Garbage collection - Java heap terminology young old and permanent generations

Questions

I m trying to understand how the concepts of young , old and permanent generations 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?

Answers

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.

https://blogs.oracle.com/jonthecollector/entry/presenting_the_permanent_generation https://blogs.oracle.com/jonthecollector/entry/presenting_the_permanent_generation

http://docs.oracle.com/javase/7/docs/technotes/guides/management/jconsole.html http://docs.oracle.com/javase/7/docs/technotes/guides/management/jconsole.html

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.

Source

License : cc by-sa 3.0

http://stackoverflow.com/questions/2129044/java-heap-terminology-young-old-and-permanent-generations

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