 improve performance for code which stores objects into arrays frequently.
420 It is safe to use this option if you are sure your code will never throw an
421 @code{ArrayStoreException}.
424 With @command{gcj} there are two options for writing native methods: CNI
425 and JNI@. By default @command{gcj} assumes you are using CNI@. If you are
426 compiling a class with native methods, and these methods are implemented
427 using JNI, then you must use @code{-fjni}. This option causes
428 @command{gcj} to generate stubs which will invoke the underlying JNI
432 Don't recognize the @code{assert} keyword. This is for compatibility
433 with older versions of the language specification.
435 @item -fno-optimize-static-class-initialization
436 When the optimization level is greater or equal to @code{-O2},
437 @command{gcj} will try to optimize the way calls into the runtime are made
438 to initialize static classes upon their first use (this optimization
439 isn't carried out if @code{-C} was specified.) When compiling to native
440 code, @code{-fno-optimize-static-class-initialization} will turn this
441 optimization off, regardless of the optimization level in use.
445 @node Configure-time Options
446 @section Configure-time Options
448 Some @command{gcj} code generations options affect the resulting ABI, and
449 so can only be meaningfully given when @code{libgcj}, the runtime
450 package, is configured. @code{libgcj} puts the appropriate options from
451 this group into a @samp{spec} file which is read by @command{gcj}. These
452 options are listed here for completeness; if you are using @code{libgcj}
453 then you won't want to touch these options.
457 This enables the use of the Boehm GC bitmap marking code. In particular
458 this causes @command{gcj} to put an object marking descriptor into each
461 @item -fhash-synchronization
462 By default, synchronization data (the data used for @code{synchronize},
463 @code{wait}, and @code{notify}) is pointed to by a word in each object.
464 With this option @command{gcj} assumes that this information is stored in a
465 hash table and not in the object itself.
467 @item -fuse-divide-subroutine
468 On some systems, a library routine is called to perform integer
469 division. This is required to get exception handling correct when
472 @item -fcheck-references
473 On some systems it's necessary to insert inline checks whenever
474 accessing an object via a reference. On other systems you won't need
475 this because null pointer accesses are caught automatically by the
482 @chapter Compatibility with the Java Platform
484 As we believe it is important that the Java platform not be fragmented,
485 @command{gcj} and @code{libgcj} try to conform to the relevant Java
486 specifications. However, limited manpower and incomplete and unclear
487 documentation work against us. So, there are caveats to using
496 @section Standard features not yet supported
498 This list of compatibility issues is by no means complete.
502 @command{gcj} implements the JDK 1.2 language. It supports inner classes
503 and the new 1.4 @code{assert} keyword. It does not yet support the Java 2
504 @code{strictfp} keyword (it recognizes the keyword but ignores it).
507 @code{libgcj} is largely compatible with the JDK 1.2 libraries.
508 However, @code{libgcj} is missing many packages, most notably
509 @code{java.awt}. There are also individual missing classes and methods.
510 We currently do not have a list showing differences between
511 @code{libgcj} and the Java 2 platform.
514 Sometimes the @code{libgcj} implementation of a method or class differs
515 from the JDK implementation. This is not always a bug. Still, if it
516 affects you, it probably makes sense to report it so that we can discuss
517 the appropriate response.
521 @section Extra features unique to gcj
523 The main feature of @command{gcj} is that it can compile programs written in
524 the Java programming language to native code. Most extensions that have been
525 added are to facilitate this functionality.
529 @command{gcj} makes it easy and efficient to mix code written in Java and C++.
530 @xref{About CNI}, for more info on how to use this in your programs.
533 When you compile your classes into a shared library they can be automatically
534 loaded by the @code{libgcj} system classloader. When trying to load a class
535 @code{gnu.pkg.SomeClass} the system classloader will first try to load the
536 shared library @file{lib-gnu-pkg-SomeClass.so}, if that fails to load the
537 class then it will try to load @file{lib-gnu-pkg.so} and finally when the
538 class is still not loaded it will try to load @file{lib-gnu.so}. Note that
539 all @samp{.}s will be transformed into @samp{-}s and that searching
540 for inner classes starts with their outermost outer class. If the class
541 cannot be found this way the system classloader tries to use
542 the @code{libgcj} bytecode interpreter to load the class from the standard
547 @chapter Invoking gcjh
549 @c man title gcjh generate header files from Java class files
551 @c man begin DESCRIPTION gcjh
553 The @code{gcjh} program is used to generate header files from class
554 files. It can generate both CNI and JNI header files, as well as stub
555 implementation files which can be used as a basis for implementing the
556 required native methods.
561 @c man begin SYNOPSIS gcjh
562 gcjh [@option{-stubs}] [@option{-jni}]
563 [@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
564 [@option{-preprend} @var{text}]
565 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
566 [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
567 [@option{-o} @var{file}] [@option{-td} @var{dir}]
568 [@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
569 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
570 @var{classname}@dots{}
572 @c man begin SEEALSO gcjh
573 gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
574 and the Info entries for @file{gcj} and @file{gcc}.
578 @c man begin OPTIONS gcjh
582 This causes @code{gcjh} to generate stub files instead of header files.
583 By default the stub file will be named after the class, with a suffix of
584 @samp{.cc}. In JNI mode, the default output file will have the suffix
588 This tells @code{gcjh} to generate a JNI header or stub. By default,
589 CNI headers are generated.
591 @item -add @var{text}
592 Inserts @var{text} into the class body. This is ignored in JNI mode.
594 @item -append @var{text}
595 Inserts @var{text} into the header file after the class declaration.
596 This is ignored in JNI mode.
598 @item -friend @var{text}
599 Inserts @var{text} into the class as a @code{friend} declaration.
600 This is ignored in JNI mode.
602 @item -prepend @var{text}
603 Inserts @var{text} into the header file before the class declaration.
604 This is ignored in JNI mode.
606 @item --classpath=@var{path}
607 @itemx --CLASSPATH=@var{path}
608 @itemx -I@var{directory}
609 @itemx -d @var{directory}
611 These options are all identical to the corresponding @command{gcj} options.
614 Sets the output file name. This cannot be used if there is more than
615 one class on the command line.
617 @item -td @var{directory}
618 Sets the name of the directory to use for temporary files.
621 Print all dependencies to stdout; suppress ordinary output.
624 Print non-system dependencies to stdout; suppress ordinary output.
627 Print all dependencies to stdout.
630 Print non-system dependencies to stdout.
633 Print help about @code{gcjh} and exit. No further processing is done.
636 Print version information for @code{gcjh} and exit. No further
640 Print extra information while running.
643 All remaining options are considered to be names of classes.
647 @node Invoking jv-scan
648 @chapter Invoking jv-scan
650 @c man title jv-scan print information about Java source file
652 @c man begin DESCRIPTION jv-scan
654 The @code{jv-scan} program can be used to print information about a Java
655 source file (@file{.java} file).
660 @c man begin SYNOPSIS jv-scan
661 jv-scan [@option{--no-assert}] [@option{--complexity}]
662 [@option{--encoding}=@var{name}] [@option{--print-main}]
663 [@option{--list-class}] [@option{--list-filename}]
664 [@option{--version}] [@option{--help}]
665 [@option{-o} @var{file}] @var{inputfile}@dots{}
667 @c man begin SEEALSO jv-scan
668 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
669 and the Info entries for @file{gcj} and @file{gcc}.
673 @c man begin OPTIONS jv-scan
677 Don't recognize the @code{assert} keyword, for backwards compatibility
678 with older versions of the language specification.
681 This prints a complexity measure, related to cyclomatic complexity, for
684 @item --encoding=@var{name}
685 This works like the corresponding @command{gcj} option.
688 This prints the name of the class in this file containing a @code{main}
692 This lists the names of all classes defined in the input files.
694 @item --list-filename
695 If @code{--list-class} is given, this option causes @code{jv-scan} to
696 also print the name of the file in which each class was found.
699 Print output to the named file.
702 Print help, then exit.
705 Print version number, then exit.
710 @node Invoking jcf-dump
711 @chapter Invoking jcf-dump
713 @c man title jcf-dump print information about Java class files
716 @c man begin SYNOPSIS jcf-dump
717 jcf-dump [@option{-c}] [@option{--javap}]
718 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
719 [@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
720 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
721 @var{classname}@dots{}
723 @c man begin SEEALSO jcf-dump
724 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
725 and the Info entries for @file{gcj} and @file{gcc}.
729 @c man begin DESCRIPTION jcf-dump
731 This is a class file examiner, similar to @code{javap}. It will print
732 information about a number of classes, which are specified by class name
737 @c man begin OPTIONS jcf-dump
741 Disassemble method bodies. By default method bodies are not printed.
744 Generate output in @code{javap} format. The implementation of this
745 feature is very incomplete.
747 @item --classpath=@var{path}
748 @itemx --CLASSPATH=@var{path}
749 @itemx -I@var{directory}
751 These options as the same as the corresponding @command{gcj} options.
754 Print help, then exit.
757 Print version number, then exit.
760 Print extra information while running.
766 @chapter Invoking gij
768 @c man title gij GNU interpreter for Java bytecode
771 @c man begin SYNOPSIS gij
772 gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
774 gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
775 [@option{-cp} @var{path}] [@option{-classpath} @var{path}]
776 [@option{-D}@var{name}[=@var{value}]@dots{}]
777 [@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
778 [@option{--version}] [@option{--help}]
780 @c man begin SEEALSO gij
781 gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
782 and the Info entries for @file{gcj} and @file{gcc}.
786 @c man begin DESCRIPTION gij
788 @code{gij} is a Java bytecode interpreter included with @code{libgcj}.
789 @code{gij} is not available on every platform; porting it requires a
790 small amount of assembly programming which has not been done for all the
791 targets supported by @command{gcj}.
793 The primary argument to @code{gij} is the name of a class or, with
794 @code{-jar}, a jar file. Options before this argument are interpreted
795 by @code{gij}; remaining options are passed to the interpreted program.
797 If a class name is specified and this class does not have a @code{main}
798 method with the appropriate signature (a @code{static void} method with
799 a @code{String[]} as its sole argument), then @code{gij} will print an
802 If a jar file is specified then @code{gij} will use information in it to
803 determine which class' @code{main} method will be invoked.
805 @code{gij} will invoke the @code{main} method with all the remaining
806 command-line options.
808 Note that @code{gij} is not limited to interpreting code. Because
809 @code{libgcj} includes a class loader which can dynamically load shared
810 objects, it is possible to give @code{gij} the name of a class which has
811 been compiled and put into a shared library on the class path.
815 @c man begin OPTIONS gij
819 @itemx -classpath @var{path}
820 Set the initial class path. The class path is used for finding
821 class and resource files. If specified, this option overrides the
822 @code{CLASSPATH} environment variable. Note that this option is
823 ignored if @code{-jar} is used.
825 @item -D@var{name}[=@var{value}]
826 This defines a system property named @var{name} with value @var{value}.
827 If @var{value} is not specified then it defaults to the empty string.
828 These system properties are initialized at the program's startup and can
829 be retrieved at runtime using the @code{java.lang.System.getProperty}
832 @item -ms=@var{number}
833 This sets the initial heap size.
835 @item -mx=@var{number}
836 This sets the maximum heap size.
839 This indicates that the name passed to @code{gij} should be interpreted
840 as the name of a jar file, not a class.
843 Print help, then exit.
846 Print version number, then exit.
851 @node Invoking jv-convert
852 @chapter Invoking jv-convert
854 @c man title jv-convert Convert file from one encoding to another
856 @c man begin SYNOPSIS jv-convert
857 @command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
860 [@option{--encoding} @var{name}]
861 [@option{--from} @var{name}]
862 [@option{--to} @var{name}]
863 [@option{-i} @var{file}] [@option{-o} @var{file}]
864 [@option{--reverse}] [@option{--help}] [@option{--version}]
868 @c man begin DESCRIPTION jv-convert
870 @command{jv-convert} is a utility included with @code{libgcj} which
871 converts a file from one encoding to another. It is similar to the Unix
872 @command{iconv} utility.
874 The encodings supported by @command{jv-convert} are platform-dependent.
875 Currently there is no way to get a list of all supported encodings.
879 @c man begin OPTIONS jv-convert
882 @item --encoding @var{name}
883 @itemx --from @var{name}
884 Use @var{name} as the input encoding. The default is the current
887 @item --to @var{name}
888 Use @var{name} as the output encoding. The default is the
889 @code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
890 non-ASCII characters.
893 Read from @var{file}. The default is to read from standard input.
896 Write to @var{file}. The default is to write to standard output.
899 Swap the input and output encodings.
902 Print a help message, then exit.
905 Print version information, then exit.
911 @chapter Invoking rmic
913 @c man title rmic Generate stubs for Remote Method Invocation
915 @c man begin SYNOPSIS rmic
916 @command{rmic} [@option{OPTION}] @dots{} @var{class} @dots{}
919 [@option{-keepgenerated}]
923 [@option{-nocompile}]
925 [@option{-d} @var{directory}]
931 @c man begin DESCRIPTION rmic
933 @command{rmic} is a utility included with @code{libgcj} which generates
934 stubs for remote objects.
936 @c FIXME: Add real information here.
937 @c This really isn't much more than the --help output.
939 Note that this program isn't yet fully compatible with the JDK
940 @command{rmic}. Some options, such as @option{-classpath}, are
941 recognized but currently ignored. We have left these options
942 undocumented for now.
944 Long options can also be given with a GNU-style leading @samp{--}. For
945 instance, @option{--help} is accepted.
949 @c man begin OPTIONS rmic
953 @itemx -keepgenerated
954 By default, @command{rmic} deletes intermediate files. Either of these
955 options causes it not to delete such files.
958 Cause @command{rmic} to create stubs and skeletons for the 1.1
962 Cause @command{rmic} to create stubs and skeletons compatible with both
963 the 1.1 and 1.2 protocol versions. This is the default.
966 Cause @command{rmic} to create stubs and skeletons for the 1.2
970 Don't compile the generated files.
973 Print information about what @command{rmic} is doing.
975 @item -d @var{directory}
976 Put output files in @var{directory}. By default the files are put in
977 the current working directory.
980 Print a help message, then exit.
983 Print version information, then exit.
989 @node Invoking rmiregistry
990 @chapter Invoking rmiregistry
992 @c man title rmiregistry Remote object registry
994 @c man begin SYNOPSIS rmiregistry
995 @command{rmic} [@option{OPTION}] @dots{} [@var{port}]
1002 @c man begin DESCRIPTION rmiregistry
1004 @command{rmiregistry} starts a remote object registry on the current
1005 host. If no port number is specified, then port 1099 is used.
1007 @c FIXME: Add real information here.
1008 @c This really isn't much more than the --help output.
1012 @c man begin OPTIONS rmiregistry
1016 Print a help message, then exit.
1019 Print version information, then exit.
1028 This documents CNI, the Cygnus Native Interface,
1029 which is is a convenient way to write Java native methods using C++.
1030 This is a more efficient, more convenient, but less portable
1031 alternative to the standard JNI (Java Native Interface).
1034 * Basic concepts:: Introduction to using CNI@.
1035 * Packages:: How packages are mapped to C++.
1036 * Primitive types:: Handling Java types in C++.
1037 * Interfaces:: How Java interfaces map to C++.
1038 * Objects and Classes:: C++ and Java classes.
1039 * Class Initialization:: How objects are initialized.
1040 * Object allocation:: How to create Java objects in C++.
1041 * Arrays:: Dealing with Java arrays in C++.
1042 * Methods:: Java methods in C++.
1043 * Strings:: Information about Java Strings.
1044 * Mixing with C++:: How CNI can interoperate with C++.
1045 * Exception Handling:: How exceptions are handled.
1046 * Synchronization:: Synchronizing between Java and C++.
1047 * Invocation:: Starting the Java runtime from C++.
1048 * Reflection:: Using reflection from C++.
1052 @node Basic concepts
1053 @section Basic concepts
1055 In terms of languages features, Java is mostly a subset
1056 of C++. Java has a few important extensions, plus a powerful standard
1057 class library, but on the whole that does not change the basic similarity.
1058 Java is a hybrid object-oriented language, with a few native types,
1059 in addition to class types. It is class-based, where a class may have
1060 static as well as per-object fields, and static as well as instance methods.
1061 Non-static methods may be virtual, and may be overloaded. Overloading is
1062 resolved at compile time by matching the actual argument types against
1063 the parameter types. Virtual methods are implemented using indirect calls
1064 through a dispatch table (virtual function table). Objects are
1065 allocated on the heap, and initialized using a constructor method.
1066 Classes are organized in a package hierarchy.
1068 All of the listed attributes are also true of C++, though C++ has
1069 extra features (for example in C++ objects may be allocated not just
1070 on the heap, but also statically or in a local stack frame). Because
1071 @command{gcj} uses the same compiler technology as G++ (the GNU
1072 C++ compiler), it is possible to make the intersection of the two
1073 languages use the same ABI (object representation and calling
1074 conventions). The key idea in CNI is that Java objects are C++
1075 objects, and all Java classes are C++ classes (but not the other way
1076 around). So the most important task in integrating Java and C++ is to
1077 remove gratuitous incompatibilities.
1079 You write CNI code as a regular C++ source file. (You do have to use
1080 a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
1082 @noindent A CNI C++ source file must have:
1085 #include <gcj/cni.h>
1088 @noindent and then must include one header file for each Java class it uses, e.g.:
1091 #include <java/lang/Character.h>
1092 #include <java/util/Date.h>
1093 #include <java/lang/IndexOutOfBoundsException.h>
1096 @noindent These header files are automatically generated by @code{gcjh}.
1099 CNI provides some functions and macros to make using Java objects and
1100 primitive types from C++ easier. In general, these CNI functions and
1101 macros start with the @code{Jv} prefix, for example the function
1102 @code{JvNewObjectArray}. This convention is used to avoid conflicts
1103 with other libraries. Internal functions in CNI start with the prefix
1104 @code{_Jv_}. You should not call these; if you find a need to, let us
1105 know and we will try to come up with an alternate solution. (This
1106 manual lists @code{_Jv_AllocBytes} as an example; CNI should instead
1107 provide a @code{JvAllocBytes} function.)
1110 @subsection Limitations
1112 Whilst a Java class is just a C++ class that doesn't mean that you are
1113 freed from the shackles of Java, a @acronym{CNI} C++ class must adhere to the
1114 rules of the Java programming language.
1116 For example: it is not possible to declare a method in a CNI class
1117 that will take a C string (@code{char*}) as an argument, or to declare a
1118 member variable of some non-Java datatype.
1124 The only global names in Java are class names, and packages. A
1125 @dfn{package} can contain zero or more classes, and also zero or more
1126 sub-packages. Every class belongs to either an unnamed package or a
1127 package that has a hierarchical and globally unique name.
1129 A Java package is mapped to a C++ @dfn{namespace}. The Java class
1130 @code{java.lang.String} is in the package @code{java.lang}, which is a
1131 sub-package of @code{java}. The C++ equivalent is the class
1132 @code{java::lang::String}, which is in the namespace @code{java::lang}
1133 which is in the namespace @code{java}.
1135 @noindent Here is how you could express this:
1138 (// @r{Declare the class(es), possibly in a header file:}
1147 class java::lang::String : public java::lang::Object
1153 @noindent The @code{gcjh} tool automatically generates the necessary namespace
1157 @subsection Leaving out package names
1159 Always using the fully-qualified name of a java class can be
1160 tiresomely verbose. Using the full qualified name also ties the code
1161 to a single package making code changes necessary should the class
1162 move from one package to another. The Java @code{package} declaration
1163 specifies that the following class declarations are in the named
1164 package, without having to explicitly name the full package
1165 qualifiers. The @code{package} declaration can be
1166 followed by zero or more @code{import} declarations, which
1167 allows either a single class or all the classes in a package to be
1168 named by a simple identifier. C++ provides something similar with the
1169 @code{using} declaration and directive.
1174 import @var{package-name}.@var{class-name};
1177 @noindent allows the program text to refer to @var{class-name} as a shorthand for
1178 the fully qualified name: @code{@var{package-name}.@var{class-name}}.
1181 @noindent To achieve the same effect C++, you have to do this:
1184 using @var{package-name}::@var{class-name};
1188 @noindent Java can also cause imports on demand, like this:
1191 import @var{package-name}.*;
1194 @noindent Doing this allows any class from the package @var{package-name} to be
1195 referred to only by its class-name within the program text.
1198 @noindent The same effect can be achieved in C++ like this:
1201 using namespace @var{package-name};
1205 @node Primitive types
1206 @section Primitive types
1208 Java provides 8 @dfn{primitives} types which represent integers, floats,
1209 characters and booleans (and also the void type). C++ has its own
1210 very similar concrete types. Such types in C++ however are not always
1211 implemented in the same way (an int might be 16, 32 or 64 bits for example)
1212 so CNI provides a special C++ type for each primitive Java type:
1214 @multitable @columnfractions .20 .25 .60
1215 @item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
1216 @item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
1217 @item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
1218 @item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
1219 @item @code{short} @tab @code{jshort} @tab 16 bit signed integer
1220 @item @code{int} @tab @code{jint} @tab 32 bit signed integer
1221 @item @code{long} @tab @code{jlong} @tab 64 bit signed integer
1222 @item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
1223 @item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
1224 @item @code{void} @tab @code{void} @tab no value
1227 When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
1228 to avoid disappointment.
1231 @subsection Reference types associated with primitive types
1233 In Java each primitive type has an associated reference type,
1234 e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
1235 In order to make working with such classes easier GCJ provides the macro
1238 @deffn macro JvPrimClass type
1239 Return a pointer to the @code{Class} object corresponding to the type supplied.
1242 JvPrimClass(void) @result{} java.lang.Void.TYPE
1251 A Java class can @dfn{implement} zero or more
1252 @dfn{interfaces}, in addition to inheriting from
1253 a single base class.
1255 @acronym{CNI} allows CNI code to implement methods of interfaces.
1256 You can also call methods through interface references, with some
1259 @acronym{CNI} doesn't understand interface inheritance at all yet. So,
1260 you can only call an interface method when the declared type of the
1261 field being called matches the interface which declares that
1262 method. The workaround is to cast the interface reference to the right
1265 For example if you have:
1273 interface B extends A
1279 and declare a variable of type @code{B} in C++, you can't call
1280 @code{a()} unless you cast it to an @code{A} first.
1282 @node Objects and Classes
1283 @section Objects and Classes
1287 All Java classes are derived from @code{java.lang.Object}. C++ does
1288 not have a unique root class, but we use the C++ class
1289 @code{java::lang::Object} as the C++ version of the
1290 @code{java.lang.Object} Java class. All other Java classes are mapped
1291 into corresponding C++ classes derived from @code{java::lang::Object}.
1293 Interface inheritance (the @code{implements} keyword) is currently not
1294 reflected in the C++ mapping.
1297 @subsection Object fields
1299 Each object contains an object header, followed by the instance fields
1300 of the class, in order. The object header consists of a single
1301 pointer to a dispatch or virtual function table. (There may be extra
1302 fields @emph{in front of} the object, for example for memory
1303 management, but this is invisible to the application, and the
1304 reference to the object points to the dispatch table pointer.)
1306 The fields are laid out in the same order, alignment, and size as in
1307 C++. Specifically, 8-bite and 16-bit native types (@code{byte},
1308 @code{short}, @code{char}, and @code{boolean}) are @emph{not} widened
1309 to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
1310 to 32 bits when on the VM stack or temporary registers.
1312 If you include the @code{gcjh}-generated header for a
1313 class, you can access fields of Java classes in the @emph{natural}
1314 way. For example, given the following Java class:
1320 public Integer (int i) @{ this.i = i; @}
1321 public static zero = new Integer(0);
1328 #include <gcj/cni.h>;
1332 mult (Int *p, jint k)
1335 return Int::zero; // @r{Static member access.}
1336 return new Int(p->i * k);
1341 @subsection Access specifiers
1343 CNI does not strictly enforce the Java access
1344 specifiers, because Java permissions cannot be directly mapped
1345 into C++ permission. Private Java fields and methods are mapped
1346 to private C++ fields and methods, but other fields and methods
1347 are mapped to public fields and methods.
1351 @node Class Initialization
1352 @section Class Initialization
1354 Java requires that each class be automatically initialized at the time
1355 of the first active use. Initializing a class involves
1356 initializing the static fields, running code in class initializer
1357 methods, and initializing base classes. There may also be
1358 some implementation specific actions, such as allocating
1359 @code{String} objects corresponding to string literals in
1362 The GCJ compiler inserts calls to @code{JvInitClass} at appropriate
1363 places to ensure that a class is initialized when required. The C++
1364 compiler does not insert these calls automatically---it is the
1365 programmer's responsibility to make sure classes are initialized.
1366 However, this is fairly painless because of the conventions assumed by
1369 First, @code{libgcj} will make sure a class is initialized before an
1370 instance of that object is created. This is one of the
1371 responsibilities of the @code{new} operation. This is taken care of
1372 both in Java code, and in C++ code. When G++ sees a @code{new} of a
1373 Java class, it will call a routine in @code{libgcj} to allocate the
1374 object, and that routine will take care of initializing the class.
1375 Note however that this does not happen for Java arrays; you must
1376 allocate those using the appropriate CNI function. It follows that
1377 you can access an instance field, or call an instance (non-static)
1378 method and be safe in the knowledge that the class and all of its base
1379 classes have been initialized.
1381 Invoking a static method is also safe. This is because the
1382 Java compiler adds code to the start of a static method to make sure
1383 the class is initialized. However, the C++ compiler does not
1384 add this extra code. Hence, if you write a native static method
1385 using CNI, you are responsible for calling @code{JvInitClass}
1386 before doing anything else in the method (unless you are sure
1387 it is safe to leave it out).
1389 Accessing a static field also requires the class of the
1390 field to be initialized. The Java compiler will generate code
1391 to call @code{Jv_InitClass} before getting or setting the field.
1392 However, the C++ compiler will not generate this extra code,
1393 so it is your responsibility to make sure the class is
1394 initialized before you access a static field from C++.
1397 @node Object allocation
1398 @section Object allocation
1400 New Java objects are allocated using a
1401 @dfn{class instance creation expression}, e.g.:
1404 new @var{Type} ( ... )
1407 The same syntax is used in C++. The main difference is that
1408 C++ objects have to be explicitly deleted; in Java they are
1409 automatically deleted by the garbage collector.
1410 Using @acronym{CNI}, you can allocate a new Java object
1411 using standard C++ syntax and the C++ compiler will allocate
1412 memory from the garbage collector. If you have overloaded
1413 constructors, the compiler will choose the correct one
1414 using standard C++ overload resolution rules.
1416 @noindent For example:
1419 java::util::Hashtable *ht = new java::util::Hashtable(120);
1422 @deftypefun void* _Jv_AllocBytes (jsize @var{size})
1423 Allocates @var{size} bytes from the heap. The memory is not scanned
1424 by the garbage collector but it freed if no references to it are discovered.
1431 While in many ways Java is similar to C and C++, it is quite different
1432 in its treatment of arrays. C arrays are based on the idea of pointer
1433 arithmetic, which would be incompatible with Java's security
1434 requirements. Java arrays are true objects (array types inherit from
1435 @code{java.lang.Object}). An array-valued variable is one that
1436 contains a reference (pointer) to an array object.
1438 Referencing a Java array in C++ code is done using the
1439 @code{JArray} template, which as defined as follows:
1442 class __JArray : public java::lang::Object
1449 class JArray : public __JArray
1453 T& operator[](jint i) @{ return data[i]; @}
1458 There are a number of @code{typedef}s which correspond to @code{typedef}s
1459 from the @acronym{JNI}. Each is the type of an array holding objects
1460 of the relevant type:
1463 typedef __JArray *jarray;
1464 typedef JArray<jobject> *jobjectArray;
1465 typedef JArray<jboolean> *jbooleanArray;
1466 typedef JArray<jbyte> *jbyteArray;
1467 typedef JArray<jchar> *jcharArray;
1468 typedef JArray<jshort> *jshortArray;
1469 typedef JArray<jint> *jintArray;
1470 typedef JArray<jlong> *jlongArray;
1471 typedef JArray<jfloat> *jfloatArray;
1472 typedef JArray<jdouble> *jdoubleArray;
1476 @deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
1477 This template function can be used to get a pointer to the elements of
1478 the @code{array}. For instance, you can fetch a pointer to the
1479 integers that make up an @code{int[]} like so:
1482 extern jintArray foo;
1483 jint *intp = elements (foo);
1486 The name of this function may change in the future.
1490 @deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
1491 Here @code{klass} is the type of elements of the array and
1492 @code{init} is the initial value put into every slot in the array.
1496 @subsection Creating arrays
1498 For each primitive type there is a function which can be used to
1499 create a new array of that type. The name of the function is of the
1503 JvNew@var{Type}Array
1506 @noindent For example:
1512 @noindent can be used to create an array of Java primitive boolean types.
1514 @noindent The following function definition is the template for all such functions:
1516 @deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
1517 Create's an array @var{length} indices long.
1520 @deftypefun jsize JvGetArrayLength (jarray @var{array})
1521 Returns the length of the @var{array}.
1528 Java methods are mapped directly into C++ methods.
1529 The header files generated by @code{gcjh}
1530 include the appropriate method definitions.
1531 Basically, the generated methods have the same names and
1532 @emph{corresponding} types as the Java methods,
1533 and are called in the natural manner.
1535 @subsection Overloading
1537 Both Java and C++ provide method overloading, where multiple
1538 methods in a class have the same name, and the correct one is chosen
1539 (at compile time) depending on the argument types.
1540 The rules for choosing the correct method are (as expected) more complicated
1541 in C++ than in Java, but given a set of overloaded methods
1542 generated by @code{gcjh} the C++ compiler will choose
1545 Common assemblers and linkers are not aware of C++ overloading,
1546 so the standard implementation strategy is to encode the
1547 parameter types of a method into its assembly-level name.
1548 This encoding is called @dfn{mangling},
1549 and the encoded name is the @dfn{mangled name}.
1550 The same mechanism is used to implement Java overloading.
1551 For C++/Java interoperability, it is important that both the Java
1552 and C++ compilers use the @emph{same} encoding scheme.
1554 @subsection Static methods
1556 Static Java methods are invoked in @acronym{CNI} using the standard
1557 C++ syntax, using the @code{::} operator rather
1558 than the @code{.} operator.
1560 @noindent For example:
1563 jint i = java::lang::Math::round((jfloat) 2.3);
1566 @noindent C++ method definition syntax is used to define a static native method.
1570 #include <java/lang/Integer>
1571 java::lang::Integer*
1572 java::lang::Integer::getInteger(jstring str)
1579 @subsection Object Constructors
1581 Constructors are called implicitly as part of object allocation
1582 using the @code{new} operator.
1584 @noindent For example:
1587 java::lang::Integer *x = new java::lang::Integer(234);
1590 Java does not allow a constructor to be a native method.
1591 This limitation can be coded round however because a constructor
1592 can @emph{call} a native method.
1595 @subsection Instance methods
1597 Calling a Java instance method from a C++ @acronym{CNI} method is done
1598 using the standard C++ syntax, e.g.:
1601 // @r{First create the Java object.}
1602 java::lang::Integer *x = new java::lang::Integer(234);
1603 // @r{Now call a method.}
1604 jint prim_value = x->intValue();
1605 if (x->longValue == 0)
1609 @noindent Defining a Java native instance method is also done the natural way:
1612 #include <java/lang/Integer.h>
1615 java::lang:Integer::doubleValue()
1617 return (jdouble) value;
1622 @subsection Interface methods
1624 In Java you can call a method using an interface reference. This is
1625 supported, but not completely. @xref{Interfaces}.
1633 @acronym{CNI} provides a number of utility functions for
1634 working with Java Java @code{String} objects.
1635 The names and interfaces are analogous to those of @acronym{JNI}.
1638 @deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
1639 Returns a Java @code{String} object with characters from the C string
1640 @var{chars} up to the index @var{len} in that array.
1643 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
1644 Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
1648 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
1649 As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
1652 @deftypefun jstring JvNewStringUTF (const char* @var{bytes})
1653 Returns a @code{String} which is made up of the UTF encoded characters
1654 present in the C string @var{bytes}.
1657 @deftypefun jchar* JvGetStringChars (jstring @var{str})
1658 Returns a pointer to an array of characters making up the @code{String} @var{str}.
1661 @deftypefun int JvGetStringUTFLength (jstring @var{str})
1662 Returns the number of bytes required to encode the contents of the
1663 @code{String} @var{str} in UTF-8.
1666 @deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
1667 Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
1668 the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
1670 Note that @var{buf} is a buffer, not a C string. It is @emph{not}
1675 @node Mixing with C++
1676 @section Interoperating with C/C++
1678 Because @acronym{CNI} is designed to represent Java classes and methods it
1679 cannot be mixed readily with C/C++ types.
1681 One important restriction is that Java classes cannot have non-Java
1682 type instance or static variables and cannot have methods which take
1683 non-Java types as arguments or return non-Java types.
1685 @noindent None of the following is possible with CNI:
1689 class ::MyClass : public java::lang::Object
1691 char* variable; // @r{char* is not a valid Java type.}
1696 ::SomeClass::someMethod (char *arg)
1701 @} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
1704 @noindent Of course, it is ok to use C/C++ types within the scope of a method:
1709 ::SomeClass::otherMethod (jstring str)
1718 But this restriction can cause a problem so @acronym{CNI} includes the
1719 @code{gnu.gcj.RawData} class. The @code{RawData} class is a
1720 @dfn{non-scanned reference} type. In other words variables declared
1721 of type @code{RawData} can contain any data and are not checked by the
1722 compiler in any way.
1724 This means that you can put C/C++ data structures (including classes)
1725 in your @acronym{CNI} classes, as long as you use the appropriate cast.
1727 @noindent Here are some examples:
1731 class ::MyClass : public java::lang::Object
1733 gnu.gcj.RawData string;
1736 gnu.gcj.RawData getText ();
1740 ::MyClass::MyClass ()
1747 ::MyClass::getText ()
1753 ::MyClass::printText ()
1755 printf("%s\n", (char*) string);
1760 @node Exception Handling
1761 @section Exception Handling
1763 While C++ and Java share a common exception handling framework,
1764 things are not yet perfectly integrated. The main issue is that the
1765 run-time type information facilities of the two
1766 languages are not integrated.
1768 Still, things work fairly well. You can throw a Java exception from
1769 C++ using the ordinary @code{throw} construct, and this
1770 exception can be caught by Java code. Similarly, you can catch an
1771 exception thrown from Java using the C++ @code{catch}
1774 @noindent Here is an example:
1778 throw new java::lang::IndexOutOfBoundsException();
1781 Normally, G++ will automatically detect when you are writing C++
1782 code that uses Java exceptions, and handle them appropriately.
1783 However, if C++ code only needs to execute destructors when Java
1784 exceptions are thrown through it, GCC will guess incorrectly. Sample
1788 struct S @{ ~S(); @};
1790 extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
1799 The usual effect of an incorrect guess is a link failure, complaining of
1800 a missing routine called @code{__gxx_personality_v0}.
1802 You can inform the compiler that Java exceptions are to be used in a
1803 translation unit, irrespective of what it might think, by writing
1804 @code{#pragma GCC java_exceptions} at the head of the
1805 file. This @code{#pragma} must appear before any
1806 functions that throw or catch exceptions, or run destructors when
1807 exceptions are thrown through them.
1809 @node Synchronization
1810 @section Synchronization
1812 Each Java object has an implicit monitor.
1813 The Java VM uses the instruction @code{monitorenter} to acquire
1814 and lock a monitor, and @code{monitorexit} to release it.
1816 The corresponding CNI macros are @code{JvMonitorEnter} and
1817 @code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
1818 and @code{MonitorExit}).
1821 The Java source language does not provide direct access to these primitives.
1822 Instead, there is a @code{synchronized} statement that does an
1823 implicit @code{monitorenter} before entry to the block,
1824 and does a @code{monitorexit} on exit from the block.
1825 Note that the lock has to be released even when the block is abnormally
1826 terminated by an exception, which means there is an implicit
1827 @code{try finally} surrounding synchronization locks.
1829 From C++, it makes sense to use a destructor to release a lock.
1830 @acronym{CNI} defines the following utility class:
1833 class JvSynchronize() @{
1835 JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
1836 ~JvSynchronize() @{ JvMonitorExit(obj); @}
1849 @noindent might become this C++ code:
1853 JvSynchronize dummy (OBJ);
1858 Java also has methods with the @code{synchronized} attribute.
1859 This is equivalent to wrapping the entire method body in a
1860 @code{synchronized} statement.
1861 (Alternatively, an implementation could require the caller to do
1862 the synchronization. This is not practical for a compiler, because
1863 each virtual method call would have to test at run-time if
1864 synchronization is needed.) Since in @command{gcj}
1865 the @code{synchronized} attribute is handled by the
1866 method implementation, it is up to the programmer
1867 of a synchronized native method to handle the synchronization
1868 (in the C++ implementation of the method).
1869 In other words, you need to manually add @code{JvSynchronize}
1870 in a @code{native synchronized} method.
1875 CNI permits C++ applications to make calls into Java classes, in addition to
1876 allowing Java code to call into C++. Several functions, known as the
1877 @dfn{invocation API}, are provided to support this.
1879 @deftypefun jint JvCreateJavaVM (void* @var{vm_args})
1880 Initializes the Java runtime. This function performs essential initialization
1881 of the threads interface, garbage collector, exception handling and other key
1882 aspects of the runtime. It must be called once by an application with
1883 a non-Java @code{main()} function, before any other Java or CNI calls are made.
1884 It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
1885 once provided it is only called from a single thread.
1886 The @var{vmargs} parameter can be used to specify initialization parameters
1887 for the Java runtime. It may be @code{NULL}.
1888 This function returns @code{0} upon success, or @code{-1} if the runtime is
1889 already initialized.
1891 @emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
1892 used in a future release.
1895 @deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{group})
1896 Registers an existing thread with the Java runtime. This must be called once
1897 from each thread, before that thread makes any other Java or CNI calls. It
1898 must be called after @code{JvCreateJavaVM}.
1899 @var{name} specifies a name for the thread. It may be @code{NULL}, in which
1900 case a name will be generated.
1901 @var{group} is the ThreadGroup in which this thread will be a member. If it
1902 is @code{NULL}, the thread will be a member of the main thread group.
1903 The return value is the Java @code{Thread} object that represents the thread.
1904 It is safe to call @code{JvAttachCurrentThread()} more than once from the same
1905 thread. If the thread is already attached, the call is ignored and the current
1906 thread object is returned.
1909 @deftypefun jint JvDetachCurrentThread ()
1910 Unregisters a thread from the Java runtime. This should be called by threads
1911 that were attached using @code{JvAttachCurrentThread()}, after they have
1912 finished making calls to Java code. This ensures that any resources associated
1913 with the thread become eligible for garbage collection.
1914 This function returns @code{0} upon success, or @code{-1} if the current thread
1918 @subsection Handling uncaught exceptions
1920 If an exception is thrown from Java code called using the invocation API, and
1921 no handler for the exception can be found, the runtime will abort the
1922 application. In order to make the application more robust, it is recommended
1923 that code which uses the invocation API be wrapped by a top-level try/catch
1924 block that catches all Java exceptions.
1928 The following code demonstrates the use of the invocation API. In this
1929 example, the C++ application initializes the Java runtime and attaches
1930 itself. The @code{java.lang.System} class is initialized in order to
1931 access its @code{out} field, and a Java string is printed. Finally, the thread
1932 is detached from the runtime once it has finished making Java calls. Everything
1933 is wrapped with a try/catch block to provide a default handler for any uncaught
1936 The example can be compiled with @command{c++ test.cc -lgcj}.
1940 #include <gcj/cni.h>
1941 #include <java/lang/System.h>
1942 #include <java/io/PrintStream.h>
1943 #include <java/lang/Throwable.h>
1945 int main(int argc, char *argv)
1947 using namespace java::lang;
1951 JvCreateJavaVM(NULL);
1952 JvAttachCurrentThread(NULL, NULL);
1954 String *message = JvNewStringLatin1("Hello from C++");
1955 JvInitClass(&System::class$);
1956 System::out->println(message);
1958 JvDetachCurrentThread();
1960 catch (Throwable *t)
1962 System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
1963 t->printStackTrace();
1971 Reflection is possible with CNI code, it functions similarly to how it
1972 functions with JNI@.
1974 @c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
1975 The types @code{jfieldID} and @code{jmethodID}
1978 @noindent The functions:
1981 @item @code{JvFromReflectedField},
1982 @item @code{JvFromReflectedMethod},
1983 @item @code{JvToReflectedField}
1984 @item @code{JvToFromReflectedMethod}
1987 @noindent will be added shortly, as will other functions corresponding to JNI@.
1990 @node System properties
1991 @chapter System properties
1993 The runtime behavior of the @code{libgcj} library can be modified by setting
1994 certain system properties. These properties can be compiled into the program
1995 using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
1996 setting them explicitly in the program by calling the
1997 @code{java.lang.System.setProperty()} method. Some system properties are only
1998 used for informational purposes (like giving a version number or a user name).
1999 A program can inspect the current value of a property by calling the
2000 @code{java.lang.System.getProperty()} method.
2003 * Standard Properties:: Standard properties supported by @code{libgcj}
2004 * GNU Classpath Properties:: Properties found in Classpath based libraries
2005 * libgcj Runtime Properties:: Properties specific to @code{libgcj}
2008 @node Standard Properties
2009 @section Standard Properties
2011 The following properties are normally found in all implementations of the core
2012 libraries for the Java language.
2017 The @code{libgcj} version number.
2020 Set to @samp{The Free Software Foundation, Inc.}
2022 @item java.vendor.url
2023 Set to @uref{http://gcc.gnu.org/java/}.
2026 The directory where @code{gcj} was installed. Taken from the @code{--prefix}
2027 option given to @command{configure}.
2029 @item java.class.version
2030 The class format version number supported by the libgcj byte code interpreter.
2031 (Currently @samp{46.0})
2033 @item java.vm.specification.version
2034 The Virtual Machine Specification version implemented by @code{libgcj}.
2035 (Currently @samp{1.0})
2037 @item java.vm.specification.vendor
2038 The name of the Virtual Machine specification designer.
2040 @item java.vm.specification.name
2041 The name of the Virtual Machine specification
2042 (Set to @samp{Java Virtual Machine Specification}).
2044 @item java.vm.version
2045 The @command{gcj} version number.
2047 @item java.vm.vendor
2048 Set to @samp{The Free Software Foundation, Inc.}
2051 Set to @samp{GNU libgcj}.
2053 @item java.specification.version
2054 The Runtime Environment specification version implemented by @code{libgcj}.
2055 (Currently set to @samp{1.3})
2057 @item java.specification.vendor
2058 The Runtime Environment specification designer.
2060 @item java.specification.name
2061 The name of the Runtime Environment specification
2062 (Set to @samp{Java Platform API Specification}).
2064 @item java.class.path
2065 The paths (jar files, zip files and directories) used for finding class files.
2067 @item java.library.path
2068 Directory path used for finding native libraries. Currently not set.
2070 @item java.io.tmpdir
2071 The directory used to put temporary files in.
2074 Name of the Just In Time compiler to use by the byte code interpreter.
2075 Currently not used in @code{libgcj}.
2078 Directories containing jar files with extra libraries. Will be used when
2079 resolving classes. Currently not used in @code{libgcj}.
2081 @item java.protocol.handler.pkgs
2082 A @samp{|} separated list of package names that is used to find classes that
2083 implement handlers for @code{java.net.URL}.
2085 @item java.rmi.server.codebase
2086 A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
2087 to load classes from.
2090 A list of class names that will be loaded by the @code{java.sql.DriverManager}
2093 @item file.separator
2094 The separator used in when directories are included in a filename
2095 (normally @samp{/} or @samp{\} ).
2098 The default character encoding used when converting platform native files to
2099 Unicode (usually set to @samp{8859_1}).
2101 @item path.separator
2102 The standard separator used when a string contains multiple paths
2103 (normally @samp{:} or @samp{;}), the string is usually not a valid character
2104 to use in normal directory names.)
2106 @item line.separator
2107 The default line separator used on the platform (normally @samp{\n}, @samp{\r}
2108 or a combination of those two characters).
2110 @item policy.provider
2111 The class name used for the default policy provider returned by
2112 @code{java.security.Policy.getPolicy}.
2115 The name of the user running the program. Can be the full name, the login name
2116 or empty if unknown.
2119 The default directory to put user specific files in.
2122 The current working directory from which the program was started.
2125 The default language as used by the @code{java.util.Locale} class.
2128 The default region as used by the @code{java.util.Local} class.
2131 The default variant of the language and region local used.
2134 The default timezone as used by the @code{java.util.TimeZone} class.
2137 The operating system/kernel name that the program runs on.
2140 The hardware that we are running on.
2143 The version number of the operating system/kernel.
2145 @item awt.appletWarning
2146 The string to display when an untrusted applet is displayed.
2147 Returned by @code{java.awt.Window.getWarningString()} when the window is
2151 The class name used for initializing the default @code{java.awt.Toolkit}.
2152 Defaults to @code{gnu.awt.gtk.GtkToolkit}.
2156 @node GNU Classpath Properties
2157 @section GNU Classpath Properties
2159 @code{libgcj} is based on the GNU Classpath (Essential Libraries for Java) a
2160 GNU project to create free core class libraries for use with virtual machines
2161 and compilers for the Java language. The following properties are common to
2162 libraries based on GNU Classpath.
2166 @item gcj.dumpobject
2167 Enables printing serialization debugging by the @code{java.io.ObjectInput} and
2168 @code{java.io.ObjectOutput} classes when set to something else then the empty
2169 string. Only used when running a debug build of the library.
2173 @node libgcj Runtime Properties
2174 @section libgcj Runtime Properties
2176 The following properties are specific to the @code{libgcj} runtime and will
2177 normally not be found in other core libraries for the java language.
2181 @item java.fullversion
2182 The combination of @code{java.vm.name} and @code{java.vm.version}.
2185 Same as @code{java.fullversion}.
2188 Used by the @code{java.net.DatagramSocket} class when set to something else
2189 then the empty string. When set all newly created @code{DatagramSocket}s will
2190 try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
2191 the normal @code{java.net.PlainDatagramSocketImpl}.
2193 @item gnu.gcj.progname
2194 The name that was used to invoked the program.
2196 @item gnu.gcj.runtime.NameFinder.demangle
2197 Whether names in a stack trace should be demangled. Defaults to @code{true}.
2199 @item gnu.gcj.runtime.NameFinder.sanitize
2200 Whether calls to initialize exceptions and starting the runtime system
2201 should be removed from the stack trace. Only done when names are
2202 demangled. Defaults to @code{true}.
2204 @item gnu.gcj.runtime.NameFinder.remove_unknown
2205 Whether calls to unknown functions (class and method names are unknown)
2206 should be removed from the stack trace. Only done when the stack is
2207 sanitized. Ignored if this means no stack trace information would be
2208 available anymore. Defaults to @code{true}.
2210 @item gnu.gcj.runtime.NameFinder.remove_interpreter
2211 Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
2212 and functions starting with @samp{ffi_}) should be removed from the stack
2213 trace. Only done when the stack is sanitized. Defaults to @code{true}.
2216 @item gnu.gcj.runtime.NameFinder.use_addr2line
2217 Whether an external process (@command{addr2line} or @command{addr2name.awk})
2218 should be used as fallback to convert the addresses to function names when
2219 the runtime is unable to do it through @code{dladdr}.
2227 While writing @command{gcj} and @code{libgcj} we have, of course, relied
2228 heavily on documentation from Sun Microsystems. In particular we have
2229 used The Java Language Specification (both first and second editions),
2230 the Java Class Libraries (volumes one and two), and the Java Virtual
2231 Machine Specification. In addition we've used the online documentation
2232 at @uref{http://java.sun.com/}.
2234 The current @command{gcj} home page is
2235 @uref{http://gcc.gnu.org/java/}.
2237 For more information on gcc, see @uref{http://gcc.gnu.org/}.
2239 Some @code{libgcj} testing is done using the Mauve test suite. This is
2240 a free software Java class library test suite which is being written
2241 because the JCK is not free. See
2242 @uref{http://sources.redhat.com/mauve/} for more information.