java线程池keepAliveTime的含义说明

 // runState is stored in the high-order bits
 private static final int RUNNING = -1 <

/**
  * Performs blocking or timed wait for a task, depending on
  * current configuration settings, or returns null if this worker
  * must exit because of any of:
  * 1. There are more than maximumPoolSize workers (due to
  * a call to setMaximumPoolSize).
  * 2. The pool is stopped.
  * 3. The pool is shutdown and the queue is empty.
  * 4. This worker timed out waiting for a task, and timed-out
  * workers are subject to termination (that is,
  * {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
  * both before and after the timed wait.
  *
  * @return task, or null if the worker must exit, in which case
  *   workerCount is decremented
  */
 private Runnable getTask() {
  boolean timedOut = false; // Did the last poll() time out&＃63;
  retry:
  for (;;) {
   int c = ctl.get();
   int rs = runStateOf(c);
   // Check if queue empty only if necessary.
   if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
    decrementWorkerCount();
    return null;
   }
   boolean timed;  // Are workers subject to culling&＃63;
   for (;;) {
    int wc = workerCountOf(c);
    timed = allowCoreThreadTimeOut || wc > corePoolSize;
    //默认allowCoreThreadTimeOut为false，除非程序指定
    //（1）当没有超过核心线程时，默认allowCoreThreadTimeOut为false时
    //timed值为false，始终break掉，不会销毁线程
    //（2）当超过核心线程数，默认allowCoreThreadTimeOut为false时
    //timed值为true，如果超过最大值，则销毁；如果timeout过，则销毁
    // 如果allowCoreThreadTimeOut为true，则timed始终为true
    if (wc <= maximumPoolSize && ! (timedOut && timed))
     break;
    if (compareAndDecrementWorkerCount(c))
     return null;
    c = ctl.get(); // Re-read ctl
    if (runStateOf(c) != rs)
     continue retry;
    // else CAS failed due to workerCount change; retry inner loop
   }
   try {
    Runnable r = timed &＃63;
     workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
     workQueue.take();
    if (r != null)
     return r;
    timedOut = true;
   } catch (InterruptedException retry) {
    timedOut = false;
   }
  }
 }

workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS)

if (wc <= maximumPoolSize && ! (timedOut && timed))
     break;
    if (compareAndDecrementWorkerCount(c))
     return null;

 /**
  * Timeout in nanoseconds for idle threads waiting for work.
  * Threads use this timeout when there are more than corePoolSize
  * present or if allowCoreThreadTimeOut. Otherwise they wait
  * forever for new work.
  */
 private volatile long keepAliveTime;
 /**
  * If false (default), core threads stay alive even when idle.
  * If true, core threads use keepAliveTime to time out waiting
  * for work.
  */
 private volatile boolean allowCoreThreadTimeOut;

/**
  * Creates a thread pool that creates new threads as needed, but
  * will reuse previously constructed threads when they are
  * available. These pools will typically improve the performance
  * of programs that execute many short-lived asynchronous tasks.
  * Calls to execute will reuse previously constructed
  * threads if available. If no existing thread is available, a new
  * thread will be created and added to the pool. Threads that have
  * not been used for sixty seconds are terminated and removed from
  * the cache. Thus, a pool that remains idle for long enough will
  * not consume any resources. Note that pools with similar
  * properties but different details (for example, timeout parameters)
  * may be created using {@link ThreadPoolExecutor} constructors.
  *
  * @return the newly created thread pool
  */
 public static ExecutorService newCachedThreadPool() {
  return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
          60L, TimeUnit.SECONDS,
          new SynchronousQueue());
 }
 /**
  * Creates a thread pool that creates new threads as needed, but
  * will reuse previously constructed threads when they are
  * available, and uses the provided
  * ThreadFactory to create new threads when needed.
  * @param threadFactory the factory to use when creating new threads
  * @return the newly created thread pool
  * @throws NullPointerException if threadFactory is null
  */
 public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
  return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
          60L, TimeUnit.SECONDS,
          new SynchronousQueue(),
          threadFactory);
 }

public E poll(long timeout, TimeUnit unit) throws InterruptedException {
  return pollFirst(timeout, unit);
 }

public E pollFirst(long timeout, TimeUnit unit)
  throws InterruptedException {
  long nanos = unit.toNanos(timeout);
  final ReentrantLock lock = this.lock;
  lock.lockInterruptibly();
  try {
   E x;
   while ( (x = unlinkFirst()) == null) {
    if (nanos <= 0)
     return null;
    nanos = notEmpty.awaitNanos(nanos);
   }
   return x;
  } finally {
   lock.unlock();
  }
 }