JDK vs JRE vs JVM: What's the Difference?

Expected Interview Answer

JDK, JRE, and JVM as nested layers

Step-by-Step Explanation

1. Step 1

   Start with the JVM

   It is the abstract execution engine that loads .class files and runs bytecode on the underlying OS and hardware.

2. Step 2

   JVM gives platform independence

   Because bytecode is the same everywhere, only the JVM implementation needs to differ per OS, not your compiled code.

3. Step 3

   JRE wraps the JVM with libraries

   The runtime environment adds the standard library classes your program calls into at runtime.

4. Step 4

   JDK wraps the JRE with build tools

   javac compiles .java source into bytecode, and other tools like jar and javadoc support packaging and documentation.

5. Step 5

   Match the kit to the role

   Developers need the JDK to compile; historically, users only needed the JRE to run compiled apps (modern distributions often ship a JDK everywhere).

Code Example

// HelloWorld.java
public class HelloWorld {
    public static void main(String[] args) {
        System.out.println("Running on the JVM!");
    }
}

// Terminal:
// javac HelloWorld.java   -> uses the JDK's compiler to produce HelloWorld.class
// java HelloWorld         -> JRE/JVM loads the bytecode and executes it
// Output: Running on the JVM!

Follow-up Questions

• How does the JVM achieve platform independence?
• What is the difference between JIT compilation and interpretation?
• What tools does the JDK provide besides javac?
• Can you run a .class file without a JDK installed?
• What is the role of the class loader in the JVM?