Understanding the Java JVM Memory Model and Its Partitions

Introduction

The Java Virtual Machine (JVM) manages a runtime data area called the "runtime data area" when executing Java programs, and understanding where objects and variables are stored is essential.

1. Memory Model & Partitions

The runtime data area is divided into the following sections:

Java Virtual Machine Stack (stack area)

Native Method Stack

Java Heap (heap area)

Method Area

Program Counter

Each partition serves a specific purpose in the execution of Java code.

2. Java Heap

Overview of the Java heap, which stores all class instances and arrays.

3. Java Virtual Machine Stack

Details about the JVM stack, which holds frames for method invocations, local variables, and operand stacks.

4. Native Method Stack

The native method stack is similar to the JVM stack but serves native (non‑Java) method calls.

5. Method Area

The method area stores class structures, static variables, and the runtime constant pool.

It also contains a runtime constant pool, which holds literals and symbolic references.

6. Program Counter

The program counter is a small memory area that records the address of the next instruction to be executed for each thread.

7. Additional Knowledge: Direct Memory

Definition: Memory allocated outside the Java heap via native functions, introduced with NIO in JDK 1.4.

Features: Not limited by heap size.

Use case: Suitable for high‑frequency I/O operations, accessed through a DirectByteBuffer object to avoid copying between Java heap and native memory.

Possible exception: OutOfMemoryError when total memory usage exceeds physical limits.

8. Summary

The article has thoroughly explained the JVM memory model and its partitions, covering the heap, stacks, method area, program counter, and direct memory, providing a solid foundation for understanding Java runtime memory management.

Author: Carson_Ho
Link: juejin.im/post/6844903677279338509
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