WeakHashMap的并发应用

第一步：生成一个全局WeakHashMap变量

private static WeakHashMap lockMap = new WeakHashMap(50);

第二步：从MAP中取对象。如果有就取出，如果没有就新建放到MAP中

 /**
  * 得到同步锁，用完丢弃即可
  * 
  * @param key
  * @return
  */
 protected static Object getSynLockObject(String key) {
  synchronized (lockMap) {
  Object o = lockMap.get(key);
  if (o == null) {
  o = new Object();
  lockMap.put(key, o);
  }
  return o;
  }
 }

第三步：锁住不同的对象，这样可以对同一对象进行同步。不同对象可以并发

 public boolean processPaymentResult(YeepayResult result) {
  long orderSeqId = result.getOrderSeqId();
  // 只要这个Key不丢，同步锁就能起作用，所以要显式的引用这个key值
  String synLockKey = String.valueOf(orderSeqId);
  Object synLock = PayCenter.getSynLockObject(synLockKey);
  /**
  * 对同一个订单的操作需要进行同步，防止出错
  */
  synchronized (synLock) {
  return synchroProcessPaymentResult(result);
  }
 }

例：并发情况下，对数据进行加1操作，插入数据库

	private Map lockMap = new WeakHashMap(); //confirm every thread get lock object is the same object
	private String lockPrefix = this.getClass().getName()+"incrementSequenceNum";
	private int incrementSequenceNum(String sID, String tID, TopicIndexItem cacheTopic){
		String syn = lockPrefix + sID+tID;
		String value = lockMap.get(syn);
		if(value == null){
			synchronized(lockPrefix){
				if(lockMap.get(syn)==null){
					lockMap.put(syn, syn);
				}
			}
			value = lockMap.get(syn);
		}
		//syn = syn.intern();
		int sequenceNum = 0;
		synchronized(value){
			sequenceNum = DB.getTopicSequenceNum(sID, tID)+1;
			DB.incrementTopicSequenceNum(sID, tID);
			if(cacheTopic!=null){
				cacheTopic.setMaxSequenceNum(sequenceNum);
			}
		}
		return sequenceNum;
	}