A Java compiler is a compiler for the Java programming language. The most common form of output from a Java compiler is Java class files containing platform-neutral Java bytecode. There exist also compilers emitting optimized native machine code for a particular hardware/operating system combination.

Most Java-to-bytecode compilers, Jikes being a well known exception, do virtually no optimization, leaving this until run time to be done by the JRE^{[citation needed]}.

The Java Virtual Machine (JVM) loads the class files and either interprets the bytecode or just-in-time compiles it to machine code and then possibly optimizes it using dynamic compilation.

The very first Java compiler developed by Sun Microsystems was written in C using some libraries from C++.^{[citation needed]}

Major Java compilers

As of 2010, the following are major Java compilers:

• javac, included in JDK from Sun Microsystems, open-sourced since 13 November 2006.
• GNU Compiler for Java, a part of the GNU Compiler Collection, which compiles C, Fortran, Pascal and other programming languages besides Java. It can also generate native code using the back-end of GCC.
• ECJ, the Eclipse Compiler for Java, is an open source incremental compiler used by the Eclipse project.

Java decompilers

A Java decompiler tries to reverse the process of compiling, that is it tries to decompile Java bytecode files (*.class) back to Java source files (*.java).

• DJ Java Decompiler [1] (Uses JAD) 10 use trial version
• JAD, written in C++, free for non-commercial use.
• JD [2], younger project, includes GUI, optional Eclipse-plugin.
• JODE, decompiler and optimizer, open-source.
• Mocha, one of the first decompilers for Java.

External links

• Sun's OpenJDK javac page
• JSR 199 Java Compiler API Java Specification Request for invoking the Java compiler from a Java program
• A Formal Introduction to the Compilation of Java, Stephan Diehl, "Software - Practice and Experience", Vol. 28(3), pages 297-327, March 1998.