什么是烫发空间？

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.

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.

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

Simple (and oversimplified) answer: it's where the jvm stores its own bookkeeping data, as opposed to your data.

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.

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.

Perm Gen stands for permanent generation which holds the meta-data information about the classes.

1. 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.
2. 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.
3. JAVA8 has introduced the concept called meta-space generation, hence permgen is no longer needed when you use jdk 1.8 versions.

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.

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.

• 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