GCJ(1) GNU GCJ(1)
gcj - Ahead-of-time compiler for the Java language
gcj [-Idir...] [-d dir...]
[-C] [--resource resource-name] [-d directory]
As gcj is just another front end to gcc, it supports many of the same
options as gcc. This manual only documents the options specific to
Input and output files
A gcj command is like a gcc command, in that it consists of a number of
options and file names. The following kinds of input file names are
Java source files.
Java bytecode files.
An archive containing one or more ".class" files, all of which are
compiled. The archive may be compressed. Files in an archive
which don’t end with .class are treated as resource files; they are
compiled into the resulting object file as core: URLs.
A file containing a whitespace-separated list of input file names.
(Currently, these must all be ".java" source files, but that may
change.) Each named file is compiled, just as if it had been on
the command line.
Libraries to use when linking. See the gcc manual.
You can specify more than one input file on the gcj command line, in
which case they will all be compiled. If you specify a "-o FILENAME"
option, all the input files will be compiled together, producing a sin-
gle output file, named FILENAME. This is allowed even when using "-S"
or "-c", but not when using "-C" or "--resource". (This is an exten-
sion beyond the what plain gcc allows.) (If more than one input file
is specified, all must currently be ".java" files, though we hope to
gcj has options to control where it looks to find files it needs. For
instance, gcj might need to load a class that is referenced by the file
it has been asked to compile. Like other compilers for the Java
language, gcj has a notion of a class path. There are several options
and environment variables which can be used to manipulate the class
path. When gcj looks for a given class, it searches the class path
looking for matching .class or .java file. gcj comes with a built-in
class path which points at the installed libgcj.jar, a file which con-
tains all the standard classes.
In the below, a directory or path component can refer either to an
actual directory on the filesystem, or to a .zip or .jar file, which
gcj will search as if it is a directory.
All directories specified by "-I" are kept in order and prepended
to the class path constructed from all the other options. Unless
compatibility with tools like "javac" is important, we recommend
always using "-I" instead of the other options for manipulating the
This sets the class path to path, a colon-separated list of paths
(on Windows-based systems, a semicolon-separate list of paths).
This does not override the builtin (‘‘boot’’) search path.
Deprecated synonym for "--classpath".
Where to find the standard builtin classes, such as
For each directory in the path, place the contents of that direc-
tory at the end of the class path.
This is an environment variable which holds a list of paths.
The final class path is constructed like so:
· First come all directories specified via "-I".
· If --classpath is specified, its value is appended. Otherwise, if
the "CLASSPATH" environment variable is specified, then its value
is appended. Otherwise, the current directory (".") is appended.
· If "--bootclasspath" was specified, append its value. Otherwise,
append the built-in system directory, libgcj.jar.
· Finally, if "--extdirs" was specified, append the contents of the
specified directories at the end of the class path. Otherwise,
append the contents of the built-in extdirs at "$(pre-
The classfile built by gcj for the class "java.lang.Object" (and placed
in "libgcj.jar") contains a special zero length attribute
"gnu.gcj.gcj-compiled". The compiler looks for this attribute when
loading "java.lang.Object" and will report an error if it isn’t found,
unless it compiles to bytecode (the option
"-fforce-classes-archive-check" can be used to override this behavior
in this particular case.)
This forces the compiler to always check for the special zero
length attribute "gnu.gcj.gcj-compiled" in "java.lang.Object" and
issue an error if it isn’t found.
The Java programming language uses Unicode throughout. In an effort to
integrate well with other locales, gcj allows .java files to be written
using almost any encoding. gcj knows how to convert these encodings
into its internal encoding at compile time.
You can use the "--encoding=NAME" option to specify an encoding (of a
particular character set) to use for source files. If this is not
specified, the default encoding comes from your current locale. If
your host system has insufficient locale support, then gcj assumes the
default encoding to be the UTF-8 encoding of Unicode.
To implement "--encoding", gcj simply uses the host platform’s "iconv"
conversion routine. This means that in practice gcj is limited by the
capabilities of the host platform.
The names allowed for the argument "--encoding" vary from platform to
platform (since they are not standardized anywhere). However, gcj
implements the encoding named UTF-8 internally, so if you choose to use
this for your source files you can be assured that it will work on
gcj implements several warnings. As with other generic gcc warnings,
if an option of the form "-Wfoo" enables a warning, then "-Wno-foo"
will disable it. Here we’ve chosen to document the form of the warning
which will have an effect -- the default being the opposite of what is
With this flag, gcj will warn about redundant modifiers. For
instance, it will warn if an interface method is declared "public".
This causes gcj to warn about empty statements. Empty statements
have been deprecated.
This option will cause gcj not to warn when a source file is newer
than its matching class file. By default gcj will warn about this.
Warn if a deprecated class, method, or field is referred to.
This is the same as gcc’s "-Wunused".
This is the same as "-Wredundant-modifiers -Wextraneous-semicolon
To turn a Java application into an executable program, you need to link
it with the needed libraries, just as for C or C++. The linker by
default looks for a global function named "main". Since Java does not
have global functions, and a collection of Java classes may have more
than one class with a "main" method, you need to let the linker know
which of those "main" methods it should invoke when starting the appli-
cation. You can do that in any of these ways:
· Specify the class containing the desired "main" method when you
link the application, using the "--main" flag, described below.
· Link the Java package(s) into a shared library (dll) rather than an
executable. Then invoke the application using the "gij" program,
making sure that "gij" can find the libraries it needs.
· Link the Java packages(s) with the flag "-lgij", which links in the
"main" routine from the "gij" command. This allows you to select
the class whose "main" method you want to run when you run the
application. You can also use other "gij" flags, such as "-D"
flags to set properties. Using the "-lgij" library (rather than
the "gij" program of the previous mechanism) has some advantages:
it is compatible with static linking, and does not require config-
uring or installing libraries.
These "gij" options relate to linking an executable:
This option is used when linking to specify the name of the class
whose "main" method should be invoked when the resulting executable
This option can only be used with "--main". It defines a system
property named name with value value. If value is not specified
then it defaults to the empty string. These system properties are
initialized at the program’s startup and can be retrieved at run-
time using the "java.lang.System.getProperty" method.
Create an application whose command-line processing is that of the
This option is an alternative to using "--main"; you cannot use
In addition to the many gcc options controlling code generation, gcj
has several options specific to itself.
-C This option is used to tell gcj to generate bytecode (.class files)
rather than object code.
This option is used to tell gcj to compile the contents of a given
file to object code so it may be accessed at runtime with the core
protocol handler as core:/resource-name. Note that resource-name
is the name of the resource as found at runtime; for instance, it
could be used in a call to "ResourceBundle.getBundle". The actual
file name to be compiled this way must be specified separately.
When used with "-C", this causes all generated .class files to be
put in the appropriate subdirectory of directory. By default they
will be put in subdirectories of the current working directory.
By default, gcj generates code which checks the bounds of all array
indexing operations. With this option, these checks are omitted,
which can improve performance for code that uses arrays exten-
sively. Note that this can result in unpredictable behavior if the
code in question actually does violate array bounds constraints.
It is safe to use this option if you are sure that your code will
never throw an "ArrayIndexOutOfBoundsException".
Don’t generate array store checks. When storing objects into
arrays, a runtime check is normally generated in order to ensure
that the object is assignment compatible with the component type of
the array (which may not be known at compile-time). With this
option, these checks are omitted. This can improve performance for
code which stores objects into arrays frequently. It is safe to
use this option if you are sure your code will never throw an
With gcj there are two options for writing native methods: CNI and
JNI. By default gcj assumes you are using CNI. If you are compil-
ing a class with native methods, and these methods are implemented
using JNI, then you must use "-fjni". This option causes gcj to
generate stubs which will invoke the underlying JNI methods.
Don’t recognize the "assert" keyword. This is for compatibility
with older versions of the language specification.
When the optimization level is greater or equal to "-O2", gcj will
try to optimize the way calls into the runtime are made to initial-
ize static classes upon their first use (this optimization isn’t
carried out if "-C" was specified.) When compiling to native code,
"-fno-optimize-static-class-initialization" will turn this opti-
mization off, regardless of the optimization level in use.
Don’t include code for checking assertions in the compiled code.
If "=class-or-package" is missing disables assertion code genera-
tion for all classes, unless overridden by a more specific
"--enable-assertions" flag. If class-or-package is a class name,
only disables generating assertion checks within the named class or
its inner classes. If class-or-package is a package name, disables
generating assertion checks within the named package or a subpack-
By default, assertions are enabled when generating class files or
when not optimizing, and disabled when generating optimized bina-
Generates code to check assertions. The option is perhaps mis-
named, as you still need to turn on assertion checking at run-time,
and we don’t support any easy way to do that. So this flag isn’t
very useful yet, except to partially override "--disable-asser-
gcj has a special binary compatibility ABI, which is enabled by the
"-findirect-dispatch" option. In this mode, the code generated by
gcj honors the binary compatibility guarantees in the Java Language
Specification, and the resulting object files do not need to be
directly linked against their dependencies. Instead, all dependen-
cies are looked up at runtime. This allows free mixing of inter-
preted and compiled code.
Note that, at present, "-findirect-dispatch" can only be used when
compiling .class files. It will not work when compiling from
source. CNI also does not yet work with the binary compatibility
ABI. These restrictions will be lifted in some future release.
However, if you compile CNI code with the standard ABI, you can
call it from code built with the binary compatibility ABI.
Some gcj code generations options affect the resulting ABI, and so can
only be meaningfully given when "libgcj", the runtime package, is con-
figured. "libgcj" puts the appropriate options from this group into a
spec file which is read by gcj. These options are listed here for com-
pleteness; if you are using "libgcj" then you won’t want to touch these
This enables the use of the Boehm GC bitmap marking code. In par-
ticular this causes gcj to put an object marking descriptor into
By default, synchronization data (the data used for "synchronize",
"wait", and "notify") is pointed to by a word in each object. With
this option gcj assumes that this information is stored in a hash
table and not in the object itself.
On some systems, a library routine is called to perform integer
division. This is required to get exception handling correct when
dividing by zero.
On some systems it’s necessary to insert inline checks whenever
accessing an object via a reference. On other systems you won’t
need this because null pointer accesses are caught automatically by
gcc(1), gcjh(1), gjnih(1), gij(1), jv-scan(1), jcf-dump(1), gfdl(7),
and the Info entries for gcj and gcc.
Copyright (c) 2001, 2002, 2003, 2004, 2005 Free Software Foundation,
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.2 or
any later version published by the Free Software Foundation; with the
Invariant Sections being ‘‘GNU General Public License’’, the Front-
Cover texts being (a) (see below), and with the Back-Cover Texts being
(b) (see below). A copy of the license is included in the man page
(a) The FSF’s Front-Cover Text is:
A GNU Manual
(b) The FSF’s Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-4.0.2 2005-11-25 GCJ(1)
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