Java Memory Concepts: Difference between revisions
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* a ''method area'' that stores per-class structures such as a [[#Runtime_Constant_Pool|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. | * a ''method area'' that stores per-class structures such as a [[#Runtime_Constant_Pool|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. | ||
* a ''thread stack storage area''. For more details, see the "[[#Thread_Stack_Memory_Management|Thread Stack Memory Management]]" section. | * a ''thread stack storage area''. For more details, see the "[[#Thread_Stack_Memory_Management|Thread Stack Memory Management]]" section. | ||
* an area where the JIT compiler stores native machine code translated from Java bytecode. | |||
=Memory Manager= | =Memory Manager= | ||
Revision as of 18:01, 9 May 2017
Internal
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.
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:
- 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.
- a thread stack storage area. For more details, see the "Thread Stack Memory Management" section.
- an area where the JIT compiler stores native machine code translated from Java bytecode.
Memory Manager
Memory Pool
Runtime Constant Pool
Garbage Collector
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").