Read ScheduledThreadPoolExecutor source code

//A timed task that can be delayed to execute
//Look at the constructor first The most important difference in the constructor is that a delayed queue is passed in to DelayedWorkQueue

/**
Summary: The thread pool inherits ThreadPoolExecutor. At the same time, it encapsulates a DelayedWorkQueue and
ScheduledFutureTask. DelayedWorkQueue is used to manage the addition and acquisition of tasks. ScheduledFutureTask
Used to manage the execution of tasks.
*/

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);
    }

//create and execute task
 public void execute(Runnable command) {
        schedule(command, 0, TimeUnit.NANOSECONDS);
    }


//Delayed execution of Runnable tasks
public ScheduledFuture<?> schedule(Runnable command,
                                       long delay,
                                       TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
	// Decorate the Task, which is equivalent to converting the task into a RunnableScheduledFuture
        RunnableScheduledFuture<?> t = decorateTask(command,
            new ScheduledFutureTask<Void>(command, null,
                                          triggerTime(delay, unit)));
        delayedExecute(t);
        return t;
    }


 private void delayedExecute(RunnableScheduledFuture<?> task) {
        //The thread is closed and no longer processing tasks
        if (isShutdown())
            reject(task);
        else {
	    // add the task to the queue
            super.getQueue().add(task);
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }
    
    //create thread
    void ensurePrestart() {
        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize)
	    //Use corePoolSize to decide whether to create a thread
            addWorker(null, true);
        else if (wc == 0)
	    //Use maximumPoolSize to decide whether to create a thread
            addWorker(null, false);
    }


    //Look at the thread execution method
     public void run() {
            boolean periodic = isPeriodic();
            if (!canRunInCurrentRunState(periodic))
                cancel(false);
	    // execute directly without delay
            else if (!periodic)
                ScheduledFutureTask.super.run();
	    // execute first
            else if (ScheduledFutureTask.super.runAndReset()) {
	        //Set the next time to execute
                setNextRunTime();
                reExecutePeriodic(outerTask);
            }
        }


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

//Delayed execution of Callable task
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;
    }


// execute the task immediately
public Future<?> submit(Runnable task) {
        return schedule(task, 0, TimeUnit.NANOSECONDS);
    }
// execute the task immediately
public <T> Future<T> submit(Runnable task, T result) {
        return schedule(Executors.callable(task, result),
                        0, TimeUnit.NANOSECONDS);
    }
// execute the task immediately
 public <T> Future<T> submit(Callable<T> task) {
        return schedule(task, 0, TimeUnit.NANOSECONDS);
    }


//Execute after every initialDelay+period. The next time is initialDelay+period*2 and so on
 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;
    }


//equivalent to time decrement
 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;
    }