@Bean
public ExecutorService emailTaskPool() {
    return new ThreadPoolExecutor(2, 4,
            0L, TimeUnit.MILLISECONDS,
            new LinkedBlockingQueue<Runnable>(), new SysThreadFactory("email-task"));
}

@Override
public void sendEmail(EmailDTO emailDTO) {
    emailTaskPool.submit(() -> {
        try {
            System.out.printf("sending email .... emailDto is %s \n", emailDTO);
            Thread.sleep(1000);
            System.out.println("sended success");
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    });
}

/**
 * @throws RejectedExecutionException {@inheritDoc}
 * @throws NullPointerException       {@inheritDoc}
 */
public Future<?> submit(Runnable task) {
    if (task == null) throw new NullPointerException();
    RunnableFuture<Void> ftask = newTaskFor(task, null);
    execute(ftask);
    return ftask;
}

Future future = emailTaskPool.submit(() -> {
          try {
              System.out.printf("sending email .... emailDto is %s \n", emailDTO);
              Thread.sleep(1000);
              System.out.println("sended success");
          } catch (InterruptedException e) {
              e.printStackTrace();
          }
      });
      //todo something
      future.get();
}

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        int c = ctl.get();
        //工作线程数小于核心线程的时候，可以填写worker线程
        if (workerCountOf(c) < corePoolSize) {
              //新增工作线程的时候会加锁
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //如果线程池的状态正常，切任务放入就绪队列正常
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                //如果当前线程池处于关闭状态，则抛出拒绝异常
                reject(command);
            //如果工作线程数超过了核心线程数，那么就需要考虑新增工作线程
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        //如果新增的工作线程已经达到了最大线程数限制的条件下，需要触发拒绝策略的抛出
        else if (!addWorker(command, false))
            reject(command);
    }

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);
        //确保当前线程池没有进入到一个销毁状态中
        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;
        for (;;) {
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
              // 如果传入的core属性是false，则这里需要比对maximumPoolSize参数
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
                //通过cas操作去增加线程池的工作线程数亩
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }
    }
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
       //真正需要指定的任务是firstTask，它会被注入到worker对象当中
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
        //加入了锁
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
                int rs = runStateOf(ctl.get());
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    //workers是一个hashset集合，会往里面新增工作线程    
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                //worker本身是一个线程，但是worker对象内部还有一个线程的参数，
                //这个t才是真正的任务内容
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        //如果worker线程创建好了，但是内部的真正任务还没有启动，此时突然整个
        //线程池的状态被关闭了，那么这时候workerStarted就会为false，然后将
        //工作线程的数目做自减调整。
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}