[GitHub] chickenlj commented on issue #1634: 为什么要在AbstractMonitorFactory#getMonitor这个地方要用上锁

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chickenlj commented on issue #1634: 
为什么要在AbstractMonitorFactory#getMonitor这个地方要用上锁
URL: 
https://github.com/apache/incubator-dubbo/issues/1634#issuecomment-383270026
 
 
   I think you should look at this `LOCK` as a whole, we have null check before 
entering this lock:
   ```java
   Monitor monitor = MONITORS.get(key);
           Future<Monitor> future = FUTURES.get(key);
           if (monitor != null || future != null) {
               return monitor;
           }
   
           LOCK.lock();
           try {
               monitor = MONITORS.get(key);
               future = FUTURES.get(key);
               if (monitor != null || future != null) {
                   return monitor;
               }
   
               final URL monitorUrl = url;
               final ListenableFutureTask<Monitor> listenableFutureTask = 
ListenableFutureTask.create(new MonitorCreator(monitorUrl));
               listenableFutureTask.addListener(new MonitorListener(key));
               executor.execute(listenableFutureTask);
               FUTURES.put(key, listenableFutureTask);
   
               return null;
           } finally {
               // unlock
               LOCK.unlock();
           }
   ```

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