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Java heap terminology: young, old and permanent generations?

I’m trying to understand What the concepts of young, old and permanent generations are in the Java heap terminology, and more specifically the interactions between the three generations.

My questions are:

  • 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?

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Answer

This seems like a common misunderstanding. In Oracle’s JVM, the permanent generation is not part of the heap. It’s a separate space for class definitions and related data. In Java 6 and earlier, interned strings were also stored in the permanent generation. In Java 7, interned strings are stored in the main object heap.

Here is a good post on permanent generation.

I like the descriptions given for each space in Oracle’s guide on JConsole:

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.
  • 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.

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