3.7 线程池使用

3.7 线程池

       线程池指的是可重复利用的线程集合，线程池的作用即可以同时执行多个任务并且线程可以重复使用，增加多任务处理效率。

3.7.1 ThreadPoolExecutor创建线程池

       阿里开发规范不要显示创建线程池，也不要用excutors静态方法创建线程池，而是用ThreadPoolExecutor。

       使用示例：

static ThreadFactory myThreadFactory = new
ThreadFactory() {
    @Override
    public Thread newThread(Runnable runnable) {
        return new Thread(runnable);
    }
};

public static ExecutorService pool = new ThreadPoolExecutor(1, 12,
        0L, TimeUnit.MILLISECONDS,
        new LinkedBlockingQueue<Runnable>(1024), myThreadFactory, new ThreadPoolExecutor.AbortPolicy());

       ThreadPoolExecutor构造方法中各个参数的含义：

       corePoolSize：线程池拥有的线程数量，不管空闲不空闲一直都在的。
       maximumPoolSize: 线程池中允许的最大线程数量。
       keepAliveTime：当线程池中线程       最大的闲置时间，到了限制时间会销毁非核心线程。
       unit：闲置时间单位，秒，分，时。
       workQueue：任务无法获得线程进行处理时进入该队列等待。
       threadFactory：线程工厂，具体的创建线程的方法，开放创建线程时操作。
       handler：当任务无法提交，等待队列也满时，对任务的处理方法。

        源码注释：

/**
 * 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
 */

       注意阿里开发规范在线程池的相关规范不一定就是最好的，重要的看实际情况，直接使用Excutors里有些相关操作也是极好的。

3.7.2 Executors创建线程池

       使用方式，Executors还提供了很多其他类型的线程池，此处只提下newFixedThreadPool这种方式。使用Executors本质上还是通过ThreadPoolExecutor创建的线程池，不过ThreadPoolExecutor的构造参数做了限制。

ExecutorService threadPool = Executors.newFixedThreadPool(5);

       newFixedThreadPool（）创建方式源码：

public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());
}