按照java內存的結構,發生內存溢出的地方常在於堆、棧、方法區、直接內存。
1、堆溢出
堆溢出原因莫過於對象太多導致,看代碼。
- package baby.oom;
- import java.util.ArrayList;
- import java.util.List;
- /**
- * 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)
- *
- *
- */
2、棧溢出
根據JAVA虛擬機規範描述:
如果線程請求的棧深度大於虛擬機所允許的最大深度,將拋出StackOverflowError
如果虛擬機在擴展棧時無法申請到足夠的內存空間,將拋出OutOfMemoryError。
實驗表明:
在單線程下,無論是由於棧幀太大還是虛擬機棧容量太小,當內存無法分配的時候,虛擬機拋出的都是StackOverflowError。
通過不斷的建立新線程的方式可以產生內存溢出溢出。爲每個線程的棧分配的內存越大,反而越容易產生內存溢出異常。
如果是建立過多線程導致的內存溢出,在不能減少線程數量或者更換64位虛擬機的情況下,就只能通過減少最大堆和減少棧容量來換取更多的線程。
假設32位windows系統虛擬機最大設爲2G,虛擬機提供了參數來控制java堆和方法區這兩部分最大值,剩餘的內存爲2G - Xmx- MaxPermSize,如果虛擬機本身進程內存大小不算在內,省下的內存就有虛擬機和本地方法棧瓜分了。每個線程分配到的棧容量越大,可以建立的線程數量自然就越少。
- package baby.oom;
- /**
- * 棧異常
- * 如果線程請求的棧深度大於虛擬機所允許的最大深度,將拋出StackOverflowError
- * 如果虛擬機在擴展棧時無法申請到足夠的內存空間,將拋出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)
- 默認情況下,即不加Xss限制,輸出的length爲8956,加了Xss128k length位2403
- */
- package baby.oom;
- /**
- * VM Args:-Xss2M (這時候不妨設大些)
- * @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
3、方法區溢出
當運行時常量池過大或者類過多時就會導致方法區溢出。
- package baby.oom;
- 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回收常量池行爲
- List<String> list = new ArrayList<String>();
- // 10MB的PermSize在integer範圍內足夠產生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)
- */
- package baby.oom;
- import java.lang.reflect.Method;
- import net.sf.cglib.proxy.Enhancer;
- import net.sf.cglib.proxy.MethodInterceptor;
- import net.sf.cglib.proxy.MethodProxy;
- /**
- * 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
- */
4、直接內存溢出
雖然使用DerictByteBuffer分配內存也會拋出內存溢出異常,但它拋出異常時並沒有真正向操作系統申請分配,而是通過計算得知內存無法分配,於是手動拋出異常,真正申請分配內存的方法是unsafe.allocateMemory()。
- package baby.oom;
- import java.lang.reflect.Field;
- import sun.misc.Unsafe;
- /**
- * VM Args:-Xmx20M -XX:MaxDirectMemorySize=10M
- * @author
- * Eclipse 默認把這些受訪問限制的API設成了ERROR。
- 解決辦法:將Windows->Preferences->Java-Complicer->Errors/Warnings->Deprecated and restricted API,中的Forbidden references(access rules)設置爲Warning,即可以編譯通過。
- */
- 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)
- */