深入理解java虚拟机:java内存溢出实战
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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。
1、堆溢出
堆溢出原因莫过于对象太多导致,看代码。
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package baby.oom;
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import java.util.ArrayList;
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import java.util.List;
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/**
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* java 堆溢出
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* VM Args:-Xms20m -Xmx20m -XX:+HeapDumpOnOutOfMemoryError
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* @author
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*/
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public class HeapOOM {
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static class OOMObject {
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}
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public static void main(String[] args) {
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List<OOMObject> list = new ArrayList<OOMObject>();
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while (true) {
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list.add(new OOMObject());
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/*System.out.println("total(k):"+Runtime.getRuntime().totalMemory()/1024+
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" freeMemory(k):"+Runtime.getRuntime().freeMemory()/1024+
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" maxMemory(k):"+Runtime.getRuntime().maxMemory()/1024+
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" availableProcessors:"+Runtime.getRuntime().availableProcessors());*/
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}
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}
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}
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/**
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* java.lang.OutOfMemoryError: Java heap space
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Dumping heap to java_pid1820.hprof ...
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Heap dump file created [24787111 bytes in 0.346 secs]
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Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
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at java.util.Arrays.copyOf(Arrays.java:2760)
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at java.util.Arrays.copyOf(Arrays.java:2734)
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at java.util.ArrayList.ensureCapacity(ArrayList.java:167)
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at java.util.ArrayList.add(ArrayList.java:351)
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at baby.oom.HeapOOM.main(HeapOOM.java:19)
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*
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*
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*/
2、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。
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package baby.oom;
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/**
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* 栈异常
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* 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
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* 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
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* VM Args:-Xss128k
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* @author
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*/
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public class JavaVMStackSOF {
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private int stackLength = 1;
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public void stackLeak() {
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stackLength++;
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stackLeak();
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}
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public static void main(String[] args) throws Throwable {
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JavaVMStackSOF oom = new JavaVMStackSOF();
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try {
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oom.stackLeak();
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} catch (Throwable e) {
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System.out.println("stack length:" + oom.stackLength);
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throw e;
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}
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}
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}
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/**
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*
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* stack length:2403
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Exception in thread "main" java.lang.StackOverflowError
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at baby.oom.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:11)
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at baby.oom.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:12)
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at baby.oom.JavaVMStackSOF.stackLeak(JavaVMStackSOF.java:12)
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默认情况下,即不加Xss限制,输出的length为8956,加了Xss128k length位2403
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*/
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package baby.oom;
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/**
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* VM Args:-Xss2M (这时候不妨设大些)
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* @author
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*/
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public class JavaVMStackOOM {
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int i=0;
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private void dontStop() {
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while (true) {
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}
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}
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public void stackLeakByThread() {
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while (true) {
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Thread thread = new Thread(new Runnable() {
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public void run() {
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dontStop();
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}
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});
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i++;
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System.out.println("i="+i);
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thread.start();
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}
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}
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public static void main(String[] args) throws Throwable {
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JavaVMStackOOM oom = new JavaVMStackOOM();
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try {
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oom.stackLeakByThread();
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} catch (Throwable e) {
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System.out.println("thread num:" + oom.i);
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throw e;
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}
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}
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}
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//i=391
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//thread num:391
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//Exception in thread "main" java.lang.OutOfMemoryError: unable to create new native thread
3、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。
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package baby.oom;
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import java.util.ArrayList;
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import java.util.List;
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/**
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* VM Args:-XX:PermSize=10M -XX:MaxPermSize=10M
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* @author
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*/
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public class RuntimeConstantPoolOOM {
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public static void main(String[] args) {
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// 使用List保持着常量池引用,避免Full GC回收常量池行为
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List<String> list = new ArrayList<String>();
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// 10MB的PermSize在integer范围内足够产生OOM了
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int i = 0;
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while (true) {
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list.add(String.valueOf(i++).intern());
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}
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}
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}
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/**
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Exception in thread "main" java.lang.OutOfMemoryError: PermGen space
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at java.lang.String.intern(Native Method)
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at baby.oom.RuntimeConstantPoolOOM.main(RuntimeConstantPoolOOM.java:18)
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*/
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package baby.oom;
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import java.lang.reflect.Method;
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import net.sf.cglib.proxy.Enhancer;
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import net.sf.cglib.proxy.MethodInterceptor;
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import net.sf.cglib.proxy.MethodProxy;
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/**
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* VM Args: -XX:PermSize=10M -XX:MaxPermSize=10M
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* @author
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*/
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public class JavaMethodAreaOOM {
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public static void main(String[] args) {
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while (true) {
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Enhancer enhancer = new Enhancer(www.mcyllpt.com/);
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enhancer.setSuperclass(OOMObject.class);
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enhancer.setUseCache(false);
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enhancer.setCallback(new MethodInterceptor() {
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public Object intercept(Object obj, Method method, Object[] arg, MethodProxy proxy) throws Throwable {
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// TODO Auto-generated method stub
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return proxy.invokeSuper(obj, arg);
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}
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});
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enhancer.create();
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}
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}
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static class www.120xh.cn OOMObject {
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}
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}
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/*
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* Exception in thread "main" net.sf.cglib.core.CodeGenerationException: java.lang.reflect.InvocationTargetException-->null
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at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:237)
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at net.sf.cglib.proxy.Enhancer.createHelper(Enhancer.java:377)
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at net.sf.cglib.proxy.Enhancer.create(Enhancer.java:285)
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at baby.oom.JavaMethodAreaOOM.main(JavaMethodAreaOOM.java:28)
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Caused by: java.lang.reflect.InvocationTargetException
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at sun.reflect.GeneratedMethodAccessor1.invoke(Unknown Source)
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at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
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at java.lang.reflect.Method.invoke(Method.java:www.huayiyul.com 597)
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at net.sf.cglib.core.ReflectUtils.defineClass(ReflectUtils.java:384)
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at net.sf.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:219)
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... 3 more
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Caused by: java.lang.OutOfMemoryError: PermGen space
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at java.lang.ClassLoader.defineClass1(Native Method)
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at java.lang.ClassLoader.defineClassCond(ClassLoader.java:631)
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at java.lang.ClassLoader.defineClass(ClassLoader.java:615)
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... 8 more
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*/
4、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()。
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package baby.oom;
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import java.lang.reflect.Field;
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import sun.misc.Unsafe;
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/**
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* VM Args:-Xmx20M -XX:MaxDirectMemorySize=10M
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* @author
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* Eclipse 默认把这些受访问限制的API设成了ERROR。
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解决办法:将Windows->Preferences->www.yigouyule2.cn 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。
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*/
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public class DirectMemoryOOM {
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private static final int _1MB = 1024 * 1024;
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public static void main(String[]www.078881.cn args) throws Exception {
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Field unsafeField = Unsafe.class.getDeclaredFields()[0];
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unsafeField.setAccessible(true);
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Unsafe unsafe = (Unsafe) unsafeField.get(null);
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while (true) www.bomaoyule.cn/{
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unsafe.allocateMemory(_1MB);
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}
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}
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}
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/**
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Exception in thread "main" java.lang.OutOfMemoryError
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at sun.misc.Unsafe.allocateMemory(Native Method)
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at baby.oom.DirectMemoryOOM.main(DirectMemoryOOM.java:20)
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