MoreExecutors工具类使用

MoreExecutors是guava提供的工具类，是对jdk自带的Executors工具类的扩展，主要方法如下：

1、addDelayedShutDown()方法的两个重载：

public static void addDelayedShutDownHook(ExecutorService service, Duration terminationTimeout)

public static void addDelayedShutDownHook(ExecutorService service, long terminationTimeout, TimeUnit unit)

给线程池增加一个关闭钩子，在线程池中的线程是守护线程(daemon thread)时有用，用户线程(user thread)执行完后，jvm不会立即关闭，而是等待一定时间。正常情况下，只要用户线程一结束，jvm就会立即关闭，而不管守护线程任务是否执行完毕。从这里也可以看出，尽量不要把自定义线程搞成守护线程，不然是作死。

示例：

    public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setDaemon(true).setNameFormat("async-pool-%d").build();
        ExecutorService executorService = new ThreadPoolExecutor(10, 20, 0, TimeUnit.MINUTES, new LinkedBlockingQueue<>(3000), threadFactory);
        executorService.submit(() -> {
            try {
                Thread.sleep(2000);
                System.out.println(Thread.currentThread().getName() + "@666");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        System.out.println(Thread.currentThread().getName() + "@888");
        MoreExecutors.addDelayedShutdownHook(executorService, 3000, TimeUnit.MILLISECONDS);
    }

上例中，如果没有调用MoreExecutors.addDelayedShutdownHook()方法的话，只会打印888，不会打印666。因为打印888的线程是用户线程，打印666的线程是守护线程(setDaemon(true))，用户线程一执行完，jvm就关闭了，所以不会有2000ms之后的666打印。假如创建ThreadFactory实例时，没有调用setDaemon(true)，即创建的线程是非守护线程，那么会先打印main@888，2000ms后会打印async-pool-0@666，因为main线程和async-pool-0线程都是用户线程。调用MoreExecutors.addDelayedShutdownHook()方法后，jvm会在用户线程结束后等待一段时间再关闭，这段之间守护线程可以工作，到了时间，jvm关闭，守护线程也完蛋了，事情干多少是多少，没干完就没干完。

2、getExitingExecutorService()方法的三个重载：把一个ThreadPoolExecutor实例转成一个应用结束后自动退出的ExecutorService实例。

public static ExecutorService getExitingExecutorService(ThreadPoolExecutor executor)

public static ExecutorService getExitingExecutorService(ThreadPoolExecutor executor, Duration terminationTimeout)

public static ExecutorService getExitingExecutorService(ThreadPoolExecutor executor, long terminationTimeout, TimeUnit timeUnit)

示例：

    public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("async-pool-%d").build();
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 20, 0, TimeUnit.MINUTES, new LinkedBlockingQueue<>(3000), threadFactory);
        ExecutorService executorService = MoreExecutors.getExitingExecutorService(threadPoolExecutor);
        executorService.submit(() -> {
            try {
                Thread.sleep(110000);
                System.out.println(Thread.currentThread().getName() + "@666");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        });
        System.out.println(Thread.currentThread().getName() + "@888");
    }

在用户线程执行完后，jvm会至多等待一定时间(默认是120s)，以期线程池线程执行结束。这里不要求线程池中线程是守护线程，因为不管是不是守护线程，getExitingExecutorService()方法都会把这个线程池包装成守护线程的线程池，并且加个默认120s的关闭钩子。 

3、getExitingScheduledExecutorService()方法的三个重载：道理同getExitingExecutorService()方法差不多

public static ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor)

public static ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor, Duration terminationTimeout)

public static ScheduledExecutorService getExitingScheduledExecutorService(ScheduledThreadPoolExecutor executor, long terminationTimeout, TimeUnit timeUnit)

4、listeningDecorator()方法的两个重载：

public static ListeningExecutorService listeningDecorator(ExecutorService delegate)：把一个ExecutorService实例转成一个ListeningExecutorService实例

示例：

    public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("async-pool-%d").build();
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(10, 20, 0, TimeUnit.MINUTES, new LinkedBlockingQueue<>(3000), threadFactory);
        ListeningExecutorService listeningExecutorService = MoreExecutors.listeningDecorator(threadPoolExecutor);
        ListenableFuture future = listeningExecutorService.submit(() -> {
            try {
                Thread.sleep(4000);
                System.out.println(Thread.currentThread().getName() + "@666");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, 1);
        Futures.addCallback(future, new FutureCallback() {

            @Override
            public void onSuccess(Object result) {
                System.out.println(Thread.currentThread().getName() + "@" + result);
            }

            @Override
            public void onFailure(Throwable t) {
                System.out.println(Thread.currentThread().getName() + "@" + t.getMessage());
            }
        }, threadPoolExecutor);
        System.out.println(Thread.currentThread().getName() + "@888");
    }

ListeningExecutorService实例的submit()方法返回值是一个ListenableFuture实例，利用Futures工具类可以给这个实例添加callback回调。

public static ListeningScheduledExecutorService listeningDecorator(ScheduledExecutorService delegate)：把一个ScheduledExecutorService实例转成一个ListeningScheduledExecutorService实例

5、directExector()方法：direct是直接的意思。

public static Executor directExecutor()：返回一个Executor实例，具体是DirectExecutor类型，DirectExecutor是一个实现了Executor接口的枚举类。调用execute(Runnable command)方法时，在当前线程执行任务，而不会另起一个线程。这个方法没卵用，在当前线程执行的话，直接写代码就好了，干嘛还整这个。

6、newDirectExecutorService()方法

public static ListeningExecutorService newDirectExecutorService()：返回一个ListeningExecutorService实例，具体是DirectExecutorService类型，DirectExecutorService是MoreExecutors的内部类，继承了AbstractListeningExecutorService。和DirectExecutor实例类似，调用DirectExecutorService实例的submit()方法时，会在当前线程执行任务，而不会另起一个线程。同样，没卵用。这里还要吐槽一下DirectExecutorService类名的不规范，妈的，明明是ListeningExecutorService实现类，就不能叫DirectListeningExecutorService吗。

7、newSequentialExecutor()方法：

public static Executor newSequentialExecutor(Executor delegate)：把一个Executor实例包装成一个顺序执行的Executor实例，具体是SequentialExecutor类型。线程池按照任务添加顺序执行任务。

示例：

    public static void main(String[] args) {
        ThreadFactory threadFactory = new ThreadFactoryBuilder().setNameFormat("async-pool-%d").build();
        ThreadPoolExecutor threadPoolExecutor =
                new ThreadPoolExecutor(10, 20, 0, TimeUnit.MINUTES, new LinkedBlockingQueue<>(3000), threadFactory);
        Executor executor = MoreExecutors.newSequentialExecutor(threadPoolExecutor);
        for (int i = 0; i < 10; i++) {
            int d = i;
            executor.execute(() -> {
                try {
                    Thread.sleep(d * 1000);
                    System.out.println(
                            Thread.currentThread().getName() + "@" + d + ", now= " + System.currentTimeMillis());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }
    }

线程池按照任务添加顺序执行任务。上一个任务执行不完，下一个任务就不会开始。活活把一个线程池包装成了一个单线程的线程池，鸡肋。

8、platformThreadFactory()方法：

MoreExecutors.platformThreadFactory()：返回默认的ThreadFactory实例。内部调用Executors.defaultThreadFactory()，返回DefaultThreadFactory实例。DefaultThreadFactory是Executors的内部类，实现了ThreadFactory接口。DefaultThreadFactory对于的线程名形如pool-0-thread-0，pool-0-thread-1，我们自定义线程池时线程的名字绝对不可能这么没有意义，所以platformThreadFactory()这个方法也是鸡肋。