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.
Contents
Of the 256 possible byte-long opcodes, as of 2015, 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.
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 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.
Example
Consider the following Java code:
A Java compiler might translate the Java code above into byte code as follows, assuming the above was put in a method:
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:
Some projects provide Java assemblers to enable writing Java bytecode by hand. Assembly code may be also generated by machine, for example by a compiler targeting a Java virtual machine. Notable Java assemblers include:
Others have developed compilers, for different programming languages, to target the Java virtual machine, such as:
Execution
There are several machines available today, both free and commercial products.
If executing Java bytecode in a Java virtual machine is undesirable, a developer can also compile Java source code or bytecode directly to native machine code with tools such as the GNU Compiler for Java (GCJ). Some processors can execute Java bytecode natively. Such processors are termed Java processors.
Support for dynamic languages
The Java virtual machine provides some support for dynamically typed languages. Most of the extant JVM instruction set is statically typed - in the sense that method calls have their signatures type-checked at compile time, without a mechanism to defer this decision to run time, or to choose the method dispatch by an alternative approach.
JSR 292 (Supporting Dynamically Typed Languages on the Java™ Platform) added a new invokedynamic instruction at the JVM level, to allow method invocation relying on dynamic type checking (instead of the extant statically type-checked invokevirtual instruction). The Da Vinci Machine is a prototype virtual machine implementation that hosts JVM extensions aimed at supporting dynamic languages. All JVMs supporting JSE 7 also include the invokedynamic opcode.