Detect when idle ThreadPoolExecutor thread is killed

Question

I am using ThreadPoolExecutor to manage the number of threads, I can catch the event of creating a new Thread for ThreadPool via ThreadFactory->newThread() But I do now know how to catch the event of killing Thread which stays idle for 2minutes as following configuration.

I hae searched a listener method but could not find.

public abstract class ThreadPoolEventProcessor<E>  implements ThreadFactory{

   private BlockingQueue<Runnable> taskQueue;
   private ThreadPoolExecutor executor;

   protected ThreadPoolEventProcessor(int coreThreadSize, int maxQueueSize) {
      taskQueue = new LinkedBlockingQueue<Runnable>(maxQueueSize);
      executor = new ThreadPoolExecutor(coreThreadSize, coreThreadSize * 5, 2L, TimeUnit.MINUTES, taskQueue,this); 
      executor.prestartAllCoreThreads();      
   }

  public Thread newThread(Runnable r) {
      return new Thread(r, getWorkerName());
    }

Answer 1

The ThreadPoolExecutor does not kill threads. It will retrieve new threads from the ThreadFactory and have them run a Worker. All this worker does is loop, attempting to retrieve a Runnable from an underlying BlockingQueue.

If it gets one, it invokes run on it.

If the allowCoreThreadTimeOut is true and you have more workers than the core amount, then the keepAliveTime value is used to poll that underlying BlockingQueue. If the poll returns null, then the worker is (potentially) removed. Some extra cleanup happens and the various method invocations are popped from the stack as the methods return, until eventually the Worker#run() method terminates and the containing thread finishes.

Nowhere in that flow does ThreadPoolExecutor offer any hooks for notifications.

You can poll ThreadPoolExecutor#getPoolSize() and ThreadPoolExecutor#getLargestPoolSize() for information periodically.