[Java] JVM

JVM

- It is a specification where working of Java Virtual Machine is specified

- An implementation Its implementation is known as JRE

- Runtime Instance whenever you write java command on the command prompt to run the java class, and instance of JVM is created

1. What it does?

1_1 The JVM performs following operation

- Loads code

- Verifies code

- Executes code

- provides runtime environment

1_2. JVM provides definitions for the

- Memory area

- Class file format

- Register set

- Garbage-collected heap

- Fatal error reporting etc

2. Internal Architecture of JVM

1) Classloader

- Classloader is a subsystem of JVM that is used to load class file

2) Class(Method) Area

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

3) Heap

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

4) Stack

- 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

5) Program Counter Register

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

6) Native Method Stack

- It contains all the native methods used in the application 

7) Execution Engine

- It contains

: A virtual processor

: Interpreter, Read bytecode stream then execute the instructions

: 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 the time needed for compilation.