读ScheduledThreadPoolExecutor源码

//一个可以延迟执行的定时任务
//先看构造函数 构造函数最重要的区别是传入了DelayedWorkQueue一个延迟的queue

/**
总结：该线程池继承了ThreadPoolExecutor。同时内部封装了一个DelayedWorkQueue和
ScheduledFutureTask。DelayedWorkQueue用于管理任务的新增和获取。ScheduledFutureTask
用于管理任务的执行。
*/

public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue());
    }

public ScheduledThreadPoolExecutor(int corePoolSize,
                                       ThreadFactory threadFactory) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), threadFactory);
    }

 public ScheduledThreadPoolExecutor(int corePoolSize,
                                       RejectedExecutionHandler handler) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), handler);
    }

public ScheduledThreadPoolExecutor(int corePoolSize,
                                       ThreadFactory threadFactory,
                                       RejectedExecutionHandler handler) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), threadFactory, handler);
    }

//创建并执行任务
 public void execute(Runnable command) {
        schedule(command, 0, TimeUnit.NANOSECONDS);
    }


//延迟执行Runnable任务
public ScheduledFuture<?> schedule(Runnable command,
                                       long delay,
                                       TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
	//装饰Task,相当于将任务转化为RunnableScheduledFuture
        RunnableScheduledFuture<?> t = decorateTask(command,
            new ScheduledFutureTask<Void>(command, null,
                                          triggerTime(delay, unit)));
        delayedExecute(t);
        return t;
    }


 private void delayedExecute(RunnableScheduledFuture<?> task) {
        //线程关闭不再处理任务
        if (isShutdown())
            reject(task);
        else {
	    //将任务加入到队列中
            super.getQueue().add(task);
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }

    //创建线程
    void ensurePrestart() {
        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize)
	    //以corePoolSize来决定是否创建线程
            addWorker(null, true);
        else if (wc == 0)
	    //以maximumPoolSize来决定是否创建线程
            addWorker(null, false);
    }


    //看线程执行方法
     public void run() {
            boolean periodic = isPeriodic();
            if (!canRunInCurrentRunState(periodic))
                cancel(false);
	    //不需要延迟直接执行
            else if (!periodic)
                ScheduledFutureTask.super.run();
	    //先执行
            else if (ScheduledFutureTask.super.runAndReset()) {
	        //设置下一次需要执行的时间
                setNextRunTime();
                reExecutePeriodic(outerTask);
            }
        }


 void reExecutePeriodic(RunnableScheduledFuture<?> task) {
        if (canRunInCurrentRunState(true)) {
            super.getQueue().add(task);
            if (!canRunInCurrentRunState(true) && remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }

//延迟执行Callable任务
public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                           long delay,
                                           TimeUnit unit) {
        if (callable == null || unit == null)
            throw new NullPointerException();
        RunnableScheduledFuture<V> t = decorateTask(callable,
            new ScheduledFutureTask<V>(callable,
                                       triggerTime(delay, unit)));
        delayedExecute(t);
        return t;
    }


//立即执行任务
public Future<?> submit(Runnable task) {
        return schedule(task, 0, TimeUnit.NANOSECONDS);
    }
//立即执行任务
public <T> Future<T> submit(Runnable task, T result) {
        return schedule(Executors.callable(task, result),
                        0, TimeUnit.NANOSECONDS);
    }
//立即执行任务
 public <T> Future<T> submit(Callable<T> task) {
        return schedule(task, 0, TimeUnit.NANOSECONDS);
    }


//每隔initialDelay+period后执行。下一次则是initialDelay+period*2以此类推
 public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (period <= 0)
            throw new IllegalArgumentException();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(period));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
    }


//相当于时间递减
 public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                     long initialDelay,
                                                     long delay,
                                                     TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (delay <= 0)
            throw new IllegalArgumentException();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(-delay));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
    }