Java bytecode

Java bytecode is the instruction set of the Java virtual machine. Each bytecode is composed of one, or in some cases two bytes that represent the instruction (opcode), along with zero or more bytes for passing parameters.

Of the 255 possible byte-long opcodes, as of 2015^[update], 198 are in use (~78%), 54 are reserved for future use (~21%), and 3 instructions (~1%) are set aside as permanently unimplemented.

The Java bytecode system does not directly support floating point operations beyond 32 bits, except indirectly via bytecodes that enable use of 64-bit and 80-bit intermediate IEEE floating point operations.

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 assembly helps the C or C++ programmer."

Instructions fall into a number of broad groups:

There are also a few instructions for a number of more specialized tasks such as exception throwing, synchronization, etc.

Many instructions have prefixes and/or suffixes referring to the types of operands they operate on. These are as follows:

For example, "iadd" will add two integers, while "dadd" will add two doubles. The "const", "load", and "store" instructions may also take a suffix of the form "_n", where n is a number from 0–3 for "load" and "store". The maximum n for "const" differs by type.

The "const" instructions push a value of the specified type onto the stack. For example, "iconst_5" will push an integer 5, while "dconst_1" will push a double 1. There is also an "aconst_null", which pushes "null". The n for the "load" and "store" instructions specifies the location in the variable table to load from or store to. The "aload_0" instruction pushes the object in variable 0 onto the stack (this is usually the "this" object). "istore_1" stores the integer on the top of the stack into variable 1. For variables with higher numbers the suffix is dropped and operands must be used.

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