The GNU Compiler for Java (GCJ or gcj) is a free software compiler for the Java programming language and a part of the GNU Compiler Collection.
GCJ can compile Java source code to either Java Virtual Machine bytecode, or directly to machine code for any of a number of CPU architectures. It can also compile class files containing bytecode or entire JARs containing such files into machine code.
Read more about GNU Compiler For Java: History, Performance, CNI (Compiled Native Interface)