JVM bytecode

JVM bytecode is the instruction set architecture (ISA) of the Java virtual machine (JVM), the language to which Java and other JVM-compatible source code is compiled. Each instruction is represented by one byte, hence the name bytecode, making it a compact form of data. Due to the nature of virtual machines and bytecode, a JVM bytecode program is runnable on any machine with a compatible JVM, without the long process of compiling from source code. JVM bytecode is used at runtime either interpreted by a JVM or compiled to machine code via just-in-time (JIT) compilation and run as a native application. As JVM bytecode is designed for cross-platform software compatibility and security, a JVM bytecode application tends to run consistently across various hardware and software configurations. In general, a Java programmer does not need to understand JVM bytecode or even be aware of it. 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." The bytecode comprises various instruction types, including data manipulation, control transfer, object creation and manipulation, and method invocation, all integral to Java's object-oriented programming model. The JVM is both a stack machine and a register machine. Each frame for a method call has an "operand stack" and an array of "local variables".