JVM method area

Understanding of method area

《Java Virtual machine specification 》 It is clearly stated in :" Although all method areas are logically part of the heap , But some simple implementations may not choose to do garbage collection or compression .” But for the HotSpotJVM for , The method area also has an alias called Non-Heap ( Non heap ), The purpose is to separate it from the pile .

therefore , The method area is regarded as a block independent of Java Heap memory space .

We can check the existence of the method area by testing the code .

/** * @author zal * @date 2022 year 02 month 19 Japan  20:49 * @Description  The default value of the test setting method area size parameter , Set up JVM Parameter is  -Xms600m -Xmx600m */
public class MethodAreaDemo {
    
    public static void main(String[] args) {
    
        System.out.println("start..");
        try{
    
            Thread.sleep(100000);
        }catch (InterruptedException e){
    
            e.printStackTrace();
        }
        System.out.println("end...");
    }
}

After running the program, you can use javaVisualVM Look at the memory distribution .

So we can say that the method area is an independent space of the heap .

• Method area (Method Area） And Java Pile up , Is an area of memory Shared by each thread .
• Methods in JVM Created at startup , And its actual physical memory space neutralizes Java The heap area can also be discontinuous .
• The size of the method area , Like heap space , You can choose to be fixed size or scalable .
• The size of the method area determines how many classes the system can hold , If the system defines too many classes , Cause method area overflow , Virtual machines also throw out memory overflow errors : java.lang.outOfMemoryError：PermGen space perhaps java.lang.outOfMemoryError: Metaspace
• close JVM This will free the memory in this area .

Set method area size

• jdk7 before , The permanent generation size can be used -XX:Permsize and -XX:MaxPermsize

• jdk8 in the future , The size of meta space can be -XX:MetaspaceSize and -XXMaxMatespaceSize Appoint

The internal structure of the method area

1） Type information

• The full class name of the type
• The full valid name of the immediate parent of the type
• Type modifier
• A sequence table of type direct interfaces

2） Domain information

• JVM Information about all fields of the type and the order in which the fields are declared must be stored in the method area
• Relevant information includes ： Domain name 、 Domain type 、 Domain modifier

3） Methods information

• Method name
• Return type of method
• Number and type of method parameters
• Method modifier
• Bytecode of method
• Anomaly table

4）non-final Class variables

• Static variables are associated with classes , Load as class loads
• Class variables are shared by all instances of a class , Even if there is no class instance , You can also visit