Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode instruction or opcode is one byte in length, however not all of the possible 256 instructions are used. In fact, Sun Microsystems, the original creators of the Java programming language, the Java virtual machine and other components of the Java Runtime Environment (JRE), have set aside a number of values to be permanently unimplemented.^[1]

Contents 1 Relation to Java 2 Generation 3 Execution 4 Support for dynamic languages 5 See also 6 References 7 External links

Relation to Java

A Java programmer does not need to be aware of or understand Java bytecode at all. However, as suggested in the IBM developerWorks journal, "Understanding bytecode and what bytecode is likely to be generated by a Java compiler helps the Java programmer in the same way that knowledge of assembler helps the C or C++ programmer."^[2].

Generation

The most common language targeting Java Virtual Machine by producing Java bytecode is Java. Originally only one compiler existed, the javac compiler from Sun Microsystems, which compiles Java source code to Java bytecode; but because all the specifications for Java bytecode are now available, other parties have supplied compilers that produce Java bytecode. Examples of other compilers include:

• Jikes, compiles from Java to Java bytecode (developed by IBM, implemented in C++)
• Espresso, compiles from Java to Java bytecode (Java 1.0 only)
• GCJ, the Gnu Compiler for Java, compiles from Java to Java bytecode; it is also able to compile to native machine code and is available as part of the