線程池有以下的優點：

• 重用線程池中的線程，避免因為線程的創建和銷毀所帶來的性能上的開銷
• 能有效控制線程池的最大并發數，避免大量的線程之間因為互相搶占系統資源而導致阻塞現象
• 能夠對線程池就行簡單的管理，并提供定時執行以及指定間隔循環

線程池介紹

ThreadPoolExecutor是線程池的真正實現

  /**
     * 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,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

ThreadPoolExecutor執行任務是遵循的規則

image.png

參數設置

private static final int CPU_COUNT = Runtime.getRuntime().availableProcessors();
    private static final int CORE_POOL_SIZE = CPU_COUNT + 1;
    private static final int MAXIMUM_POOL_SIZE = CPU_COUNT * 2 + 1;
    private static final int KEEP_ALIVE = 1;

    private static final ThreadFactory sThreadFactory = new ThreadFactory() {
        private final AtomicInteger mCount = new AtomicInteger(1);

        public Thread newThread(Runnable r) {
            return new Thread(r, "AsyncTask #" + mCount.getAndIncrement());
        }
    };

    private static final BlockingQueue<Runnable> sPoolWorkQueue =
            new LinkedBlockingQueue<Runnable>(128);

    /**
     * An {@link Executor} that can be used to execute tasks in parallel.
     */
    public static final Executor THREAD_POOL_EXECUTOR
            = new ThreadPoolExecutor(CORE_POOL_SIZE, MAXIMUM_POOL_SIZE, KEEP_ALIVE,
                    TimeUnit.SECONDS, sPoolWorkQueue, sThreadFactory);

線程池的分類

·FixedThreadPool：
數量固定的核心線程池，沒有超時機制。任務隊列大小沒有限制。
·CachedThreadPool：
線程數量不定，只有非核心賢臣，最大線程數位Integer.MAX_VALUE。它比較適合執行大量的耗時較少的任務
·ScheduledThreadPool：
核心線程數量固定，非核心線程數量沒有限制。適合執行定時任務和具有固定周期的重復任務。
·SingleThreadExecutor：
只有一個核心線程