27-扩展ThreadPoolExecutor

扩展ThreadPoolExecutor

ThreadPoolExecutor是可扩展的，通过查看源码可以发现，它提供了几个可以在子类化中改写的方法：beforeExecute,afterExecute,terminated。源码片段如下所示：

protected void beforeExecute(Thread t, Runnable r) { }  
protected void afterExecute(Runnable r, Throwable t) { }  
protected void terminated() { }  

可以注意到，这三个方法都是protected的空方法，摆明了是让子类扩展的嘛。

在执行任务的线程中将调用beforeExecute和afterExecute等方法，在这些方法中还可以添加日志、计时、监视或者统计信息收集的功能。无论任务是从run中正常返回，还是抛出一个异常而返回，afterExecute都会被调用。如果任务在完成后带有一个Error，那么就不会调用afterExecute。如果beforeExecute抛出一个RuntimeException，那么任务将不被执行，并且afterExecute也不会被调用。

在线程池完成关闭时调用terminated，也就是在所有任务都已经完成并且所有工作者线程也已经关闭后，terminated可以用来释放Executor在其生命周期里分配的各种资源，此外还可以执行发送通知、记录日志或者收集finalize统计信息等操作。

下面就以给线程池添加统计信息为例：

public class TimingThreadPool extends ThreadPoolExecutor {
    private final ThreadLocal startTime = new ThreadLocal();
    private final Logger log = Logger.getAnonymousLogger();
    private final AtomicLong numTasks = new AtomicLong();
    private final AtomicLong totalTime = new AtomicLong();

    public TimingThreadPool(int corePoolSize, int maximumPoolSize,
            long keepAliveTime, TimeUnit unit, BlockingQueue workQueue) {
        super(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue);
    }

    protected void beforeExecute(Thread t, Runnable r) {
        super.beforeExecute(t, r);
        log.info(String.format("Thread %s: start %s", t, r));
        startTime.set(System.nanoTime());
    }

    protected void afterExecute(Runnable r, Throwable t) {
        try {
            long endTime = System.nanoTime();
            long taskTime = endTime - startTime.get();
            numTasks.incrementAndGet();
            totalTime.addAndGet(taskTime);
            log.info(String.format("Thread %s: end %s, time=%dns", t, r,
                    taskTime));
        } finally {
            super.afterExecute(r, t);
        }
    }

    protected void terminated() {
        try {
            log.info(String.format("Terminated: avg time=%dns", totalTime.get()
                    / numTasks.get()));
        } finally {
            super.terminated();
        }
    }
}

可以看到TimingThreadPool重写了父类的三个方法。

下面写一个测试类，参考运行效果：

public class CheckTimingThreadPool {
    public static void main(String[] args) {
        ThreadPoolExecutor exec = new TimingThreadPool(0, Integer.MAX_VALUE,
                60L, TimeUnit.SECONDS, new SynchronousQueue());
        exec.execute(new DoSomething(5));
        exec.execute(new DoSomething(4));
        exec.execute(new DoSomething(3));
        exec.execute(new DoSomething(2));
        exec.execute(new DoSomething(1));
        exec.shutdown();
    }
}

class DoSomething implements Runnable {
    private int sleepTime;

    public DoSomething(int sleepTime) {
        this.sleepTime = sleepTime;
    }

    @Override
    public void run() {
        System.out.println(Thread.currentThread().getName() + " is running.");
        try {
            TimeUnit.SECONDS.sleep(sleepTime);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

运行结果：

一月 20, 2018 5:34:51 下午 concurrent.TimingThreadPool beforeExecute
INFO: Thread Thread[pool-1-thread-5,5,main]: start concurrent.DoSomething@63425e17
pool-1-thread-5 is running.
一月 20, 2018 5:34:51 下午 concurrent.TimingThreadPool beforeExecute
INFO: Thread Thread[pool-1-thread-2,5,main]: start concurrent.DoSomething@156237e0
pool-1-thread-2 is running.
一月 20, 2018 5:34:51 下午 concurrent.TimingThreadPool beforeExecute
INFO: Thread Thread[pool-1-thread-3,5,main]: start concurrent.DoSomething@156237e0
pool-1-thread-3 is running.
一月 20, 2018 5:34:51 下午 concurrent.TimingThreadPool beforeExecute
INFO: Thread Thread[pool-1-thread-1,5,main]: start concurrent.DoSomething@156237e0
pool-1-thread-1 is running.
一月 20, 2018 5:34:51 下午 concurrent.TimingThreadPool beforeExecute
INFO: Thread Thread[pool-1-thread-4,5,main]: start concurrent.DoSomething@1a2f6aea
pool-1-thread-4 is running.
一月 20, 2018 5:34:52 下午 concurrent.TimingThreadPool afterExecute
INFO: Thread null: end concurrent.DoSomething@63425e17, time=1000557327ns
一月 20, 2018 5:34:53 下午 concurrent.TimingThreadPool afterExecute
INFO: Thread null: end concurrent.DoSomething@1a2f6aea, time=1999474354ns
一月 20, 2018 5:34:54 下午 concurrent.TimingThreadPool afterExecute
INFO: Thread null: end concurrent.DoSomething@156237e0, time=2999558243ns
一月 20, 2018 5:34:55 下午 concurrent.TimingThreadPool afterExecute
INFO: Thread null: end concurrent.DoSomething@156237e0, time=3999593049ns
一月 20, 2018 5:34:56 下午 concurrent.TimingThreadPool afterExecute
INFO: Thread null: end concurrent.DoSomething@156237e0, time=4999608220ns
一月 20, 2018 5:34:56 下午 concurrent.TimingThreadPool terminated
INFO: Terminated: avg time=2999758238ns

可以看到，在测试类CheckTimingThreadPool中通过execute了五个线程，然后分别对这五个线程进行统计，最后统计出各个线程的耗时平均时间。