2.2 Extra features unique to gcj

The main feature of gcj is that it can compile programs written in the Java programming language to native code. Most extensions that have been added are to facilitate this functionality.

• gcj makes it easy and efficient to mix code written in Java and C++. See About CNI, for more info on how to use this in your programs.
• When you compile your classes into a shared library using -findirect-dispatch then add them to the system-wide classmap.db file using gcj-dbtool, they will be automatically loaded by the libgcj system classloader. This is the new, preferred classname-to-library resolution mechanism. See Invoking gcj-dbtool, for more information on using the classmap database.
• The old classname-to-library lookup mechanism is still supported through the gnu.gcj.runtime.VMClassLoader.library_control property, but it is deprecated and will likely be removed in some future release. When trying to load a class gnu.pkg.SomeClass the system classloader will first try to load the shared library lib-gnu-pkg-SomeClass.so, if that fails to load the class then it will try to load lib-gnu-pkg.so and finally when the class is still not loaded it will try to load lib-gnu.so. Note that all ‘.’s will be transformed into ‘-’s and that searching for inner classes starts with their outermost outer class. If the class cannot be found this way the system classloader tries to use the libgcj bytecode interpreter to load the class from the standard classpath. This process can be controlled to some degree via the gnu.gcj.runtime.VMClassLoader.library_control property; See libgcj Runtime Properties.
• libgcj includes a special ‘gcjlib’ URL type. A URL of this form is like a jar URL, and looks like ‘gcjlib:/path/to/shared/library.so!/path/to/resource’. An access to one of these URLs causes the shared library to be dlopen()d, and then the resource is looked for in that library. These URLs are most useful when used in conjunction with java.net.URLClassLoader. Note that, due to implementation limitations, currently any such URL can be accessed by only one class loader, and libraries are never unloaded. This means some care must be exercised to make sure that a gcjlib URL is not accessed by more than one class loader at once. In a future release this limitation will be lifted, and such libraries will be mapped privately.
• A program compiled by gcj will examine the GCJ_PROPERTIES environment variable and change its behavior in some ways. In particular GCJ_PROPERTIES holds a list of assignments to global properties, such as would be set with the -D option to java. For instance, ‘java.compiler=gcj’ is a valid (but currently meaningless) setting.