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What are heap memory generation in Java?

This is a very frequently asked question in Java Tuning. Other related question are like 

  • What is the young generation?
  • What is the old generation?
  • What is the permanent generation?
  • How does the three generations interact/relate to each other?

For the HotSpot Java VM, the memory pools for serial garbage collection are the following.

  • Eden Space (heap): The pool from which memory is initially allocated for most objects.
  • Survivor Space (heap): The pool containing objects that have survived the garbage collection of the Eden space.
  • Tenured Generation (heap): The pool containing objects that have existed for some time in the survivor space.
  • Permanent Generation (non-heap): The pool containing all the reflective data of the virtual machine itself, such as class and method objects. With Java VMs that use class data sharing, this generation is divided into read-only and read-write areas.
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Code Cache (non-heap): The HotSpot Java VM also includes a code cache, containing memory that is used for compilation and storage of native code.
Java uses generational garbage collection. This means that if you have an object foo (which is an instance of some class), the more garbage collection events it survives (if there are still references to it), the further it gets promoted. It starts in the young generation (which itself is divided into multiple spaces - Eden and Survivor) and would eventually end up in the tenured generation if it survived long enough.
If you want to read how garbage collector works then please follow the link :

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