JVM (Java Virtual Machine) is an abstract machine. It is a specification that provides runtime environment in which java bytecode can be executed.
JVMs are available for many hardware and software platforms (i.e. JVM is platform dependent).
The JVM performs following operation:
JVM provides definitions for the:
Let’s understand the internal architecture of JVM. It contains classloader, memory area, execution engine etc.
Classloader is a subsystem of JVM which is used to load class files. Whenever we run the java program, it is loaded first by the classloader. There are three built-in classloaders in Java.
//Let’s see an example to print the classloader name
public class ClassLoaderExample
public static void main(String args)
// Let’s print the classloader name of current class.
//Application/System classloader will load this class
//If we print the classloader name of String, it will print null because it is an
//in-built class which is found in rt.jar, so it is loaded by Bootstrap classloader
These are the internal classloaders provided by Java. If you want to create your own classloader, you need to extend the ClassLoader class.
Class(Method) Area stores per-class structures such as the runtime constant pool, field and method data, the code for methods.
It is the runtime data area in which objects are allocated.
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.
PC (program counter) register contains the address of the Java virtual machine instruction currently being executed.
It contains all the native methods used in the application.
Java Native Interface (JNI) is a framework which provides an interface to communicate with another application written in another language like C, C++, Assembly etc. Java uses JNI framework to send output to the Console or interact with OS libraries.