概览

原文地址 http://blog.csdn.net/qq_25806863/article/details/71159452

通过上一篇对ThreadPoolExecutor的构造方法分析可以感受到，通过ThreadPoolExecutor来创建线程池是比较复杂的，参数比较多，考虑因素也多。

因此java自己提供了一个工厂类Executors，里面提供了一些方法，用来创建常用的几种ThreadPoolExecutor线程池。

下面是方法概览：

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


public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }

分析

FixedThreadPool

FixedThreadPool调用的是ThreadPoolExecutor的构造方法。有下面两种使用方式：

//不带工厂的
public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());
}
//带工厂的
public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>(),
                                      threadFactory);
    }

通过对ThreadPoolExecutor构造参数的分析可以看出来，这个线程池的最大线程数就是核心线程数，也就是没有非核心线程的存在。这些线程都是核心线程，即使在闲置状态也不会被回收，除非线程池关闭了，所以超时机制并没有用。他的任务队列是无界的LinkedBlockingQueue，因此超过核心线程数量的任务会放在队列中排队。

这样的线程池优点很明显，只会创建固定数量的线程，然后这些线程一直存活重用，不会有额外的创建和销毁线程的开销，能更快的执行任务。

测试

ExecutorService executor = Executors.newFixedThreadPool(2);
Runnable myRunnable = new Runnable() {
    @Override
    public void run() {
        try {
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + " run time: "+System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }
};
for (int i = 0; i < 6; i++) {
    executor.execute(myRunnable);
}

创建一个固定大小为2的FixedThreadPool，然后添加6个任务，输出是：

pool-1-thread-2 run time: 1493863106021
pool-1-thread-1 run time: 1493863106021
pool-1-thread-2 run time: 1493863108022
pool-1-thread-1 run time: 1493863108022
pool-1-thread-1 run time: 1493863110026
pool-1-thread-2 run time: 1493863110027

可以看到这个线程池只会创建2个线程，其他的都在排队。

线程工厂

public static void main(String[] args) throws InterruptedException {
     ExecutorService executor = Executors.newFixedThreadPool(2,new MyFactory());
     Runnable myRunnable = new Runnable() {
         @Override
         public void run() {
             try {
                 Thread.sleep(2000);
                 System.out.println(Thread.currentThread().getName() + " run time: "+System.currentTimeMillis());
             } catch (InterruptedException e) {
                 e.printStackTrace();
             }

         }
     };
     for (int i = 0; i < 6; i++) {
         executor.execute(myRunnable);
     }
 }

static class MyFactory implements ThreadFactory{

     @Override
     public Thread newThread(@NonNull Runnable r) {
         Thread thread = new Thread(r);
         thread.setName("哈哈"+ UUID.randomUUID().toString().substring(0,3));
         return thread;
     }
 }

创建一个工厂，自定义设置线程名字，然后在newFixedThreadPool时传入工厂，看输出

哈哈fea run time: 1493863498244
哈哈491 run time: 1493863498244
哈哈fea run time: 1493863500250
哈哈491 run time: 1493863500250
哈哈fea run time: 1493863502253
哈哈491 run time: 1493863502253

下面的工厂参数都是这样，就不在写了。

CachedThreadPool

CachedThreadPool也是用的ThreadPoolExecutor的构造方法

public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>());
}

public static ScheduledExecutorService newSingleThreadScheduledExecutor(ThreadFactory threadFactory) {
        return new DelegatedScheduledExecutorService
            (new ScheduledThreadPoolExecutor(1, threadFactory));
}

分析这个参数可以看出，这个线程池没有核心线程，所有的线程都会在限制60秒后被回收。而且最大线程数为Integer.MAX_VALUE，相当于无限大。因为任务队列是SynchronousQueue，不会保存任何任务，所以当有新任务时，如果当前线程都在活动着，就会新建一个线程来执行任务。

这样的线程池的特点就是，适合执行大量的耗时短的任务。而且当所有任务执行完后，闲置超过60秒就会全部回收，这是线程池里就没有任何线程，不会占用系统资源。

测试

ExecutorService executor = Executors.newCachedThreadPool();
        Runnable myRunnable = new Runnable() {
    @Override
    public void run() {
        try {
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + " run time: "+System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
};
for (int i = 0; i < 3; i++) {
    executor.execute(myRunnable);
}
Thread.sleep(30000);
System.out.println("睡眠30秒后");
for (int i = 0; i < 3; i++) {
    executor.execute(myRunnable);
}
Thread.sleep(65000);
System.out.println("再睡眠65秒后");
for (int i = 0; i < 3; i++) {
    executor.execute(myRunnable);
}

先运行三个任务，30秒后再添加3个任务，再过65秒再添加3个任务。

pool-1-thread-1 run time: 1493864065635
pool-1-thread-2 run time: 1493864065635
pool-1-thread-3 run time: 1493864065639
睡眠30秒后
pool-1-thread-3 run time: 1493864095637
pool-1-thread-2 run time: 1493864095637
pool-1-thread-1 run time: 1493864095637
再睡眠65秒后
pool-1-thread-5 run time: 1493864160643
pool-1-thread-4 run time: 1493864160643
pool-1-thread-6 run time: 1493864160643

可以看到一开始创建了3个线程，30秒后重用了这三个线程。但是再过65秒后，者三个线程就因为超时被回收了，所以新建了三个线程。

SingleThreadExecutor

SingleThreadExecutor使用new FinalizableDelegatedExecutorService来创建线程池。但是其实通过一个委托调用了ThreadPoolExecutor的构造方法

public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

public static ExecutorService newSingleThreadExecutor(ThreadFactory threadFactory) {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>(),
                                    threadFactory));
}

从ThreadPoolExecutor的构造参数依然可以看出，这个线程池仅有一个核心线程，其他的任务都在任务队列中排队。

这样的线程池会让所有的任务都在同一个线程中执行，避免的同步问题。

测试

ExecutorService executor = Executors.newSingleThreadExecutor();
        Runnable myRunnable = new Runnable() {
    @Override
    public void run() {
        try {
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + " run time: "+System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
};
for (int i = 0; i < 3; i++) {
    executor.execute(myRunnable);
}

Thread.sleep(65000);
System.out.println("睡眠65秒后");
for (int i = 0; i < 3; i++) {
    executor.execute(myRunnable);
}

pool-1-thread-1 run time: 1493864476076
pool-1-thread-1 run time: 1493864478078
pool-1-thread-1 run time: 1493864480082
睡眠65秒后
pool-1-thread-1 run time: 1493864541080
pool-1-thread-1 run time: 1493864543083
pool-1-thread-1 run time: 1493864545087

自始至终都只有一个线程。而且不会被回收。

ScheduledThreadPool

ScheduledThreadPool使用new ScheduledThreadPoolExecutor来创建

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
}

public static ScheduledExecutorService newScheduledThreadPool(
            int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
}

看一下具体使用的构造参数：

public ScheduledThreadPoolExecutor(int corePoolSize,
                                   ThreadFactory threadFactory) {
    super(corePoolSize, Integer.MAX_VALUE,
          DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
          new DelayedWorkQueue(), threadFactory);
}

可以看到，这个线程池的核心线程数是固定的，但是总线程数是无限的，然而因为DelayedWorkQueue是个无界队列，所以这个值没有意义，超过核心线程的任务都会放在队列中。

ScheduledThreadPoolExecutor主要是用来执行定时任务和有周期性的重复任务。

这里就举个不恰当的例子，不是ScheduledThreadPoolExecutor的用法

ExecutorService executor = Executors.newScheduledThreadPool(3);
        Runnable myRunnable = new Runnable() {
    @Override
    public void run() {
        try {
            Thread.sleep(2000);
            System.out.println(Thread.currentThread().getName() + " run time: "+System.currentTimeMillis());
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
};
for (int i = 0; i < 6; i++) {
    executor.execute(myRunnable);
}

pool-1-thread-3 run time: 1493866372444
pool-1-thread-1 run time: 1493866372444
pool-1-thread-2 run time: 1493866372444
pool-1-thread-2 run time: 1493866374450
pool-1-thread-1 run time: 1493866374450
pool-1-thread-3 run time: 1493866374450

设置的是3，就只会创建3个线程。

Java多线程-线程池Executors

概览

分析

FixedThreadPool

测试

线程工厂

CachedThreadPool

测试

SingleThreadExecutor

测试

ScheduledThreadPool

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