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
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:
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
single output file, named FILENAME. This is allowed even when using
"-S" or "-c", but not when using "-C" or "--resource". (This is an
extension beyond the what plain gcc allows.) (If more than one input
file is specified, all must currently be ".java" files, though we
hope to fix this.)
Input Optionsgcj 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 contains all the standard classes.
In the text 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 class path.
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
directory 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
* 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
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.
This option is used to choose the source version accepted by gcj.
The default is 1.5.
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 every host.
Warningsgcj 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 listed.
With this flag, gcj will warn about redundant modifiers. For
instance, it will warn if an interface method is declared
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
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
application. 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 configuring 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 is run.
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
runtime using the "java.lang.System.getProperty" method.
Create an application whose command-line processing is that of
the "gij" command.
This option is an alternative to using "--main"; you cannot use
This option causes linking to be done against a static version of
the libgcj runtime library. This option is only available if
corresponding linker support exists.
Caution: Static linking of libgcj may cause essential parts of
libgcj to be omitted. Some parts of libgcj use reflection to
load classes at runtime. Since the linker does not see these
references at link time, it can omit the referred to classes.
The result is usually (but not always) a "ClassNotFoundException"
being thrown at runtime. Caution must be used when using this
option. For more details see:
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.
This can be used with -C to choose the version of bytecode
emitted by gcj. The default is 1.5. When not generating
bytecode, this option has no effect.
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
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
extensively. 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
compiling 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
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
initialize 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 optimization off, regardless of the optimization level
Don't include code for checking assertions in the compiled code.
If "=class-or-package" is missing disables assertion code
generation 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
By default, assertions are enabled when generating class files or
when not optimizing, and disabled when generating optimized
Generates code to check assertions. The option is perhaps
misnamed, 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
-findirect-dispatchgcj 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 dependencies are looked up at runtime. This allows
free mixing of interpreted 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
However, if you compile CNI code with the standard ABI, you can
call it from code built with the binary compatibility ABI.
This option can be use to tell "libgcj" that the compiled classes
should be loaded by the bootstrap loader, not the system class
loader. By default, if you compile a class and link it into an
executable, it will be treated as if it was loaded using the
system class loader. This is convenient, as it means that things
like "Class.forName()" will search CLASSPATH to find the desired
This option causes the code generated by gcj to contain a reduced
amount of the class meta-data used to support runtime reflection.
The cost of this savings is the loss of the ability to use
certain reflection capabilities of the standard Java runtime
environment. When set all meta-data except for that which is
needed to obtain correct runtime semantics is eliminated.
For code that does not use reflection (i.e. serialization, RMI,
CORBA or call methods in the "java.lang.reflect" package),
"-freduced-reflection" will result in proper operation with a
savings in executable code size.
JNI ("-fjni") and the binary compatibility ABI
("-findirect-dispatch") do not work properly without full
reflection meta-data. Because of this, it is an error to use
these options with "-freduced-reflection".
Caution: If there is no reflection meta-data, code that uses a
"SecurityManager" may not work properly. Also calling
"Class.forName()" may fail if the calling method has no
Some gcj code generations options affect the resulting ABI, and so
can only be meaningfully given when "libgcj", the runtime package, is
configured. "libgcj" puts the appropriate options from this group
into a spec file which is read by gcj. These options are listed here
for completeness; if you are using "libgcj" then you won't want to
touch these options.
This enables the use of the Boehm GC bitmap marking code. In
particular this causes gcj to put an object marking descriptor
into each vtable.
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 the processor.
On some systems, GCC can generate code for built-in atomic
operations. Use this option to force gcj to use these builtins
when compiling Java code. Where this capability is present it
should be automatically detected, so you won't usually need to
use this option.
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