When one of my colleague asked me What is 'Java Generations'?
I don’t know how many
Java techies know about Java Generations. We never think about memory
management and garbage collection.
This article is very useful
who really face these question:
1) How heap and non-heap works in Java?
2)
What
are the Generations in Java? 1) How heap and non-heap works in Java?
3) How Garbage collection works with Objects?
At
initialization, a maximum address space is virtually reserved but not allocated
to physical memory unless it is needed.
The
complete address space reserved for object memory can be divided into the young and tenured generations.
The
young
generation consists of eden
plus two survivor
spaces. Objects are initially allocated in eden. One survivor space is empty at any time, and
serves as a destination of the next, copying collection of any live objects in
eden and the other survivor space. Objects are copied between survivor spaces
in this way until they are old enough to be tenured, or copied to the tenured generation.
Other
virtual machines, including the production virtual machine for the J2SE
Platform version 1.2 for the Solaris Operating System, used two equally sized
spaces for copying rather than one large eden plus two small spaces. This means
the options for sizing the young
generation are not directly comparable.
A
third generation closely related to the tenured
generation is the permanent
generation. The permanent generation
is special because it holds data needed by the virtual machine to describe
objects that do not have equivalence at the Java language level. For example
objects describing classes and methods are stored in the permanent generation.
Summary:
- 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.
Source:
http://www.oracle.com/technetwork/java/gc-tuning-5-138395.html
