The relationship between method area, perpetual generation and meta space

What is the method area 、 Forever 、 Meta space

as everyone knows , Method area Is a very important part of the runtime data area , Is used to store class information 、 Constant pool 、 Method data 、 Code compiled by the compiler in time .

Forever yes jdk1.7 A previous landing implementation of the method area , In the heap , Used to store class information and string constant pool , Also contains SE Library classes and methods . stay jdk1.7 Move the string constant pool from the permanent generation to the heap , For ever jdk8 Removed .

Meta space Similar to the permanent generation , It is a landing implementation of the method area , The data stored in it is much purer than the permanent generation , It's the metadata of the class . The location of the meta space does not exist in the virtual machine , It's in local memory .

Why replace permanent generation with meta space

The disadvantages of the permanent generation

1. It takes up the space of the heap
2. The permanent generation space is too large and takes up too much heap memory , Too small, limited storage information
3. Permanent GC It will trigger the heap GC

Why can it be designed like this before ？

At that time or 32 It's a plane , And I can't see clearly , Up to now 64 It's a plane , The disadvantages are exposed .

Meta space solves the problem of permanent generation ,jdk1.8 Start to replace permanent storage information

Problems in meta space

Meta space is allocated according to class loader , That is, the class loader loads a class , The meta space allocated to this class is actually allocated to the class loader , Different class loaders occupy different spaces , They do not share class information , If there are a lot of classloaders in the program , And they load very few classes , Then it may cause a lot of space waste .

Space separation for a long time may also cause memory space fragmentation .