JDK, JRE, and JVM difference in Java - Step-by-Step Execution

Concept Flow - JDK, JRE, and JVM difference

Write Java Code

↓

Use JDK to Compile

↓

Bytecode (.class files)

↓

Use JRE to Run

↓

JVM Executes Bytecode

↓

Program Output

Java code is written and compiled by JDK into bytecode, which is run by JRE using JVM to produce output.

Execution Sample

Java

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

This Java program prints 'Hello World' to the screen.

Execution Table

Step	Component	Action	Input	Output
1	Developer	Writes Java source code	Hello.java	Java code text
2	JDK (javac)	Compiles Java code	Hello.java	Hello.class (bytecode)
3	JRE	Prepares runtime environment	Hello.class	Ready to run bytecode
4	JVM	Loads and executes bytecode	Hello.class	Prints 'Hello World'
5	Program	Ends execution	N/A	Program stops

💡 Program ends after printing output.

Variable Tracker

Component	State Before	State After
Java Source Code	Not written	Hello.java with code
Bytecode	Not compiled	Hello.class file created
Runtime Environment	Not ready	JRE ready to run bytecode
Execution	Not started	Program output displayed

Key Moments - 3 Insights

Why do we need both JDK and JRE?

JDK includes tools to write and compile code (see Step 2), while JRE only runs compiled code (Step 3). You need JDK to create programs and JRE to run them.

Is JVM part of JDK or JRE?

JVM is inside JRE (Step 4). JRE provides JVM to run bytecode, and JDK includes JRE plus tools to compile.

What does JVM actually do?

JVM reads the bytecode (.class files) and runs it on your computer (Step 4), turning code into actions like printing text.

Visual Quiz - 3 Questions

Look at the execution table, which component compiles the Java code?

A JVM

B JRE

C JDK

D Developer

💡 Hint

Check Step 2 in the execution table where compilation happens.

At which step does the JVM execute the bytecode?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look at Step 4 in the execution table where execution happens.

If you only have JRE installed, what can you NOT do?

A Compile Java source code

B Execute bytecode

C Run Java programs

D Print output

💡 Hint

Refer to key moment about difference between JDK and JRE.

Concept Snapshot

JDK = Java Development Kit: tools to write and compile Java code.
JRE = Java Runtime Environment: runs compiled Java bytecode.
JVM = Java Virtual Machine: part of JRE that executes bytecode.
Write code -> Compile with JDK -> Run with JRE/JVM -> Output.
JDK includes JRE; JRE includes JVM.

Full Transcript

Java programs start as source code written by developers. The JDK compiles this code into bytecode files. The JRE provides the environment to run these bytecode files. Inside the JRE, the JVM executes the bytecode, turning it into actions like printing text. The JDK is needed to create and compile programs, while the JRE is enough to run them. JVM is the engine inside JRE that runs the code.