JDK JRE JVM Explained Example in Java

Introduction

Before writing advanced Java programs, it helps to understand how source code becomes running output. Three terms appear constantly: JDK, JRE and JVM.

Comparison

Execution Flow

Hello.java  --javac-->  Hello.class (bytecode)  --JVM-->  Program Output
     ^                        ^                          ^
   JDK tool                 JRE/JVM                   Running process

Example Program

public class JdkDemo {
    public static void main(String[] args) {
        System.out.println("Java Version : " + System.getProperty("java.version"));
        System.out.println("JVM Name     : " + System.getProperty("java.vm.name"));
        System.out.println("JRE Vendor   : " + System.getProperty("java.vendor"));
        System.out.println("OS Name      : " + System.getProperty("os.name"));

        int a = 10, b = 20;
        System.out.println("Sum = " + (a + b));
    }
}

Sample Output

Java Version : 21.0.2
JVM Name     : OpenJDK 64-Bit Server VM
JRE Vendor   : Oracle Corporation
OS Name      : Windows 11
Sum = 30

How It Works

1. You write .java source and compile with javac (part of the JDK).
2. The compiler produces platform-independent bytecode in .class files.
3. The JVM (inside the JRE) interprets or JIT-compiles bytecode and runs main.
4. System.getProperty queries runtime details exposed by the JVM.

Best Practices

• Install the JDK (not JRE alone) on development machines.
• Set JAVA_HOME and add %JAVA_HOME%\bin to PATH on Windows.
• Match your project’s Java version with the JDK used in CI and production.

Common Mistakes

• Confusing JRE with JDK and failing to find javac.
• Mixing bytecode compiled with a newer JDK than the runtime JVM supports.