The Java Environment - Overview

A Java program is run differently than a traditional executable program.

Traditional programs are invoked through the operating system.

• They occupy their own memory space.
• They are tracked as an individual process by the OS.

Traditional programs are compiled from source code into a machine and OS-specific binary executable file.

• To run the program in different environments, the source code would be compiled for that specific target environment.

When you run a Java program, the OS is actually running the Java Runtime Engine, orJRE, as an executable program; it processes your compiled code through the Java Virtual Machine (usually referred to as the JVM).

A Java source code file is compiled into a bytecode file.

• You can consider bytecode as a sort of generic machine language.
• The compiled bytecode is read by the JVM as a data file.
• The JVM interprets the bytecode at runtime, performing a final mapping of bytecode to machine language for whatever platform it is running on.
• Thus, Java programs are portable - they can be written in any environment, compiled in any environment, and run in any environment (as long as a JVM is available for that environment).
• The JVM manages its own memory area and allocates it to your program as necessary.
• Although this involves more steps at runtime, Java is still very efficient, much more so than completely interpreted languages like JavaScript, since the time-consuming parsing of the source code is done in advance.