What is perm space

Question

While learning about java memory profiling  I keep seeing the term  perm space  in addition to  heap    I know what the heap is - what s perm space

User · Answer

Permgen space is always known as method area When the classloader subsystem will load the the class file byte code  to the method area permGen   It contains all the class metadata eg  Fully qualified name of your class  Fully qualified name of the immediate parent class  variable info  constructor info  constant pool infor etc

User · Answer

Perm Gen stands for permanent generation which holds the meta-data information about the classes     Suppose if you create a class name A  it s instance variable will be  stored in heap memory and class A along with static classloaders will be stored in permanent generation  Garbage collectors will find it difficult to clear or free the memory space stored in permanent generation memory  Hence it is always recommended to keep the permgen memory settings to the advisable limit  JAVA8 has introduced the concept called meta-space generation  hence permgen is no longer needed when you use jdk 1 8 versions

User · Answer

PermGen Space stands for memory allocation for Permanent generation All Java immutable objects come under this category  like String which is created with literals or with String intern   methods and for loading the classes into memory  PermGen Space speeds up our String equality searching

User · Answer

The permgen space is the area of heap that holds all the reflective data of the virtual machine itself  such as class and method objects

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What exists under PremGen   Class Area comes under PremGen area  Static fields are also developed at class loading time  so they also exist in PremGen  Constant Pool area having all immutable fields that are pooled like String are kept here  In addition to that  class data loaded by class loaders  Object arrays  internal objects used by jvm are also located

User · Answer

JVM has an internal representation of Java objects and those internal representations are stored in the heap  in the young generation or the tenured generation   JVM also has an internal representation of the Java classes and those are stored in the permanent generation

User · Answer

Perm space is used to keep informations for loaded classes and few other advanced features like String Pool for highly optimized string equality testing   which usually get created by String intern   methods   As your application number of classes  will grow this space shall get filled quickly  since the garbage collection on this Space is not much effective to clean up as required  you quickly get Out of Memory   perm gen space error  After then  no application shall run on that machine effectively even after having a huge empty JVM   Before starting your application you should java -XX MaxPermSize to get rid of this error

User · Answer

Simple  and oversimplified  answer  it s where the jvm stores its own bookkeeping data  as opposed to your data

User · Answer

It holds stuff like class definitions  string pool  etc   I guess you could call it meta-data

User · Answer

It stands for permanent generation      The permanent generation is special   because it holds meta-data describing   user classes  classes that are not   part of the Java language   Examples   of such meta-data are objects   describing classes and methods and   they are stored in the Permanent   Generation  Applications with large   code-base can quickly fill up this   segment of the heap which will cause   java lang OutOfMemoryError  PermGen no   matter how high your -Xmx and how much   memory you have on the machine

[java] What is perm space?

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