Java · Theory

JVM: Java Virtual Machine

Theory

9/270

JVM: Java Virtual Machine

JVM is a virtual machine on your computer. It takes .class bytecode and turns it into real CPU work. That is why the same Java program can run on different operating systems.

Objects live on the heap. Each method call uses a stack frame. Unused objects can be cleaned by garbage collection. Classes are loaded when needed, then checked.

JVM is not another name for JDK. JDK is the full toolkit (write + compile). JVM is only the runtime engine inside.

Let's take this on the board with one tiny Main class — no extra files. JVM: Java Virtual Machine — reads JVM properties — proves your runtime is available. Start from main, go line by line, and stop at each print. That output is the proof for JVM: Virtual Machine.

Diagram

.class  (bytecode)
      │
      ▼
     JVM
      │
      ▼
    CPU / output

Exam tip

Say: JVM runs bytecode — that is how Java stays portable. Then contrast with JDK.

Example

// JVM / runtime info
public class Main {
  public static void main(String[] args) {
    System.out.println("java.version = " + System.getProperty("java.version"));
    System.out.println("java.home = " + System.getProperty("java.home"));
    System.out.println("os.name = " + System.getProperty("os.name"));
  }
}

JVM: Java Virtual Machine — reads JVM properties — proves your runtime is available.

Short notes

DefJVM runs bytecode and turns it into real CPU work.
RuleSame bytecode can run on any OS that has a JVM.
RememberHeap = objects. Stack = method calls. GC clears unused objects.
TrapJVM ≠ JDK. JDK is the toolkit. JVM is only the engine.

Java · Theory

JVM: Java Virtual Machine

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Theory

9/270

JVM: Java Virtual Machine

JVM is a virtual machine on your computer. It takes .class bytecode and turns it into real CPU work. That is why the same Java program can run on different operating systems.

Objects live on the heap. Each method call uses a stack frame. Unused objects can be cleaned by garbage collection. Classes are loaded when needed, then checked.

JVM is not another name for JDK. JDK is the full toolkit (write + compile). JVM is only the runtime engine inside.

Diagram

.class  (bytecode)
      │
      ▼
     JVM
      │
      ▼
    CPU / output

Exam tip

Say: JVM runs bytecode — that is how Java stays portable. Then contrast with JDK.

Example

// JVM / runtime info
public class Main {
  public static void main(String[] args) {
    System.out.println("java.version = " + System.getProperty("java.version"));
    System.out.println("java.home = " + System.getProperty("java.home"));
    System.out.println("os.name = " + System.getProperty("os.name"));
  }
}

JVM: Java Virtual Machine — reads JVM properties — proves your runtime is available.

Short notes

DefJVM runs bytecode and turns it into real CPU work.
RuleSame bytecode can run on any OS that has a JVM.
RememberHeap = objects. Stack = method calls. GC clears unused objects.
TrapJVM ≠ JDK. JDK is the toolkit. JVM is only the engine.

JVM: Java Virtual Machine

Example

Short notes

Questions

JVM: Java Virtual Machine

Example

Short notes

Questions