从几个sample来学习JAVA堆、方法区、JAVA栈和本地方法栈

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：

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：

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：

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：

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：

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。