jvm设置方法区大小的参数

jdk7及以前：

通过-XX:PermSize 来设置永久代初始分配空间，默认值是20.75m
-XX:MaxPermSize来设定永久代最大可分配空间，32位是64m，64位是82m

jdk8及之后：

通过-XX:metaspaceSize 来设置永久代初始分配空间，默认值是21m
-XX:MaxmetaspaceSize来设定永久代最大可分配空间，值为-1

由于没有上限，因此当本机内存耗尽时，会抛出oom的错误，对于起始值21m来说，如果所使用的内存超过这个值，则会触发full gc卸载没用的类，之后将会重置这个初始值，新的初始值的高低在于gc后释放了多少的空间，释放得少则提升这个初始值，释放得多则降低这个初始值。因此在实际的开发场景中，为了减少full gc的频率，会将这个初始值设置大一些。

代码示例：
通过for循环不停创建类，直到溢出为止。
设置jvm参数：-XX:metaspaceSize=10m -XX:MaxmetaspaceSize=10m

package com.lydon.test;


import jdk.internal.org.objectweb.asm.ClassWriter;
import jdk.internal.org.objectweb.asm.Opcodes;


public class OOMTest extends ClassLoader{


    public static void main(String[] args) {
        int j=0;
        OOMTest test =new OOMTest();


        try{
            for (int i = 0; i <100000 ; i++) {
                ClassWriter classWriter=new ClassWriter(0);
                //指明版本号，修饰符，类型，包名，父类，接口
                classWriter.visit(Opcodes.V1_8,Opcodes.ACC_PUBLIC,"Class"+i,"com.lydon.test","java/lang/Object",null);
                byte[] code=classWriter.toByteArray();
                test.defineClass("Class"+i,code,0,code.length);
                j++;
            }
        }catch (Exception e){
            e.printStackTrace();
        }finally {
            System.out.println(j);
        }
    }
}

输出结果：

D:Javajdk1.8.0_121binjava.exe -XX:metaspaceSize=10m -XX:MaxmetaspaceSize=10m "-javaagent:D:IntelliJ IDEA 2021.1libidea_rt.jar=63745:D:IntelliJ IDEA 2021.1bin" -Dfile.encoding=UTF-8 -classpath D:Javajdk1.8.0_121jrelibcharsets.jar;D:Javajdk1.8.0_121jrelibdeploy.jar;D:Javajdk1.8.0_121jrelibextaccess-bridge-64.jar;D:Javajdk1.8.0_121jrelibextcldrdata.jar;D:Javajdk1.8.0_121jrelibextdnsns.jar;D:Javajdk1.8.0_121jrelibextjaccess.jar;D:Javajdk1.8.0_121jrelibextjfxrt.jar;D:Javajdk1.8.0_121jrelibextlocaledata.jar;D:Javajdk1.8.0_121jrelibextnashorn.jar;D:Javajdk1.8.0_121jrelibextsunec.jar;D:Javajdk1.8.0_121jrelibextsunjce_provider.jar;D:Javajdk1.8.0_121jrelibextsunmscapi.jar;D:Javajdk1.8.0_121jrelibextsunpkcs11.jar;D:Javajdk1.8.0_121jrelibextzipfs.jar;D:Javajdk1.8.0_121jrelibjavaws.jar;D:Javajdk1.8.0_121jrelibjce.jar;D:Javajdk1.8.0_121jrelibjfr.jar;D:Javajdk1.8.0_121jrelibjfxswt.jar;D:Javajdk1.8.0_121jrelibjsse.jar;D:Javajdk1.8.0_121jrelibmanagement-agent.jar;D:Javajdk1.8.0_121jrelibplugin.jar;D:Javajdk1.8.0_121jrelibresources.jar;D:Javajdk1.8.0_121jrelibrt.jar;D:workspacejvm-demochapters09targetclasses com.lydon.test.OOMTest
8271
Exception in thread "main" java.lang.OutOfMemoryError: metaspace
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:642)
    at com.lydon.test.OOMTest.main(OOMTest.java:18)


Process finished with exit code 1