Java heap terminology young old and permanent generations

Question

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

User · Accepted 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.

User · Answer

What is the young generation    The Young Generation is where all new objects are allocated and aged  When the young generation fills up  this causes a minor garbage collection  A young generation full of dead objects is collected very quickly  Some survived objects are aged and eventually move to the old generation      What is the old generation    The Old Generation is used to store long surviving objects  Typically  a threshold is set for young generation object and when that age is met  the object gets moved to the old generation  Eventually the old generation needs to be collected  This event is called a major garbage collection     What is the permanent generation    The Permanent generation contains metadata required by the JVM to describe the classes and methods used in the application  The permanent generation is populated by the JVM at runtime based on classes in use by the application   PermGen has been replaced with Metaspace since Java 8 release   PermSize  amp  MaxPermSize parameters will be ignored now     How does the three generations interact relate to each other      Image source  amp  oracle technetwork tutorial article  http   www oracle com webfolder technetwork tutorials obe java gc01 index html   The General Garbage Collection Process  in above article explains the interactions between them with many diagrams    Have a look at summary diagram

User · Answer

Memory in SunHotSpot JVM is organized into three generations  young generation  old generation and permanent generation     Young Generation   the newly created objects are allocated to the young gen  Old Generation   If the new object requests for a larger heap space  it gets allocated directly into the old gen  Also objects which have survived a few GC cycles gets promoted to the old gen i e long lived objects house in old gen   Permanent Generation   The permanent generation holds objects that the JVM finds convenient to have the garbage collector manage  such as objects describing classes and methods  as well as the classes and methods themselves    FYI  The permanent gen is not considered a part of the Java heap   How does the three generations interact relate to each other        Objects except the large ones  are first allocated to the young generation  If an object remain alive after x no  of garbage collection cycles it gets promoted to the old tenured gen  Hence we can say that the young gen contains the short lived objects while the old gen contains the objects having a long life  The permanent gen does not interact with the other two generations

User · Answer

The Heap is divided into young and old generations as follows     Young Generation   It is place where lived for short period and divided into two spaces    Eden Space   When object created using new keyword memory allocated on this space  Survivor Space   This is the pool which contains objects which have survived after java garbage collection from Eden space    Old Generation   This pool basically contains tenured and virtual     reserved  space and will be holding those objects which survived    after garbage collection from Young Generation    Tenured Space  This memory pool contains objects which survived after multiple garbage collection means object which survived after garbage collection from Survivor space    Permanent Generation   This memory pool as name also says contain permanent class metadata and descriptors information so PermGen space always reserved for classes and those that is tied to the classes for example static members      Java8 Update  PermGen is replaced with Metaspace which is very similar    Main difference is that Metaspace re-sizes dynamically  i e   It can expand at runtime    Java Metaspace space  unbounded  default    Code Cache  Virtual or reserved    If you are using HotSpot Java VM this includes code cache area that containing memory which will be used for compilation and storage of native code     Courtesy

User · Answer

The Java virtual machine is organized into three generations  a young generation  an old generation  and a permanent generation  Most objects are initially allocated in the young generation  The old generation contains objects that have survived some number of young generation collections  as well as some large objects that may be allocated directly in the old generation  The permanent generation holds objects that the JVM finds convenient to have the garbage collector manage  such as objects describing classes and methods  as well as the classes and methods themselves

[java] Java heap terminology: young, old and permanent generations?

Examples related to java

Examples related to garbage-collection

Examples related to jvm

Examples related to heap-memory