1. private Runnable getTask() {
  2.     boolean timedOut = false; // Did the last poll() time out?
  4.     for (;;) {
  5.         int c = ctl.get();
  6.         int rs = runStateOf(c);
  8.         // Check if queue empty only if necessary.
  9.         if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
  10.             decrementWorkerCount();
  11.             return null;
  12.         }
  14.         int wc = workerCountOf(c);
  16.         // 削减条件
  17.         boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
  19.         // 这里是线程个数是否删除的关键
  20.         if ((wc > maximumPoolSize || (timed && timedOut))
  21.             && (wc > 1 || workQueue.isEmpty())) {
  22.             if (compareAndDecrementWorkerCount(c))
  23.                 return null;
  24.             continue;
  25.         }
  27.         try {
  28.             // 这里才是最核心的所在
  29.             Runnable r = timed ?
  30.                 workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
  31.             workQueue.take();
  32.             if (r != null)
  33.                 return r;
  34.             timedOut = true;
  35.         } catch (InterruptedException retry) {
  36.             timedOut = false;
  37.         }
  38.     }
  39. }

结论：

1.workerCount不可能一直maxSize
2.线程池中的可用线程大于1时候，只有满足削减条件（削减条件：workerCount > coreSize或者允许核心线程超时），并且超时了，才会削减线程个数
3.线程池中的可用线程为1，只有满足削减条件（削减条件：workerCount > coreSize或者允许核心线程超时），而且队列为空，才会减少线程个数为0