CachedThreadPool是怎样实现线程复用的？

Question

按我的理解，假设我不断给线程池提交新的同样的Runnable，线程池会复用已经运行完但没销毁的线程，来减少创建线程的开销。 但是，如果我们多次调用线程的start()方法，程序会崩溃的，而且Thread类也没有类似SetRunnable之类的方法，那么请问线程池是怎样做到的呢？ 另外，当需要重复开启同样的线程时，最佳实践是什么？谢谢！

Answer 1

我猜题主是使用 Executors.newCachedThreadPool();这个方法来创建线程池的吧，不知道你有没有点进去这个方法看看里面是怎么来创建线程池的。
我就来帮你看下源码。
创建线程池最终调用的是这个方法。

/**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler)

来看下这几个参数
corePoolSize ：核心线程数量，就算线程的状态是闲置也不会销毁
maximumPoolSize： 最大线程数量
keepAliveTime ：超出核心线程的数量可以闲置多久，超过这个时间就销毁了
unit ：keepAliveTime的单位
workQueue： 从泛型可以看出来里面装的都是Runnable，而且是一个阻塞的队列
ThreadFactory： 线程工厂
RejectedExecutionHandler： 任务拒绝策略，有四种可选

        1、直接丢弃（DiscardPolicy）
        2、丢弃队列中最老的任务(DiscardOldestPolicy)。
        3、抛异常(AbortPolicy)
        4、将任务分给调用线程来执行(CallerRunsPolicy)。

然后再看execute方法

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

但是，如果我们多次调用线程的start()方法，程序会崩溃的，而且Thread类也没有类似SetRunnable之类的方法，那么请问线程池是怎样做到的呢？

线程池是先找到一个闲置的线程，然后从workQueue这个阻塞队列中拿到你提交的任务（Runnable）然后去跑。（理想状态）

另外，当需要重复开启同样的线程时，最佳实践是什么？

当然是使用线程池咯，可以设置核心线程数量为1。