📦 1️⃣ JVM (Java Virtual Machine)
What is it?
✅ The JVM is the heart of Java.
✅ It’s responsible for actually executing the bytecode (the compiled .class files).
✅ The JVM is platform-dependent (each OS has its own implementation), but the bytecode is platform-independent — that’s the “Write Once, Run Anywhere” magic.
What it does:
✅ Loads the bytecode
✅ Verifies it (security checks)
✅ Interprets/Compiles (JIT) it into native machine code
✅ Manages Memory (Garbage Collection)
📦 2️⃣ JRE (Java Runtime Environment)
What is it?
✅ The JRE is basically the JVM + essential libraries (like java.util, java.io, etc.)
✅ It’s enough if you only want to run Java programs, not develop them.
JRE contains:
✔️ JVM
✔️ Core Libraries (like collections, IO, networking, threading)
📦 3️⃣ JDK (Java Development Kit)
What is it?
✅ The JDK is the full package — it’s what developers install.
✅ It contains everything the JRE has, plus development tools.
JDK contains:
✔️ JRE (JVM + core libraries)
✔️ Compiler (javac) — to compile source code into bytecode
✔️ Debugger (jdb)
✔️ Other tools — jar, javadoc, javap, etc.
🚀 Simple Analogy
| Thing | Real-world analogy |
|---|---|
| JVM | Engine that runs the program |
| JRE | Car ready to drive (engine + necessary parts) |
| JDK | Garage with tools to build and fix the car (includes the whole car itself) |
🎯 Summary Chart
| Component | Contains | Needed for |
|---|---|---|
| JVM | Executes bytecode | Running programs |
| JRE | JVM + Core Libraries | Running programs |
| JDK | JRE + Compiler + Tools | Developing + Running programs |
⚠️ Quick Tip
| Use this if you want to: | Install |
|---|---|
| Just run Java programs | JRE (or JDK if JRE is removed in newer versions) |
| Develop Java programs | JDK |