MemoryMXBean和MemoryPoolMXBean--OutOfMemoryError Warning System

MemoryMXBean可以通过ManagementFactory.getMemoryMXBean()获取，也可以通过平台MBeanServer的ObjectName为java.lang:type=Memory获取

MemoryPoolMXBean可以通过ManagementFactory.getMemoryPoolMXBean()获取，也可以通过平台MBeanServer的ObjectName为java.lang:type=MemoryPool,name={pool's name}获取

MemoryMXBean的实现MemoryImpl是一个NotificationEmitter类的子类，可以通过addNotificationListener来添加监控；可以监控到MemoryNotificationInfo中定义的两种通知（MEMORY_THRESHOLD_EXCEDED和MEMORY_COLLECTION_THRESHOLD_EXCEEDED，二者区别是是否对不使用的对象经过回收）。

MemoryPoolMXBean可以检测到堆和非堆内存的所有信息，可以清楚的看到Code Cache、Eden Space、Survivor Space、Tenured Gen、Perm Gen等虚拟机内存的信息，通过getUsage()和getPeakUsage()获取信息，通过轮询和通知的方式对阈值进行监控。

注意，我在eclipse中对MemoryMXBean.addNotificationListener中添加的NotificationListener的handleNotification方法设置断点，并未起效，原因可能是Low Memory Detector不支持Java Debug协议，eclipse无法在上面设置断点造成的。

OOME预防可以通过上述工具完成。

首先设置VM arguments Xmx和Xms

代码如下

import javax.management.*;import java.lang.management.*;import java.util.*;/** * This memory warning system will call the listener when we * exceed the percentage of available memory specified.  There * should only be one instance of this object created, since the * usage threshold can only be set to one number. */public class MemoryWarningSystem {  private final Collection<Listener> listeners =      new ArrayList<Listener>();  public interface Listener {    public void memoryUsageLow(long usedMemory, long maxMemory);  }  public MemoryWarningSystem() {    MemoryMXBean mbean = ManagementFactory.getMemoryMXBean();    NotificationEmitter emitter = (NotificationEmitter) mbean;    emitter.addNotificationListener(new NotificationListener() {      public void handleNotification(Notification n, Object hb) {        if (n.getType().equals(            MemoryNotificationInfo.MEMORY_THRESHOLD_EXCEEDED)) {          long maxMemory = tenuredGenPool.getUsage().getMax();          long usedMemory = tenuredGenPool.getUsage().getUsed();          for (Listener listener : listeners) {            listener.memoryUsageLow(usedMemory, maxMemory);          }        }      }    }, null, null);  }  public boolean addListener(Listener listener) {    return listeners.add(listener);  }  public boolean removeListener(Listener listener) {    return listeners.remove(listener);  }  private static final MemoryPoolMXBean tenuredGenPool =      findTenuredGenPool();  public static void setPercentageUsageThreshold(double percentage) {    if (percentage <= 0.0 || percentage > 1.0) {      throw new IllegalArgumentException("Percentage not in range");    }    long maxMemory = tenuredGenPool.getUsage().getMax();    long warningThreshold = (long) (maxMemory * percentage);    tenuredGenPool.setUsageThreshold(warningThreshold);  }  /**   * Tenured Space Pool can be determined by it being of type   * HEAP and by it being possible to set the usage threshold.   */  private static MemoryPoolMXBean findTenuredGenPool() {    for (MemoryPoolMXBean pool :        ManagementFactory.getMemoryPoolMXBeans()) {      // I don't know whether this approach is better, or whether      // we should rather check for the pool name "Tenured Gen"?      if (pool.getType() == MemoryType.HEAP &&          pool.isUsageThresholdSupported()) {        return pool;      }    }    throw new AssertionError("Could not find tenured space");  }}

使用MemTest测试

import java.util.*;public class MemTest {  public static void main(String[] args) {    MemoryWarningSystem.setPercentageUsageThreshold(0.6);    MemoryWarningSystem mws = new MemoryWarningSystem();    mws.addListener(new MemoryWarningSystem.Listener() {      public void memoryUsageLow(long usedMemory, long maxMemory) {        System.out.println("Memory usage low!!!");        double percentageUsed = ((double) usedMemory) / maxMemory;        System.out.println("percentageUsed = " + percentageUsed);        MemoryWarningSystem.setPercentageUsageThreshold(0.8);      }    });    Collection<Double> numbers = new LinkedList<Double>();    while (true) {      numbers.add(Math.random());    }  }}

产生结果如下

Memory usage low!!!percentageUsed = 0.6281726667795322Memory usage low!!!percentageUsed = 0.8504659318016649Exception in thread "main" java.lang.OutOfMemoryError: Java heap space

来源：http://www.javaspecialists.co.za/archive/Issue092.html

参考：http://java-boy.iteye.com/blog/464953