JVM - heap, non heap memory and other terminologies

Java has only two types of memory when it comes to JVM. Heap memory and Non-heap memory. All the other memory jargons you hear are logical part of either of these two.

Heap Memory
In lay man's term class instances and arrays are stored in heap memory. Heap memory is also called as shared memory. As this is the place where multiple threads will share the same data.

Some points to consider:

The Java virtual machine has a heap that is shared among all Java virtual machine threads. The heap is the runtime data area from which memory for all class instances and arrays is allocated.

The heap is created on virtual machine start-up. Heap storage for objects is reclaimed by an automatic storage management system (known as a garbage collector); objects are never explicitly deallocated.

The heap may be of a fixed size or may be expanded as required by the computation and may be contracted if a larger heap becomes unnecessary. The memory for the heap does not need to be contiguous.

A Java virtual machine implementation may provide the programmer or the user control over the initial size of the heap, as well as, if the heap can be dynamically expanded or contracted, control over the maximum and minimum heap size.

Non-heap Memory
It comprises of ‘Method Area’ and other memory required for internal processing. So here the major player is ‘Method Area’.

Method Area
As given in the last line, method area is part of non-heap memory. It stores per-class structures, code for methods and constructors. Per-class structure means runtime constants and static fields.

The above three (heap memory, non-heap memory and method area) are the main jargon when it comes to memory and JVM. There are some other technical jargon you might have heard and I will summarize them below.

Memory Pool
Memory pools are created by JVM memory managers during runtime. Memory pool may belong to either heap or non-heap memory.

Runtime Constant Pool
A run time constant pool is a per-class or per-interface run time representation of the constant_pool table in a class file. Each runtime constant pool is allocated from the Java virtual machine’s method area.

Java Stacks or Frames
Java stacks are created private to a thread. Every thread will have a program counter (PC) and a java stack. PC will use the java stack to store the intermediate values, dynamic linking, return values for methods and dispatch exceptions. This is used in the place of registers.
Definition of stack:

Each Java virtual machine thread has a private Java virtual machine stack, created at the same time as the thread. A Java virtual machine stack stores frames. A Java virtual machine stack is analogous to the stack of a conventional language such as C: it holds local variables and partial results, and plays a part in method invocation and return. Because the Java virtual machine stack is never manipulated directly except to push and pop frames, frames may be heap allocated.

See more on frames


Memory Generations
HotSpot VM’s garbage collector uses generational garbage collection. It separates the JVM’s memory into and they are called young generation and old generation.

Young Generation
Young generation memory consists of two parts, Eden space and survivor space. Shortlived objects will be available in Eden space. Every object starts its life from Eden space. When GC happens, if an object is still alive and it will be moved to survivor space and other dereferenced objects will be removed.

Old Generation – Tenured and PermGen
Old generation memory has two parts, tenured generation and permanent generation (PermGen). PermGen is a popular term. We used to error like PermGen space not sufficient.
GC moves live objects from survivor space to tenured generation. The permanent generation contains meta data of the virtual machine, class and method objects.

Discussion:
Java specification doesn’t give hard and fast rules about the design of JVM with respect to memory. So it is completely left to the JVM implementers. The types of memory and which kind of variable / objects and where they will be stored is specific to the JVM implementation.


Key Takeaways

• Local Variables are stored in Frames during runtime.
• Static Variables are stored in Method Area.
• Arrays are stored in heap memory.

References:

• MemoryPoolMXBean provides you api to explore the memory usage, threshold notifications, peak memory usage and memory usage monitoring.
• Java Docs API for JConsole
• Threads and Locks chapter of Java Language Specification talks lot about java memory