Java Memory Concepts

Internal

• Java Memory

Heap

The heap is the runtime data area in the JVM where all class instances and arrays are allocated. The heap is allocated when the JVM starts, and it may be of a fixed or variable size. The objects that are no longer in use are automatically reclaimed by a garbage collector. Metrics reflecting heap usage are exposed by the Memory MBean, which can be obtained from the platform MBean server. The heap usually consists of several memory pools, depending on what garbage collector is in use:

Eden

Eden statistics are exposed by the "java.lang:type=MemoryPool,name=PS Eden Space" MemoryPool MBean.

Survivor Space

Survivor Space statistics are exposed by the "java.lang:type=MemoryPool,name=PS Survivor Space" MemoryPool MBean.

Old Generation

Old Generation statistics are exposed by the "java.lang:type=MemoryPool,name=PS Old Gen" MemoryPool MBean.

Non-Heap Memory

Anything else that is not a class instance or an array is allocated by the JVM in memory areas different from heap. These memory areas are referred collectively as non-heap memory. The non-heap memory include:

• "Metaspace" - a method area that stores per-class structures such as a runtime constant pool, per-class field and method definition data, and the code for method and constructors. The method area is shared among all threads. The method area is created at the JVM startup.
• "Code Cache" - an area where the JIT compiler stores native machine code translated from Java bytecode.
• "Compressed Class Space"
• a thread stack storage area. For more details, see the "Thread Stack Memory Management" section.

Memory Pool

A memory pool represents a memory area managed by the JVM, and it can belong either to the heap or the non-heap memory. A memory pool is managed by one or more memory managers. Statistics about specific memory pools are exposed by the corresponding Memory Pool MBean. Examples of memory pools:

• Eden (heap)
• Survivor Space (heap)
• Old Generation (heap)

Memory Manager

A memory manager is the JVM runtime component that manages one or more memory pools.

The garbage collectors are a type of memory manager.

Other non-heap memory managers:

• The CodeCache manager - manages the Code Cache memory pool.
• The Metaspace manager - manages the Metaspace memory pool.

The memory managers are exposed over JMX by MemoryManager MBeans and the Garbage Collector MBean.

Garbage Collector

A set of one or more memory managers responsible for reclaiming memory that is occupied by unreachable objects.

The parallel collector, for example, consists of two distinct memory managers:

• "Scavenge" memory manager, that interacts with the Eden and the Survivor Space memory pools.
• "Mark and Sweep" memory manager, that interacts with all three heap memory pools (Eden, Survivor Space and Old Generation).

Thread Stack Memory Management

The default thread stack size on 64-bit systems is 1024K. 64k is the least amount of stack space allowed per thread.

It can be modified with with the -Xss option:

java ... -Xss<size> ...

where "<size>" represents the amount of memory and the measure unit (ex "2048k").