1.2 Input Options

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 contains 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.

-Idir

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.

--classpath=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.

--CLASSPATH=path

Deprecated synonym for --classpath.

--bootclasspath=path

Where to find the standard builtin classes, such as java.lang.String.

--extdirs=path

For each directory in the path, place the contents of that directory at the end of the class path.

CLASSPATH

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 $(prefix)/share/java/ext.

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.)

-fforce-classes-archive-check

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.