GCJ

GCJ is a bandend for the popular GNU C Compiler (GCC) that can compile Java Source Code into either Java Bytecode or native machine code.

This is useful for a number of reasons:

Java applications compiled with GCJ do not need to be deployed with a Java Runtime Environment. This means that there is no potential problems with users running the wrong JRE.
Your application does not need to wait for the HotSpot dynamic compiler to translate all of its critical code segments into machine code -- they are loaded off of disk in that format.
Perhaps the most important benefit is that the Java Native Interface is no longer relevant. Because your Java code becomes ordinary machine code just like C and C++ code do, GCJ can manage communication between your Java and C/C++ code via something it calls the Compiled Native Interface (CNI). This does not have the same level of overhead that is comes with use of the JNI; thus, language integration with GCJ is far more efficient.