What is Java Virtual Machine (JVM).

JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime
environment in which java bytecode(.class file) can be executed.JVMs are available for many hardware and software platforms (i.e. JVM is plateform dependent:).

For different OS there are different JVMs

The JVM performs four main tasks:

> Loads code
> Verifies code
> Executes code
> Provides runtime environment

JVM provides definitions for the:

> Memory area
> Class file format
> Register set
> Garbage-collected heap
> Fatal error reporting etc.

Internal Architecture of JVM:

Let's understand the internal architecture of JVM.

It contains  three things:

1> Classloader
2> Memory area

  2.1>Class(method) area
  2.2>Heap area
  2.3>Stack area
  2.4>Program Counter Register area
  2.5>Native Mthod Stack area
 
3> Execution engine.

Below one by one Explanation of each of the above:

1> Classloader:

Classloader is a subsystem of JVM that is used to load class files.

2.1> Class(method) Area:

Class(Method) Area stores per-class structures such as the runtime
constant pool, field and method data,the code for methods.

2.2> Heap Area:

It is the runtime data area in which objects are allocated.

2.3> Stack Area

Java Stack stores frames.It holds local variables and partial  results,and plays a part in method invocation and return.

Each thread has a private JVM stack, created at the same time as thread.A new frame is created each time a method is invoked. A frame is destroyed when its method invocation
completes.

2.4> Program Counter Register Area:

PC (program counter) register. It contains the address of the Java  virtual machine instruction currently being executed.

2.5> Native Method Stack Area:

It contains all the native methods used in the application.

3> Execution Engine:

It contains:

1) A virtual processor:
2) Interpreter:Read bytecode stream then execute the instructions.
3) Just-In-Time(JIT) compiler:It is used to improve the performance. JIT compiles parts of the byte code that have similar functionality at the same time, and hence reduces the amount of time needed for compilation.Here the term compiler refers to a translator from the  instruction set of a Java virtual machine (JVM) to the instruction set of a specific CPU.