Method area of JVM

JVM The method area

Java Virtual machine memory region model

** Yellow area ：** The area of memory shared by all threads , There will be garbage collection .

** Grey areas ：** Thread private does not generate garbage collection .

The method area is part of the runtime data area , So what is the specific purpose of the method area ？ What information can be stored in the method area ？

What is the method area

The method area, like the heap, is shared by all threads , In some places, the method area is called Non-Heap（ Non heap ）, The purpose is to distinguish the method area from the heap memory , stay JDK1.7 Many developers in call the method area the permanent generation , But in essence, the two are not equivalent , Just because HotSpot The virtual machine design team will GC Generational collection extends to the method area , Or use permanent generation to implement method areas , such HotSpot The garbage collector can manage the permanent generation memory just like the heap memory , The rest are not HotSpot There is no concept of permanent generation for virtual machines .

HotSpot Method area evolution

Methods in jdk7 And before , Custom calls it the method area forever , The structure is as follows

stay jdk1.8 Have adopted the Metaspace（ Metadata space ） To replace the Perm（ The permanent zone ）, And separate from the heap space , The structure is as follows

It must be noted that Metaspace（ Metadata space ） No longer stored in the heap , Not used JVM The memory space is the memory of the machine , In this way, the metadata is only limited by the local memory size ,OOM There will be no problem .

Why? JDK1.8 The method area needs to be changed from JVM Forever （ Indirect memory ） Move to the meta space （ Direct memory ） Well ？

• JVM The permanent generation has a capacity limit , If the storage capacity of such information is limited, it is easy to trigger OOM, And use meta space storage , Limited by local memory size only OOM It's very unlikely that .

• Because it's direct memory ,JVM Will be in IO Higher performance in operation , Because it acts directly on the local operating system IO operation , Heap memory does IO operation , First you need to copy the data to direct memory , recycling IO Handle , Direct memory does not require this layer of conversion .

Stack and method area interaction

For example, we often build business objects , With Car Object as an example

When we create a Car After the instance object , Will be in the heap 、 Stack 、 The method area memory space writes data respectively

• Car： Is an object template , by Class object , Store in method area .

• car： It belongs to reference data , Stored in stack space .

• new Car()： You need to make room in the heap , Store in a pile .

What data is stored in the method area

The data stored in the method area is mainly the class information loaded by the class loader , Runtime constant pool, etc .

There are two points to note

• Static variables , stay JDK1.8 Previously, static variables were stored in the permanent memory ,JDK1.8 Later, the meta space is used to replace the permanent generation ,== The static variables are transferred from the permanent generation to the heap space ==.

• Constant pool , stay JDK1.7 Previously, the runtime constant pool logic included that the string constant pool is stored in the permanent generation ,JDK1.7 Moved the string constant pool to heap memory , Note that there is no mention of the runtime constant pool , That is, the runtime constant pool still exists in the permanent generation ,JDK1.8 Remove the permanent generation and replace it with a meta space , At this time, the runtime constant pool exists in the meta space .

So what does this kind of information include ？

• Class information

• The full name of the class

• The full name of the immediate parent class of the class

• The direct implementation interface of class has sequence table

• Type identification （ Class or interface ）

• Class access modifier （public、private wait ）

• Type constant pool

• An ordered collection of constants used by this type , Including direct constants （ character string 、 floating-point 、 integer ）, And for other types 、 Field 、 Symbolic reference to method .

• Field information

• Field type 、 name 、 Access modifiers, etc

• Methods information

• Class

• Method return type 、 Method access modifier 、 Method name and so on

• Class variables

• References to class loaders

• Point to Class Instance reference

• Method table