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.
435 @item --disable-assertions[=@var{class-or-package}]
436 Don't include code for checking assertions in the compiled code.
437 If @code{=@var{class-or-package}} is missing disables assertion code
438 generation for all classes, unless overridden by a more
439 specific @code{--enable-assertions} flag.
440 If @var{class-or-package} is a class name, only disables generating
441 assertion checks within the named class or its inner classes.
442 If @var{class-or-package} is a package name, disables generating
443 assertion checks within the named package or a subpackage.
445 By default, assertions are enabled when generating class files
446 or when not optimizing, and disabled when generating optimized binaries.
448 @item --enable-assertions[=@var{class-or-package}]
449 Generates code to check assertions. The option is perhaps misnamed,
450 as you still need to turn on assertion checking at run-time,
451 and we don't support any easy way to do that.
452 So this flag isn't very useful yet, except to partially override
453 @code{--disable-assertions}.
458 @node Configure-time Options
459 @section Configure-time Options
461 Some @command{gcj} code generations options affect the resulting ABI, and
462 so can only be meaningfully given when @code{libgcj}, the runtime
463 package, is configured. @code{libgcj} puts the appropriate options from
464 this group into a @samp{spec} file which is read by @command{gcj}. These
465 options are listed here for completeness; if you are using @code{libgcj}
466 then you won't want to touch these options.
470 This enables the use of the Boehm GC bitmap marking code. In particular
471 this causes @command{gcj} to put an object marking descriptor into each
474 @item -fhash-synchronization
475 By default, synchronization data (the data used for @code{synchronize},
476 @code{wait}, and @code{notify}) is pointed to by a word in each object.
477 With this option @command{gcj} assumes that this information is stored in a
478 hash table and not in the object itself.
480 @item -fuse-divide-subroutine
481 On some systems, a library routine is called to perform integer
482 division. This is required to get exception handling correct when
485 @item -fcheck-references
486 On some systems it's necessary to insert inline checks whenever
487 accessing an object via a reference. On other systems you won't need
488 this because null pointer accesses are caught automatically by the
495 @chapter Compatibility with the Java Platform
497 As we believe it is important that the Java platform not be fragmented,
498 @command{gcj} and @code{libgcj} try to conform to the relevant Java
499 specifications. However, limited manpower and incomplete and unclear
500 documentation work against us. So, there are caveats to using
509 @section Standard features not yet supported
511 This list of compatibility issues is by no means complete.
515 @command{gcj} implements the JDK 1.2 language. It supports inner classes
516 and the new 1.4 @code{assert} keyword. It does not yet support the Java 2
517 @code{strictfp} keyword (it recognizes the keyword but ignores it).
520 @code{libgcj} is largely compatible with the JDK 1.2 libraries.
521 However, @code{libgcj} is missing many packages, most notably
522 @code{java.awt}. There are also individual missing classes and methods.
523 We currently do not have a list showing differences between
524 @code{libgcj} and the Java 2 platform.
527 Sometimes the @code{libgcj} implementation of a method or class differs
528 from the JDK implementation. This is not always a bug. Still, if it
529 affects you, it probably makes sense to report it so that we can discuss
530 the appropriate response.
533 @command{gcj} does not currently allow for piecemeal replacement of
534 components within @code{libgcj}. Unfortunately, programmers often want
535 to use newer versions of certain packages, such as those provided by
536 the Apache Software Foundation's Jakarta project. This has forced us
537 to place the @code{org.w3c.dom} and @code{org.xml.sax} packages into
538 their own libraries, separate from @code{libgcj}. If you intend to
539 use these classes, you must link them explicitly with
540 @code{-l-org-w3c-dom} and @code{-l-org-xml-sax}. Future versions of
541 @command{gcj} may not have this restriction.
545 @section Extra features unique to gcj
547 The main feature of @command{gcj} is that it can compile programs written in
548 the Java programming language to native code. Most extensions that have been
549 added are to facilitate this functionality.
553 @command{gcj} makes it easy and efficient to mix code written in Java and C++.
554 @xref{About CNI}, for more info on how to use this in your programs.
557 When you compile your classes into a shared library they can be automatically
558 loaded by the @code{libgcj} system classloader. When trying to load a class
559 @code{gnu.pkg.SomeClass} the system classloader will first try to load the
560 shared library @file{lib-gnu-pkg-SomeClass.so}, if that fails to load the
561 class then it will try to load @file{lib-gnu-pkg.so} and finally when the
562 class is still not loaded it will try to load @file{lib-gnu.so}. Note that
563 all @samp{.}s will be transformed into @samp{-}s and that searching
564 for inner classes starts with their outermost outer class. If the class
565 cannot be found this way the system classloader tries to use
566 the @code{libgcj} bytecode interpreter to load the class from the standard
567 classpath. This process can be controlled to some degree via the
568 @code{gnu.gcj.runtime.VMClassLoader.library_control} property;
569 @xref{libgcj Runtime Properties}.
572 @code{libgcj} includes a special @samp{gcjlib} URL type. A URL of
573 this form is like a @code{jar} URL, and looks like
574 @samp{gcjlib:/path/to/shared/library.so!/path/to/resource}. An access
575 to one of these URLs causes the shared library to be @code{dlopen()}d,
576 and then the resource is looked for in that library. These URLs are
577 most useful when used in conjunction with @code{java.net.URLClassLoader}.
578 Note that, due to implementation limitations, currently any such URL
579 can be accessed by only one class loader, and libraries are never
580 unloaded. This means some care must be exercised to make sure that
581 a @code{gcjlib} URL is not accessed by more than one class loader at once.
582 In a future release this limitation will be lifted, and such
583 libraries will be mapped privately.
586 A program compiled by @command{gcj} will examine the
587 @env{GCJ_PROPERTIES} environment variable and change its behavior in
588 some ways. In particular @env{GCJ_PROPERTIES} holds a list of
589 assignments to global properties, such as would be set with the
590 @option{-D} option to @command{java}. For instance,
591 @samp{java.compiler=gcj} is a valid (but currently meaningless)
593 @cindex GCJ_PROPERTIES
594 @vindex GCJ_PROPERTIES
600 @chapter Invoking gcjh
602 @c man title gcjh generate header files from Java class files
604 @c man begin DESCRIPTION gcjh
606 The @code{gcjh} program is used to generate header files from class
607 files. It can generate both CNI and JNI header files, as well as stub
608 implementation files which can be used as a basis for implementing the
609 required native methods.
614 @c man begin SYNOPSIS gcjh
615 gcjh [@option{-stubs}] [@option{-jni}]
616 [@option{-add} @var{text}] [@option{-append} @var{text}] [@option{-friend} @var{text}]
617 [@option{-preprend} @var{text}]
618 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
619 [@option{-I}@var{dir}@dots{}] [@option{-d} @var{dir}@dots{}]
620 [@option{-o} @var{file}] [@option{-td} @var{dir}]
621 [@option{-M}] [@option{-MM}] [@option{-MD}] [@option{-MMD}]
622 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
623 @var{classname}@dots{}
625 @c man begin SEEALSO gcjh
626 gcc(1), gcj(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
627 and the Info entries for @file{gcj} and @file{gcc}.
631 @c man begin OPTIONS gcjh
635 This causes @code{gcjh} to generate stub files instead of header files.
636 By default the stub file will be named after the class, with a suffix of
637 @samp{.cc}. In JNI mode, the default output file will have the suffix
641 This tells @code{gcjh} to generate a JNI header or stub. By default,
642 CNI headers are generated.
644 @item -add @var{text}
645 Inserts @var{text} into the class body. This is ignored in JNI mode.
647 @item -append @var{text}
648 Inserts @var{text} into the header file after the class declaration.
649 This is ignored in JNI mode.
651 @item -friend @var{text}
652 Inserts @var{text} into the class as a @code{friend} declaration.
653 This is ignored in JNI mode.
655 @item -prepend @var{text}
656 Inserts @var{text} into the header file before the class declaration.
657 This is ignored in JNI mode.
659 @item --classpath=@var{path}
660 @itemx --CLASSPATH=@var{path}
661 @itemx -I@var{directory}
662 @itemx -d @var{directory}
664 These options are all identical to the corresponding @command{gcj} options.
667 Sets the output file name. This cannot be used if there is more than
668 one class on the command line.
670 @item -td @var{directory}
671 Sets the name of the directory to use for temporary files.
674 Print all dependencies to stdout; suppress ordinary output.
677 Print non-system dependencies to stdout; suppress ordinary output.
680 Print all dependencies to stdout.
683 Print non-system dependencies to stdout.
686 Print help about @code{gcjh} and exit. No further processing is done.
689 Print version information for @code{gcjh} and exit. No further
693 Print extra information while running.
696 All remaining options are considered to be names of classes.
700 @node Invoking jv-scan
701 @chapter Invoking jv-scan
703 @c man title jv-scan print information about Java source file
705 @c man begin DESCRIPTION jv-scan
707 The @code{jv-scan} program can be used to print information about a Java
708 source file (@file{.java} file).
713 @c man begin SYNOPSIS jv-scan
714 jv-scan [@option{--no-assert}] [@option{--complexity}]
715 [@option{--encoding}=@var{name}] [@option{--print-main}]
716 [@option{--list-class}] [@option{--list-filename}]
717 [@option{--version}] [@option{--help}]
718 [@option{-o} @var{file}] @var{inputfile}@dots{}
720 @c man begin SEEALSO jv-scan
721 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
722 and the Info entries for @file{gcj} and @file{gcc}.
726 @c man begin OPTIONS jv-scan
730 Don't recognize the @code{assert} keyword, for backwards compatibility
731 with older versions of the language specification.
734 This prints a complexity measure, related to cyclomatic complexity, for
737 @item --encoding=@var{name}
738 This works like the corresponding @command{gcj} option.
741 This prints the name of the class in this file containing a @code{main}
745 This lists the names of all classes defined in the input files.
747 @item --list-filename
748 If @code{--list-class} is given, this option causes @code{jv-scan} to
749 also print the name of the file in which each class was found.
752 Print output to the named file.
755 Print help, then exit.
758 Print version number, then exit.
763 @node Invoking jcf-dump
764 @chapter Invoking jcf-dump
766 @c man title jcf-dump print information about Java class files
769 @c man begin SYNOPSIS jcf-dump
770 jcf-dump [@option{-c}] [@option{--javap}]
771 [@option{--classpath}=@var{path}] [@option{--CLASSPATH}=@var{path}]
772 [@option{-I}@var{dir}@dots{}] [@option{-o} @var{file}]
773 [@option{--version}] [@option{--help}] [@option{-v}] [@option{--verbose}]
774 @var{classname}@dots{}
776 @c man begin SEEALSO jcf-dump
777 gcc(1), gcj(1), gcjh(1), gij(1), jcf-dump(1), gfdl(7),
778 and the Info entries for @file{gcj} and @file{gcc}.
782 @c man begin DESCRIPTION jcf-dump
784 This is a class file examiner, similar to @code{javap}. It will print
785 information about a number of classes, which are specified by class name
790 @c man begin OPTIONS jcf-dump
794 Disassemble method bodies. By default method bodies are not printed.
796 @item --print-constants
797 Print the constant pool. When printing a reference to a constant
798 also print its index in the constant pool.
801 Generate output in @code{javap} format. The implementation of this
802 feature is very incomplete.
804 @item --classpath=@var{path}
805 @itemx --CLASSPATH=@var{path}
806 @itemx -I@var{directory}
808 These options as the same as the corresponding @command{gcj} options.
811 Print help, then exit.
814 Print version number, then exit.
817 Print extra information while running.
818 Implies @code{--print-constants}.
824 @chapter Invoking gij
826 @c man title gij GNU interpreter for Java bytecode
829 @c man begin SYNOPSIS gij
830 gij [@option{OPTION}] @dots{} @var{JARFILE} [@var{ARGS}@dots{}]
832 gij [@option{-jar}] [@option{OPTION}] @dots{} @var{CLASS} [@var{ARGS}@dots{}]
833 [@option{-cp} @var{path}] [@option{-classpath} @var{path}]
834 [@option{-D}@var{name}[=@var{value}]@dots{}]
835 [@option{-ms=}@var{number}] [@option{-mx=}@var{number}]
836 [@option{-X@var{argument}]
837 [@option{--showversion}] [@option{--version}] [@option{--help}][@option{-?}]
839 @c man begin SEEALSO gij
840 gcc(1), gcj(1), gcjh(1), jv-scan(1), jcf-dump(1), gfdl(7),
841 and the Info entries for @file{gcj} and @file{gcc}.
845 @c man begin DESCRIPTION gij
847 @code{gij} is a Java bytecode interpreter included with @code{libgcj}.
848 @code{gij} is not available on every platform; porting it requires a
849 small amount of assembly programming which has not been done for all the
850 targets supported by @command{gcj}.
852 The primary argument to @code{gij} is the name of a class or, with
853 @code{-jar}, a jar file. Options before this argument are interpreted
854 by @code{gij}; remaining options are passed to the interpreted program.
856 If a class name is specified and this class does not have a @code{main}
857 method with the appropriate signature (a @code{static void} method with
858 a @code{String[]} as its sole argument), then @code{gij} will print an
861 If a jar file is specified then @code{gij} will use information in it to
862 determine which class' @code{main} method will be invoked.
864 @code{gij} will invoke the @code{main} method with all the remaining
865 command-line options.
867 Note that @code{gij} is not limited to interpreting code. Because
868 @code{libgcj} includes a class loader which can dynamically load shared
869 objects, it is possible to give @code{gij} the name of a class which has
870 been compiled and put into a shared library on the class path.
874 @c man begin OPTIONS gij
878 @itemx -classpath @var{path}
879 Set the initial class path. The class path is used for finding
880 class and resource files. If specified, this option overrides the
881 @code{CLASSPATH} environment variable. Note that this option is
882 ignored if @code{-jar} is used.
884 @item -D@var{name}[=@var{value}]
885 This defines a system property named @var{name} with value @var{value}.
886 If @var{value} is not specified then it defaults to the empty string.
887 These system properties are initialized at the program's startup and can
888 be retrieved at runtime using the @code{java.lang.System.getProperty}
891 @item -ms=@var{number}
892 This sets the initial heap size.
894 @item -mx=@var{number}
895 This sets the maximum heap size.
898 @itemx -X@var{argument}
899 Supplying @code{-X} by itself will cause @code{gij} to list all the
900 supported @code{-X} options. Currently there are none. Unrecognized
901 @code{-X} options are ignored, for compatibility with other runtimes.
904 This indicates that the name passed to @code{gij} should be interpreted
905 as the name of a jar file, not a class.
909 Print help, then exit.
912 Print version number and continue.
915 Print version number, then exit.
920 @node Invoking jv-convert
921 @chapter Invoking jv-convert
923 @c man title jv-convert Convert file from one encoding to another
925 @c man begin SYNOPSIS jv-convert
926 @command{jv-convert} [@option{OPTION}] @dots{} [@var{INPUTFILE} [@var{OUTPUTFILE}]]
929 [@option{--encoding} @var{name}]
930 [@option{--from} @var{name}]
931 [@option{--to} @var{name}]
932 [@option{-i} @var{file}] [@option{-o} @var{file}]
933 [@option{--reverse}] [@option{--help}] [@option{--version}]
937 @c man begin DESCRIPTION jv-convert
939 @command{jv-convert} is a utility included with @code{libgcj} which
940 converts a file from one encoding to another. It is similar to the Unix
941 @command{iconv} utility.
943 The encodings supported by @command{jv-convert} are platform-dependent.
944 Currently there is no way to get a list of all supported encodings.
948 @c man begin OPTIONS jv-convert
951 @item --encoding @var{name}
952 @itemx --from @var{name}
953 Use @var{name} as the input encoding. The default is the current
956 @item --to @var{name}
957 Use @var{name} as the output encoding. The default is the
958 @code{JavaSrc} encoding; this is ASCII with @samp{\u} escapes for
959 non-ASCII characters.
962 Read from @var{file}. The default is to read from standard input.
965 Write to @var{file}. The default is to write to standard output.
968 Swap the input and output encodings.
971 Print a help message, then exit.
974 Print version information, then exit.
980 @chapter Invoking rmic
982 @c man title rmic Generate stubs for Remote Method Invocation
984 @c man begin SYNOPSIS rmic
985 @command{rmic} [@option{OPTION}] @dots{} @var{class} @dots{}
988 [@option{-keepgenerated}]
992 [@option{-nocompile}]
994 [@option{-d} @var{directory}]
1000 @c man begin DESCRIPTION rmic
1002 @command{rmic} is a utility included with @code{libgcj} which generates
1003 stubs for remote objects.
1005 @c FIXME: Add real information here.
1006 @c This really isn't much more than the --help output.
1008 Note that this program isn't yet fully compatible with the JDK
1009 @command{rmic}. Some options, such as @option{-classpath}, are
1010 recognized but currently ignored. We have left these options
1011 undocumented for now.
1013 Long options can also be given with a GNU-style leading @samp{--}. For
1014 instance, @option{--help} is accepted.
1018 @c man begin OPTIONS rmic
1022 @itemx -keepgenerated
1023 By default, @command{rmic} deletes intermediate files. Either of these
1024 options causes it not to delete such files.
1027 Cause @command{rmic} to create stubs and skeletons for the 1.1
1031 Cause @command{rmic} to create stubs and skeletons compatible with both
1032 the 1.1 and 1.2 protocol versions. This is the default.
1035 Cause @command{rmic} to create stubs and skeletons for the 1.2
1039 Don't compile the generated files.
1042 Print information about what @command{rmic} is doing.
1044 @item -d @var{directory}
1045 Put output files in @var{directory}. By default the files are put in
1046 the current working directory.
1049 Print a help message, then exit.
1052 Print version information, then exit.
1058 @node Invoking rmiregistry
1059 @chapter Invoking rmiregistry
1061 @c man title rmiregistry Remote object registry
1063 @c man begin SYNOPSIS rmiregistry
1064 @command{rmic} [@option{OPTION}] @dots{} [@var{port}]
1067 [@option{--version}]
1071 @c man begin DESCRIPTION rmiregistry
1073 @command{rmiregistry} starts a remote object registry on the current
1074 host. If no port number is specified, then port 1099 is used.
1076 @c FIXME: Add real information here.
1077 @c This really isn't much more than the --help output.
1081 @c man begin OPTIONS rmiregistry
1085 Print a help message, then exit.
1088 Print version information, then exit.
1097 This documents CNI, the Compiled Native Interface,
1098 which is is a convenient way to write Java native methods using C++.
1099 This is a more efficient, more convenient, but less portable
1100 alternative to the standard JNI (Java Native Interface).
1103 * Basic concepts:: Introduction to using CNI@.
1104 * Packages:: How packages are mapped to C++.
1105 * Primitive types:: Handling Java types in C++.
1106 * Interfaces:: How Java interfaces map to C++.
1107 * Objects and Classes:: C++ and Java classes.
1108 * Class Initialization:: How objects are initialized.
1109 * Object allocation:: How to create Java objects in C++.
1110 * Arrays:: Dealing with Java arrays in C++.
1111 * Methods:: Java methods in C++.
1112 * Strings:: Information about Java Strings.
1113 * Mixing with C++:: How CNI can interoperate with C++.
1114 * Exception Handling:: How exceptions are handled.
1115 * Synchronization:: Synchronizing between Java and C++.
1116 * Invocation:: Starting the Java runtime from C++.
1117 * Reflection:: Using reflection from C++.
1121 @node Basic concepts
1122 @section Basic concepts
1124 In terms of languages features, Java is mostly a subset
1125 of C++. Java has a few important extensions, plus a powerful standard
1126 class library, but on the whole that does not change the basic similarity.
1127 Java is a hybrid object-oriented language, with a few native types,
1128 in addition to class types. It is class-based, where a class may have
1129 static as well as per-object fields, and static as well as instance methods.
1130 Non-static methods may be virtual, and may be overloaded. Overloading is
1131 resolved at compile time by matching the actual argument types against
1132 the parameter types. Virtual methods are implemented using indirect calls
1133 through a dispatch table (virtual function table). Objects are
1134 allocated on the heap, and initialized using a constructor method.
1135 Classes are organized in a package hierarchy.
1137 All of the listed attributes are also true of C++, though C++ has
1138 extra features (for example in C++ objects may be allocated not just
1139 on the heap, but also statically or in a local stack frame). Because
1140 @command{gcj} uses the same compiler technology as G++ (the GNU
1141 C++ compiler), it is possible to make the intersection of the two
1142 languages use the same ABI (object representation and calling
1143 conventions). The key idea in CNI is that Java objects are C++
1144 objects, and all Java classes are C++ classes (but not the other way
1145 around). So the most important task in integrating Java and C++ is to
1146 remove gratuitous incompatibilities.
1148 You write CNI code as a regular C++ source file. (You do have to use
1149 a Java/CNI-aware C++ compiler, specifically a recent version of G++.)
1151 @noindent A CNI C++ source file must have:
1154 #include <gcj/cni.h>
1157 @noindent and then must include one header file for each Java class it uses, e.g.:
1160 #include <java/lang/Character.h>
1161 #include <java/util/Date.h>
1162 #include <java/lang/IndexOutOfBoundsException.h>
1165 @noindent These header files are automatically generated by @code{gcjh}.
1168 CNI provides some functions and macros to make using Java objects and
1169 primitive types from C++ easier. In general, these CNI functions and
1170 macros start with the @code{Jv} prefix, for example the function
1171 @code{JvNewObjectArray}. This convention is used to avoid conflicts
1172 with other libraries. Internal functions in CNI start with the prefix
1173 @code{_Jv_}. You should not call these; if you find a need to, let us
1174 know and we will try to come up with an alternate solution. (This
1175 manual lists @code{_Jv_AllocBytes} as an example; CNI should instead
1176 provide a @code{JvAllocBytes} function.)
1179 @subsection Limitations
1181 Whilst a Java class is just a C++ class that doesn't mean that you are
1182 freed from the shackles of Java, a @acronym{CNI} C++ class must adhere to the
1183 rules of the Java programming language.
1185 For example: it is not possible to declare a method in a CNI class
1186 that will take a C string (@code{char*}) as an argument, or to declare a
1187 member variable of some non-Java datatype.
1193 The only global names in Java are class names, and packages. A
1194 @dfn{package} can contain zero or more classes, and also zero or more
1195 sub-packages. Every class belongs to either an unnamed package or a
1196 package that has a hierarchical and globally unique name.
1198 A Java package is mapped to a C++ @dfn{namespace}. The Java class
1199 @code{java.lang.String} is in the package @code{java.lang}, which is a
1200 sub-package of @code{java}. The C++ equivalent is the class
1201 @code{java::lang::String}, which is in the namespace @code{java::lang}
1202 which is in the namespace @code{java}.
1204 @noindent Here is how you could express this:
1207 (// @r{Declare the class(es), possibly in a header file:}
1216 class java::lang::String : public java::lang::Object
1222 @noindent The @code{gcjh} tool automatically generates the necessary namespace
1226 @subsection Leaving out package names
1228 Always using the fully-qualified name of a java class can be
1229 tiresomely verbose. Using the full qualified name also ties the code
1230 to a single package making code changes necessary should the class
1231 move from one package to another. The Java @code{package} declaration
1232 specifies that the following class declarations are in the named
1233 package, without having to explicitly name the full package
1234 qualifiers. The @code{package} declaration can be
1235 followed by zero or more @code{import} declarations, which
1236 allows either a single class or all the classes in a package to be
1237 named by a simple identifier. C++ provides something similar with the
1238 @code{using} declaration and directive.
1243 import @var{package-name}.@var{class-name};
1246 @noindent allows the program text to refer to @var{class-name} as a shorthand for
1247 the fully qualified name: @code{@var{package-name}.@var{class-name}}.
1250 @noindent To achieve the same effect C++, you have to do this:
1253 using @var{package-name}::@var{class-name};
1257 @noindent Java can also cause imports on demand, like this:
1260 import @var{package-name}.*;
1263 @noindent Doing this allows any class from the package @var{package-name} to be
1264 referred to only by its class-name within the program text.
1267 @noindent The same effect can be achieved in C++ like this:
1270 using namespace @var{package-name};
1274 @node Primitive types
1275 @section Primitive types
1277 Java provides 8 @dfn{primitives} types which represent integers, floats,
1278 characters and booleans (and also the void type). C++ has its own
1279 very similar concrete types. Such types in C++ however are not always
1280 implemented in the same way (an int might be 16, 32 or 64 bits for example)
1281 so CNI provides a special C++ type for each primitive Java type:
1283 @multitable @columnfractions .20 .25 .60
1284 @item @strong{Java type} @tab @strong{C/C++ typename} @tab @strong{Description}
1285 @item @code{char} @tab @code{jchar} @tab 16 bit Unicode character
1286 @item @code{boolean} @tab @code{jboolean} @tab logical (true or false) values
1287 @item @code{byte} @tab @code{jbyte} @tab 8-bit signed integer
1288 @item @code{short} @tab @code{jshort} @tab 16 bit signed integer
1289 @item @code{int} @tab @code{jint} @tab 32 bit signed integer
1290 @item @code{long} @tab @code{jlong} @tab 64 bit signed integer
1291 @item @code{float} @tab @code{jfloat} @tab 32 bit IEEE floating point number
1292 @item @code{double} @tab @code{jdouble} @tab 64 bit IEEE floating point number
1293 @item @code{void} @tab @code{void} @tab no value
1296 When referring to a Java type You should always use these C++ typenames (e.g.: @code{jint})
1297 to avoid disappointment.
1300 @subsection Reference types associated with primitive types
1302 In Java each primitive type has an associated reference type,
1303 e.g.: @code{boolean} has an associated @code{java.lang.Boolean} class.
1304 In order to make working with such classes easier GCJ provides the macro
1307 @deffn macro JvPrimClass type
1308 Return a pointer to the @code{Class} object corresponding to the type supplied.
1311 JvPrimClass(void) @result{} java.lang.Void.TYPE
1320 A Java class can @dfn{implement} zero or more
1321 @dfn{interfaces}, in addition to inheriting from
1322 a single base class.
1324 @acronym{CNI} allows CNI code to implement methods of interfaces.
1325 You can also call methods through interface references, with some
1328 @acronym{CNI} doesn't understand interface inheritance at all yet. So,
1329 you can only call an interface method when the declared type of the
1330 field being called matches the interface which declares that
1331 method. The workaround is to cast the interface reference to the right
1334 For example if you have:
1342 interface B extends A
1348 and declare a variable of type @code{B} in C++, you can't call
1349 @code{a()} unless you cast it to an @code{A} first.
1351 @node Objects and Classes
1352 @section Objects and Classes
1356 All Java classes are derived from @code{java.lang.Object}. C++ does
1357 not have a unique root class, but we use the C++ class
1358 @code{java::lang::Object} as the C++ version of the
1359 @code{java.lang.Object} Java class. All other Java classes are mapped
1360 into corresponding C++ classes derived from @code{java::lang::Object}.
1362 Interface inheritance (the @code{implements} keyword) is currently not
1363 reflected in the C++ mapping.
1366 @subsection Object fields
1368 Each object contains an object header, followed by the instance fields
1369 of the class, in order. The object header consists of a single
1370 pointer to a dispatch or virtual function table. (There may be extra
1371 fields @emph{in front of} the object, for example for memory
1372 management, but this is invisible to the application, and the
1373 reference to the object points to the dispatch table pointer.)
1375 The fields are laid out in the same order, alignment, and size as in
1376 C++. Specifically, 8-bite and 16-bit native types (@code{byte},
1377 @code{short}, @code{char}, and @code{boolean}) are @emph{not} widened
1378 to 32 bits. Note that the Java VM does extend 8-bit and 16-bit types
1379 to 32 bits when on the VM stack or temporary registers.
1381 If you include the @code{gcjh}-generated header for a
1382 class, you can access fields of Java classes in the @emph{natural}
1383 way. For example, given the following Java class:
1389 public Integer (int i) @{ this.i = i; @}
1390 public static zero = new Integer(0);
1397 #include <gcj/cni.h>;
1401 mult (Int *p, jint k)
1404 return Int::zero; // @r{Static member access.}
1405 return new Int(p->i * k);
1410 @subsection Access specifiers
1412 CNI does not strictly enforce the Java access
1413 specifiers, because Java permissions cannot be directly mapped
1414 into C++ permission. Private Java fields and methods are mapped
1415 to private C++ fields and methods, but other fields and methods
1416 are mapped to public fields and methods.
1420 @node Class Initialization
1421 @section Class Initialization
1423 Java requires that each class be automatically initialized at the time
1424 of the first active use. Initializing a class involves
1425 initializing the static fields, running code in class initializer
1426 methods, and initializing base classes. There may also be
1427 some implementation specific actions, such as allocating
1428 @code{String} objects corresponding to string literals in
1431 The GCJ compiler inserts calls to @code{JvInitClass} at appropriate
1432 places to ensure that a class is initialized when required. The C++
1433 compiler does not insert these calls automatically---it is the
1434 programmer's responsibility to make sure classes are initialized.
1435 However, this is fairly painless because of the conventions assumed by
1438 First, @code{libgcj} will make sure a class is initialized before an
1439 instance of that object is created. This is one of the
1440 responsibilities of the @code{new} operation. This is taken care of
1441 both in Java code, and in C++ code. When G++ sees a @code{new} of a
1442 Java class, it will call a routine in @code{libgcj} to allocate the
1443 object, and that routine will take care of initializing the class.
1444 Note however that this does not happen for Java arrays; you must
1445 allocate those using the appropriate CNI function. It follows that
1446 you can access an instance field, or call an instance (non-static)
1447 method and be safe in the knowledge that the class and all of its base
1448 classes have been initialized.
1450 Invoking a static method is also safe. This is because the
1451 Java compiler adds code to the start of a static method to make sure
1452 the class is initialized. However, the C++ compiler does not
1453 add this extra code. Hence, if you write a native static method
1454 using CNI, you are responsible for calling @code{JvInitClass}
1455 before doing anything else in the method (unless you are sure
1456 it is safe to leave it out).
1458 Accessing a static field also requires the class of the
1459 field to be initialized. The Java compiler will generate code
1460 to call @code{Jv_InitClass} before getting or setting the field.
1461 However, the C++ compiler will not generate this extra code,
1462 so it is your responsibility to make sure the class is
1463 initialized before you access a static field from C++.
1466 @node Object allocation
1467 @section Object allocation
1469 New Java objects are allocated using a
1470 @dfn{class instance creation expression}, e.g.:
1473 new @var{Type} ( ... )
1476 The same syntax is used in C++. The main difference is that
1477 C++ objects have to be explicitly deleted; in Java they are
1478 automatically deleted by the garbage collector.
1479 Using @acronym{CNI}, you can allocate a new Java object
1480 using standard C++ syntax and the C++ compiler will allocate
1481 memory from the garbage collector. If you have overloaded
1482 constructors, the compiler will choose the correct one
1483 using standard C++ overload resolution rules.
1485 @noindent For example:
1488 java::util::Hashtable *ht = new java::util::Hashtable(120);
1491 @deftypefun void* _Jv_AllocBytes (jsize @var{size})
1492 Allocates @var{size} bytes from the heap. The memory is not scanned
1493 by the garbage collector but it freed if no references to it are discovered.
1500 While in many ways Java is similar to C and C++, it is quite different
1501 in its treatment of arrays. C arrays are based on the idea of pointer
1502 arithmetic, which would be incompatible with Java's security
1503 requirements. Java arrays are true objects (array types inherit from
1504 @code{java.lang.Object}). An array-valued variable is one that
1505 contains a reference (pointer) to an array object.
1507 Referencing a Java array in C++ code is done using the
1508 @code{JArray} template, which as defined as follows:
1511 class __JArray : public java::lang::Object
1518 class JArray : public __JArray
1522 T& operator[](jint i) @{ return data[i]; @}
1527 There are a number of @code{typedef}s which correspond to @code{typedef}s
1528 from the @acronym{JNI}. Each is the type of an array holding objects
1529 of the relevant type:
1532 typedef __JArray *jarray;
1533 typedef JArray<jobject> *jobjectArray;
1534 typedef JArray<jboolean> *jbooleanArray;
1535 typedef JArray<jbyte> *jbyteArray;
1536 typedef JArray<jchar> *jcharArray;
1537 typedef JArray<jshort> *jshortArray;
1538 typedef JArray<jint> *jintArray;
1539 typedef JArray<jlong> *jlongArray;
1540 typedef JArray<jfloat> *jfloatArray;
1541 typedef JArray<jdouble> *jdoubleArray;
1545 @deftypemethod {template<class T>} T* elements (JArray<T> @var{array})
1546 This template function can be used to get a pointer to the elements of
1547 the @code{array}. For instance, you can fetch a pointer to the
1548 integers that make up an @code{int[]} like so:
1551 extern jintArray foo;
1552 jint *intp = elements (foo);
1555 The name of this function may change in the future.
1559 @deftypefun jobjectArray JvNewObjectArray (jsize @var{length}, jclass @var{klass}, jobject @var{init})
1560 Here @code{klass} is the type of elements of the array and
1561 @code{init} is the initial value put into every slot in the array.
1565 @subsection Creating arrays
1567 For each primitive type there is a function which can be used to
1568 create a new array of that type. The name of the function is of the
1572 JvNew@var{Type}Array
1575 @noindent For example:
1581 @noindent can be used to create an array of Java primitive boolean types.
1583 @noindent The following function definition is the template for all such functions:
1585 @deftypefun jbooleanArray JvNewBooleanArray (jint @var{length})
1586 Create's an array @var{length} indices long.
1589 @deftypefun jsize JvGetArrayLength (jarray @var{array})
1590 Returns the length of the @var{array}.
1597 Java methods are mapped directly into C++ methods.
1598 The header files generated by @code{gcjh}
1599 include the appropriate method definitions.
1600 Basically, the generated methods have the same names and
1601 @emph{corresponding} types as the Java methods,
1602 and are called in the natural manner.
1604 @subsection Overloading
1606 Both Java and C++ provide method overloading, where multiple
1607 methods in a class have the same name, and the correct one is chosen
1608 (at compile time) depending on the argument types.
1609 The rules for choosing the correct method are (as expected) more complicated
1610 in C++ than in Java, but given a set of overloaded methods
1611 generated by @code{gcjh} the C++ compiler will choose
1614 Common assemblers and linkers are not aware of C++ overloading,
1615 so the standard implementation strategy is to encode the
1616 parameter types of a method into its assembly-level name.
1617 This encoding is called @dfn{mangling},
1618 and the encoded name is the @dfn{mangled name}.
1619 The same mechanism is used to implement Java overloading.
1620 For C++/Java interoperability, it is important that both the Java
1621 and C++ compilers use the @emph{same} encoding scheme.
1623 @subsection Static methods
1625 Static Java methods are invoked in @acronym{CNI} using the standard
1626 C++ syntax, using the @code{::} operator rather
1627 than the @code{.} operator.
1629 @noindent For example:
1632 jint i = java::lang::Math::round((jfloat) 2.3);
1635 @noindent C++ method definition syntax is used to define a static native method.
1639 #include <java/lang/Integer>
1640 java::lang::Integer*
1641 java::lang::Integer::getInteger(jstring str)
1648 @subsection Object Constructors
1650 Constructors are called implicitly as part of object allocation
1651 using the @code{new} operator.
1653 @noindent For example:
1656 java::lang::Integer *x = new java::lang::Integer(234);
1659 Java does not allow a constructor to be a native method.
1660 This limitation can be coded round however because a constructor
1661 can @emph{call} a native method.
1664 @subsection Instance methods
1666 Calling a Java instance method from a C++ @acronym{CNI} method is done
1667 using the standard C++ syntax, e.g.:
1670 // @r{First create the Java object.}
1671 java::lang::Integer *x = new java::lang::Integer(234);
1672 // @r{Now call a method.}
1673 jint prim_value = x->intValue();
1674 if (x->longValue == 0)
1678 @noindent Defining a Java native instance method is also done the natural way:
1681 #include <java/lang/Integer.h>
1684 java::lang:Integer::doubleValue()
1686 return (jdouble) value;
1691 @subsection Interface methods
1693 In Java you can call a method using an interface reference. This is
1694 supported, but not completely. @xref{Interfaces}.
1702 @acronym{CNI} provides a number of utility functions for
1703 working with Java Java @code{String} objects.
1704 The names and interfaces are analogous to those of @acronym{JNI}.
1707 @deftypefun jstring JvNewString (const char* @var{chars}, jsize @var{len})
1708 Returns a Java @code{String} object with characters from the C string
1709 @var{chars} up to the index @var{len} in that array.
1712 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes}, jsize @var{len})
1713 Returns a Java @code{String} made up of @var{len} bytes from @var{bytes}.
1717 @deftypefun jstring JvNewStringLatin1 (const char* @var{bytes})
1718 As above but the length of the @code{String} is @code{strlen(@var{bytes})}.
1721 @deftypefun jstring JvNewStringUTF (const char* @var{bytes})
1722 Returns a @code{String} which is made up of the UTF encoded characters
1723 present in the C string @var{bytes}.
1726 @deftypefun jchar* JvGetStringChars (jstring @var{str})
1727 Returns a pointer to an array of characters making up the @code{String} @var{str}.
1730 @deftypefun int JvGetStringUTFLength (jstring @var{str})
1731 Returns the number of bytes required to encode the contents of the
1732 @code{String} @var{str} in UTF-8.
1735 @deftypefun jsize JvGetStringUTFRegion (jstring @var{str}, jsize @var{start}, jsize @var{len}, char* @var{buf})
1736 Puts the UTF-8 encoding of a region of the @code{String} @var{str} into
1737 the buffer @code{buf}. The region to fetch is marked by @var{start} and @var{len}.
1739 Note that @var{buf} is a buffer, not a C string. It is @emph{not}
1744 @node Mixing with C++
1745 @section Interoperating with C/C++
1747 Because @acronym{CNI} is designed to represent Java classes and methods it
1748 cannot be mixed readily with C/C++ types.
1750 One important restriction is that Java classes cannot have non-Java
1751 type instance or static variables and cannot have methods which take
1752 non-Java types as arguments or return non-Java types.
1754 @noindent None of the following is possible with CNI:
1758 class ::MyClass : public java::lang::Object
1760 char* variable; // @r{char* is not a valid Java type.}
1765 ::SomeClass::someMethod (char *arg)
1770 @} // @r{@code{uint} is not a valid Java type, neither is @code{char*}}
1773 @noindent Of course, it is ok to use C/C++ types within the scope of a method:
1778 ::SomeClass::otherMethod (jstring str)
1787 But this restriction can cause a problem so @acronym{CNI} includes the
1788 @code{gnu.gcj.RawData} class. The @code{RawData} class is a
1789 @dfn{non-scanned reference} type. In other words variables declared
1790 of type @code{RawData} can contain any data and are not checked by the
1791 compiler in any way.
1793 This means that you can put C/C++ data structures (including classes)
1794 in your @acronym{CNI} classes, as long as you use the appropriate cast.
1796 @noindent Here are some examples:
1800 class ::MyClass : public java::lang::Object
1802 gnu.gcj.RawData string;
1805 gnu.gcj.RawData getText ();
1809 ::MyClass::MyClass ()
1816 ::MyClass::getText ()
1822 ::MyClass::printText ()
1824 printf("%s\n", (char*) string);
1829 @node Exception Handling
1830 @section Exception Handling
1832 While C++ and Java share a common exception handling framework,
1833 things are not yet perfectly integrated. The main issue is that the
1834 run-time type information facilities of the two
1835 languages are not integrated.
1837 Still, things work fairly well. You can throw a Java exception from
1838 C++ using the ordinary @code{throw} construct, and this
1839 exception can be caught by Java code. Similarly, you can catch an
1840 exception thrown from Java using the C++ @code{catch}
1843 @noindent Here is an example:
1847 throw new java::lang::IndexOutOfBoundsException();
1850 Normally, G++ will automatically detect when you are writing C++
1851 code that uses Java exceptions, and handle them appropriately.
1852 However, if C++ code only needs to execute destructors when Java
1853 exceptions are thrown through it, GCC will guess incorrectly. Sample
1857 struct S @{ ~S(); @};
1859 extern void bar(); // @r{Is implemented in Java and may throw exceptions.}
1868 The usual effect of an incorrect guess is a link failure, complaining of
1869 a missing routine called @code{__gxx_personality_v0}.
1871 You can inform the compiler that Java exceptions are to be used in a
1872 translation unit, irrespective of what it might think, by writing
1873 @code{#pragma GCC java_exceptions} at the head of the
1874 file. This @code{#pragma} must appear before any
1875 functions that throw or catch exceptions, or run destructors when
1876 exceptions are thrown through them.
1878 @node Synchronization
1879 @section Synchronization
1881 Each Java object has an implicit monitor.
1882 The Java VM uses the instruction @code{monitorenter} to acquire
1883 and lock a monitor, and @code{monitorexit} to release it.
1885 The corresponding CNI macros are @code{JvMonitorEnter} and
1886 @code{JvMonitorExit} (JNI has similar methods @code{MonitorEnter}
1887 and @code{MonitorExit}).
1890 The Java source language does not provide direct access to these primitives.
1891 Instead, there is a @code{synchronized} statement that does an
1892 implicit @code{monitorenter} before entry to the block,
1893 and does a @code{monitorexit} on exit from the block.
1894 Note that the lock has to be released even when the block is abnormally
1895 terminated by an exception, which means there is an implicit
1896 @code{try finally} surrounding synchronization locks.
1898 From C++, it makes sense to use a destructor to release a lock.
1899 @acronym{CNI} defines the following utility class:
1902 class JvSynchronize() @{
1904 JvSynchronize(jobject o) @{ obj = o; JvMonitorEnter(o); @}
1905 ~JvSynchronize() @{ JvMonitorExit(obj); @}
1918 @noindent might become this C++ code:
1922 JvSynchronize dummy (OBJ);
1927 Java also has methods with the @code{synchronized} attribute.
1928 This is equivalent to wrapping the entire method body in a
1929 @code{synchronized} statement.
1930 (Alternatively, an implementation could require the caller to do
1931 the synchronization. This is not practical for a compiler, because
1932 each virtual method call would have to test at run-time if
1933 synchronization is needed.) Since in @command{gcj}
1934 the @code{synchronized} attribute is handled by the
1935 method implementation, it is up to the programmer
1936 of a synchronized native method to handle the synchronization
1937 (in the C++ implementation of the method).
1938 In other words, you need to manually add @code{JvSynchronize}
1939 in a @code{native synchronized} method.
1944 CNI permits C++ applications to make calls into Java classes, in addition to
1945 allowing Java code to call into C++. Several functions, known as the
1946 @dfn{invocation API}, are provided to support this.
1948 @deftypefun jint JvCreateJavaVM (void* @var{vm_args})
1949 Initializes the Java runtime. This function performs essential initialization
1950 of the threads interface, garbage collector, exception handling and other key
1951 aspects of the runtime. It must be called once by an application with
1952 a non-Java @code{main()} function, before any other Java or CNI calls are made.
1953 It is safe, but not recommended, to call @code{JvCreateJavaVM()} more than
1954 once provided it is only called from a single thread.
1955 The @var{vmargs} parameter can be used to specify initialization parameters
1956 for the Java runtime. It may be @code{NULL}.
1957 This function returns @code{0} upon success, or @code{-1} if the runtime is
1958 already initialized.
1960 @emph{Note:} In GCJ 3.1, the @code{vm_args} parameter is ignored. It may be
1961 used in a future release.
1964 @deftypefun java::lang::Thread* JvAttachCurrentThread (jstring @var{name}, java::lang::ThreadGroup* @var{group})
1965 Registers an existing thread with the Java runtime. This must be called once
1966 from each thread, before that thread makes any other Java or CNI calls. It
1967 must be called after @code{JvCreateJavaVM}.
1968 @var{name} specifies a name for the thread. It may be @code{NULL}, in which
1969 case a name will be generated.
1970 @var{group} is the ThreadGroup in which this thread will be a member. If it
1971 is @code{NULL}, the thread will be a member of the main thread group.
1972 The return value is the Java @code{Thread} object that represents the thread.
1973 It is safe to call @code{JvAttachCurrentThread()} more than once from the same
1974 thread. If the thread is already attached, the call is ignored and the current
1975 thread object is returned.
1978 @deftypefun jint JvDetachCurrentThread ()
1979 Unregisters a thread from the Java runtime. This should be called by threads
1980 that were attached using @code{JvAttachCurrentThread()}, after they have
1981 finished making calls to Java code. This ensures that any resources associated
1982 with the thread become eligible for garbage collection.
1983 This function returns @code{0} upon success, or @code{-1} if the current thread
1987 @subsection Handling uncaught exceptions
1989 If an exception is thrown from Java code called using the invocation API, and
1990 no handler for the exception can be found, the runtime will abort the
1991 application. In order to make the application more robust, it is recommended
1992 that code which uses the invocation API be wrapped by a top-level try/catch
1993 block that catches all Java exceptions.
1997 The following code demonstrates the use of the invocation API. In this
1998 example, the C++ application initializes the Java runtime and attaches
1999 itself. The @code{java.lang.System} class is initialized in order to
2000 access its @code{out} field, and a Java string is printed. Finally, the thread
2001 is detached from the runtime once it has finished making Java calls. Everything
2002 is wrapped with a try/catch block to provide a default handler for any uncaught
2005 The example can be compiled with @command{c++ test.cc -lgcj}.
2009 #include <gcj/cni.h>
2010 #include <java/lang/System.h>
2011 #include <java/io/PrintStream.h>
2012 #include <java/lang/Throwable.h>
2014 int main(int argc, char *argv)
2016 using namespace java::lang;
2020 JvCreateJavaVM(NULL);
2021 JvAttachCurrentThread(NULL, NULL);
2023 String *message = JvNewStringLatin1("Hello from C++");
2024 JvInitClass(&System::class$);
2025 System::out->println(message);
2027 JvDetachCurrentThread();
2029 catch (Throwable *t)
2031 System::err->println(JvNewStringLatin1("Unhandled Java exception:"));
2032 t->printStackTrace();
2040 Reflection is possible with CNI code, it functions similarly to how it
2041 functions with JNI@.
2043 @c clean this up... I mean, what are the types jfieldID and jmethodID in JNI?
2044 The types @code{jfieldID} and @code{jmethodID}
2047 @noindent The functions:
2050 @item @code{JvFromReflectedField},
2051 @item @code{JvFromReflectedMethod},
2052 @item @code{JvToReflectedField}
2053 @item @code{JvToFromReflectedMethod}
2056 @noindent will be added shortly, as will other functions corresponding to JNI@.
2059 @node System properties
2060 @chapter System properties
2062 The runtime behavior of the @code{libgcj} library can be modified by setting
2063 certain system properties. These properties can be compiled into the program
2064 using the @code{-D@var{name}[=@var{value}]} option to @command{gcj} or by
2065 setting them explicitly in the program by calling the
2066 @code{java.lang.System.setProperty()} method. Some system properties are only
2067 used for informational purposes (like giving a version number or a user name).
2068 A program can inspect the current value of a property by calling the
2069 @code{java.lang.System.getProperty()} method.
2072 * Standard Properties:: Standard properties supported by @code{libgcj}
2073 * GNU Classpath Properties:: Properties found in Classpath based libraries
2074 * libgcj Runtime Properties:: Properties specific to @code{libgcj}
2077 @node Standard Properties
2078 @section Standard Properties
2080 The following properties are normally found in all implementations of the core
2081 libraries for the Java language.
2086 The @code{libgcj} version number.
2089 Set to @samp{The Free Software Foundation, Inc.}
2091 @item java.vendor.url
2092 Set to @uref{http://gcc.gnu.org/java/}.
2095 The directory where @code{gcj} was installed. Taken from the @code{--prefix}
2096 option given to @command{configure}.
2098 @item java.class.version
2099 The class format version number supported by the libgcj byte code interpreter.
2100 (Currently @samp{46.0})
2102 @item java.vm.specification.version
2103 The Virtual Machine Specification version implemented by @code{libgcj}.
2104 (Currently @samp{1.0})
2106 @item java.vm.specification.vendor
2107 The name of the Virtual Machine specification designer.
2109 @item java.vm.specification.name
2110 The name of the Virtual Machine specification
2111 (Set to @samp{Java Virtual Machine Specification}).
2113 @item java.vm.version
2114 The @command{gcj} version number.
2116 @item java.vm.vendor
2117 Set to @samp{The Free Software Foundation, Inc.}
2120 Set to @samp{GNU libgcj}.
2122 @item java.specification.version
2123 The Runtime Environment specification version implemented by @code{libgcj}.
2124 (Currently set to @samp{1.3})
2126 @item java.specification.vendor
2127 The Runtime Environment specification designer.
2129 @item java.specification.name
2130 The name of the Runtime Environment specification
2131 (Set to @samp{Java Platform API Specification}).
2133 @item java.class.path
2134 The paths (jar files, zip files and directories) used for finding class files.
2136 @item java.library.path
2137 Directory path used for finding native libraries.
2139 @item java.io.tmpdir
2140 The directory used to put temporary files in.
2143 Name of the Just In Time compiler to use by the byte code interpreter.
2144 Currently not used in @code{libgcj}.
2147 Directories containing jar files with extra libraries. Will be used when
2148 resolving classes. Currently not used in @code{libgcj}.
2150 @item java.protocol.handler.pkgs
2151 A @samp{|} separated list of package names that is used to find classes that
2152 implement handlers for @code{java.net.URL}.
2154 @item java.rmi.server.codebase
2155 A list of URLs that is used by the @code{java.rmi.server.RMIClassLoader}
2156 to load classes from.
2159 A list of class names that will be loaded by the @code{java.sql.DriverManager}
2162 @item file.separator
2163 The separator used in when directories are included in a filename
2164 (normally @samp{/} or @samp{\} ).
2167 The default character encoding used when converting platform native files to
2168 Unicode (usually set to @samp{8859_1}).
2170 @item path.separator
2171 The standard separator used when a string contains multiple paths
2172 (normally @samp{:} or @samp{;}), the string is usually not a valid character
2173 to use in normal directory names.)
2175 @item line.separator
2176 The default line separator used on the platform (normally @samp{\n}, @samp{\r}
2177 or a combination of those two characters).
2179 @item policy.provider
2180 The class name used for the default policy provider returned by
2181 @code{java.security.Policy.getPolicy}.
2184 The name of the user running the program. Can be the full name, the login name
2185 or empty if unknown.
2188 The default directory to put user specific files in.
2191 The current working directory from which the program was started.
2194 The default language as used by the @code{java.util.Locale} class.
2197 The default region as used by the @code{java.util.Local} class.
2200 The default variant of the language and region local used.
2203 The default timezone as used by the @code{java.util.TimeZone} class.
2206 The operating system/kernel name that the program runs on.
2209 The hardware that we are running on.
2212 The version number of the operating system/kernel.
2214 @item awt.appletWarning
2215 The string to display when an untrusted applet is displayed.
2216 Returned by @code{java.awt.Window.getWarningString()} when the window is
2220 The class name used for initializing the default @code{java.awt.Toolkit}.
2221 Defaults to @code{gnu.awt.gtk.GtkToolkit}.
2223 @item http.proxyHost
2224 Name of proxy host for http connections.
2226 @item http.proxyPort
2227 Port number to use when a proxy host is in use.
2231 @node GNU Classpath Properties
2232 @section GNU Classpath Properties
2234 @code{libgcj} is based on the GNU Classpath (Essential Libraries for Java) a
2235 GNU project to create free core class libraries for use with virtual machines
2236 and compilers for the Java language. The following properties are common to
2237 libraries based on GNU Classpath.
2241 @item gcj.dumpobject
2242 Enables printing serialization debugging by the @code{java.io.ObjectInput} and
2243 @code{java.io.ObjectOutput} classes when set to something else then the empty
2244 string. Only used when running a debug build of the library.
2246 @item gnu.classpath.vm.shortname
2247 This is a succint name of the virtual machine. For @code{libgcj},
2248 this will always be @samp{libgcj}.
2250 @item gnu.classpath.home.url
2251 A base URL used for finding system property files (e.g.,
2252 @file{classpath.security}). By default this is a @samp{file:} URL
2253 pointing to the @file{lib} directory under @samp{java.home}.
2257 @node libgcj Runtime Properties
2258 @section libgcj Runtime Properties
2260 The following properties are specific to the @code{libgcj} runtime and will
2261 normally not be found in other core libraries for the java language.
2265 @item java.fullversion
2266 The combination of @code{java.vm.name} and @code{java.vm.version}.
2269 Same as @code{java.fullversion}.
2272 Used by the @code{java.net.DatagramSocket} class when set to something else
2273 then the empty string. When set all newly created @code{DatagramSocket}s will
2274 try to load a class @code{java.net.[impl.prefix]DatagramSocketImpl} instead of
2275 the normal @code{java.net.PlainDatagramSocketImpl}.
2277 @item gnu.gcj.progname
2278 The name that was used to invoked the program.
2280 @item gnu.gcj.runtime.NameFinder.demangle
2281 Whether names in a stack trace should be demangled. Defaults to @code{true}.
2283 @item gnu.gcj.runtime.NameFinder.sanitize
2284 Whether calls to initialize exceptions and starting the runtime system
2285 should be removed from the stack trace. Only done when names are
2286 demangled. Defaults to @code{true}.
2288 @item gnu.gcj.runtime.NameFinder.remove_unknown
2289 Whether calls to unknown functions (class and method names are unknown)
2290 should be removed from the stack trace. Only done when the stack is
2291 sanitized. Ignored if this means no stack trace information would be
2292 available anymore. Defaults to @code{true}.
2294 @item gnu.gcj.runtime.NameFinder.remove_interpreter
2295 Whether runtime interpreter calls (methods in the @code{_Jv_InterpMethod} class
2296 and functions starting with @samp{ffi_}) should be removed from the stack
2297 trace. Only done when the stack is sanitized. Defaults to @code{true}.
2300 @item gnu.gcj.runtime.NameFinder.use_addr2line
2301 Whether an external process (@command{addr2line} or @command{addr2name.awk})
2302 should be used as fallback to convert the addresses to function names when
2303 the runtime is unable to do it through @code{dladdr}.
2305 @item gnu.gcj.runtime.VMClassLoader.library_control
2306 This controls how shared libraries are automatically loaded by the
2307 built-in class loader. By default, or if this property is set to
2308 @samp{full}, a full search is done for each requested class. If this
2309 property is set to @samp{cache}, then any failed lookups are cached
2310 and not tried again. If this property is set to @samp{never}, then
2311 lookups are never done. For more information, @xref{Extensions}.
2319 While writing @command{gcj} and @code{libgcj} we have, of course, relied
2320 heavily on documentation from Sun Microsystems. In particular we have
2321 used The Java Language Specification (both first and second editions),
2322 the Java Class Libraries (volumes one and two), and the Java Virtual
2323 Machine Specification. In addition we've used the online documentation
2324 at @uref{http://java.sun.com/}.
2326 The current @command{gcj} home page is
2327 @uref{http://gcc.gnu.org/java/}.
2329 For more information on gcc, see @uref{http://gcc.gnu.org/}.
2331 Some @code{libgcj} testing is done using the Mauve test suite. This is
2332 a free software Java class library test suite which is being written
2333 because the JCK is not free. See
2334 @uref{http://sources.redhat.com/mauve/} for more information.