The Java Memory Model (JMM) defines how the JVM organizes memory and ensures thread-safe visibility, ordering, and atomicity in concurrent programs. It guarantees that operations like volatile writes, synchronized blocks, and Thread.start()/join() establish happens-before relationships, ensuring consistent behavior across hardware and JVM implementations.

At runtime, the JVM memory is divided into distinct areas:

• Heap – Stores all objects and arrays, split into Young Generation (Eden + Survivor spaces) for short-lived objects and Old Generation for long-lived ones. Managed by the Garbage Collector (GC).

• Method Area / Metaspace – Holds class metadata, static variables, and runtime constants. In Java 8+, Metaspace resides in native memory.

• JVM Stack – Thread-local memory storing method frames, local variables, and references.

• PC Register – Tracks the current bytecode instruction for each thread.

• Native Method Stack – Supports execution of non-Java (JNI) methods.

Garbage Collection Process GC automatically reclaims memory for unreachable objects. Common algorithms include: