最近在看《深入理解Java虚拟机》，书中给了几个例子，比较好的说明了几种OOM（OutOfMemory）产生的过程，大部分的程序员在写程序时不会太关注Java运行时数据区域的结构：
感觉有必要通过几个实在的例子来加深对这几个区域的了解
1）Java堆
所有对象的实例分配都在Java堆上分配内存，堆大小由-Xmx和-Xms来调节，sample如下所示：
[java] view plaincopyprint?
- public class HeapOOM {
- static class OOMObject{}
- /**
- * @param args
- */
- public static void main(String[] args) {
- List<OOMObject> list = new ArrayList<OOMObject>();
- while(true){
- list.add(new OOMObject());
- }
- }
- }
public class HeapOOM {
static class OOMObject{}
/**
* @param args
*/
public static void main(String[] args) {
List<OOMObject> list = new ArrayList<OOMObject>();
while(true){
list.add(new OOMObject());
}
}
}
加上JVM参数-verbose:gc -Xms10M -Xmx10M -XX:+PrintGCDetails -XX:SurvivorRatio=8 -XX:+HeapDumpOnOutOfMemoryError，就能很快报出OOM：
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
并且能自动生成Dump。
2）方法区
方法区是存放虚拟机加载类的相关信息，如类、静态变量和常量，大小由-XX:PermSize和-XX:MaxPermSize来调节，类太多有可能撑爆永久带：
[java] view plaincopyprint?
- public class MethodAreaOOM {
- static class OOMOjbect{}
- /**
- * @param args
- */
- public static void main(String[] args) {
- // TODO Auto-generated method stub
- while(true){
- Enhancer eh = new Enhancer();
- eh.setSuperclass(OOMOjbect.class);
- eh.setUseCache(false);
- eh.setCallback(new MethodInterceptor(){
- @Override
- public Object intercept(Object arg0, Method arg1,
- Object[] arg2, MethodProxy arg3) throws Throwable {
- // TODO Auto-generated method stub
- return arg3.invokeSuper(arg0, arg2);
- }
- });
- eh.create();
- }
- }
- }
public class MethodAreaOOM {
static class OOMOjbect{}
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
while(true){
Enhancer eh = new Enhancer();
eh.setSuperclass(OOMOjbect.class);
eh.setUseCache(false);
eh.setCallback(new MethodInterceptor(){
@Override
public Object intercept(Object arg0, Method arg1,
Object[] arg2, MethodProxy arg3) throws Throwable {
// TODO Auto-generated method stub
return arg3.invokeSuper(arg0, arg2);
}
});
eh.create();
}
}
}
加上永久带的JVM参数：-XX:PermSize=10M -XX:MaxPermSize=10M，运行后会报如下异常：
Exception in thread "main" java.lang.OutOfMemoryError: PermGen space
静态变量或常量也会有可能撑爆方法区：
[java] view plaincopyprint?
- public class ConstantOOM {
- /**
- * @param args
- */
- public static void main(String[] args) {
- // TODO Auto-generated method stub
- List<String> list = new ArrayList<String>();
- int i=0;
- while(true){
- list.add(String.valueOf(i++).intern());
- }
- }
- }
public class ConstantOOM {
/**
* @param args
*/
public static void main(String[] args) {
// TODO Auto-generated method stub
List<String> list = new ArrayList<String>();
int i=0;
while(true){
list.add(String.valueOf(i++).intern());
}
}
}
同样加上JVM参数：-XX:PermSize=10M -XX:MaxPermSize=10M，运行后报如下异常：
Exception in thread "main" java.lang.OutOfMemoryError: PermGen space
3）Java栈和本地方法栈
栈是存放线程调用方法时存储局部变量表，操作，方法出口等与方法执行相关的信息，栈大小由Xss来调节，方法调用层次太多会撑爆这个区域，samples如下所示：
[java] view plaincopyprint?
- package com.cutesource;
- public class StackOOM {
- /**
- * @param args
- */
- private int stackLength = 1;
- public void stackLeak(){
- stackLength++;
- stackLeak();
- }
- public static void main(String[] args) throws Throwable{
- // TODO Auto-generated method stub
- StackOOM oom = new StackOOM();
- try{
- oom.stackLeak();
- }catch(Throwable err){
- System.out.println("Stack length:" + oom.stackLength);
- throw err;
- }
- }
- }
package com.cutesource;
public class StackOOM {
/**
* @param args
*/
private int stackLength = 1;
public void stackLeak(){
stackLength++;
stackLeak();
}
public static void main(String[] args) throws Throwable{
// TODO Auto-generated method stub
StackOOM oom = new StackOOM();
try{
oom.stackLeak();
}catch(Throwable err){
System.out.println("Stack length:" + oom.stackLength);
throw err;
}
}
}
设置JVM参数：-Xss128k，报出异常：
Exception in thread "main" java.lang.StackOverflowError
打印出Stack length:1007，这里可以看出，在我的机器上128k的栈容量能承载深度为1007的方法调用。当然报这样的错很少见，一般只会出现无限循环的递归中，另外，线程太多也会占满栈区域：
[java] view plaincopyprint?
- package com.cutesource;
- public class StackOOM {
- /**
- * @param args
- */
- private int stackLength = 1;
- private void dontStop(){
- while(true){
- try{Thread.sleep(1000);}catch(Exception err){}
- }
- }
- public void stackLeakByThread(){
- while(true){
- Thread t = new Thread(new Runnable(){
- @Override
- public void run() {
- // TODO Auto-generated method stub
- dontStop();
- }
- });
- t.start();
- stackLength++;
- }
- }
- public static void main(String[] args) throws Throwable{
- // TODO Auto-generated method stub
- StackOOM oom = new StackOOM();
- try{
- oom.stackLeakByThread();
- }catch(Throwable err){
- System.out.println("Stack length:" + oom.stackLength);
- throw err;
- }
- }
- }
package com.cutesource;
public class StackOOM {
/**
* @param args
*/
private int stackLength = 1;
private void dontStop(){
while(true){
try{Thread.sleep(1000);}catch(Exception err){}
}
}
public void stackLeakByThread(){
while(true){
Thread t = new Thread(new Runnable(){
@Override
public void run() {
// TODO Auto-generated method stub
dontStop();
}
});
t.start();
stackLength++;
}
}
public static void main(String[] args) throws Throwable{
// TODO Auto-generated method stub
StackOOM oom = new StackOOM();
try{
oom.stackLeakByThread();
}catch(Throwable err){
System.out.println("Stack length:" + oom.stackLength);
throw err;
}
}
}
报出异常：Exception in thread "main" java.lang.OutOfMemoryError:unable to create new native thread
不过在windows上运行这个例子要小心，会出现系统假死的情况，有可能需要重启机器才行。
以上几个例子虽然比较简单，但能很好帮助普通的程序员更加直观的了解Java堆，方法区，Java栈和本地方法栈。