1 \input texinfo @c -*-texinfo-*-
3 @settitle Guide to GNU gcj
5 @include gcc-common.texi
7 @c Note: When reading this manual you'll find lots of strange
8 @c circumlocutions like ``compiler for the Java language''.
9 @c This is necessary due to Sun's restrictions on the use of
12 @c When this manual is copyrighted.
13 @set copyrights-gcj 2001, 2002
16 @set which-gcj GCC-@value{version-GCC}
19 @c man begin COPYRIGHT
20 Copyright @copyright{} @value{copyrights-gcj} Free Software Foundation, Inc.
22 Permission is granted to copy, distribute and/or modify this document
23 under the terms of the GNU Free Documentation License, Version 1.2 or
24 any later version published by the Free Software Foundation; with the
25 Invariant Sections being ``GNU General Public License'', the Front-Cover
26 texts being (a) (see below), and with the Back-Cover Texts being (b)
27 (see below). A copy of the license is included in the
30 ``GNU Free Documentation License''.
32 @c man begin COPYRIGHT
37 @c man begin COPYRIGHT
39 (a) The FSF's Front-Cover Text is:
43 (b) The FSF's Back-Cover Text is:
45 You have freedom to copy and modify this GNU Manual, like GNU
46 software. Copies published by the Free Software Foundation raise
47 funds for GNU development.
53 @dircategory Programming
55 * Gcj: (gcj). Ahead-of-time compiler for the Java language
58 @dircategory Individual utilities
60 * gcjh: (gcj)Invoking gcjh.
61 Generate header files from Java class files
62 * jv-scan: (gcj)Invoking jv-scan.
63 Print information about Java source files
64 * jcf-dump: (gcj)Invoking jcf-dump.
65 Print information about Java class files
66 * gij: (gcj)Invoking gij. GNU interpreter for Java bytecode
67 * jv-convert: (gcj)Invoking jv-convert.
68 Convert file from one encoding to another
69 * rmic: (gcj)Invoking rmic.
70 Generate stubs for Remote Method Invocation.
71 * rmiregistry: (gcj)Invoking rmiregistry.
72 The remote object registry.
84 @vskip 0pt plus 1filll
85 For the @value{which-gcj} Version*
87 Published by the Free Software Foundation @*
88 59 Temple Place - Suite 330@*
89 Boston, MA 02111-1307, USA@*
100 This manual describes how to use @command{gcj}, the GNU compiler for the
101 Java programming language. @command{gcj} can generate both @file{.class}
102 files and object files, and it can read both Java source code and
106 * Copying:: The GNU General Public License
107 * GNU Free Documentation License::
108 How you can share and copy this manual
109 * Invoking gcj:: Compiler options supported by @command{gcj}
110 * Compatibility:: Compatibility between gcj and other tools for Java
111 * Invoking gcjh:: Generate header files from class files
112 * Invoking jv-scan:: Print information about source files
113 * Invoking jcf-dump:: Print information about class files
114 * Invoking gij:: Interpreting Java bytecodes
115 * Invoking jv-convert:: Converting from one encoding to another
116 * Invoking rmic:: Generate stubs for Remote Method Invocation.
117 * Invoking rmiregistry:: The remote object registry.
118 * About CNI:: Description of the Compiled Native Interface
119 * System properties:: Modifying runtime behavior of the libgcj library
120 * Resources:: Where to look for more information
130 @chapter Invoking gcj
132 @c man title gcj Ahead-of-time compiler for the Java language
135 @c man begin SYNOPSIS gcj
136 gcj [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
137 [@option{--CLASSPATH}=@var{path}] [@option{--classpath}=@var{path}]
138 [@option{-f}@var{option}@dots{}] [@option{--encoding}=@var{name}]
139 [@option{--main}=@var{classname}] [@option{-D}@var{name}[=@var{value}]@dots{}]
140 [@option{-C}] [@option{--resource} @var{resource-name}] [@option{-d} @var{directory}]
141 [@option{-W}@var{warn}@dots{}]
142 @var{sourcefile}@dots{}
144 @c man begin SEEALSO gcj
145 gcc(1), gcjh(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
146 and the Info entries for @file{gcj} and @file{gcc}.
150 @c man begin DESCRIPTION gcj
152 As @command{gcj} is just another front end to @command{gcc}, it supports many
153 of the same options as gcc. @xref{Option Summary, , Option Summary,
154 gcc, Using the GNU Compiler Collection (GCC)}. This manual only documents the
155 options specific to @command{gcj}.
160 * Input and output files::
161 * Input Options:: How gcj finds files
162 * Encodings:: Options controlling source file encoding
163 * Warnings:: Options controlling warnings specific to gcj
164 * Code Generation:: Options controlling the output of gcj
165 * Configure-time Options:: Options you won't use
168 @c man begin OPTIONS gcj
170 @node Input and output files
171 @section Input and output files
173 A @command{gcj} command is like a @command{gcc} command, in that it
174 consists of a number of options and file names. The following kinds
175 of input file names are supported:
178 @item @var{file}.java
180 @item @var{file}.class
183 @itemx @var{file}.jar
184 An archive containing one or more @code{.class} files, all of
185 which are compiled. The archive may be compressed. Files in
186 an archive which don't end with @samp{.class} are treated as
187 resource files; they are copmiled into the resulting object file
188 as @samp{core:} URLs.
190 A file containing a whitespace-separated list of input file names.
191 (Currently, these must all be @code{.java} source files, but that
193 Each named file is compiled, just as if it had been on the command line.
194 @item @var{library}.a
195 @itemx @var{library}.so
196 @itemx -l@var{libname}
197 Libraries to use when linking. See the @command{gcc} manual.
200 You can specify more than one input file on the @command{gcj} command line,
201 in which case they will all be compiled. If you specify a
202 @code{-o @var{FILENAME}}
203 option, all the input files will be compiled together, producing a
204 single output file, named @var{FILENAME}.
205 This is allowed even when using @code{-S} or @code{-c},
206 but not when using @code{-C} or @code{--resource}.
207 (This is an extension beyond the what plain @command{gcc} allows.)
208 (If more than one input file is specified, all must currently
209 be @code{.java} files, though we hope to fix this.)
212 @section Input Options
216 @command{gcj} has options to control where it looks to find files it needs.
217 For instance, @command{gcj} might need to load a class that is referenced
218 by the file it has been asked to compile. Like other compilers for the
219 Java language, @command{gcj} has a notion of a @dfn{class path}. There are
220 several options and environment variables which can be used to
221 manipulate the class path. When @command{gcj} looks for a given class, it
222 searches the class path looking for matching @file{.class} or
223 @file{.java} file. @command{gcj} comes with a built-in class path which
224 points at the installed @file{libgcj.jar}, a file which contains all the
227 In the below, a directory or path component can refer either to an
228 actual directory on the filesystem, or to a @file{.zip} or @file{.jar}
229 file, which @command{gcj} will search as if it is a directory.
233 All directories specified by @code{-I} are kept in order and prepended
234 to the class path constructed from all the other options. Unless
235 compatibility with tools like @code{javac} is important, we recommend
236 always using @code{-I} instead of the other options for manipulating the
239 @item --classpath=@var{path}
240 This sets the class path to @var{path}, a colon-separated list of paths
241 (on Windows-based systems, a semicolon-separate list of paths).
242 This does not override the builtin (``boot'') search path.
244 @item --CLASSPATH=@var{path}
245 Deprecated synonym for @code{--classpath}.
247 @item --bootclasspath=@var{path}
248 Where to find the standard builtin classes, such as @code{java.lang.String}.
250 @item --extdirs=@var{path}
251 For each directory in the @var{path}, place the contents of that
252 directory at the end of the class path.
255 This is an environment variable which holds a list of paths.
258 The final class path is constructed like so:
262 First come all directories specified via @code{-I}.
265 If @option{--classpath} is specified, its value is appended.
266 Otherwise, if the @code{CLASSPATH} environment variable is specified,
267 then its value is appended.
268 Otherwise, the current directory (@code{"."}) is appended.
271 If @code{--bootclasspath} was specified, append its value.
272 Otherwise, append the built-in system directory, @file{libgcj.jar}.
275 Finally, if @code{--extdirs} was specified, append the contents of the
276 specified directories at the end of the class path. Otherwise, append
277 the contents of the built-in extdirs at @code{$(prefix)/share/java/ext}.
280 The classfile built by @command{gcj} for the class @code{java.lang.Object}
281 (and placed in @code{libgcj.jar}) contains a special zero length
282 attribute @code{gnu.gcj.gcj-compiled}. The compiler looks for this
283 attribute when loading @code{java.lang.Object} and will report an error
284 if it isn't found, unless it compiles to bytecode (the option
285 @code{-fforce-classes-archive-check} can be used to override this
286 behavior in this particular case.)
289 @item -fforce-classes-archive-check
290 This forces the compiler to always check for the special zero length
291 attribute @code{gnu.gcj.gcj-compiled} in @code{java.lang.Object} and
292 issue an error if it isn't found.
298 The Java programming language uses Unicode throughout. In an effort to
299 integrate well with other locales, @command{gcj} allows @file{.java} files
300 to be written using almost any encoding. @command{gcj} knows how to
301 convert these encodings into its internal encoding at compile time.
303 You can use the @code{--encoding=@var{NAME}} option to specify an
304 encoding (of a particular character set) to use for source files. If
305 this is not specified, the default encoding comes from your current
306 locale. If your host system has insufficient locale support, then
307 @command{gcj} assumes the default encoding to be the @samp{UTF-8} encoding
310 To implement @code{--encoding}, @command{gcj} simply uses the host
311 platform's @code{iconv} conversion routine. This means that in practice
312 @command{gcj} is limited by the capabilities of the host platform.
314 The names allowed for the argument @code{--encoding} vary from platform
315 to platform (since they are not standardized anywhere). However,
316 @command{gcj} implements the encoding named @samp{UTF-8} internally, so if
317 you choose to use this for your source files you can be assured that it
318 will work on every host.
324 @command{gcj} implements several warnings. As with other generic
325 @command{gcc} warnings, if an option of the form @code{-Wfoo} enables a
326 warning, then @code{-Wno-foo} will disable it. Here we've chosen to
327 document the form of the warning which will have an effect -- the
328 default being the opposite of what is listed.
331 @item -Wredundant-modifiers
332 With this flag, @command{gcj} will warn about redundant modifiers. For
333 instance, it will warn if an interface method is declared @code{public}.
335 @item -Wextraneous-semicolon
336 This causes @command{gcj} to warn about empty statements. Empty statements
337 have been deprecated.
339 @item -Wno-out-of-date
340 This option will cause @command{gcj} not to warn when a source file is
341 newer than its matching class file. By default @command{gcj} will warn
344 @item -Wno-deprecated
345 Warn if a deprecated class, method, or field is referred to.
348 This is the same as @command{gcc}'s @code{-Wunused}.
351 This is the same as @code{-Wredundant-modifiers -Wextraneous-semicolon
356 @node Code Generation
357 @section Code Generation
359 In addition to the many @command{gcc} options controlling code generation,
360 @command{gcj} has several options specific to itself.
363 @item --main=@var{CLASSNAME}
364 This option is used when linking to specify the name of the class whose
365 @code{main} method should be invoked when the resulting executable is
366 run. @footnote{The linker by default looks for a global function named
367 @code{main}. Since Java does not have global functions, and a
368 collection of Java classes may have more than one class with a
369 @code{main} method, you need to let the linker know which of those
370 @code{main} methods it should invoke when starting the application.}
372 @item -D@var{name}[=@var{value}]
373 This option can only be used with @code{--main}. It defines a system
374 property named @var{name} with value @var{value}. If @var{value} is not
375 specified then it defaults to the empty string. These system properties
376 are initialized at the program's startup and can be retrieved at runtime
377 using the @code{java.lang.System.getProperty} method.
380 This option is used to tell @command{gcj} to generate bytecode
381 (@file{.class} files) rather than object code.
383 @item --resource @var{resource-name}
384 This option is used to tell @command{gcj} to compile the contents of a
385 given file to object code so it may be accessed at runtime with the core
386 protocol handler as @samp{core:/@var{resource-name}}. Note that
387 @var{resource-name} is the name of the resource as found at runtime; for
388 instance, it could be used in a call to @code{ResourceBundle.getBundle}.
389 The actual file name to be compiled this way must be specified
392 @item -d @var{directory}
393 When used with @code{-C}, this causes all generated @file{.class} files
394 to be put in the appropriate subdirectory of @var{directory}. By
395 default they will be put in subdirectories of the current working
398 @item -fno-bounds-check
399 By default, @command{gcj} generates code which checks the bounds of all
400 array indexing operations. With this option, these checks are omitted, which
401 can improve performance for code that uses arrays extensively. Note that this
402 can result in unpredictable behavior if the code in question actually does
403 violate array bounds constraints. It is safe to use this option if you are
404 sure that your code will never throw an @code{ArrayIndexOutOfBoundsException}.
406 @item -fno-store-check
407 Don't generate array store checks. When storing objects into arrays, a runtime
408 check is normally generated in order to ensure that the object is assignment
409 compatible with the component type of the array (which may not be known
410 at compile-time). With this option, these checks are omitted. This can
411 improve performance for code which stores objects into arrays frequently.
412 It is safe to use this option if you are sure your code will never throw an
413 @code{ArrayStoreException}.
416 With @command{gcj} there are two options for writing native methods: CNI
417 and JNI@. By default @command{gcj} assumes you are using CNI@. If you are
418 compiling a class with native methods, and these methods are implemented
419 using JNI, then you must use @code{-fjni}. This option causes
420 @command{gcj} to generate stubs which will invoke the underlying JNI
424 Don't recognize the @code{assert} keyword. This is for compatibility
425 with older versions of the language specification.
427 @item -fno-optimize-static-class-initialization
428 When the optimization level is greater or equal to @code{-O2},
429 @command{gcj} will try to optimize the way calls into the runtime are made
430 to initialize static classes upon their first use (this optimization
431 isn't carried out if @code{-C} was specified.) When compiling to native
432 code, @code{-fno-optimize-static-class-initialization} will turn this
433 optimization off, regardless of the optimization level in use.
437 @node Configure-time Options
438 @section Configure-time Options
440 Some @command{gcj} code generations options affect the resulting ABI, and
441 so can only be meaningfully given when @code{libgcj}, the runtime
442 package, is configured. @code{libgcj} puts the appropriate options from
443 this group into a @samp{spec} file which is read by @command{gcj}. These
444 options are listed here for completeness; if you are using @code{libgcj}
445 then you won't want to touch these options.
449 This enables the use of the Boehm GC bitmap marking code. In particular
450 this causes @command{gcj} to put an object marking descriptor into each
453 @item -fhash-synchronization
454 By default, synchronization data (the data used for @code{synchronize},
455 @code{wait}, and @code{notify}) is pointed to by a word in each object.
456 With this option @command{gcj} assumes that this information is stored in a
457 hash table and not in the object itself.
459 @item -fuse-divide-subroutine
460 On some systems, a library routine is called to perform integer
461 division. This is required to get exception handling correct when
464 @item -fcheck-references
465 On some systems it's necessary to insert inline checks whenever
466 accessing an object via a reference. On other systems you won't need
467 this because null pointer accesses are caught automatically by the
474 @chapter Compatibility with the Java Platform
476 As we believe it is important that the Java platform not be fragmented,
477 @command{gcj} and @code{libgcj} try to conform to the relevant Java
478 specifications. However, limited manpower and incomplete and unclear
479 documentation work against us. So, there are caveats to using
488 @section Standard features not yet supported
490 This list of compatibility issues is by no means complete.
494 @command{gcj} implements the JDK 1.2 language. It supports inner classes
495 and the new 1.4 @code{assert} keyword. It does not yet support the Java 2
496 @code{strictfp} keyword (it recognizes the keyword but ignores it).
499 @code{libgcj} is largely compatible with the JDK 1.2 libraries.
500 However, @code{libgcj} is missing many packages, most notably
501 @code{java.awt}. There are also individual missing classes and methods.
502 We currently do not have a list showing differences between
503 @code{libgcj} and the Java 2 platform.
506 Sometimes the @code{libgcj} implementation of a method or class differs
507 from the JDK implementation. This is not always a bug. Still, if it
508 affects you, it probably makes sense to report it so that we can discuss
509 the appropriate response.
512 @command{gcj} does not currently allow for piecemeal replacement of
513 components within @code{libgcj}. Unfortunately, programmers often want
514 to use newer versions of certain packages, such as those provided by
515 the Apache Software Foundation's Jakarta project. This has forced us
516 to place the @code{org.w3c.dom} and @code{org.xml.sax} packages into
517 their own libraries, separate from @code{libgcj}. If you intend to
518 use these classes, you must link them explicitly with
519 @code{-l-org-w3c-dom} and @code{-l-org-xml-sax}. Future versions of
520 @command{gcj} may not have this restriction.
524 @section Extra features unique to gcj
526 The main feature of @command{gcj} is that it can compile programs written in
527 the Java programming language to native code. Most extensions that have been
528 added are to facilitate this functionality.
532 @command{gcj} makes it easy and efficient to mix code written in Java and C++.
533 @xref{About CNI}, for more info on how to use this in your programs.
536 When you compile your classes into a shared library they can be automatically
537 loaded by the @code{libgcj} system classloader. When trying to load a class
538 @code{gnu.pkg.SomeClass} the system classloader will first try to load the
539 shared library @file{lib-gnu-pkg-SomeClass.so}, if that fails to load the
540 class then it will try to load @file{lib-gnu-pkg.so} and finally when the
541 class is still not loaded it will try to load @file{lib-gnu.so}. Note that
542 all @samp{.}s will be transformed into @samp{-}s and that searching
543 for inner classes starts with their outermost outer class. If the class
544 cannot be found this way the system classloader tries to use
545 the @code{libgcj} bytecode interpreter to load the class from the standard
546 classpath. This process can be controlled to some degree via the
547 @code{gnu.gcj.runtime.VMClassLoader.library_control} property;
548 @xref{libgcj Runtime Properties}.
552 @chapter Invoking gcjh
554 @c man title gcjh generate header files from Java class files
556 @c man begin DESCRIPTION gcjh
558 The @code{gcjh} program is used to generate header files from class
559 files. It can generate both CNI and JNI header files, as well as stub
560 implementation files which can be used as a basis for implementing the
561 required native methods.
566 @c man begin SYNOPSIS gcjh
567 gcjh [@option{-stubs}] [@option{-jni}]
568 [@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
569 [@option{-preprend} @var{text}]
570 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
571 [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
572 [@option{-o} @var{file}] [@option{-td} @var{dir}]
573 [@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
574 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
575 @var{classname}@dots{}
577 @c man begin SEEALSO gcjh
578 gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
579 and the Info entries for @file{gcj} and @file{gcc}.
583 @c man begin OPTIONS gcjh
587 This causes @code{gcjh} to generate stub files instead of header files.
588 By default the stub file will be named after the class, with a suffix of
589 @samp{.cc}. In JNI mode, the default output file will have the suffix
593 This tells @code{gcjh} to generate a JNI header or stub. By default,
594 CNI headers are generated.
596 @item -add @var{text}
597 Inserts @var{text} into the class body. This is ignored in JNI mode.
599 @item -append @var{text}
600 Inserts @var{text} into the header file after the class declaration.
601 This is ignored in JNI mode.
603 @item -friend @var{text}
604 Inserts @var{text} into the class as a @code{friend} declaration.
605 This is ignored in JNI mode.
607 @item -prepend @var{text}
608 Inserts @var{text} into the header file before the class declaration.
609 This is ignored in JNI mode.
611 @item --classpath=@var{path}
612 @itemx --CLASSPATH=@var{path}
613 @itemx -I@var{directory}
614 @itemx -d @var{directory}
616 These options are all identical to the corresponding @command{gcj} options.
619 Sets the output file name. This cannot be used if there is more than
620 one class on the command line.
622 @item -td @var{directory}
623 Sets the name of the directory to use for temporary files.
626 Print all dependencies to stdout; suppress ordinary output.
629 Print non-system dependencies to stdout; suppress ordinary output.
632 Print all dependencies to stdout.
635 Print non-system dependencies to stdout.
638 Print help about @code{gcjh} and exit. No further processing is done.
641 Print version information for @code{gcjh} and exit. No further
645 Print extra information while running.
648 All remaining options are considered to be names of classes.
652 @node Invoking jv-scan
653 @chapter Invoking jv-scan
655 @c man title jv-scan print information about Java source file
657 @c man begin DESCRIPTION jv-scan
659 The @code{jv-scan} program can be used to print information about a Java
660 source file (@file{.java} file).
665 @c man begin SYNOPSIS jv-scan
666 jv-scan [@option{--no-assert}] [@option{--complexity}]
667 [@option{--encoding}=@var{name}] [@option{--print-main}]
668 [@option{--list-class}] [@option{--list-filename}]
669 [@option{--version}] [@option{--help}]
670 [@option{-o} @var{file}] @var{inputfile}@dots{}
672 @c man begin SEEALSO jv-scan
673 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
674 and the Info entries for @file{gcj} and @file{gcc}.
678 @c man begin OPTIONS jv-scan
682 Don't recognize the @code{assert} keyword, for backwards compatibility
683 with older versions of the language specification.
686 This prints a complexity measure, related to cyclomatic complexity, for
689 @item --encoding=@var{name}
690 This works like the corresponding @command{gcj} option.
693 This prints the name of the class in this file containing a @code{main}
697 This lists the names of all classes defined in the input files.
699 @item --list-filename
700 If @code{--list-class} is given, this option causes @code{jv-scan} to
701 also print the name of the file in which each class was found.
704 Print output to the named file.
707 Print help, then exit.
710 Print version number, then exit.
715 @node Invoking jcf-dump
716 @chapter Invoking jcf-dump
718 @c man title jcf-dump print information about Java class files
721 @c man begin SYNOPSIS jcf-dump
722 jcf-dump [@option{-c}] [@option{--javap}]
723 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
724 [@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
725 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
726 @var{classname}@dots{}
728 @c man begin SEEALSO jcf-dump
729 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
730 and the Info entries for @file{gcj} and @file{gcc}.
734 @c man begin DESCRIPTION jcf-dump
736 This is a class file examiner, similar to @code{javap}. It will print
737 information about a number of classes, which are specified by class name
742 @c man begin OPTIONS jcf-dump
746 Disassemble method bodies. By default method bodies are not printed.
749 Generate output in @code{javap} format. The implementation of this
750 feature is very incomplete.
752 @item --classpath=@var{path}
753 @itemx --CLASSPATH=@var{path}
754 @itemx -I@var{directory}
756 These options as the same as the corresponding @command{gcj} options.
759 Print help, then exit.
762 Print version number, then exit.
765 Print extra information while running.
771 @chapter Invoking gij
773 @c man title gij GNU interpreter for Java bytecode
776 @c man begin SYNOPSIS gij
777 gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
779 gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
780 [@option{-cp} @var{path}] [@option{-classpath} @var{path}]
781 [@option{-D}@var{name}[=@var{value}]@dots{}]
782 [@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
783 [@option{-X@var{argument}]
784 [@option{--showversion}] [@option{--version}] [@option{--help}][@option{-?}]
786 @c man begin SEEALSO gij
787 gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
788 and the Info entries for @file{gcj} and @file{gcc}.
792 @c man begin DESCRIPTION gij
794 @code{gij} is a Java bytecode interpreter included with @code{libgcj}.
795 @code{gij} is not available on every platform; porting it requires a
796 small amount of assembly programming which has not been done for all the
797 targets supported by @command{gcj}.
799 The primary argument to @code{gij} is the name of a class or, with
800 @code{-jar}, a jar file. Options before this argument are interpreted
801 by @code{gij}; remaining options are passed to the interpreted program.
803 If a class name is specified and this class does not have a @code{main}
804 method with the appropriate signature (a @code{static void} method with
805 a @code{String[]} as its sole argument), then @code{gij} will print an
808 If a jar file is specified then @code{gij} will use information in it to
809 determine which class' @code{main} method will be invoked.
811 @code{gij} will invoke the @code{main} method with all the remaining
812 command-line options.
814 Note that @code{gij} is not limited to interpreting code. Because
815 @code{libgcj} includes a class loader which can dynamically load shared
816 objects, it is possible to give @code{gij} the name of a class which has
817 been compiled and put into a shared library on the class path.
821 @c man begin OPTIONS gij
825 @itemx -classpath @var{path}
826 Set the initial class path. The class path is used for finding
827 class and resource files. If specified, this option overrides the
828 @code{CLASSPATH} environment variable. Note that this option is
829 ignored if @code{-jar} is used.
831 @item -D@var{name}[=@var{value}]
832 This defines a system property named @var{name} with value @var{value}.
833 If @var{value} is not specified then it defaults to the empty string.
834 These system properties are initialized at the program's startup and can
835 be retrieved at runtime using the @code{java.lang.System.getProperty}
838 @item -ms=@var{number}
839 This sets the initial heap size.
841 @item -mx=@var{number}
842 This sets the maximum heap size.
845 @itemx -X@var{argument}
846 Supplying @code{-X} by itself will cause @code{gij} to list all the
847 supported @code{-X} options. Currently there are none. Unrecognized
848 @code{-X} options are ignored, for compatibility with other runtimes.
851 This indicates that the name passed to @code{gij} should be interpreted
852 as the name of a jar file, not a class.
856 Print help, then exit.
859 Print version number and continue.
862 Print version number, then exit.
867 @node Invoking jv-convert
868 @chapter Invoking jv-convert
870 @c man title jv-convert Convert file from one encoding to another
872 @c man begin SYNOPSIS jv-convert
873 @command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
876 [@option{--encoding} @var{name}]
877 [@option{--from} @var{name}]
878 [@option{--to} @var{name}]
879 [@option{-i} @var{file}] [@option{-o} @var{file}]
880 [@option{--reverse}] [@option{--help}] [@option{--version}]
884 @c man begin DESCRIPTION jv-convert
886 @command{jv-convert} is a utility included with @code{libgcj} which
887 converts a file from one encoding to another. It is similar to the Unix
888 @command{iconv} utility.
890 The encodings supported by @command{jv-convert} are platform-dependent.
891 Currently there is no way to get a list of all supported encodings.
895 @c man begin OPTIONS jv-convert
898 @item --encoding @var{name}
899 @itemx --from @var{name}
900 Use @var{name} as the input encoding. The default is the current
903 @item --to @var{name}
904 Use @var{name} as the output encoding. The default is the
905 @code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
906 non-ASCII characters.
909 Read from @var{file}. The default is to read from standard input.
912 Write to @var{file}. The default is to write to standard output.
915 Swap the input and output encodings.
918 Print a help message, then exit.
921 Print version information, then exit.
927 @chapter Invoking rmic
929 @c man title rmic Generate stubs for Remote Method Invocation
931 @c man begin SYNOPSIS rmic
932 @command{rmic} [@option{OPTION}] @dots{} @var{class} @dots{}
935 [@option{-keepgenerated}]
939 [@option{-nocompile}]
941 [@option{-d} @var{directory}]
947 @c man begin DESCRIPTION rmic
949 @command{rmic} is a utility included with @code{libgcj} which generates
950 stubs for remote objects.
952 @c FIXME: Add real information here.
953 @c This really isn't much more than the --help output.
955 Note that this program isn't yet fully compatible with the JDK
956 @command{rmic}. Some options, such as @option{-classpath}, are
957 recognized but currently ignored. We have left these options
958 undocumented for now.
960 Long options can also be given with a GNU-style leading @samp{--}. For
961 instance, @option{--help} is accepted.
965 @c man begin OPTIONS rmic
969 @itemx -keepgenerated
970 By default, @command{rmic} deletes intermediate files. Either of these
971 options causes it not to delete such files.
974 Cause @command{rmic} to create stubs and skeletons for the 1.1
978 Cause @command{rmic} to create stubs and skeletons compatible with both
979 the 1.1 and 1.2 protocol versions. This is the default.
982 Cause @command{rmic} to create stubs and skeletons for the 1.2
986 Don't compile the generated files.
989 Print information about what @command{rmic} is doing.
991 @item -d @var{directory}
992 Put output files in @var{directory}. By default the files are put in
993 the current working directory.
996 Print a help message, then exit.
999 Print version information, then exit.
1005 @node Invoking rmiregistry
1006 @chapter Invoking rmiregistry
1008 @c man title rmiregistry Remote object registry
1010 @c man begin SYNOPSIS rmiregistry
1011 @command{rmic} [@option{OPTION}] @dots{} [@var{port}]
1014 [@option{--version}]
1018 @c man begin DESCRIPTION rmiregistry
1020 @command{rmiregistry} starts a remote object registry on the current
1021 host. If no port number is specified, then port 1099 is used.
1023 @c FIXME: Add real information here.
1024 @c This really isn't much more than the --help output.
1028 @c man begin OPTIONS rmiregistry
1032 Print a help message, then exit.
1035 Print version information, then exit.
1044 This documents CNI, the Compiled Native Interface,
1045 which is is a convenient way to write Java native methods using C++.
1046 This is a more efficient, more convenient, but less portable
1047 alternative to the standard JNI (Java Native Interface).
1050 * Basic concepts:: Introduction to using CNI@.
1051 * Packages:: How packages are mapped to C++.
1052 * Primitive types:: Handling Java types in C++.
1053 * Interfaces:: How Java interfaces map to C++.
1054 * Objects and Classes:: C++ and Java classes.
1055 * Class Initialization:: How objects are initialized.
1056 * Object allocation:: How to create Java objects in C++.
1057 * Arrays:: Dealing with Java arrays in C++.
1058 * Methods:: Java methods in C++.
1059 * Strings:: Information about Java Strings.
1060 * Mixing with C++:: How CNI can interoperate with C++.
1061 * Exception Handling:: How exceptions are handled.
1062 * Synchronization:: Synchronizing between Java and C++.
1063 * Invocation:: Starting the Java runtime from C++.
1064 * Reflection:: Using reflection from C++.
1068 @node Basic concepts
1069 @section Basic concepts
1071 In terms of languages features, Java is mostly a subset
1072 of C++. Java has a few important extensions, plus a powerful standard
1073 class library, but on the whole that does not change the basic similarity.
1074 Java is a hybrid object-oriented language, with a few native types,
1075 in addition to class types. It is class-based, where a class may have
1076 static as well as per-object fields, and static as well as instance methods.
1077 Non-static methods may be virtual, and may be overloaded. Overloading is
1078 resolved at compile time by matching the actual argument types against
1079 the parameter types. Virtual methods are implemented using indirect calls
1080 through a dispatch table (virtual function table). Objects are
1081 allocated on the heap, and initialized using a constructor method.
1082 Classes are organized in a package hierarchy.
1084 All of the listed attributes are also true of C++, though C++ has
1085 extra features (for example in C++ objects may be allocated not just
1086 on the heap, but also statically or in a local stack frame). Because
1087 @command{gcj} uses the same compiler technology as G++ (the GNU
1088 C++ compiler), it is possible to make the intersection of the two
1089 languages use the same ABI (object representation and calling
1090 conventions). The key idea in CNI is that Java objects are C++
1091 objects, and all Java classes are C++ classes (but not the other way
1092 around). So the most important task in integrating Java and C++ is to
1093 remove gratuitous incompatibilities.
1095 You write CNI code as a regular C++ source file. (You do have to use
1096 a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
1098 @noindent A CNI C++ source file must have:
1101 #include <gcj/cni.h>
1104 @noindent and then must include one header file for each Java class it uses, e.g.:
1107 #include <java/lang/Character.h>
1108 #include <java/util/Date.h>
1109 #include <java/lang/IndexOutOfBoundsException.h>
1112 @noindent These header files are automatically generated by @code{gcjh}.
1115 CNI provides some functions and macros to make using Java objects and
1116 primitive types from C++ easier. In general, these CNI functions and
1117 macros start with the @code{Jv} prefix, for example the function
1118 @code{JvNewObjectArray}. This convention is used to avoid conflicts
1119 with other libraries. Internal functions in CNI start with the prefix
1120 @code{_Jv_}. You should not call these; if you find a need to, let us
1121 know and we will try to come up with an alternate solution. (This
1122 manual lists @code{_Jv_AllocBytes} as an example; CNI should instead
1123 provide a @code{JvAllocBytes} function.)
1126 @subsection Limitations
1128 Whilst a Java class is just a C++ class that doesn't mean that you are
1129 freed from the shackles of Java, a @acronym{CNI} C++ class must adhere to the
1130 rules of the Java programming language.
1132 For example: it is not possible to declare a method in a CNI class
1133 that will take a C string (@code{char*}) as an argument, or to declare a
1134 member variable of some non-Java datatype.
1140 The only global names in Java are class names, and packages. A
1141 @dfn{package} can contain zero or more classes, and also zero or more
1142 sub-packages. Every class belongs to either an unnamed package or a
1143 package that has a hierarchical and globally unique name.
1145 A Java package is mapped to a C++ @dfn{namespace}. The Java class
1146 @code{java.lang.String} is in the package @code{java.lang}, which is a
1147 sub-package of @code{java}. The C++ equivalent is the class
1148 @code{java::lang::String}, which is in the namespace @code{java::lang}
1149 which is in the namespace @code{java}.
1151 @noindent Here is how you could express this:
1154 (// @r{Declare the class(es), possibly in a header file:}
1163 class java::lang::String : public java::lang::Object
1169 @noindent The @code{gcjh} tool automatically generates the necessary namespace
1173 @subsection Leaving out package names
1175 Always using the fully-qualified name of a java class can be
1176 tiresomely verbose. Using the full qualified name also ties the code
1177 to a single package making code changes necessary should the class
1178 move from one package to another. The Java @code{package} declaration
1179 specifies that the following class declarations are in the named
1180 package, without having to explicitly name the full package
1181 qualifiers. The @code{package} declaration can be
1182 followed by zero or more @code{import} declarations, which
1183 allows either a single class or all the classes in a package to be
1184 named by a simple identifier. C++ provides something similar with the
1185 @code{using} declaration and directive.
1190 import @var{package-name}.@var{class-name};
1193 @noindent allows the program text to refer to @var{class-name} as a shorthand for
1194 the fully qualified name: @code{@var{package-name}.@var{class-name}}.
1197 @noindent To achieve the same effect C++, you have to do this:
1200 using @var{package-name}::@var{class-name};
1204 @noindent Java can also cause imports on demand, like this:
1207 import @var{package-name}.*;
1210 @noindent Doing this allows any class from the package @var{package-name} to be
1211 referred to only by its class-name within the program text.
1214 @noindent The same effect can be achieved in C++ like this:
1217 using namespace @var{package-name};
1221 @node Primitive types
1222 @section Primitive types
1224 Java provides 8 @dfn{primitives} types which represent integers, floats,
1225 characters and booleans (and also the void type). C++ has its own
1226 very similar concrete types. Such types in C++ however are not always
1227 implemented in the same way (an int might be 16, 32 or 64 bits for example)
1228 so CNI provides a special C++ type for each primitive Java type:
1230 @multitable @columnfractions .20 .25 .60
1231 @item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
1232 @item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
1233 @item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
1234 @item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
1235 @item @code{short} @tab @code{jshort} @tab 16 bit signed integer
1236 @item @code{int} @tab @code{jint} @tab 32 bit signed integer
1237 @item @code{long} @tab @code{jlong} @tab 64 bit signed integer
1238 @item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
1239 @item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
1240 @item @code{void} @tab @code{void} @tab no value
1243 When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
1244 to avoid disappointment.
1247 @subsection Reference types associated with primitive types
1249 In Java each primitive type has an associated reference type,
1250 e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
1251 In order to make working with such classes easier GCJ provides the macro
1254 @deffn macro JvPrimClass type
1255 Return a pointer to the @code{Class} object corresponding to the type supplied.
1258 JvPrimClass(void) @result{} java.lang.Void.TYPE
1267 A Java class can @dfn{implement} zero or more
1268 @dfn{interfaces}, in addition to inheriting from
1269 a single base class.
1271 @acronym{CNI} allows CNI code to implement methods of interfaces.
1272 You can also call methods through interface references, with some
1275 @acronym{CNI} doesn't understand interface inheritance at all yet. So,
1276 you can only call an interface method when the declared type of the
1277 field being called matches the interface which declares that
1278 method. The workaround is to cast the interface reference to the right
1281 For example if you have:
1289 interface B extends A
1295 and declare a variable of type @code{B} in C++, you can't call
1296 @code{a()} unless you cast it to an @code{A} first.
1298 @node Objects and Classes
1299 @section Objects and Classes
1303 All Java classes are derived from @code{java.lang.Object}. C++ does
1304 not have a unique root class, but we use the C++ class
1305 @code{java::lang::Object} as the C++ version of the
1306 @code{java.lang.Object} Java class. All other Java classes are mapped
1307 into corresponding C++ classes derived from @code{java::lang::Object}.
1309 Interface inheritance (the @code{implements} keyword) is currently not
1310 reflected in the C++ mapping.
1313 @subsection Object fields
1315 Each object contains an object header, followed by the instance fields
1316 of the class, in order. The object header consists of a single
1317 pointer to a dispatch or virtual function table. (There may be extra
1318 fields @emph{in front of} the object, for example for memory
1319 management, but this is invisible to the application, and the
1320 reference to the object points to the dispatch table pointer.)
1322 The fields are laid out in the same order, alignment, and size as in
1323 C++. Specifically, 8-bite and 16-bit native types (@code{byte},
1324 @code{short}, @code{char}, and @code{boolean}) are @emph{not} widened
1325 to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
1326 to 32 bits when on the VM stack or temporary registers.
1328 If you include the @code{gcjh}-generated header for a
1329 class, you can access fields of Java classes in the @emph{natural}
1330 way. For example, given the following Java class:
1336 public Integer (int i) @{ this.i = i; @}
1337 public static zero = new Integer(0);
1344 #include <gcj/cni.h>;
1348 mult (Int *p, jint k)
1351 return Int::zero; // @r{Static member access.}
1352 return new Int(p->i * k);
1357 @subsection Access specifiers
1359 CNI does not strictly enforce the Java access
1360 specifiers, because Java permissions cannot be directly mapped
1361 into C++ permission. Private Java fields and methods are mapped
1362 to private C++ fields and methods, but other fields and methods
1363 are mapped to public fields and methods.
1367 @node Class Initialization
1368 @section Class Initialization
1370 Java requires that each class be automatically initialized at the time
1371 of the first active use. Initializing a class involves
1372 initializing the static fields, running code in class initializer
1373 methods, and initializing base classes. There may also be
1374 some implementation specific actions, such as allocating
1375 @code{String} objects corresponding to string literals in
1378 The GCJ compiler inserts calls to @code{JvInitClass} at appropriate
1379 places to ensure that a class is initialized when required. The C++
1380 compiler does not insert these calls automatically---it is the
1381 programmer's responsibility to make sure classes are initialized.
1382 However, this is fairly painless because of the conventions assumed by
1385 First, @code{libgcj} will make sure a class is initialized before an
1386 instance of that object is created. This is one of the
1387 responsibilities of the @code{new} operation. This is taken care of
1388 both in Java code, and in C++ code. When G++ sees a @code{new} of a
1389 Java class, it will call a routine in @code{libgcj} to allocate the
1390 object, and that routine will take care of initializing the class.
1391 Note however that this does not happen for Java arrays; you must
1392 allocate those using the appropriate CNI function. It follows that
1393 you can access an instance field, or call an instance (non-static)
1394 method and be safe in the knowledge that the class and all of its base
1395 classes have been initialized.
1397 Invoking a static method is also safe. This is because the
1398 Java compiler adds code to the start of a static method to make sure
1399 the class is initialized. However, the C++ compiler does not
1400 add this extra code. Hence, if you write a native static method
1401 using CNI, you are responsible for calling @code{JvInitClass}
1402 before doing anything else in the method (unless you are sure
1403 it is safe to leave it out).
1405 Accessing a static field also requires the class of the
1406 field to be initialized. The Java compiler will generate code
1407 to call @code{Jv_InitClass} before getting or setting the field.
1408 However, the C++ compiler will not generate this extra code,
1409 so it is your responsibility to make sure the class is
1410 initialized before you access a static field from C++.
1413 @node Object allocation
1414 @section Object allocation
1416 New Java objects are allocated using a
1417 @dfn{class instance creation expression}, e.g.:
1420 new @var{Type} ( ... )
1423 The same syntax is used in C++. The main difference is that
1424 C++ objects have to be explicitly deleted; in Java they are
1425 automatically deleted by the garbage collector.
1426 Using @acronym{CNI}, you can allocate a new Java object
1427 using standard C++ syntax and the C++ compiler will allocate
1428 memory from the garbage collector. If you have overloaded
1429 constructors, the compiler will choose the correct one
1430 using standard C++ overload resolution rules.
1432 @noindent For example:
1435 java::util::Hashtable *ht = new java::util::Hashtable(120);
1438 @deftypefun void* _Jv_AllocBytes (jsize @var{size})
1439 Allocates @var{size} bytes from the heap. The memory is not scanned
1440 by the garbage collector but it freed if no references to it are discovered.
1447 While in many ways Java is similar to C and C++, it is quite different
1448 in its treatment of arrays. C arrays are based on the idea of pointer
1449 arithmetic, which would be incompatible with Java's security
1450 requirements. Java arrays are true objects (array types inherit from
1451 @code{java.lang.Object}). An array-valued variable is one that
1452 contains a reference (pointer) to an array object.
1454 Referencing a Java array in C++ code is done using the
1455 @code{JArray} template, which as defined as follows:
1458 class __JArray : public java::lang::Object
1465 class JArray : public __JArray
1469 T& operator[](jint i) @{ return data[i]; @}
1474 There are a number of @code{typedef}s which correspond to @code{typedef}s
1475 from the @acronym{JNI}. Each is the type of an array holding objects
1476 of the relevant type:
1479 typedef __JArray *jarray;
1480 typedef JArray<jobject> *jobjectArray;
1481 typedef JArray<jboolean> *jbooleanArray;
1482 typedef JArray<jbyte> *jbyteArray;
1483 typedef JArray<jchar> *jcharArray;
1484 typedef JArray<jshort> *jshortArray;
1485 typedef JArray<jint> *jintArray;
1486 typedef JArray<jlong> *jlongArray;
1487 typedef JArray<jfloat> *jfloatArray;
1488 typedef JArray<jdouble> *jdoubleArray;
1492 @deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
1493 This template function can be used to get a pointer to the elements of
1494 the @code{array}. For instance, you can fetch a pointer to the
1495 integers that make up an @code{int[]} like so:
1498 extern jintArray foo;
1499 jint *intp = elements (foo);
1502 The name of this function may change in the future.
1506 @deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
1507 Here @code{klass} is the type of elements of the array and
1508 @code{init} is the initial value put into every slot in the array.
1512 @subsection Creating arrays
1514 For each primitive type there is a function which can be used to
1515 create a new array of that type. The name of the function is of the
1519 JvNew@var{Type}Array
1522 @noindent For example:
1528 @noindent can be used to create an array of Java primitive boolean types.
1530 @noindent The following function definition is the template for all such functions:
1532 @deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
1533 Create's an array @var{length} indices long.
1536 @deftypefun jsize JvGetArrayLength (jarray @var{array})
1537 Returns the length of the @var{array}.
1544 Java methods are mapped directly into C++ methods.
1545 The header files generated by @code{gcjh}
1546 include the appropriate method definitions.
1547 Basically, the generated methods have the same names and
1548 @emph{corresponding} types as the Java methods,
1549 and are called in the natural manner.
1551 @subsection Overloading
1553 Both Java and C++ provide method overloading, where multiple
1554 methods in a class have the same name, and the correct one is chosen
1555 (at compile time) depending on the argument types.
1556 The rules for choosing the correct method are (as expected) more complicated
1557 in C++ than in Java, but given a set of overloaded methods
1558 generated by @code{gcjh} the C++ compiler will choose
1561 Common assemblers and linkers are not aware of C++ overloading,
1562 so the standard implementation strategy is to encode the
1563 parameter types of a method into its assembly-level name.
1564 This encoding is called @dfn{mangling},
1565 and the encoded name is the @dfn{mangled name}.
1566 The same mechanism is used to implement Java overloading.
1567 For C++/Java interoperability, it is important that both the Java
1568 and C++ compilers use the @emph{same} encoding scheme.
1570 @subsection Static methods
1572 Static Java methods are invoked in @acronym{CNI} using the standard
1573 C++ syntax, using the @code{::} operator rather
1574 than the @code{.} operator.
1576 @noindent For example:
1579 jint i = java::lang::Math::round((jfloat) 2.3);
1582 @noindent C++ method definition syntax is used to define a static native method.
1586 #include <java/lang/Integer>
1587 java::lang::Integer*
1588 java::lang::Integer::getInteger(jstring str)
1595 @subsection Object Constructors
1597 Constructors are called implicitly as part of object allocation
1598 using the @code{new} operator.
1600 @noindent For example:
1603 java::lang::Integer *x = new java::lang::Integer(234);
1606 Java does not allow a constructor to be a native method.
1607 This limitation can be coded round however because a constructor
1608 can @emph{call} a native method.
1611 @subsection Instance methods
1613 Calling a Java instance method from a C++ @acronym{CNI} method is done
1614 using the standard C++ syntax, e.g.:
1617 // @r{First create the Java object.}
1618 java::lang::Integer *x = new java::lang::Integer(234);
1619 // @r{Now call a method.}
1620 jint prim_value = x->intValue();
1621 if (x->longValue == 0)
1625 @noindent Defining a Java native instance method is also done the natural way:
1628 #include <java/lang/Integer.h>
1631 java::lang:Integer::doubleValue()
1633 return (jdouble) value;
1638 @subsection Interface methods
1640 In Java you can call a method using an interface reference. This is
1641 supported, but not completely. @xref{Interfaces}.
1649 @acronym{CNI} provides a number of utility functions for
1650 working with Java Java @code{String} objects.
1651 The names and interfaces are analogous to those of @acronym{JNI}.
1654 @deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
1655 Returns a Java @code{String} object with characters from the C string
1656 @var{chars} up to the index @var{len} in that array.
1659 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
1660 Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
1664 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
1665 As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
1668 @deftypefun jstring JvNewStringUTF (const char* @var{bytes})
1669 Returns a @code{String} which is made up of the UTF encoded characters
1670 present in the C string @var{bytes}.
1673 @deftypefun jchar* JvGetStringChars (jstring @var{str})
1674 Returns a pointer to an array of characters making up the @code{String} @var{str}.
1677 @deftypefun int JvGetStringUTFLength (jstring @var{str})
1678 Returns the number of bytes required to encode the contents of the
1679 @code{String} @var{str} in UTF-8.
1682 @deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
1683 Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
1684 the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
1686 Note that @var{buf} is a buffer, not a C string. It is @emph{not}
1691 @node Mixing with C++
1692 @section Interoperating with C/C++
1694 Because @acronym{CNI} is designed to represent Java classes and methods it
1695 cannot be mixed readily with C/C++ types.
1697 One important restriction is that Java classes cannot have non-Java
1698 type instance or static variables and cannot have methods which take
1699 non-Java types as arguments or return non-Java types.
1701 @noindent None of the following is possible with CNI:
1705 class ::MyClass : public java::lang::Object
1707 char* variable; // @r{char* is not a valid Java type.}
1712 ::SomeClass::someMethod (char *arg)
1717 @} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
1720 @noindent Of course, it is ok to use C/C++ types within the scope of a method:
1725 ::SomeClass::otherMethod (jstring str)
1734 But this restriction can cause a problem so @acronym{CNI} includes the
1735 @code{gnu.gcj.RawData} class. The @code{RawData} class is a
1736 @dfn{non-scanned reference} type. In other words variables declared
1737 of type @code{RawData} can contain any data and are not checked by the
1738 compiler in any way.
1740 This means that you can put C/C++ data structures (including classes)
1741 in your @acronym{CNI} classes, as long as you use the appropriate cast.
1743 @noindent Here are some examples:
1747 class ::MyClass : public java::lang::Object
1749 gnu.gcj.RawData string;
1752 gnu.gcj.RawData getText ();
1756 ::MyClass::MyClass ()
1763 ::MyClass::getText ()
1769 ::MyClass::printText ()
1771 printf("%s\n", (char*) string);
1776 @node Exception Handling
1777 @section Exception Handling
1779 While C++ and Java share a common exception handling framework,
1780 things are not yet perfectly integrated. The main issue is that the
1781 run-time type information facilities of the two
1782 languages are not integrated.
1784 Still, things work fairly well. You can throw a Java exception from
1785 C++ using the ordinary @code{throw} construct, and this
1786 exception can be caught by Java code. Similarly, you can catch an
1787 exception thrown from Java using the C++ @code{catch}
1790 @noindent Here is an example:
1794 throw new java::lang::IndexOutOfBoundsException();
1797 Normally, G++ will automatically detect when you are writing C++
1798 code that uses Java exceptions, and handle them appropriately.
1799 However, if C++ code only needs to execute destructors when Java
1800 exceptions are thrown through it, GCC will guess incorrectly. Sample
1804 struct S @{ ~S(); @};
1806 extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
1815 The usual effect of an incorrect guess is a link failure, complaining of
1816 a missing routine called @code{__gxx_personality_v0}.
1818 You can inform the compiler that Java exceptions are to be used in a
1819 translation unit, irrespective of what it might think, by writing
1820 @code{#pragma GCC java_exceptions} at the head of the
1821 file. This @code{#pragma} must appear before any
1822 functions that throw or catch exceptions, or run destructors when
1823 exceptions are thrown through them.
1825 @node Synchronization
1826 @section Synchronization
1828 Each Java object has an implicit monitor.
1829 The Java VM uses the instruction @code{monitorenter} to acquire
1830 and lock a monitor, and @code{monitorexit} to release it.
1832 The corresponding CNI macros are @code{JvMonitorEnter} and
1833 @code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
1834 and @code{MonitorExit}).
1837 The Java source language does not provide direct access to these primitives.
1838 Instead, there is a @code{synchronized} statement that does an
1839 implicit @code{monitorenter} before entry to the block,
1840 and does a @code{monitorexit} on exit from the block.
1841 Note that the lock has to be released even when the block is abnormally
1842 terminated by an exception, which means there is an implicit
1843 @code{try finally} surrounding synchronization locks.
1845 From C++, it makes sense to use a destructor to release a lock.
1846 @acronym{CNI} defines the following utility class:
1849 class JvSynchronize() @{
1851 JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
1852 ~JvSynchronize() @{ JvMonitorExit(obj); @}
1865 @noindent might become this C++ code:
1869 JvSynchronize dummy (OBJ);
1874 Java also has methods with the @code{synchronized} attribute.
1875 This is equivalent to wrapping the entire method body in a
1876 @code{synchronized} statement.
1877 (Alternatively, an implementation could require the caller to do
1878 the synchronization. This is not practical for a compiler, because
1879 each virtual method call would have to test at run-time if
1880 synchronization is needed.) Since in @command{gcj}
1881 the @code{synchronized} attribute is handled by the
1882 method implementation, it is up to the programmer
1883 of a synchronized native method to handle the synchronization
1884 (in the C++ implementation of the method).
1885 In other words, you need to manually add @code{JvSynchronize}
1886 in a @code{native synchronized} method.
1891 CNI permits C++ applications to make calls into Java classes, in addition to
1892 allowing Java code to call into C++. Several functions, known as the
1893 @dfn{invocation API}, are provided to support this.
1895 @deftypefun jint JvCreateJavaVM (void* @var{vm_args})
1896 Initializes the Java runtime. This function performs essential initialization
1897 of the threads interface, garbage collector, exception handling and other key
1898 aspects of the runtime. It must be called once by an application with
1899 a non-Java @code{main()} function, before any other Java or CNI calls are made.
1900 It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
1901 once provided it is only called from a single thread.
1902 The @var{vmargs} parameter can be used to specify initialization parameters
1903 for the Java runtime. It may be @code{NULL}.
1904 This function returns @code{0} upon success, or @code{-1} if the runtime is
1905 already initialized.
1907 @emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
1908 used in a future release.
1911 @deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{group})
1912 Registers an existing thread with the Java runtime. This must be called once
1913 from each thread, before that thread makes any other Java or CNI calls. It
1914 must be called after @code{JvCreateJavaVM}.
1915 @var{name} specifies a name for the thread. It may be @code{NULL}, in which
1916 case a name will be generated.
1917 @var{group} is the ThreadGroup in which this thread will be a member. If it
1918 is @code{NULL}, the thread will be a member of the main thread group.
1919 The return value is the Java @code{Thread} object that represents the thread.
1920 It is safe to call @code{JvAttachCurrentThread()} more than once from the same
1921 thread. If the thread is already attached, the call is ignored and the current
1922 thread object is returned.
1925 @deftypefun jint JvDetachCurrentThread ()
1926 Unregisters a thread from the Java runtime. This should be called by threads
1927 that were attached using @code{JvAttachCurrentThread()}, after they have
1928 finished making calls to Java code. This ensures that any resources associated
1929 with the thread become eligible for garbage collection.
1930 This function returns @code{0} upon success, or @code{-1} if the current thread
1934 @subsection Handling uncaught exceptions
1936 If an exception is thrown from Java code called using the invocation API, and
1937 no handler for the exception can be found, the runtime will abort the
1938 application. In order to make the application more robust, it is recommended
1939 that code which uses the invocation API be wrapped by a top-level try/catch
1940 block that catches all Java exceptions.
1944 The following code demonstrates the use of the invocation API. In this
1945 example, the C++ application initializes the Java runtime and attaches
1946 itself. The @code{java.lang.System} class is initialized in order to
1947 access its @code{out} field, and a Java string is printed. Finally, the thread
1948 is detached from the runtime once it has finished making Java calls. Everything
1949 is wrapped with a try/catch block to provide a default handler for any uncaught
1952 The example can be compiled with @command{c++ test.cc -lgcj}.
1956 #include <gcj/cni.h>
1957 #include <java/lang/System.h>
1958 #include <java/io/PrintStream.h>
1959 #include <java/lang/Throwable.h>
1961 int main(int argc, char *argv)
1963 using namespace java::lang;
1967 JvCreateJavaVM(NULL);
1968 JvAttachCurrentThread(NULL, NULL);
1970 String *message = JvNewStringLatin1("Hello from C++");
1971 JvInitClass(&System::class$);
1972 System::out->println(message);
1974 JvDetachCurrentThread();
1976 catch (Throwable *t)
1978 System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
1979 t->printStackTrace();
1987 Reflection is possible with CNI code, it functions similarly to how it
1988 functions with JNI@.
1990 @c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
1991 The types @code{jfieldID} and @code{jmethodID}
1994 @noindent The functions:
1997 @item @code{JvFromReflectedField},
1998 @item @code{JvFromReflectedMethod},
1999 @item @code{JvToReflectedField}
2000 @item @code{JvToFromReflectedMethod}
2003 @noindent will be added shortly, as will other functions corresponding to JNI@.
2006 @node System properties
2007 @chapter System properties
2009 The runtime behavior of the @code{libgcj} library can be modified by setting
2010 certain system properties. These properties can be compiled into the program
2011 using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
2012 setting them explicitly in the program by calling the
2013 @code{java.lang.System.setProperty()} method. Some system properties are only
2014 used for informational purposes (like giving a version number or a user name).
2015 A program can inspect the current value of a property by calling the
2016 @code{java.lang.System.getProperty()} method.
2019 * Standard Properties:: Standard properties supported by @code{libgcj}
2020 * GNU Classpath Properties:: Properties found in Classpath based libraries
2021 * libgcj Runtime Properties:: Properties specific to @code{libgcj}
2024 @node Standard Properties
2025 @section Standard Properties
2027 The following properties are normally found in all implementations of the core
2028 libraries for the Java language.
2033 The @code{libgcj} version number.
2036 Set to @samp{The Free Software Foundation, Inc.}
2038 @item java.vendor.url
2039 Set to @uref{http://gcc.gnu.org/java/}.
2042 The directory where @code{gcj} was installed. Taken from the @code{--prefix}
2043 option given to @command{configure}.
2045 @item java.class.version
2046 The class format version number supported by the libgcj byte code interpreter.
2047 (Currently @samp{46.0})
2049 @item java.vm.specification.version
2050 The Virtual Machine Specification version implemented by @code{libgcj}.
2051 (Currently @samp{1.0})
2053 @item java.vm.specification.vendor
2054 The name of the Virtual Machine specification designer.
2056 @item java.vm.specification.name
2057 The name of the Virtual Machine specification
2058 (Set to @samp{Java Virtual Machine Specification}).
2060 @item java.vm.version
2061 The @command{gcj} version number.
2063 @item java.vm.vendor
2064 Set to @samp{The Free Software Foundation, Inc.}
2067 Set to @samp{GNU libgcj}.
2069 @item java.specification.version
2070 The Runtime Environment specification version implemented by @code{libgcj}.
2071 (Currently set to @samp{1.3})
2073 @item java.specification.vendor
2074 The Runtime Environment specification designer.
2076 @item java.specification.name
2077 The name of the Runtime Environment specification
2078 (Set to @samp{Java Platform API Specification}).
2080 @item java.class.path
2081 The paths (jar files, zip files and directories) used for finding class files.
2083 @item java.library.path
2084 Directory path used for finding native libraries.
2086 @item java.io.tmpdir
2087 The directory used to put temporary files in.
2090 Name of the Just In Time compiler to use by the byte code interpreter.
2091 Currently not used in @code{libgcj}.
2094 Directories containing jar files with extra libraries. Will be used when
2095 resolving classes. Currently not used in @code{libgcj}.
2097 @item java.protocol.handler.pkgs
2098 A @samp{|} separated list of package names that is used to find classes that
2099 implement handlers for @code{java.net.URL}.
2101 @item java.rmi.server.codebase
2102 A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
2103 to load classes from.
2106 A list of class names that will be loaded by the @code{java.sql.DriverManager}
2109 @item file.separator
2110 The separator used in when directories are included in a filename
2111 (normally @samp{/} or @samp{\} ).
2114 The default character encoding used when converting platform native files to
2115 Unicode (usually set to @samp{8859_1}).
2117 @item path.separator
2118 The standard separator used when a string contains multiple paths
2119 (normally @samp{:} or @samp{;}), the string is usually not a valid character
2120 to use in normal directory names.)
2122 @item line.separator
2123 The default line separator used on the platform (normally @samp{\n}, @samp{\r}
2124 or a combination of those two characters).
2126 @item policy.provider
2127 The class name used for the default policy provider returned by
2128 @code{java.security.Policy.getPolicy}.
2131 The name of the user running the program. Can be the full name, the login name
2132 or empty if unknown.
2135 The default directory to put user specific files in.
2138 The current working directory from which the program was started.
2141 The default language as used by the @code{java.util.Locale} class.
2144 The default region as used by the @code{java.util.Local} class.
2147 The default variant of the language and region local used.
2150 The default timezone as used by the @code{java.util.TimeZone} class.
2153 The operating system/kernel name that the program runs on.
2156 The hardware that we are running on.
2159 The version number of the operating system/kernel.
2161 @item awt.appletWarning
2162 The string to display when an untrusted applet is displayed.
2163 Returned by @code{java.awt.Window.getWarningString()} when the window is
2167 The class name used for initializing the default @code{java.awt.Toolkit}.
2168 Defaults to @code{gnu.awt.gtk.GtkToolkit}.
2170 @item http.proxyHost
2171 Name of proxy host for http connections.
2173 @item http.proxyPort
2174 Port number to use when a proxy host is in use.
2178 @node GNU Classpath Properties
2179 @section GNU Classpath Properties
2181 @code{libgcj} is based on the GNU Classpath (Essential Libraries for Java) a
2182 GNU project to create free core class libraries for use with virtual machines
2183 and compilers for the Java language. The following properties are common to
2184 libraries based on GNU Classpath.
2188 @item gcj.dumpobject
2189 Enables printing serialization debugging by the @code{java.io.ObjectInput} and
2190 @code{java.io.ObjectOutput} classes when set to something else then the empty
2191 string. Only used when running a debug build of the library.
2193 @item gnu.classpath.vm.shortname
2194 This is a succint name of the virtual machine. For @code{libgcj},
2195 this will always be @samp{libgcj}.
2197 @item gnu.classpath.home.url
2198 A base URL used for finding system property files (e.g.,
2199 @file{classpath.security}). By default this is a @samp{file:} URL
2200 pointing to the @file{lib} directory under @samp{java.home}.
2204 @node libgcj Runtime Properties
2205 @section libgcj Runtime Properties
2207 The following properties are specific to the @code{libgcj} runtime and will
2208 normally not be found in other core libraries for the java language.
2212 @item java.fullversion
2213 The combination of @code{java.vm.name} and @code{java.vm.version}.
2216 Same as @code{java.fullversion}.
2219 Used by the @code{java.net.DatagramSocket} class when set to something else
2220 then the empty string. When set all newly created @code{DatagramSocket}s will
2221 try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
2222 the normal @code{java.net.PlainDatagramSocketImpl}.
2224 @item gnu.gcj.progname
2225 The name that was used to invoked the program.
2227 @item gnu.gcj.runtime.NameFinder.demangle
2228 Whether names in a stack trace should be demangled. Defaults to @code{true}.
2230 @item gnu.gcj.runtime.NameFinder.sanitize
2231 Whether calls to initialize exceptions and starting the runtime system
2232 should be removed from the stack trace. Only done when names are
2233 demangled. Defaults to @code{true}.
2235 @item gnu.gcj.runtime.NameFinder.remove_unknown
2236 Whether calls to unknown functions (class and method names are unknown)
2237 should be removed from the stack trace. Only done when the stack is
2238 sanitized. Ignored if this means no stack trace information would be
2239 available anymore. Defaults to @code{true}.
2241 @item gnu.gcj.runtime.NameFinder.remove_interpreter
2242 Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
2243 and functions starting with @samp{ffi_}) should be removed from the stack
2244 trace. Only done when the stack is sanitized. Defaults to @code{true}.
2247 @item gnu.gcj.runtime.NameFinder.use_addr2line
2248 Whether an external process (@command{addr2line} or @command{addr2name.awk})
2249 should be used as fallback to convert the addresses to function names when
2250 the runtime is unable to do it through @code{dladdr}.
2252 @item gnu.gcj.runtime.VMClassLoader.library_control
2253 This controls how shared libraries are automatically loaded by the
2254 built-in class loader. By default, or if this property is set to
2255 @samp{full}, a full search is done for each requested class. If this
2256 property is set to @samp{cache}, then any failed lookups are cached
2257 and not tried again. If this property is set to @samp{never}, then
2258 lookups are never done. For more information, @xref{Extensions}.
2266 While writing @command{gcj} and @code{libgcj} we have, of course, relied
2267 heavily on documentation from Sun Microsystems. In particular we have
2268 used The Java Language Specification (both first and second editions),
2269 the Java Class Libraries (volumes one and two), and the Java Virtual
2270 Machine Specification. In addition we've used the online documentation
2271 at @uref{http://java.sun.com/}.
2273 The current @command{gcj} home page is
2274 @uref{http://gcc.gnu.org/java/}.
2276 For more information on gcc, see @uref{http://gcc.gnu.org/}.
2278 Some @code{libgcj} testing is done using the Mauve test suite. This is
2279 a free software Java class library test suite which is being written
2280 because the JCK is not free. See
2281 @uref{http://sources.redhat.com/mauve/} for more information.