Q: What is the difference between JDK, JRE, and JVM?

Answer:

This is the most common Java interview opener. These three form a layered architecture.

JVM (Java Virtual Machine)

The JVM is an abstract machine that provides the runtime environment to execute Java bytecode. It does NOT understand Java source code — only .class bytecode files.

Responsibilities:

• Loading bytecode (via ClassLoader)
• Verifying bytecode (bytecode verifier)
• Executing bytecode (interpreter + JIT compiler)
• Managing memory (heap, stack, garbage collection)

Key point: The JVM is what makes Java platform-independent. The same .class file runs on any OS that has a JVM implementation (Windows, macOS, Linux).

JRE (Java Runtime Environment)

The JRE = JVM + standard class libraries (java.lang, java.util, java.io, etc.). It's everything you need to run a Java application, but you cannot compile code with it.

JDK (Java Development Kit)

The JDK = JRE + development tools (javac compiler, javadoc, jdb debugger, jconsole, etc.). It's what developers install to develop and compile Java applications.

The Relationship

┌───────────────────────────────────┐
│             JDK                   │
│  ┌─────────────────────────────┐  │
│  │           JRE               │  │
│  │  ┌───────────────────────┐  │  │
│  │  │         JVM           │  │  │
│  │  │  (bytecode execution) │  │  │
│  │  └───────────────────────┘  │  │
│  │  + Standard Libraries       │  │
│  │    (rt.jar, java.*, etc.)   │  │
│  └─────────────────────────────┘  │
│  + Development Tools              │
│    (javac, javadoc, jar, jdb)     │
└───────────────────────────────────┘

[!NOTE] Since Java 11, Oracle no longer ships a separate JRE. The JDK is the only downloadable package, and you can create custom minimal runtimes using jlink.