Java Virtual Machine

Java Virtual Machine

1. Beginner Mental Model

Think of JVM as a translator + manager between your program and the real computer. You give it bytecode (.class), it gives the OS native instructions, while handling memory + safety.

Hello.java --javac--> Hello.class (bytecode) --JVM--> runs on Windows/Mac/Linux

JDK vs JRE vs JVM: JDK = tools to develop (javac, java), JRE = runtime to run (JVM + libs), JVM = engine that runs bytecode.

2. Three Big Parts (exam diagram)

Part	Job	Beginner analogy
Class Loader	loads .class, verifies safety, links	airport security check
Runtime Memory	Method area (class info), Heap (objects via new), Stack (method calls/locals), PC register	different rooms for different things
Execution Engine	Interpreter (line-by-line) + JIT (hot code → fast native) + GC (frees unused objects)	slow careful reader + fast memorizer + cleaner

3. Flow with Code

public class Demo {
    public static void main(String[] args) {
        String s = new String("hi"); // object → Heap, variable s → Stack
        System.out.println(s);
    }
}

Try: System.gc() hints GC (not guaranteed). Deep recursion → StackOverflowError. Too many objects → OutOfMemoryError: Java heap space.

4. Why WORA Works

Same Hello.class runs on any JVM (Windows x86, Mac ARM) because each JVM translates to its own native code. JVM itself is platform-dependent, your program is not.

5. Exam Points

• Draw JVM architecture (loader, memory, engine) — 7 marks, very frequent.
• JDK vs JRE vs JVM — 3-5 marks, very frequent.
• Interpreter vs JIT vs GC roles — 5 marks, frequent.