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
490 This list of compatibility issues is by no means complete.
494 @command{gcj} implements the JDK 1.1 language. It supports inner classes,
495 though these are known to still be buggy. It does not yet support the
496 Java 2 @code{strictfp} keyword (it recognizes the keyword but ignores
500 @code{libgcj} is largely compatible with the JDK 1.2 libraries.
501 However, @code{libgcj} is missing many packages, most notably
502 @code{java.awt}. There are also individual missing classes and methods.
503 We currently do not have a list showing differences between
504 @code{libgcj} and the Java 2 platform.
507 Sometimes the @code{libgcj} implementation of a method or class differs
508 from the JDK implementation. This is not always a bug. Still, if it
509 affects you, it probably makes sense to report it so that we can discuss
510 the appropriate response.
515 @chapter Invoking gcjh
517 @c man title gcjh generate header files from Java class files
519 @c man begin DESCRIPTION gcjh
521 The @code{gcjh} program is used to generate header files from class
522 files. It can generate both CNI and JNI header files, as well as stub
523 implementation files which can be used as a basis for implementing the
524 required native methods.
529 @c man begin SYNOPSIS gcjh
530 gcjh [@option{-stubs}] [@option{-jni}]
531 [@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
532 [@option{-preprend} @var{text}]
533 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
534 [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
535 [@option{-o} @var{file}] [@option{-td} @var{dir}]
536 [@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
537 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
538 @var{classname}@dots{}
540 @c man begin SEEALSO gcjh
541 gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
542 and the Info entries for @file{gcj} and @file{gcc}.
546 @c man begin OPTIONS gcjh
550 This causes @code{gcjh} to generate stub files instead of header files.
551 By default the stub file will be named after the class, with a suffix of
552 @samp{.cc}. In JNI mode, the default output file will have the suffix
556 This tells @code{gcjh} to generate a JNI header or stub. By default,
557 CNI headers are generated.
559 @item -add @var{text}
560 Inserts @var{text} into the class body. This is ignored in JNI mode.
562 @item -append @var{text}
563 Inserts @var{text} into the header file after the class declaration.
564 This is ignored in JNI mode.
566 @item -friend @var{text}
567 Inserts @var{text} into the class as a @code{friend} declaration.
568 This is ignored in JNI mode.
570 @item -prepend @var{text}
571 Inserts @var{text} into the header file before the class declaration.
572 This is ignored in JNI mode.
574 @item --classpath=@var{path}
575 @itemx --CLASSPATH=@var{path}
576 @itemx -I@var{directory}
577 @itemx -d @var{directory}
579 These options are all identical to the corresponding @command{gcj} options.
582 Sets the output file name. This cannot be used if there is more than
583 one class on the command line.
585 @item -td @var{directory}
586 Sets the name of the directory to use for temporary files.
589 Print all dependencies to stdout; suppress ordinary output.
592 Print non-system dependencies to stdout; suppress ordinary output.
595 Print all dependencies to stdout.
598 Print non-system dependencies to stdout.
601 Print help about @code{gcjh} and exit. No further processing is done.
604 Print version information for @code{gcjh} and exit. No further
608 Print extra information while running.
611 All remaining options are considered to be names of classes.
615 @node Invoking jv-scan
616 @chapter Invoking jv-scan
618 @c man title jv-scan print information about Java source file
620 @c man begin DESCRIPTION jv-scan
622 The @code{jv-scan} program can be used to print information about a Java
623 source file (@file{.java} file).
628 @c man begin SYNOPSIS jv-scan
629 jv-scan [@option{--no-assert}] [@option{--complexity}]
630 [@option{--encoding}=@var{name}] [@option{--print-main}]
631 [@option{--list-class}] [@option{--list-filename}]
632 [@option{--version}] [@option{--help}]
633 [@option{-o} @var{file}] @var{inputfile}@dots{}
635 @c man begin SEEALSO jv-scan
636 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
637 and the Info entries for @file{gcj} and @file{gcc}.
641 @c man begin OPTIONS jv-scan
645 Don't recognize the @code{assert} keyword, for backwards compatibility
646 with older versions of the language specification.
649 This prints a complexity measure, related to cyclomatic complexity, for
652 @item --encoding=@var{name}
653 This works like the corresponding @command{gcj} option.
656 This prints the name of the class in this file containing a @code{main}
660 This lists the names of all classes defined in the input files.
662 @item --list-filename
663 If @code{--list-class} is given, this option causes @code{jv-scan} to
664 also print the name of the file in which each class was found.
667 Print output to the named file.
670 Print help, then exit.
673 Print version number, then exit.
678 @node Invoking jcf-dump
679 @chapter Invoking jcf-dump
681 @c man title jcf-dump print information about Java class files
684 @c man begin SYNOPSIS jcf-dump
685 jcf-dump [@option{-c}] [@option{--javap}]
686 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
687 [@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
688 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
689 @var{classname}@dots{}
691 @c man begin SEEALSO jcf-dump
692 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
693 and the Info entries for @file{gcj} and @file{gcc}.
697 @c man begin DESCRIPTION jcf-dump
699 This is a class file examiner, similar to @code{javap}. It will print
700 information about a number of classes, which are specified by class name
705 @c man begin OPTIONS jcf-dump
709 Disassemble method bodies. By default method bodies are not printed.
712 Generate output in @code{javap} format. The implementation of this
713 feature is very incomplete.
715 @item --classpath=@var{path}
716 @itemx --CLASSPATH=@var{path}
717 @itemx -I@var{directory}
719 These options as the same as the corresponding @command{gcj} options.
722 Print help, then exit.
725 Print version number, then exit.
728 Print extra information while running.
734 @chapter Invoking gij
736 @c man title gij GNU interpreter for Java bytecode
739 @c man begin SYNOPSIS gij
740 gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
742 gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
743 [@option{-cp} @var{path}] [@option{-classpath} @var{path}]
744 [@option{-D}@var{name}[=@var{value}]@dots{}]
745 [@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
746 [@option{--version}] [@option{--help}]
748 @c man begin SEEALSO gij
749 gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
750 and the Info entries for @file{gcj} and @file{gcc}.
754 @c man begin DESCRIPTION gij
756 @code{gij} is a Java bytecode interpreter included with @code{libgcj}.
757 @code{gij} is not available on every platform; porting it requires a
758 small amount of assembly programming which has not been done for all the
759 targets supported by @command{gcj}.
761 The primary argument to @code{gij} is the name of a class or, with
762 @code{-jar}, a jar file. Options before this argument are interpreted
763 by @code{gij}; remaining options are passed to the interpreted program.
765 If a class name is specified and this class does not have a @code{main}
766 method with the appropriate signature (a @code{static void} method with
767 a @code{String[]} as its sole argument), then @code{gij} will print an
770 If a jar file is specified then @code{gij} will use information in it to
771 determine which class' @code{main} method will be invoked.
773 @code{gij} will invoke the @code{main} method with all the remaining
774 command-line options.
776 Note that @code{gij} is not limited to interpreting code. Because
777 @code{libgcj} includes a class loader which can dynamically load shared
778 objects, it is possible to give @code{gij} the name of a class which has
779 been compiled and put into a shared library on the class path.
783 @c man begin OPTIONS gij
787 @itemx -classpath @var{path}
788 Set the initial class path. The class path is used for finding
789 class and resource files. If specified, this option overrides the
790 @code{CLASSPATH} environment variable. Note that this option is
791 ignored if @code{-jar} is used.
793 @item -D@var{name}[=@var{value}]
794 This defines a system property named @var{name} with value @var{value}.
795 If @var{value} is not specified then it defaults to the empty string.
796 These system properties are initialized at the program's startup and can
797 be retrieved at runtime using the @code{java.lang.System.getProperty}
800 @item -ms=@var{number}
801 This sets the initial heap size.
803 @item -mx=@var{number}
804 This sets the maximum heap size.
807 This indicates that the name passed to @code{gij} should be interpreted
808 as the name of a jar file, not a class.
811 Print help, then exit.
814 Print version number, then exit.
819 @node Invoking jv-convert
820 @chapter Invoking jv-convert
822 @c man title jv-convert Convert file from one encoding to another
824 @c man begin SYNOPSIS jv-convert
825 @command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
828 [@option{--encoding} @var{name}]
829 [@option{--from} @var{name}]
830 [@option{--to} @var{name}]
831 [@option{-i} @var{file}] [@option{-o} @var{file}]
832 [@option{--reverse}] [@option{--help}] [@option{--version}]
836 @c man begin DESCRIPTION jv-convert
838 @command{jv-convert} is a utility included with @code{libgcj} which
839 converts a file from one encoding to another. It is similar to the Unix
840 @command{iconv} utility.
842 The encodings supported by @command{jv-convert} are platform-dependent.
843 Currently there is no way to get a list of all supported encodings.
847 @c man begin OPTIONS jv-convert
850 @item --encoding @var{name}
851 @itemx --from @var{name}
852 Use @var{name} as the input encoding. The default is the current
855 @item --to @var{name}
856 Use @var{name} as the output encoding. The default is the
857 @code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
858 non-ASCII characters.
861 Read from @var{file}. The default is to read from standard input.
864 Write to @var{file}. The default is to write to standard output.
867 Swap the input and output encodings.
870 Print a help message, then exit.
873 Print version information, then exit.
879 @chapter Invoking rmic
881 @c man title rmic Generate stubs for Remote Method Invocation
883 @c man begin SYNOPSIS rmic
884 @command{rmic} [@option{OPTION}] @dots{} @var{class} @dots{}
887 [@option{-keepgenerated}]
891 [@option{-nocompile}]
893 [@option{-d} @var{directory}]
899 @c man begin DESCRIPTION rmic
901 @command{rmic} is a utility included with @code{libgcj} which generates
902 stubs for remote objects.
904 @c FIXME: Add real information here.
905 @c This really isn't much more than the --help output.
907 Note that this program isn't yet fully compatible with the JDK
908 @command{rmic}. Some options, such as @option{-classpath}, are
909 recognized but currently ignored. We have left these options
910 undocumented for now.
912 Long options can also be given with a GNU-style leading @samp{--}. For
913 instance, @option{--help} is accepted.
917 @c man begin OPTIONS rmic
921 @itemx -keepgenerated
922 By default, @command{rmic} deletes intermediate files. Either of these
923 options causes it not to delete such files.
926 Cause @command{rmic} to create stubs and skeletons for the 1.1
930 Cause @command{rmic} to create stubs and skeletons compatible with both
931 the 1.1 and 1.2 protocol versions. This is the default.
934 Cause @command{rmic} to create stubs and skeletons for the 1.2
938 Don't compile the generated files.
941 Print information about what @command{rmic} is doing.
943 @item -d @var{directory}
944 Put output files in @var{directory}. By default the files are put in
945 the current working directory.
948 Print a help message, then exit.
951 Print version information, then exit.
957 @node Invoking rmiregistry
958 @chapter Invoking rmiregistry
960 @c man title rmiregistry Remote object registry
962 @c man begin SYNOPSIS rmiregistry
963 @command{rmic} [@option{OPTION}] @dots{} [@var{port}]
970 @c man begin DESCRIPTION rmiregistry
972 @command{rmiregistry} starts a remote object registry on the current
973 host. If no port number is specified, then port 1099 is used.
975 @c FIXME: Add real information here.
976 @c This really isn't much more than the --help output.
980 @c man begin OPTIONS rmiregistry
984 Print a help message, then exit.
987 Print version information, then exit.
996 This documents CNI, the Cygnus Native Interface,
997 which is is a convenient way to write Java native methods using C++.
998 This is a more efficient, more convenient, but less portable
999 alternative to the standard JNI (Java Native Interface).
1002 * Basic concepts:: Introduction to using CNI@.
1003 * Packages:: How packages are mapped to C++.
1004 * Primitive types:: Handling Java types in C++.
1005 * Interfaces:: How Java interfaces map to C++.
1006 * Objects and Classes:: C++ and Java classes.
1007 * Class Initialization:: How objects are initialized.
1008 * Object allocation:: How to create Java objects in C++.
1009 * Arrays:: Dealing with Java arrays in C++.
1010 * Methods:: Java methods in C++.
1011 * Strings:: Information about Java Strings.
1012 * Mixing with C++:: How CNI can interoperate with C++.
1013 * Exception Handling:: How exceptions are handled.
1014 * Synchronization:: Synchronizing between Java and C++.
1015 * Invocation:: Starting the Java runtime from C++.
1016 * Reflection:: Using reflection from C++.
1020 @node Basic concepts
1021 @section Basic concepts
1023 In terms of languages features, Java is mostly a subset
1024 of C++. Java has a few important extensions, plus a powerful standard
1025 class library, but on the whole that does not change the basic similarity.
1026 Java is a hybrid object-oriented language, with a few native types,
1027 in addition to class types. It is class-based, where a class may have
1028 static as well as per-object fields, and static as well as instance methods.
1029 Non-static methods may be virtual, and may be overloaded. Overloading is
1030 resolved at compile time by matching the actual argument types against
1031 the parameter types. Virtual methods are implemented using indirect calls
1032 through a dispatch table (virtual function table). Objects are
1033 allocated on the heap, and initialized using a constructor method.
1034 Classes are organized in a package hierarchy.
1036 All of the listed attributes are also true of C++, though C++ has
1037 extra features (for example in C++ objects may be allocated not just
1038 on the heap, but also statically or in a local stack frame). Because
1039 @command{gcj} uses the same compiler technology as G++ (the GNU
1040 C++ compiler), it is possible to make the intersection of the two
1041 languages use the same ABI (object representation and calling
1042 conventions). The key idea in CNI is that Java objects are C++
1043 objects, and all Java classes are C++ classes (but not the other way
1044 around). So the most important task in integrating Java and C++ is to
1045 remove gratuitous incompatibilities.
1047 You write CNI code as a regular C++ source file. (You do have to use
1048 a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
1050 @noindent A CNI C++ source file must have:
1053 #include <gcj/cni.h>
1056 @noindent and then must include one header file for each Java class it uses, e.g.:
1059 #include <java/lang/Character.h>
1060 #include <java/util/Date.h>
1061 #include <java/lang/IndexOutOfBoundsException.h>
1064 @noindent These header files are automatically generated by @code{gcjh}.
1067 CNI provides some functions and macros to make using Java objects and
1068 primitive types from C++ easier. In general, these CNI functions and
1069 macros start with the @code{Jv} prefix, for example the function
1070 @code{JvNewObjectArray}. This convention is used to avoid conflicts
1071 with other libraries. Internal functions in CNI start with the prefix
1072 @code{_Jv_}. You should not call these; if you find a need to, let us
1073 know and we will try to come up with an alternate solution. (This
1074 manual lists @code{_Jv_AllocBytes} as an example; CNI should instead
1075 provide a @code{JvAllocBytes} function.)
1078 @subsection Limitations
1080 Whilst a Java class is just a C++ class that doesn't mean that you are
1081 freed from the shackles of Java, a @acronym{CNI} C++ class must adhere to the
1082 rules of the Java programming language.
1084 For example: it is not possible to declare a method in a CNI class
1085 that will take a C string (@code{char*}) as an argument, or to declare a
1086 member variable of some non-Java datatype.
1092 The only global names in Java are class names, and packages. A
1093 @dfn{package} can contain zero or more classes, and also zero or more
1094 sub-packages. Every class belongs to either an unnamed package or a
1095 package that has a hierarchical and globally unique name.
1097 A Java package is mapped to a C++ @dfn{namespace}. The Java class
1098 @code{java.lang.String} is in the package @code{java.lang}, which is a
1099 sub-package of @code{java}. The C++ equivalent is the class
1100 @code{java::lang::String}, which is in the namespace @code{java::lang}
1101 which is in the namespace @code{java}.
1103 @noindent Here is how you could express this:
1106 (// @r{Declare the class(es), possibly in a header file:}
1115 class java::lang::String : public java::lang::Object
1121 @noindent The @code{gcjh} tool automatically generates the necessary namespace
1125 @subsection Leaving out package names
1127 Always using the fully-qualified name of a java class can be
1128 tiresomely verbose. Using the full qualified name also ties the code
1129 to a single package making code changes necessary should the class
1130 move from one package to another. The Java @code{package} declaration
1131 specifies that the following class declarations are in the named
1132 package, without having to explicitly name the full package
1133 qualifiers. The @code{package} declaration can be
1134 followed by zero or more @code{import} declarations, which
1135 allows either a single class or all the classes in a package to be
1136 named by a simple identifier. C++ provides something similar with the
1137 @code{using} declaration and directive.
1142 import @var{package-name}.@var{class-name};
1145 @noindent allows the program text to refer to @var{class-name} as a shorthand for
1146 the fully qualified name: @code{@var{package-name}.@var{class-name}}.
1149 @noindent To achieve the same effect C++, you have to do this:
1152 using @var{package-name}::@var{class-name};
1156 @noindent Java can also cause imports on demand, like this:
1159 import @var{package-name}.*;
1162 @noindent Doing this allows any class from the package @var{package-name} to be
1163 referred to only by its class-name within the program text.
1166 @noindent The same effect can be achieved in C++ like this:
1169 using namespace @var{package-name};
1173 @node Primitive types
1174 @section Primitive types
1176 Java provides 8 @dfn{primitives} types which represent integers, floats,
1177 characters and booleans (and also the void type). C++ has its own
1178 very similar concrete types. Such types in C++ however are not always
1179 implemented in the same way (an int might be 16, 32 or 64 bits for example)
1180 so CNI provides a special C++ type for each primitive Java type:
1182 @multitable @columnfractions .20 .25 .60
1183 @item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
1184 @item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
1185 @item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
1186 @item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
1187 @item @code{short} @tab @code{jshort} @tab 16 bit signed integer
1188 @item @code{int} @tab @code{jint} @tab 32 bit signed integer
1189 @item @code{long} @tab @code{jlong} @tab 64 bit signed integer
1190 @item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
1191 @item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
1192 @item @code{void} @tab @code{void} @tab no value
1195 When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
1196 to avoid disappointment.
1199 @subsection Reference types associated with primitive types
1201 In Java each primitive type has an associated reference type,
1202 e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
1203 In order to make working with such classes easier GCJ provides the macro
1206 @deffn macro JvPrimClass type
1207 Return a pointer to the @code{Class} object corresponding to the type supplied.
1210 JvPrimClass(void) @result{} java.lang.Void.TYPE
1219 A Java class can @dfn{implement} zero or more
1220 @dfn{interfaces}, in addition to inheriting from
1221 a single base class.
1223 @acronym{CNI} allows CNI code to implement methods of interfaces.
1224 You can also call methods through interface references, with some
1227 @acronym{CNI} doesn't understand interface inheritance at all yet. So,
1228 you can only call an interface method when the declared type of the
1229 field being called matches the interface which declares that
1230 method. The workaround is to cast the interface reference to the right
1233 For example if you have:
1241 interface B extends A
1247 and declare a variable of type @code{B} in C++, you can't call
1248 @code{a()} unless you cast it to an @code{A} first.
1250 @node Objects and Classes
1251 @section Objects and Classes
1255 All Java classes are derived from @code{java.lang.Object}. C++ does
1256 not have a unique root class, but we use the C++ class
1257 @code{java::lang::Object} as the C++ version of the
1258 @code{java.lang.Object} Java class. All other Java classes are mapped
1259 into corresponding C++ classes derived from @code{java::lang::Object}.
1261 Interface inheritance (the @code{implements} keyword) is currently not
1262 reflected in the C++ mapping.
1265 @subsection Object fields
1267 Each object contains an object header, followed by the instance fields
1268 of the class, in order. The object header consists of a single
1269 pointer to a dispatch or virtual function table. (There may be extra
1270 fields @emph{in front of} the object, for example for memory
1271 management, but this is invisible to the application, and the
1272 reference to the object points to the dispatch table pointer.)
1274 The fields are laid out in the same order, alignment, and size as in
1275 C++. Specifically, 8-bite and 16-bit native types (@code{byte},
1276 @code{short}, @code{char}, and @code{boolean}) are @emph{not} widened
1277 to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
1278 to 32 bits when on the VM stack or temporary registers.
1280 If you include the @code{gcjh}-generated header for a
1281 class, you can access fields of Java classes in the @emph{natural}
1282 way. For example, given the following Java class:
1288 public Integer (int i) @{ this.i = i; @}
1289 public static zero = new Integer(0);
1296 #include <gcj/cni.h>;
1300 mult (Int *p, jint k)
1303 return Int::zero; // @r{Static member access.}
1304 return new Int(p->i * k);
1309 @subsection Access specifiers
1311 CNI does not strictly enforce the Java access
1312 specifiers, because Java permissions cannot be directly mapped
1313 into C++ permission. Private Java fields and methods are mapped
1314 to private C++ fields and methods, but other fields and methods
1315 are mapped to public fields and methods.
1319 @node Class Initialization
1320 @section Class Initialization
1322 Java requires that each class be automatically initialized at the time
1323 of the first active use. Initializing a class involves
1324 initializing the static fields, running code in class initializer
1325 methods, and initializing base classes. There may also be
1326 some implementation specific actions, such as allocating
1327 @code{String} objects corresponding to string literals in
1330 The GCJ compiler inserts calls to @code{JvInitClass} at appropriate
1331 places to ensure that a class is initialized when required. The C++
1332 compiler does not insert these calls automatically---it is the
1333 programmer's responsibility to make sure classes are initialized.
1334 However, this is fairly painless because of the conventions assumed by
1337 First, @code{libgcj} will make sure a class is initialized before an
1338 instance of that object is created. This is one of the
1339 responsibilities of the @code{new} operation. This is taken care of
1340 both in Java code, and in C++ code. When G++ sees a @code{new} of a
1341 Java class, it will call a routine in @code{libgcj} to allocate the
1342 object, and that routine will take care of initializing the class.
1343 Note however that this does not happen for Java arrays; you must
1344 allocate those using the appropriate CNI function. It follows that
1345 you can access an instance field, or call an instance (non-static)
1346 method and be safe in the knowledge that the class and all of its base
1347 classes have been initialized.
1349 Invoking a static method is also safe. This is because the
1350 Java compiler adds code to the start of a static method to make sure
1351 the class is initialized. However, the C++ compiler does not
1352 add this extra code. Hence, if you write a native static method
1353 using CNI, you are responsible for calling @code{JvInitClass}
1354 before doing anything else in the method (unless you are sure
1355 it is safe to leave it out).
1357 Accessing a static field also requires the class of the
1358 field to be initialized. The Java compiler will generate code
1359 to call @code{Jv_InitClass} before getting or setting the field.
1360 However, the C++ compiler will not generate this extra code,
1361 so it is your responsibility to make sure the class is
1362 initialized before you access a static field from C++.
1365 @node Object allocation
1366 @section Object allocation
1368 New Java objects are allocated using a
1369 @dfn{class instance creation expression}, e.g.:
1372 new @var{Type} ( ... )
1375 The same syntax is used in C++. The main difference is that
1376 C++ objects have to be explicitly deleted; in Java they are
1377 automatically deleted by the garbage collector.
1378 Using @acronym{CNI}, you can allocate a new Java object
1379 using standard C++ syntax and the C++ compiler will allocate
1380 memory from the garbage collector. If you have overloaded
1381 constructors, the compiler will choose the correct one
1382 using standard C++ overload resolution rules.
1384 @noindent For example:
1387 java::util::Hashtable *ht = new java::util::Hashtable(120);
1390 @deftypefun void* _Jv_AllocBytes (jsize @var{size})
1391 Allocates @var{size} bytes from the heap. The memory is not scanned
1392 by the garbage collector but it freed if no references to it are discovered.
1399 While in many ways Java is similar to C and C++, it is quite different
1400 in its treatment of arrays. C arrays are based on the idea of pointer
1401 arithmetic, which would be incompatible with Java's security
1402 requirements. Java arrays are true objects (array types inherit from
1403 @code{java.lang.Object}). An array-valued variable is one that
1404 contains a reference (pointer) to an array object.
1406 Referencing a Java array in C++ code is done using the
1407 @code{JArray} template, which as defined as follows:
1410 class __JArray : public java::lang::Object
1417 class JArray : public __JArray
1421 T& operator[](jint i) @{ return data[i]; @}
1426 There are a number of @code{typedef}s which correspond to @code{typedef}s
1427 from the @acronym{JNI}. Each is the type of an array holding objects
1428 of the relevant type:
1431 typedef __JArray *jarray;
1432 typedef JArray<jobject> *jobjectArray;
1433 typedef JArray<jboolean> *jbooleanArray;
1434 typedef JArray<jbyte> *jbyteArray;
1435 typedef JArray<jchar> *jcharArray;
1436 typedef JArray<jshort> *jshortArray;
1437 typedef JArray<jint> *jintArray;
1438 typedef JArray<jlong> *jlongArray;
1439 typedef JArray<jfloat> *jfloatArray;
1440 typedef JArray<jdouble> *jdoubleArray;
1444 @deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
1445 This template function can be used to get a pointer to the elements of
1446 the @code{array}. For instance, you can fetch a pointer to the
1447 integers that make up an @code{int[]} like so:
1450 extern jintArray foo;
1451 jint *intp = elements (foo);
1454 The name of this function may change in the future.
1458 @deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
1459 Here @code{klass} is the type of elements of the array and
1460 @code{init} is the initial value put into every slot in the array.
1464 @subsection Creating arrays
1466 For each primitive type there is a function which can be used to
1467 create a new array of that type. The name of the function is of the
1471 JvNew@var{Type}Array
1474 @noindent For example:
1480 @noindent can be used to create an array of Java primitive boolean types.
1482 @noindent The following function definition is the template for all such functions:
1484 @deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
1485 Create's an array @var{length} indices long.
1488 @deftypefun jsize JvGetArrayLength (jarray @var{array})
1489 Returns the length of the @var{array}.
1496 Java methods are mapped directly into C++ methods.
1497 The header files generated by @code{gcjh}
1498 include the appropriate method definitions.
1499 Basically, the generated methods have the same names and
1500 @emph{corresponding} types as the Java methods,
1501 and are called in the natural manner.
1503 @subsection Overloading
1505 Both Java and C++ provide method overloading, where multiple
1506 methods in a class have the same name, and the correct one is chosen
1507 (at compile time) depending on the argument types.
1508 The rules for choosing the correct method are (as expected) more complicated
1509 in C++ than in Java, but given a set of overloaded methods
1510 generated by @code{gcjh} the C++ compiler will choose
1513 Common assemblers and linkers are not aware of C++ overloading,
1514 so the standard implementation strategy is to encode the
1515 parameter types of a method into its assembly-level name.
1516 This encoding is called @dfn{mangling},
1517 and the encoded name is the @dfn{mangled name}.
1518 The same mechanism is used to implement Java overloading.
1519 For C++/Java interoperability, it is important that both the Java
1520 and C++ compilers use the @emph{same} encoding scheme.
1522 @subsection Static methods
1524 Static Java methods are invoked in @acronym{CNI} using the standard
1525 C++ syntax, using the @code{::} operator rather
1526 than the @code{.} operator.
1528 @noindent For example:
1531 jint i = java::lang::Math::round((jfloat) 2.3);
1534 @noindent C++ method definition syntax is used to define a static native method.
1538 #include <java/lang/Integer>
1539 java::lang::Integer*
1540 java::lang::Integer::getInteger(jstring str)
1547 @subsection Object Constructors
1549 Constructors are called implicitly as part of object allocation
1550 using the @code{new} operator.
1552 @noindent For example:
1555 java::lang::Integer *x = new java::lang::Integer(234);
1558 Java does not allow a constructor to be a native method.
1559 This limitation can be coded round however because a constructor
1560 can @emph{call} a native method.
1563 @subsection Instance methods
1565 Calling a Java instance method from a C++ @acronym{CNI} method is done
1566 using the standard C++ syntax, e.g.:
1569 // @r{First create the Java object.}
1570 java::lang::Integer *x = new java::lang::Integer(234);
1571 // @r{Now call a method.}
1572 jint prim_value = x->intValue();
1573 if (x->longValue == 0)
1577 @noindent Defining a Java native instance method is also done the natural way:
1580 #include <java/lang/Integer.h>
1583 java::lang:Integer::doubleValue()
1585 return (jdouble) value;
1590 @subsection Interface methods
1592 In Java you can call a method using an interface reference. This is
1593 supported, but not completely. @xref{Interfaces}.
1601 @acronym{CNI} provides a number of utility functions for
1602 working with Java Java @code{String} objects.
1603 The names and interfaces are analogous to those of @acronym{JNI}.
1606 @deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
1607 Returns a Java @code{String} object with characters from the C string
1608 @var{chars} up to the index @var{len} in that array.
1611 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
1612 Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
1616 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
1617 As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
1620 @deftypefun jstring JvNewStringUTF (const char* @var{bytes})
1621 Returns a @code{String} which is made up of the UTF encoded characters
1622 present in the C string @var{bytes}.
1625 @deftypefun jchar* JvGetStringChars (jstring @var{str})
1626 Returns a pointer to an array of characters making up the @code{String} @var{str}.
1629 @deftypefun int JvGetStringUTFLength (jstring @var{str})
1630 Returns the number of bytes required to encode the contents of the
1631 @code{String} @var{str} in UTF-8.
1634 @deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
1635 Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
1636 the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
1638 Note that @var{buf} is a buffer, not a C string. It is @emph{not}
1643 @node Mixing with C++
1644 @section Interoperating with C/C++
1646 Because @acronym{CNI} is designed to represent Java classes and methods it
1647 cannot be mixed readily with C/C++ types.
1649 One important restriction is that Java classes cannot have non-Java
1650 type instance or static variables and cannot have methods which take
1651 non-Java types as arguments or return non-Java types.
1653 @noindent None of the following is possible with CNI:
1657 class ::MyClass : public java::lang::Object
1659 char* variable; // @r{char* is not a valid Java type.}
1664 ::SomeClass::someMethod (char *arg)
1669 @} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
1672 @noindent Of course, it is ok to use C/C++ types within the scope of a method:
1677 ::SomeClass::otherMethod (jstring str)
1686 But this restriction can cause a problem so @acronym{CNI} includes the
1687 @code{gnu.gcj.RawData} class. The @code{RawData} class is a
1688 @dfn{non-scanned reference} type. In other words variables declared
1689 of type @code{RawData} can contain any data and are not checked by the
1690 compiler in any way.
1692 This means that you can put C/C++ data structures (including classes)
1693 in your @acronym{CNI} classes, as long as you use the appropriate cast.
1695 @noindent Here are some examples:
1699 class ::MyClass : public java::lang::Object
1701 gnu.gcj.RawData string;
1704 gnu.gcj.RawData getText ();
1708 ::MyClass::MyClass ()
1715 ::MyClass::getText ()
1721 ::MyClass::printText ()
1723 printf("%s\n", (char*) string);
1728 @node Exception Handling
1729 @section Exception Handling
1731 While C++ and Java share a common exception handling framework,
1732 things are not yet perfectly integrated. The main issue is that the
1733 run-time type information facilities of the two
1734 languages are not integrated.
1736 Still, things work fairly well. You can throw a Java exception from
1737 C++ using the ordinary @code{throw} construct, and this
1738 exception can be caught by Java code. Similarly, you can catch an
1739 exception thrown from Java using the C++ @code{catch}
1742 @noindent Here is an example:
1746 throw new java::lang::IndexOutOfBoundsException();
1749 Normally, G++ will automatically detect when you are writing C++
1750 code that uses Java exceptions, and handle them appropriately.
1751 However, if C++ code only needs to execute destructors when Java
1752 exceptions are thrown through it, GCC will guess incorrectly. Sample
1756 struct S @{ ~S(); @};
1758 extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
1767 The usual effect of an incorrect guess is a link failure, complaining of
1768 a missing routine called @code{__gxx_personality_v0}.
1770 You can inform the compiler that Java exceptions are to be used in a
1771 translation unit, irrespective of what it might think, by writing
1772 @code{#pragma GCC java_exceptions} at the head of the
1773 file. This @code{#pragma} must appear before any
1774 functions that throw or catch exceptions, or run destructors when
1775 exceptions are thrown through them.
1777 @node Synchronization
1778 @section Synchronization
1780 Each Java object has an implicit monitor.
1781 The Java VM uses the instruction @code{monitorenter} to acquire
1782 and lock a monitor, and @code{monitorexit} to release it.
1784 The corresponding CNI macros are @code{JvMonitorEnter} and
1785 @code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
1786 and @code{MonitorExit}).
1789 The Java source language does not provide direct access to these primitives.
1790 Instead, there is a @code{synchronized} statement that does an
1791 implicit @code{monitorenter} before entry to the block,
1792 and does a @code{monitorexit} on exit from the block.
1793 Note that the lock has to be released even when the block is abnormally
1794 terminated by an exception, which means there is an implicit
1795 @code{try finally} surrounding synchronization locks.
1797 From C++, it makes sense to use a destructor to release a lock.
1798 @acronym{CNI} defines the following utility class:
1801 class JvSynchronize() @{
1803 JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
1804 ~JvSynchronize() @{ JvMonitorExit(obj); @}
1817 @noindent might become this C++ code:
1821 JvSynchronize dummy (OBJ);
1826 Java also has methods with the @code{synchronized} attribute.
1827 This is equivalent to wrapping the entire method body in a
1828 @code{synchronized} statement.
1829 (Alternatively, an implementation could require the caller to do
1830 the synchronization. This is not practical for a compiler, because
1831 each virtual method call would have to test at run-time if
1832 synchronization is needed.) Since in @command{gcj}
1833 the @code{synchronized} attribute is handled by the
1834 method implementation, it is up to the programmer
1835 of a synchronized native method to handle the synchronization
1836 (in the C++ implementation of the method).
1837 In other words, you need to manually add @code{JvSynchronize}
1838 in a @code{native synchronized} method.
1843 CNI permits C++ applications to make calls into Java classes, in addition to
1844 allowing Java code to call into C++. Several functions, known as the
1845 @dfn{invocation API}, are provided to support this.
1847 @deftypefun jint JvCreateJavaVM (void* @var{vm_args})
1848 Initializes the Java runtime. This function performs essential initialization
1849 of the threads interface, garbage collector, exception handling and other key
1850 aspects of the runtime. It must be called once by an application with
1851 a non-Java @code{main()} function, before any other Java or CNI calls are made.
1852 It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
1853 once provided it is only called from a single thread.
1854 The @var{vmargs} parameter can be used to specify initialization parameters
1855 for the Java runtime. It may be @code{NULL}.
1856 This function returns @code{0} upon success, or @code{-1} if the runtime is
1857 already initialized.
1859 @emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
1860 used in a future release.
1863 @deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{group})
1864 Registers an existing thread with the Java runtime. This must be called once
1865 from each thread, before that thread makes any other Java or CNI calls. It
1866 must be called after @code{JvCreateJavaVM}.
1867 @var{name} specifies a name for the thread. It may be @code{NULL}, in which
1868 case a name will be generated.
1869 @var{group} is the ThreadGroup in which this thread will be a member. If it
1870 is @code{NULL}, the thread will be a member of the main thread group.
1871 The return value is the Java @code{Thread} object that represents the thread.
1872 It is safe to call @code{JvAttachCurrentThread()} more than once from the same
1873 thread. If the thread is already attached, the call is ignored and the current
1874 thread object is returned.
1877 @deftypefun jint JvDetachCurrentThread ()
1878 Unregisters a thread from the Java runtime. This should be called by threads
1879 that were attached using @code{JvAttachCurrentThread()}, after they have
1880 finished making calls to Java code. This ensures that any resources associated
1881 with the thread become eligible for garbage collection.
1882 This function returns @code{0} upon success, or @code{-1} if the current thread
1886 @subsection Handling uncaught exceptions
1888 If an exception is thrown from Java code called using the invocation API, and
1889 no handler for the exception can be found, the runtime will abort the
1890 application. In order to make the application more robust, it is recommended
1891 that code which uses the invocation API be wrapped by a top-level try/catch
1892 block that catches all Java exceptions.
1896 The following code demonstrates the use of the invocation API. In this
1897 example, the C++ application initializes the Java runtime and attaches
1898 itself. The @code{java.lang.System} class is initialized in order to
1899 access its @code{out} field, and a Java string is printed. Finally, the thread
1900 is detached from the runtime once it has finished making Java calls. Everything
1901 is wrapped with a try/catch block to provide a default handler for any uncaught
1904 The example can be compiled with @command{c++ test.cc -lgcj}.
1908 #include <gcj/cni.h>
1909 #include <java/lang/System.h>
1910 #include <java/io/PrintStream.h>
1911 #include <java/lang/Throwable.h>
1913 int main(int argc, char *argv)
1915 using namespace java::lang;
1919 JvCreateJavaVM(NULL);
1920 JvAttachCurrentThread(NULL, NULL);
1922 String *message = JvNewStringLatin1("Hello from C++");
1923 JvInitClass(&System::class$);
1924 System::out->println(message);
1926 JvDetachCurrentThread();
1928 catch (Throwable *t)
1930 System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
1931 t->printStackTrace();
1939 Reflection is possible with CNI code, it functions similarly to how it
1940 functions with JNI@.
1942 @c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
1943 The types @code{jfieldID} and @code{jmethodID}
1946 @noindent The functions:
1949 @item @code{JvFromReflectedField},
1950 @item @code{JvFromReflectedMethod},
1951 @item @code{JvToReflectedField}
1952 @item @code{JvToFromReflectedMethod}
1955 @noindent will be added shortly, as will other functions corresponding to JNI@.
1958 @node System properties
1959 @chapter System properties
1961 The runtime behavior of the @code{libgcj} library can be modified by setting
1962 certain system properties. These properties can be compiled into the program
1963 using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
1964 setting them explicitly in the program by calling the
1965 @code{java.lang.System.setProperty()} method. Some system properties are only
1966 used for informational purposes (like giving a version number or a user name).
1967 A program can inspect the current value of a property by calling the
1968 @code{java.lang.System.getProperty()} method.
1971 * Standard Properties:: Standard properties supported by @code{libgcj}
1972 * GNU Classpath Properties:: Properties found in Classpath based libraries
1973 * libgcj Runtime Properties:: Properties specific to @code{libgcj}
1976 @node Standard Properties
1977 @section Standard Properties
1979 The following properties are normally found in all implementations of the core
1980 libraries for the Java language.
1985 The @code{libgcj} version number.
1988 Set to @samp{The Free Software Foundation, Inc.}
1990 @item java.vendor.url
1991 Set to @uref{http://gcc.gnu.org/java/}.
1994 The directory where @code{gcj} was installed. Taken from the @code{--prefix}
1995 option given to @command{configure}.
1997 @item java.class.version
1998 The class format version number supported by the libgcj byte code interpreter.
1999 (Currently @samp{46.0})
2001 @item java.vm.specification.version
2002 The Virtual Machine Specification version implemented by @code{libgcj}.
2003 (Currently @samp{1.0})
2005 @item java.vm.specification.vendor
2006 The name of the Virtual Machine specification designer.
2008 @item java.vm.specification.name
2009 The name of the Virtual Machine specification
2010 (Set to @samp{Java Virtual Machine Specification}).
2012 @item java.vm.version
2013 The @command{gcj} version number.
2015 @item java.vm.vendor
2016 Set to @samp{The Free Software Foundation, Inc.}
2019 Set to @samp{GNU libgcj}.
2021 @item java.specification.version
2022 The Runtime Environment specification version implemented by @code{libgcj}.
2023 (Currently set to @samp{1.3})
2025 @item java.specification.vendor
2026 The Runtime Environment specification designer.
2028 @item java.specification.name
2029 The name of the Runtime Environment specification
2030 (Set to @samp{Java Platform API Specification}).
2032 @item java.class.path
2033 The paths (jar files, zip files and directories) used for finding class files.
2035 @item java.library.path
2036 Directory path used for finding native libraries. Currently not set.
2038 @item java.io.tmpdir
2039 The directory used to put temporary files in.
2042 Name of the Just In Time compiler to use by the byte code interpreter.
2043 Currently not used in @code{libgcj}.
2046 Directories containing jar files with extra libraries. Will be used when
2047 resolving classes. Currently not used in @code{libgcj}.
2049 @item java.protocol.handler.pkgs
2050 A @samp{|} separated list of package names that is used to find classes that
2051 implement handlers for @code{java.net.URL}.
2053 @item java.rmi.server.codebase
2054 A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
2055 to load classes from.
2058 A list of class names that will be loaded by the @code{java.sql.DriverManager}
2061 @item file.separator
2062 The separator used in when directories are included in a filename
2063 (normally @samp{/} or @samp{\} ).
2066 The default character encoding used when converting platform native files to
2067 Unicode (usually set to @samp{8859_1}).
2069 @item path.separator
2070 The standard separator used when a string contains multiple paths
2071 (normally @samp{:} or @samp{;}), the string is usually not a valid character
2072 to use in normal directory names.)
2074 @item line.separator
2075 The default line separator used on the platform (normally @samp{\n}, @samp{\r}
2076 or a combination of those two characters).
2078 @item policy.provider
2079 The class name used for the default policy provider returned by
2080 @code{java.security.Policy.getPolicy}.
2083 The name of the user running the program. Can be the full name, the login name
2084 or empty if unknown.
2087 The default directory to put user specific files in.
2090 The current working directory from which the program was started.
2093 The default language as used by the @code{java.util.Locale} class.
2096 The default region as used by the @code{java.util.Local} class.
2099 The default variant of the language and region local used.
2102 The default timezone as used by the @code{java.util.TimeZone} class.
2105 The operating system/kernel name that the program runs on.
2108 The hardware that we are running on.
2111 The version number of the operating system/kernel.
2113 @item awt.appletWarning
2114 The string to display when an untrusted applet is displayed.
2115 Returned by @code{java.awt.Window.getWarningString()} when the window is
2119 The class name used for initializing the default @code{java.awt.Toolkit}.
2120 Defaults to @code{gnu.awt.gtk.GtkToolkit}.
2124 @node GNU Classpath Properties
2125 @section GNU Classpath Properties
2127 @code{libgcj} is based on the GNU Classpath (Essential Libraries for Java) a
2128 GNU project to create free core class libraries for use with virtual machines
2129 and compilers for the Java language. The following properties are common to
2130 libraries based on GNU Classpath.
2134 @item gcj.dumpobject
2135 Enables printing serialization debugging by the @code{java.io.ObjectInput} and
2136 @code{java.io.ObjectOutput} classes when set to something else then the empty
2137 string. Only used when running a debug build of the library.
2141 @node libgcj Runtime Properties
2142 @section libgcj Runtime Properties
2144 The following properties are specific to the @code{libgcj} runtime and will
2145 normally not be found in other core libraries for the java language.
2149 @item java.fullversion
2150 The combination of @code{java.vm.name} and @code{java.vm.version}.
2153 Same as @code{java.fullversion}.
2156 Used by the @code{java.net.DatagramSocket} class when set to something else
2157 then the empty string. When set all newly created @code{DatagramSocket}s will
2158 try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
2159 the normal @code{java.net.PlainDatagramSocketImpl}.
2161 @item gnu.gcj.runtime.NameFinder.demangle
2162 Whether names in a stack trace should be demangled. Defaults to @code{true}.
2164 @item gnu.gcj.runtime.NameFinder.sanitize
2165 Whether calls to initialize exceptions and starting the runtime system
2166 should be removed from the stack trace. Only done when names are
2167 demangled. Defaults to @code{true}.
2169 @item gnu.gcj.runtime.NameFinder.remove_unknown
2170 Whether calls to unknown functions (class and method names are unknown)
2171 should be removed from the stack trace. Only done when the stack is
2172 sanitized. Ignored if this means no stack trace information would be
2173 available anymore. Defaults to @code{true}.
2175 @item gnu.gcj.runtime.NameFinder.remove_interpreter
2176 Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
2177 and functions starting with @samp{ffi_}) should be removed from the stack
2178 trace. Only done when the stack is sanitized. Defaults to @code{true}.
2181 @item gnu.gcj.runtime.NameFinder.use_addr2line
2182 Whether an external process (@command{addr2line} or @command{addr2name.awk})
2183 should be used as fallback to convert the addresses to function names when
2184 the runtime is unable to do it through @code{dladdr}.
2192 While writing @command{gcj} and @code{libgcj} we have, of course, relied
2193 heavily on documentation from Sun Microsystems. In particular we have
2194 used The Java Language Specification (both first and second editions),
2195 the Java Class Libraries (volumes one and two), and the Java Virtual
2196 Machine Specification. In addition we've used the online documentation
2197 at @uref{http://java.sun.com/}.
2199 The current @command{gcj} home page is
2200 @uref{http://gcc.gnu.org/java/}.
2202 For more information on gcc, see @uref{http://gcc.gnu.org/}.
2204 Some @code{libgcj} testing is done using the Mauve test suite. This is
2205 a free software Java class library test suite which is being written
2206 because the JCK is not free. See
2207 @uref{http://sources.redhat.com/mauve/} for more information.