线程池的用法及思考

Posted 2020-11-27 cnxieyang

本文介绍下Java中线程池的基本用法，基于此说明如何去确定线程池的大小。

线程池的创建方法

　　newFixedThreadPool

　　 创建固定线程数的线程池，见下创建代码

　　

/**
     * Creates a thread pool that reuses a fixed number of threads
     * operating off a shared unbounded queue.  At any point, at most
     * {@code nThreads} threads will be active processing tasks.
     * If additional tasks are submitted when all threads are active,
     * they will wait in the queue until a thread is available.
     * If any thread terminates due to a failure during execution
     * prior to shutdown, a new one will take its place if needed to
     * execute subsequent tasks.  The threads in the pool will exist
     * until it is explicitly {@link ExecutorService#shutdown shutdown}.
     *
     * @param nThreads the number of threads in the pool
     * @return the newly created thread pool
     * @throws IllegalArgumentException if {@code nThreads <= 0}
     */
    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }

　　　　从代码中可发现，corePoolSize和maxPoolSize都是相等的，取决于传入的线程数，keepAliveTime=0代表线程不进行超时回收，但使用的是无界队列，当任务数量过多时，将会导致OOM溢出。

　　newSingleThreadPool

　　创建线程数为1的线程池，创建代码如下。

　　

 /**
     * Creates an Executor that uses a single worker thread operating
     * off an unbounded queue. (Note however that if this single
     * thread terminates due to a failure during execution prior to
     * shutdown, a new one will take its place if needed to execute
     * subsequent tasks.)  Tasks are guaranteed to execute
     * sequentially, and no more than one task will be active at any
     * given time. Unlike the otherwise equivalent
     * {@code newFixedThreadPool(1)} the returned executor is
     * guaranteed not to be reconfigurable to use additional threads.
     *
     * @return the newly created single-threaded Executor
     */
    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }


/**
    * 创建线程数为1的线程池
    *
    */

　　我们发现corePoolSize和MaxPoolSize都是1，使用的还是无界队列，存在OOM

　　newCachedThreadPool

　　创建可缓存的线程池。代码如下

　　

  /**
     * 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 {@code execute} 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>());

　　 从创建代码中发现，corePoolSize核心线程数在一定时间内可以为零，当有一个新的任务时，只需要创建一个存活周期为60秒的线程，使用的是直接队列，队列任务为零，当任务过多时，会创建大量的线程，仍然会导致溢出。

　　newScheduledThreadPool

　　创建定义任务相关的线程数，在普通线程池的基础上引入了定时任务相关的操作。

　　

 /**
     * Creates a new {@code ScheduledThreadPoolExecutor} with the
     * given core pool size.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     */
    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

　　我们发现可以指定核心线程数，maxPoolSize为Integer.MAX_VALUE,非核心线程数不进行回收，使用了延迟队列，也存在OOM

 

　　workStealingPool

　　jdk1.8新增的。产生递归操作时，可使用此线程池。

 

　　综上所述，提供了多种创建线程池的方法，具体使用哪种线程池方法，需要根据实际情况来定，使用多少线程数，最后通过性能测试去确定，我们也可以从CPU密集型和IO密集型方面进行简易区分。CPU密集型的线程数为CPU核心数的1到2倍。IO密集型线程数可为CPU核心数的10倍左右。

　　参考计算方法为：线程数=cpu核心数 * （1+ 平均等待时间/平均运行时间）