Java Virtual Machine(JVM) Architecture

Java Virtual Machine(JVM) Architecture :

Every Java program have extension as .java, when we compile a java program (.java file), the compiler will generate .class file containing byte code instructions

    .java    (java code or source code)        compiler------à .class (byte code)

The byte code that is generating by compiler can be executed on any machine with the help of JVM.

The JVM will take the .class file as the input and executes the program

Class Loader Subsystem:

The sub system will take the .class file which is generated by the compiler as the input and performance the following tasks.

1. It will load the .class file into the JVM
2. Before loading the .class file ,it will verify whether  the byte code  is valid or not ,verification  is done by the byte code verifier
3. If the byte code is valid, then memory will allocated for the byte in different memory areas.
4. The various memory allocations which are used for storing the byte code are called Runtime Date Area.
5. The various  runtime data areas are:

Method Area: This area can be used for storing all the class code and method code

Heap: This area can be used for storing all the objects of the program.

Java Stack: This area can be used for storing all the methods that are under execution.

               The Java stack can be consider as a collection of stack frames where each frame will contain the state (information) of a single method

             PC Register (Program counter): This Register will contain the address of the next instruction to                                                                                           executed 

               Java Native stack: [Non java code –Native stack] -: This stack is used for storing the Non java code. The Non java code is called as Native code.

    Execution Engine: The execution engine is responsible for executing the byte code stored in different runtime data areas.

The Execution Engine contains Two Parts:

1. Interpreter
2. JIT  Compiler (Just-in-time  compiler) [Given by Host pot Technologies]

Both Interpreter and JIT compiler will execute the code parallel. Which will improve the performance of the application by executing the code faster.

[**Note]: The code that is executed by the JIT compiler will be called as hot spots.

Native Interface:  This interface will help to load the native code into the JVM .which is required for the program

Native Libraries:  It is a collection of predefined program of other languages.

OS(operating system): To execute  the java program , the JVM requires  some resource from the machine .To get the resource  from the machine ,the JVM  has to  Communicate  with the operating system .