按照java内存的结构,发生内存溢出的地方常在于堆、栈、方法区、直接内存

一、堆溢出

堆溢出原因莫过于对象太多导致,看代码:

/**
 * java 堆溢出
 * VM Args:-Xms20m -Xmx20m -XX:+HeapDumpOnOutOfMemoryError
 * @author 
 */
public class HeapOOM {

    static class OOMObject {
    }

    public static void main(String[] args) {
        List<OOMObject> list = new ArrayList<OOMObject>();
        
        while (true) {
            list.add(new OOMObject());
            /*System.out.println("total(k):"+Runtime.getRuntime().totalMemory()/1024+
                    "  freeMemory(k):"+Runtime.getRuntime().freeMemory()/1024+
                    "  maxMemory(k):"+Runtime.getRuntime().maxMemory()/1024+
                    "  availableProcessors:"+Runtime.getRuntime().availableProcessors());*/
        }
    }
}

/**
 * java.lang.OutOfMemoryError: Java heap space
Dumping heap to java_pid1820.hprof ...
Heap dump file created [24787111 bytes in 0.346 secs]
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Arrays.java:2760)
    at java.util.Arrays.copyOf(Arrays.java:2734)
    at java.util.ArrayList.ensureCapacity(ArrayList.java:167)
    at java.util.ArrayList.add(ArrayList.java:351)
    at baby.oom.HeapOOM.main(HeapOOM.java:19)
 * 
 *
*/

二、栈溢出

根据JAVA虚拟机规范描述:如果线程请求的栈深度大于虚拟机所允许的最大深度,将抛出StackOverflowError

                                             如果虚拟机在扩展栈时无法申请到足够的内存空间,将抛出OutOfMemoryError。

实验表明:在单线程下,无论是由于栈帧太大还是虚拟机栈容量太小,当内存无法分配的时候,虚拟机抛出的都是StackOverflowError。

 通过不断的建立新线程的方式可以产生内存溢出溢出。为每个线程的栈分配的内存越大,反而越容易产生内存溢出异常。如果是建立过多线程导致的内存溢出,在不能减少线程数量或者更换64位虚拟机的情况下,就只能通过减少最大堆和减少栈容量来换取更多的线程。

假设32位windows系统虚拟机最大设为2G,虚拟机提供了参数来控制java堆和方法区这两部分最大值,剩余的内存为2G - Xmx- MaxPermSize,如果虚拟机本身进程内存大小不算在内,省下的内存就有虚拟机和本地方法栈瓜分了。每个线程分配到的栈容量越大,可以建立的线程数量自然就越少。

/**
 * 栈异常
 * 如果线程请求的栈深度大于虚拟机所允许的最大深度,将抛出StackOverflowError
 * 如果虚拟机在扩展栈时无法申请到足够的内存空间,将抛出OutOfMemoryError
 * VM Args:-Xss128k
 * @author 
 */
public class JavaVMStackSOF {

    private int stackLength = 1;

    public void stackLeak() {
        stackLength++;
        stackLeak();
    }

    public static void main(String[] args) throws Throwable {
        JavaVMStackSOF oom = new JavaVMStackSOF();
        try {
            oom.stackLeak();
        } catch (Throwable e) {
            System.out.println("stack length:" + oom.stackLength);
            throw e;
        }
    }
}

/**
 * 
 * stack length:2403
Exception in thread "main" java.lang.StackOverflowError
    at baby.oom.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:11)
    at baby.oom.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:12)
    at baby.oom.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:12)
    
    默认情况下,即不加Xss限制,输出的length为8956,加了Xss128k length位2403
 */
/**
 * VM Args:-Xss2M (这时候不妨设大些)
 * @author 
 */
public class JavaVMStackOOM {
 
    int i=0;
       private void dontStop() {
              while (true) {
              }
       }
 
       public void stackLeakByThread() {
           
              while (true) {
                     Thread thread = new Thread(new Runnable() {
                            @Override
                            public void run() {
                                   dontStop();
                            }
                     });
                     i++;
                     System.out.println("i="+i);
                     thread.start();
              }
       }
 
       public static void main(String[] args) throws Throwable {
              JavaVMStackOOM oom = new JavaVMStackOOM();
             
              try {
                  oom.stackLeakByThread();
              } catch (Throwable e) {
                  System.out.println("thread num:" + oom.i);
                  throw e;
              }
       }
}
//i=391
//thread num:391
//Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread

三、方法区溢出

   当运行时常量池过大或者类过多时就会导致方法区溢出。

import java.util.ArrayList;
import java.util.List;

/**
 * VM Args:-XX:PermSize=10M -XX:MaxPermSize=10M
 * @author 
 */
public class RuntimeConstantPoolOOM {

    public static void main(String[] args) {
        // 使用List保持着常量池引用,避免Full GC回收常量池行为
        List<String> list = new ArrayList<String>();
        // 10MB的PermSize在integer范围内足够产生OOM了
        int i = 0; 
        while (true) {
            list.add(String.valueOf(i++).intern());
        }
    }
}

/**
Exception in thread "main" java.lang.OutOfMemoryError: PermGen space
    at java.lang.String.intern(Native Method)
    at baby.oom.RuntimeConstantPoolOOM.main(RuntimeConstantPoolOOM.java:18)

*/





/**
 * VM Args: -XX:PermSize=10M -XX:MaxPermSize=10M
 * @author 
 */
public class JavaMethodAreaOOM {

    public static void main(String[] args) {
        while (true) {
            Enhancer enhancer = new Enhancer();
            enhancer.setSuperclass(OOMObject.class);
            enhancer.setUseCache(false);
            enhancer.setCallback(new MethodInterceptor() {
            
                @Override
                public Object intercept(Object obj, Method method, Object[] arg, MethodProxy proxy) throws Throwable {
                    // TODO Auto-generated method stub
                    return proxy.invokeSuper(obj, arg);
                }
            });
            enhancer.create();
        }
    }

    static class OOMObject {

    }
}

/*
 * Exception in thread "main" net.sf.cglib.core.CodeGenerationException: java.lang.reflect.InvocationTargetException-->null
    at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:237)
    at net.sf.cglib.proxy.Enhancer.createHelper(Enhancer.java:377)
    at net.sf.cglib.proxy.Enhancer.create(Enhancer.java:285)
    at baby.oom.JavaMethodAreaOOM.main(JavaMethodAreaOOM.java:28)
Caused by: java.lang.reflect.InvocationTargetException
    at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)
    at net.sf.cglib.core.ReflectUtils.defineClass(ReflectUtils.java:384)
    at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:219)
    ... 3 more
Caused by: java.lang.OutOfMemoryError: PermGen space
    at java.lang.ClassLoader.defineClass1(Native Method)
    at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
    at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
    ... 8 more
 */

四、直接内存溢出
    虽然使用DerictByteBuffer分配内存也会抛出内存溢出异常,但它抛出异常时并没有真正向操作系统申请分配,而是通过计算得知内存无法分配,于是手动抛出异常,真正申请分配内存的方法是unsafe.allocateMemory()。

import java.lang.reflect.Field;   
import sun.misc.Unsafe; 
/**
 * VM Args:-Xmx20M -XX:MaxDirectMemorySize=10M
 * @author 
 * Eclipse 默认把这些受访问限制的API设成了ERROR。

解决办法:将Windows->Preferences->Java-Complicer->Errors/Warnings->Deprecated and restricted API,中的Forbidden references(access rules)设置为Warning,即可以编译通过。

 */
public class DirectMemoryOOM {

    private static final int _1MB = 1024 * 1024;

    public static void main(String[] args) throws Exception {
        
        Field unsafeField = Unsafe.class.getDeclaredFields()[0];
        unsafeField.setAccessible(true);
        Unsafe unsafe = (Unsafe) unsafeField.get(null);
        while (true) {
            unsafe.allocateMemory(_1MB);
        }
    }
}

/**
Exception in thread "main" java.lang.OutOfMemoryError
    at sun.misc.Unsafe.allocateMemory(Native Method)
    at baby.oom.DirectMemoryOOM.main(DirectMemoryOOM.java:20)
*/