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 static void main(String[] args) throws Exception{
    MemoryWarningSystem.setPercentageUsageThreshold(0.1);

    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>();
    long counter = 0;
    while (true) {
      numbers.add(Math.random());
      counter++;
      if( counter % 10000 == 0){
        Thread.sleep(100);
      }
    }
  }

  public MemoryWarningSystem() {
    MemoryMXBean mbean = ManagementFactory.getCMemoryMXBean();
    NotificationEmitter emitter = (NotificationEmitter) mbean;
    emitter.addNotificationListener(new NotificationListener() {
      public void handleNotification(Notification n, Object hb) {
        System.out.println("Notfication received:" + n.getType());
        if (n.getType().equals(
            MemoryNotificationInfo.MEMORY_THRESHOLD_EXCEEDED)) {
          System.out.println("Notfication MEMORY_THRESHOLD_EXCEEDED received:");
          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");
    }
    System.out.println("Setting threashold for pool " + tenuredGenPool.getName() + " percentage:" + percentage );
    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()) {
        System.out.println("Pool:" + pool.getName());
      }
    }

    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");
  }
}