Java Virtual Machine

To execute a Java program, you run a program called a bytecode interpreter, which in turn reads the bytecodes and executes your Java program. The Java bytecode interpreter is often also called the Java virtual machine or the Java runtime.
Java bytecodes are a special set of machine instructions that are not specific to any one processor or computer system. A platform-specific bytecode interpreter executes the Java bytecodes. The bytecode interpreter is also called the Java virtual machine or the Java runtime interpreter.

The Java virtual machine, which is a component of the runtime system, is responsible for interpreting the bytecodes and making the appropriate system level calls to the native platform. It is at this point where platform independence is achieved by Java; the bytecodes are in a generic form that is only converted to a native form when processed by the virtual machine. The JVM concept allows a layer of translation between the executable program and the machine-specific code. In a non-Java compiler, the source code is compiled into machine- specific assembly code. In doing this, the executable limits itself to the confines of that machine architecture. Compiling Java code creates an executable using JVM assembly directives. The difference of the two approaches is quite fundamental to the portability of the executable. Non-Java executables communicate directly with the platform's instruction set. Java executables communicate with the JVM instruction set, which is then translated into platform-specific instructions.