Understanding Java JVM Memory Model: All Runtime Data Areas Explained

Memory Model & Partitions

When a Java program runs, the JVM manages a runtime data area that is divided into several partitions.

Java Virtual Machine Stack

Native Method Stack

Java Heap

Method Area

Program Counter

Java Heap

The heap stores all object instances and arrays. It is the main area for dynamic memory allocation.

Additional illustration of the heap structure.

Java Virtual Machine Stack

The JVM stack stores frames for each method invocation, including local variables, the operand stack, and the return address.

Illustration of the JVM stack layout.

Native Method Stack

Similar to the JVM stack but dedicated to native (non‑Java) method calls.

Method Area

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

The constant pool stores literals and symbolic references used by the JVM.

Program Counter

The program counter (PC) register points to the current instruction being executed for each thread.

Extra Knowledge: Direct Memory

Definition: Off‑heap memory allocated via NIO’s DirectByteBuffer using native functions.

Features: Not limited by the Java heap size.

Use case: Frequent I/O operations to avoid copying between Java heap and native memory.

Exception: OutOfMemoryError when total memory usage exceeds physical limits.

Summary

This article comprehensively explains the JVM memory model and its partitions, covering the heap, stacks, method area, program counter, and direct memory.