【VisualVM 简明教程】（3）：分析PermGenOOM

PermGen Space是Oracle-Sun Hotspot才有的，同类产品Oralce JRockit, IBM J9, Taobao JVM 是没有的，在Java8中永久区(PermGen)已经变成元空间(Metaspace),这里怀旧一下。

案例：PermGen OOM

设置VM启动参数 -XX:PermSize=5m -XX:MaxPermSize=5m -XX:+HeapDumpOnOutOfMemoryError -XX:HeapDumpPath=d:\dump 运行下面代码(这里用的Maven仓库下的jar包，根据自己情况来)，通过不断的动态加载类（Spring经常干这样的事情），造成PermGen OOM. 一般JVM默认PermGen大约是80M左右(和环境也有点关系，不过八九不离十)，可以通过jmap -heap pid 查看，为什么那么小可以看下这个回答

import java.io.File;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;
import java.util.List;
 
 
public class PermGen {
 
    private static List<Object> insList = new ArrayList<Object>();
 
    public static void main(String[] args) throws ClassNotFoundException, NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, MalformedURLException, InterruptedException {
        permLeak();
    }
 
    private static void permLeak() throws ClassNotFoundException, NoSuchMethodException, SecurityException, IllegalAccessException, IllegalArgumentException, InvocationTargetException, MalformedURLException, InterruptedException{
        for (int i = 0; i < 1000 ; i++) {
            URL[] urls = getURLS();
            URLClassLoader loader = new URLClassLoader(urls,null);
            Class<?> logfClass = Class.forName("org.apache.commons.logging.LogFactory",true,loader);
            Method getLog = logfClass.getMethod("getLog", String.class);
            Object result = getLog.invoke(logfClass, "TestPermGen");
            insList.add(result);
            System.out.println(i + " : " + result);
            Thread.sleep(100);
        }
    }
 
    private static URL[] getURLS() throws MalformedURLException{
        File libDir = new File("D:\\HHSZXA\\.m2\\repository\\commons-logging\\commons-logging\\1.1.1");
        File[] subFiles = libDir.listFiles();
        int count = subFiles.length;
        URL[] urls = new URL[count];
        for (int i = 0; i < count ; i++) {
            urls[i] = subFiles[i].toURI().toURL();
        }
        return urls;
    }
 
 
}

输出：

0 : org.apache.commons.logging.impl.Jdk14Logger@2babbba1
1 : org.apache.commons.logging.impl.Jdk14Logger@5d08e435
2 : org.apache.commons.logging.impl.Jdk14Logger@30d46b95
3 : org.apache.commons.logging.impl.Jdk14Logger@4ec4f498
4 : org.apache.commons.logging.impl.Jdk14Logger@7dbc345a
5 : org.apache.commons.logging.impl.Jdk14Logger@7ee361ad
6 : org.apache.commons.logging.impl.Jdk14Logger@7d94a8eb
java.lang.OutOfMemoryError: PermGen space
Dumping heap to d:\dump\java_pid3276.hprof ...
Heap dump file created [2648239 bytes in 0.016 secs]
Exception in thread "Reference Handler" Exception in thread "main" java.lang.reflect.InvocationTargetException
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:606)
    at PermGen.permLeak(PermGen.java:25)
    at PermGen.main(PermGen.java:16)
Caused by: java.lang.OutOfMemoryError: PermGen space
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:792)
    at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
    at java.net.URLClassLoader.defineClass(URLClassLoader.java:449)
    at java.net.URLClassLoader.access$100(URLClassLoader.java:71)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:361)
    at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
    at java.security.AccessController.doPrivileged(Native Method)
    at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
    at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
    at org.apache.commons.logging.LogFactory.createFactory(LogFactory.java:1131)
    at org.apache.commons.logging.LogFactory$2.run(LogFactory.java:1065)
    at java.security.AccessController.doPrivileged(Native Method)
    at org.apache.commons.logging.LogFactory.newFactory(LogFactory.java:1062)
    at org.apache.commons.logging.LogFactory.getFactory(LogFactory.java:650)
    at org.apache.commons.logging.LogFactory.getLog(LogFactory.java:704)
    ... 6 more
java.lang.OutOfMemoryError: PermGen space
    at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:140)

通过控制台信息，可以初步判定是方法区OOM，原因是通过URLClassLoader类加载器加载某个动态类，这个动态类是由调用org.apache.commons.logging.LogFactory.getLog方法生成的，同时可以看到引起的错误的代码行数PermGen.java:25

首先看摘要