Java architecture Basics

Java comprises of the following basic parts:

• Java programming language: Code in Java is compiled to Bytecode format .class file.
• Java APIs: A set of Java .class files, which provide pre-coded functions.
• Java Virtual Machine (JVM): Code that runs the Bytecode in a particular Operating System.. Bytecode .class file is converted to machine language by JVM and then run in the local Operating System.

JVM is made up of 2 basic parts

• Class loader: Loads the .class file into RAM after converting into Operating System dependent data structures
• Execution Engine: Takes the data structures and converts to Machine Language for the OS and then runs it in the OS.

For each hardware architecture, a different Java Bytecode Interpreter is needed. Since it is an interpreter, execution is slower than program compiled to native Machine language.This is mitigated by Just-in-time compilers (JIT) which compile to Machine Code before executing.

Java Runtime Environment (JRE): Contain all that is needed to run a Java program. Is a combination of JVM and Java APIs.

Java Developmen Kit (JDK): Comprises a package of all the software needed to compile, document an execute Java programs. Consists of

• Java programming language
• JRE
• Tools and Tool APIs – java, javac, apt, jar, javap etc.

References:

• http://geekexplains.blogspot.com/2008/07/java-architecture-jre-java-platform-jvm.html
• http://stackoverflow.com/questions/7674839/is-the-jvm-a-compiler-or-an-interpreter
• https://en.wikipedia.org/wiki/Java_virtual_machine
• http://docs.oracle.com/javase/8/docs/