JDK, JRE, JVM

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1. Introduction

Java is a platform-independent programming language. This independence is achieved because Java programs do not run directly on your operating system; instead, they run inside a virtual machine — the JVM. To understand how Java works internally, you must clearly understand three important components:

• JDK – Java Development Kit
• JRE – Java Runtime Environment
• JVM – Java Virtual Machine

These three work together to allow writing, compiling, and running Java applications.

2. What is JVM? (Java Virtual Machine)

Definition

The JVM is a virtual computer inside your real computer. It is responsible for executing Java bytecode.

Key Responsibilities of JVM

1. Loads the .class file (Class Loader subsystem)
2. Verifies bytecode (Bytecode Verifier)
3. Executes bytecode (Interpreter + JIT Compiler)
4. Provides memory management (Heap, Stack, Method Area)
5. Runs garbage collection

Internal Working of JVM

When you type:

java MyProgram

This is what happens internally:

1. Class loader loads MyProgram.class
2. Bytecode is verified for safety
3. Interpreter converts bytecode → machine code
4. JIT compiler optimizes frequently used code
5. Garbage collector removes unused objects

JVM Memory Structure

JVM memory is divided into sections:

1. Method Area

Stores:

• Class metadata
• Static variables
• Constant pool
• Method code

2. Heap Area

Stores objects and instance variables.

3. Stack Area

Stores method calls, local variables, references.

4. PC Register

Tracks the current instruction.

5. Native Method Stack

Stores native (e.g., C/C++) calls.

3. What is JRE? (Java Runtime Environment)

Definition

JRE is the environment needed to run Java programs. It includes:

• JVM
• Core Java class libraries (String, Math, Collection, IO, etc.)
• Java supporting files

What JRE Does

If you only need to run Java applications (not write them), installing the JRE is enough.

JRE = JDK – (Compiler + Development tools)

Example Scenario

If you only want to run a Java-based software like Eclipse or Minecraft, you only need the JRE.

4. What is JDK? (Java Development Kit)

Definition

JDK is the complete Java development package used to build Java applications.

It includes:

• JRE (JVM + Libraries)
• Compiler → javac
• Debugger → jdb
• Documentation tool → javadoc
• Packaging tool → jar
• Profiler → jconsole, jvisualvm

In simple words:

JDK = JRE + Development Tools

Example Commands in JDK

Compile a Java program

javac Hello.java

Run the program

java Hello

Create documentation

javadoc Hello.java

5. Differences Between JDK, JRE, JVM

6. Which One Should You Install?

If you want to run Java programs → Install JRE If you want to develop Java programs → Install JDK

(because JDK already contains the JRE)

In modern Java versions, JRE is no longer distributed separately. Now JDK includes everything.

7. Real-World Example

Consider the following Java program:

class Hello {
    public static void main(String[] args) {
        System.out.println("Hello Java");
    }
}

Step-by-step execution:

1. You write code → JDK
2. Code is compiled into bytecode .class → javac
3. Bytecode is executed by JVM → java command
4. JRE provides libraries → System.out.println
5. JVM converts bytecode → machine code
6. CPU runs the final machine code

8. Why Java Is Platform Independent

• Source code → compiled once into bytecode
• Bytecode → runs on any JVM
• JVM is platform-dependent (Windows JVM, Mac JVM, Linux JVM)
• But bytecode is platform-independent

Therefore: "Write Once, Run Anywhere" = Java Philosophy

9. Interview Questions

10. Summary

• JVM runs Java programs by translating bytecode to machine code.
• JRE provides libraries + JVM needed to run Java applications.
• JDK provides everything needed to develop Java applications — including JRE + tools.
• Java is platform-independent due to its bytecode and JVM.

Written By: Shiva Srivastava