Learn the memory management of JVM 03 - Method area and meta space of JVM

Catalog

What is the method area ？

Implementation of method area

In the early JDK

JDK8

Meta space

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What is the method area ？

         Used to store... That has been loaded by the virtual machine Type information 、 Constant 、 Static variables 、 Real time compiler compiled code cache and other data .《Java Virtual machine specification 》 Zhongba Method area Described as a logical part of the heap , But it has an alias called “ Non heap ” The purpose is to Java Heap differentiation .

Notice that there is one important place here ：《Java Virtual machine specification 》 Zhongba Method area Described as a logical part of the heap , this sentence , Don't be confused . Clear two points ：

1. Logic
2. He's not a pile

Pile up , yes JVM A piece of memory in virtual machine memory , It exists physically , Will be assigned .

Method area , Is a logical part of the heap . Also means that , There is a corresponding Physical implementation .

Implementation of method area

In the early JDK

My early days here JDK The concept is JDK8 Previous .

This is the time , The implementation method area is called Forever .

This permanent generation , yes Java Part of the pile . Type information 、 Constant 、 Data in the method area such as static variables , Stored in this location .

JDK6 When ,HotSopt（ A kind of JVM virtual machine ） There is a plan to give up the permanent generation , stay JDK7, Realize the transfer of some data . To JDK8, Completely abandoned the concept of permanent generation .

JDK8

that , Replace the permanent generation . His name is Meta space .

Meta space

This meta space is quite special , It is Local memory A piece of . be not in JVM In the virtual machine .

But be careful ： Not everything in the method area is in the meta space .

Constant pool （ Including runtime constant pool and string constant pool ） and Static variables are still in the heap .

Type information （ Metadata information ） And other information is moved to the meta space .

That is, meta space mainly stores type information .

All right. , That's all for the canto .

Keep early hours , take care . King of early rising , I wish you good luck .