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2.9 JVM Memory Model Architecture

← Back to Chapter 2: Java Overview & Memory Model

The Java Virtual Machine organizes physical system memory into distinct logical runtime data areas defined by the JVM Specification.

1. JVM Runtime Data Areas Overview​

┌────────────────────────────────────────────────────────────────────────┐
│                          JVM Process Memory                            │
│                                                                        │
│  ┌──────────────────────────────────────────────────────────────────┐  │
│  │ Shared Across All Threads                                        │  │
│  │ ┌───────────────────────────────────────┐ ┌────────────────────┐ │  │
│  │ │ Heap Space                            │ │ Metaspace          │ │  │
│  │ │ • Young Gen (Eden, S0, S1)            │ │ (Class metadata,   │ │  │
│  │ │ • Old Gen (Tenured)                   │ │  static fields)    │ │  │
│  │ └───────────────────────────────────────┘ └────────────────────┘ │  │
│  └──────────────────────────────────────────────────────────────────┘  │
│                                                                        │
│  ┌──────────────────────────────────────────────────────────────────┐  │
│  │ Per-Thread Isolated Memory (Created per active Java thread)      │  │
│  │ ┌───────────────────┐ ┌───────────────────┐ ┌──────────────────┐ │  │
│  │ │ Thread Stack      │ │ PC Register       │ │ Native Method    │ │  │
│  │ │ (Stack Frames)    │ │ (Next Bytecode IP)│ │ Stack            │ │  │
│  │ └───────────────────┘ └───────────────────┘ └──────────────────┘ │  │
│  └──────────────────────────────────────────────────────────────────┘  │
└────────────────────────────────────────────────────────────────────────┘

2. Deep Dive: Memory Regions​

A. Java Heap​

  • Shared across all threads.
  • Stores all class instances, arrays, and associated instance fields.
  • Managed and reclaimed automatically by the Garbage Collector (GC).
  • Throws OutOfMemoryError: Java heap space when full.

B. Thread Stack (JVM Stack)​

  • Private to each thread. Created when the thread spawns; destroyed on termination.
  • Composed of Stack Frames. Each method invocation pushes a new frame:
    • Local Variable Array (LVA): Primitives and object reference pointers.
    • Operand Stack: Workspace where mathematical operations and bytecode evaluations occur.
    • Frame Data: Method return address and reference to runtime Constant Pool.
  • Throws StackOverflowError on deep or infinite recursion.

C. Metaspace (Native Memory, Java 8+)​

  • Replaced the old contiguous PermGen (Permanent Generation) in Java 8.
  • Located in Native OS Memory, outside the JVM heap limit.
  • Stores class metadata, bytecode definitions, runtime constant pools, and method definitions.
  • Automatically expands up to physical OS memory unless capped via -XX:MaxMetaspaceSize.

D. Program Counter (PC) Register​

  • Holds the address of the current JVM bytecode instruction being executed by the thread.
  • If executing a native method, the PC register value is undefined.