java多线程执行方法之ThreadPoolExecutor与Executors的比较

Executors有以下几种方法：

1. newFixedThreadPool() 方法：具有固定数量的线程池，线程数量始终不变。当有一个新任务提交时，线程中若有空闲进程变会执行它。若没有，则新的任务会被暂停在一个任务队列中。源码：如下

   /**
        * Creates a thread pool that reuses a fixed number of threads
        * operating off a shared unbounded queue.  At any point, at most
        * 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());
       }

    

2. newCachedThreadPool() 方法：线程数量不固定，当线程不足时便会产生新的线程。

   /**
        * Creates a thread pool that creates new threads as needed, but
        * will reuse previously constructed threads when they are
        * available, and uses the provided
        * ThreadFactory to create new threads when needed.
        * @param threadFactory the factory to use when creating new threads
        * @return the newly created thread pool
        * @throws NullPointerException if threadFactory is null
        */
       public static ExecutorService newCachedThreadPool(ThreadFactory threadFactory) {
           return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                         60L, TimeUnit.SECONDS,
                                         new SynchronousQueue(),
                                         threadFactory);
       }

    

3. newSingleThreadExecutor()方法：只有一个线程的线程池，按先进先出的顺序执行队列中的任务。

   /**
        * Creates a single-threaded executor that can schedule commands
        * to run after a given delay, or to execute periodically.
        * (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
        * newScheduledThreadPool(1) the returned executor is
        * guaranteed not to be reconfigurable to use additional threads.
        * @return the newly created scheduled executor
        */
       public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
           return new DelegatedScheduledExecutorService
               (new ScheduledThreadPoolExecutor(1));
       }

    

4. newSingleThreadScheduledExecutor() 方法：只有一个线程的线程池，但是可以指定执行时间或执行周期

   /**
        * Creates a single-threaded executor that can schedule commands
        * to run after a given delay, or to execute periodically.  (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 newScheduledThreadPool(1, threadFactory)
        * the returned executor is guaranteed not to be reconfigurable to
        * use additional threads.
        * @param threadFactory the factory to use when creating new
        * threads
        * @return a newly created scheduled executor
        * @throws NullPointerException if threadFactory is null
        */
       public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
           return new DelegatedScheduledExecutorService
               (new ScheduledThreadPoolExecutor(1, threadFactory));
       }

    

从他们的源码可以看出，

newCachedThreadPool和newFixedThreadPool最终都是调用的ThreadPoolExecutor方法并且里面的maximumPoolSize、keepAliveTimeunit，workQueue、threadFactory参数都是默认的值。

如果我们直接调用ThreadPoolExecutor方法这些值可以自己定义

/**
     * 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:

     *         {@code corePoolSize < 0}

     *         {@code keepAliveTime < 0}

     *         {@code maximumPoolSize <= 0}

     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

这是ThreadPoolExecutor源码。

下面我们来写个例子看看效果。

public class MyThreadTest implements Runnable,Comparable{


    private static ExecutorService executorService=Executors.newFixedThreadPool(100);

    private static Executor myThread=new ThreadPoolExecutor(100,100,0L, TimeUnit.SECONDS,new PriorityBlockingQueue());
    private String name;
    public MyThreadTest(String name){
        this.name=name;
    }
    public MyThreadTest(){
        name="";
    }

    @Override
    public void run(){
        try{
            Thread.sleep(100);
            System.out.println(name+" running....");
        }catch(InterruptedException e){
            e.printStackTrace();
        }
    }
    @Override
    public int compareTo(MyThreadTest o){
        int me = Integer.valueOf(name.split("_")[1]);
        int other = Integer.valueOf(o.name.split("_")[1]);
        return me-other;
    }


    public static void main(String[] args) {
        for (int i=0;i<1000;i++){
//            myThread.execute(new MyThreadTest("test_"+(i)));

            executorService.execute(new MyThreadTest("test_"+(i)));

        }
    }
}

test_56 running....
test_39 running....
test_37 running....
test_43 running....
test_57 running....
test_42 running....
test_33 running....
test_15 running....
test_97 running....
test_1 running....
test_74 running....
test_54 running....
。。。
。。。
。。。
test_948 running....
test_943 running....
test_942 running....
test_940 running....
test_941 running....
test_929 running....
test_934 running....
test_932 running....
test_926 running....
test_925 running....
test_923 running....

两者执行效率差不多，如果没有特殊要求建议使用

ExecutorService executorService=Executors.newFixedThreadPool(100);这种方式很简单快捷。如果需要进行其他特殊需求可换成

Executor myThread=new ThreadPoolExecutor(100,100,0L, TimeUnit.SECONDS,new PriorityBlockingQueue());

其中new PriorityBlockingQueue()可以自己定义queue