java 线程池keepAliveTime的含义说明

之前对线程池中属性：keepAliveTime比较模糊，而且看过之后过一段时间就会忘掉，于是就在此记录一下。

keepAliveTime的jdk中的解释为：

当线程数大于核心时，此为终止前多余的空闲线程等待新任务的最长时间。

说的让人感觉比较模糊，总结一下大概意思为：比如说线程池中最大的线程数为50，而其中只有40个线程任务在跑，相当于有10个空闲线程，这10个空闲线程不能让他一直在开着，因为线程的存在也会特别好资源的，所有就需要设置一个这个空闲线程的存活时间，这么解释应该就很清楚了。

这样以后忘记了就过来看看就OK了。

补充：线程池的状态及KeepAliveTime参数

五个状态

 // runState is stored in the high-order bits
 private static final int RUNNING = -1 << COUNT_BITS;
 private static final int SHUTDOWN = 0 << COUNT_BITS;
 private static final int STOP  = 1 << COUNT_BITS;
 private static final int TIDYING = 2 << COUNT_BITS;
 private static final int TERMINATED = 3 << COUNT_BITS;

循环getTask方法

/**
  * 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?
  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?
   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 ?
     workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
     workQueue.take();
    if (r != null)
     return r;
    timedOut = true;
   } catch (InterruptedException retry) {
    timedOut = false;
   }
  }
 }

线程池状态大于SHUTDOWN值的两种情况

1、调用shutdown方法

当线程池调用了shutdown方法，线程池的状态会首先被设置为SHUTDOWN，然后遍历线程池中所有线程，调用一次interrupt方法，如果在休眠中的线程将会激活，激活后的线程以及调用shutdown方法本身的线程都会尝试去调用tryTerminate方法，该方法将判定如果线程池中所有记录的线程数为0，则将线程状态改为TERMINATED，这个值为3，将大于SHUTDOWN状态值。

2、调用shutdownNow方法

当线程调用了shutdownNow方法后，首先将线程的状态修改为STOP，这个状态是大于SHUTDOWN值的，接下来它也会通过中断激活线程，只是它来的更暴力一些，连加锁和一些基本判断都没有，直接中断；在调用tryTerminate之前会先清空阻塞队列中所有的元素，这些元素被组装为一个List列表作为shutdownNow方法的返回值。换句话说，没有执行的任务在shutdownNow执行后的返回值中可以得到。在程序某些必要的情况下，可以通过线程池的isTerminating，isTerminated，isStopped，isShutdown来对线程做一些状态判定。

KeepAliveTime参数

workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS)

当阻塞队列中没有任务时，等待时间达到keepAliveTime毫秒值时就会被自动唤醒，而不会永远地沉睡下去。

keepAliveTime，如果是通过newCachedThreadPool的话，默认是1分钟超时，如果遇到前面所提到的瞬间冲击，那么线程池数量将瞬间快速膨胀，而且这些瞬间膨胀的线程的生命周期最少在1分钟以上。

如果设置了该参数，那么当timeout的时候，就return null，就会跳出循环，回收线程。

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

allowCoreThreadTimeout : 默认情况下核心线程不会退出，可通过将该参数设置为true，让核心线程也退出。

默认的Executors工厂，只有newCachedThreadPool，timeout为60秒，出现timeout情况下，而且线程数超过了核心线程数，会销毁销毁线程。保持在corePoolSize数（如果是cached的，corePoolSize为0）。

 /**
  * 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;

线程池最小是corePoolSize，最大是maximumPoolSize，除非设置了allowCoreThreadTimeOut和超时时间，这种情况线程数可能减少到0，最大可能是Integer.MAX_VALUE。

Core pool size is the minimum number of workers to keep alive(and not allow to time out etc) unless allowCoreThreadTimeOut is set, in which case the minimum is zero.

/**
  * 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 <tt>execute</tt> 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<Runnable>());
 }
 /**
  * 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<Runnable>(),
          threadFactory);
 }

超时timeout设置为0的话，表示不等待

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

具体如下

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();
  }
 }

以上为个人经验，希望能给大家一个参考，也希望大家多多支持我们。如有错误或未考虑完全的地方，望不吝赐教。