Internal Details of JVM

Education is not limited to just classrooms. It can be gained anytime, anywhere... - Ravi Ranjan (M.Tech-NIT)

JVM (Java Virtual Machine)

JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java byte code can be executed.

  1. Java Virtual Machine
  2. Internal Architecture of JVM

JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).

What is JVM

  1. A specification where working of Java Virtual Machine is specified. But implementation provider is independent to choose the algorithm. Its implementation has been provided by Sun and other companies.
  2. An implementation Its implementation is known as JRE (Java Runtime Environment).
  3. Runtime Instance Whenever you write java command on the command prompt to run the java class, an instance of JVM is created.

What it does

The JVM performs following operation:

  • 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 class loader, memory area, execution engine etc.

Jvm Internal

1) Class loader

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

2) Class (Method) Area

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

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) registers. 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:

1) A virtual processor

2) Interpreter: Read byte code 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.