What is JVM memory structure?

The JVM (Java Virtual Machine) Memory Structure defines how memory is divided and managed when a Java program runs. JVM allocates different memory areas for storing class data, objects, methods, and execution information.

Here are the main parts of JVM memory:

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✅ 1. Method Area (Class Area)

• Stores class-level information.

• Contains:

  • Class metadata
  • Method information
  • Static variables
  • Runtime constant pool

• Shared among all threads.

👉 Created when JVM starts.

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✅ 2. Heap Memory

• Used to store objects and instance variables.
• Shared by all threads.
• Managed by Garbage Collector (GC).

Heap is mainly divided into:

• Young Generation

  • Eden Space
  • Survivor Spaces (S0, S1)

• Old (Tenured) Generation

👉 Most memory-related errors like OutOfMemoryError occur here.

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✅ 3. Stack Memory (JVM Stack)

• Each thread has its own stack.

• Stores:

  • Method calls
  • Local variables
  • Partial results

• Memory is created when a method is called and removed after execution.

👉 Error example: StackOverflowError.

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✅ 4. Program Counter (PC) Register

• Each thread has a separate PC register.
• Stores the address of the current executing instruction.
• Helps JVM switch between threads.

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✅ 5. Native Method Stack

• Used for executing native methods (methods written in C/C++ using JNI).
• Works alongside the JVM stack.

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🧠 Simple Diagram (Conceptual)

JVM Memory
│
├── Method Area (Class Data)
├── Heap (Objects)
├── Stack (Per Thread)
├── PC Register (Per Thread)
└── Native Method Stack

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⭐ Quick Interview Tip

• Heap → Objects
• Stack → Method execution
• Method Area → Class metadata
• PC Register → Current instruction
• Native Stack → Native code execution

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