Memory Areas

The JVM manages two kinds of memory: heap and non-heap memory, both created when it starts.

Non-heap memory is all the memory the JVM allocated for purposes other than the heap. The only information JVM provides on non-heap memory is its overall size. No detailed information on non-heap memory content is available.

The Java Virtual Machine defines various run-time data areas that are used during execution of a program. Some of these data areas are created on Java Virtual Machine start-up and are destroyed only when the Java Virtual Machine exits. Other data areas are per thread. Per-thread data areas are created when a thread is created and destroyed when the thread exits.

1. (JVM) Stack: Stack memory is responsible for holding references to heap objects and for storing value types (primitive types), which hold the value itself rather than a reference to an object from the heap. Stack memory in Java is allocated per Thread. Therefore, each time a Thread is created and started, it has its own stack memory and cannot access another thread’s stack memory.

2. Heap: The heap part of memory stores the actual object in memory. There exists only one heap memory (heap) for each running JVM process. Therefore, this is a shared part of memory regardless of how many threads are running. The heap memory is periodically garbage collected by the framework.

3. Program Counter (PC) Register: The PC register holds the address of the current instruction. If the current method is native then the PC is undefined. All CPUs have a PC, typically the PC is incremented after each instruction and holds the address of the next instruction to be executed. The JVM uses the PC to keep track of where it is executing instructions, the PC will in fact be pointing at a memory address in the Method Area.

4. Method area and Runtime constant pool: JVM has a common method area across all threads. It contains per-class ...