Java多线程之ThreadPoolExecutor
线程池的作用就是使用更少的线程, 来执行更多的任务, 以达到线程的充分利用. 从ThreadPoolExecutor类的构造方法学习:
/** * Creates a new {@code ThreadPoolExecutor} with the given initial * parameters and default rejected execution handler. * * @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 * @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} is null */public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue<Runnable> workQueue) { this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue, Executors.defaultThreadFactory(), defaultHandler); }
ThreadPoolExecutor的具体执行流程有四步:
(1) 当目前执行线程的总数小于核心线程大小corePoolSize时, 提交了任务, 会立马新建一个线程来执行提交的任务.
(2) 当目前执行线程的总数达到了核心线程大小corePoolSize时, 就会将当前提交的任务存到缓存队列中
(3) 当缓存队列满了之后, 如果当前线程的总数小于maximumPoolSize时, 此时就会新建线程进行救急处理, 提交的任务会立马执行.
(4) 当线程数达到maximumPoolSize后, 就会采用拒绝策略拒绝任务