Tell your girlfriend easily: the internal principle of thread pool

A restaurant appointment

The dinner plate is especially crystal white under the light , My girlfriend took a sip of red wine , Said to me, ：“ I often hear you say thread pool , What is the principle of thread pool ？” I was in a daze , Think about what happened to my girlfriend today , Why do you suddenly ask such a professional question , But as a professional, I can't be shy in front of my girlfriend , Think about it and say ：“ Let me tell you the story of my former colleague Lao Wang ！”

Old programmer Wang

Lao Wang is a programmer who has drifted North for more than ten years , Old age , I can't afford to work overtime , There is no hope of promotion , So I took some savings in my hand , Go back home and start a business . He chose the bath industry , Open a bath center , Yes , A formal bath center . When I was in Beijing , The bathhouse I like to go to is called “ Tsinghua pool ”, He thought about it. , Call your bath center “ Thread pool ”.

Thread pool bath center

After the thread pool is opened , Lao Wang found that some customers wanted to do foot therapy , So I recruited 1 A pedicure technician , An extra business added to the revenue . With the increase of foot therapy customers , In order to make more money, I hired again 4 A pedicure technician . After a while , The business of the bath center is getting better and better , More and more people are doing foot therapy . however , Lao Wang found that the foot therapists in his shop already had 5 A pedicure technician , There's too much to recruit , I can't afford to pay any more . What can a foot technician do if he is too busy ？ Lao Wang is a wise man , Think of a way ： Line up customers , Which pedicure technician has finished , Free time comes out , Just ask another customer in the team to continue .

A busy weekend

One to the weekend , There are several times more customers coming to the bath center than usual , Customers who want pedicure have to wait too long , The customers are impatient . Lao Wang immediately responded , Another urgent recruitment from other bath centers 5 A pedicure technician , Do foot therapy for customers in the team , Greatly reduce the number of customers waiting in line . however , Sometimes business is so hot , Urgent technicians are also using , Customers also have a long queue , And new customers , Lao Wang can only say to customers with a smile on his face ：“ Come back next time , Find a good technician for you next time .”, Shut the customer out . After the weekend , We can't keep idle people in the shop , Lao Wang dismissed all the technicians who had been recruited urgently .

Lao Wang's way of Management

Lao Wang's business is booming , We will open a branch soon 、 Financing listing 、 Reach the peak of life . Since it's so successful , Let's review his management ：

If you know Lao Wang's business philosophy , The thread pool is easy to understand , hold customer Replace with Mission , hold Pedicure Technician Replace with Threads , Thread pool bath center Namely Thread pool 了 , This is the internal principle of thread pool ：

Wake up in a dream

Bell , The alarm woke me up , It turned out to be a dream , I don't have any girlfriend ？ There is an interview this morning , Get up and wash , Just set out . Think about that strange dream on the way , Don't review the internal principle of a downline Chengchi any more ！ Have a look first ThreadPoolExecutor Class execute Method ：

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    // obtain clt,clt It records the status of thread pool and the number of running threads .
    int c = ctl.get();
    // When the number of running threads is less than the number of core threads , Create threads and put them in the thread pool , And run the current task .
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        // Failed to create thread , Recapture clt.
        c = ctl.get();
    }
    // When the thread pool is running and the number of running threads is greater than the number of core threads , Put the task in the queue .
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        // Recheck that the thread pool is not running ,
        // Remove the task from the queue , And through the rejection strategy to deal with the task .
        if (! isRunning(recheck) && remove(command))
            reject(command);
        // The current number of running threads is 0 when , Create threads to join the thread pool .
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // When the number of running threads is greater than the number of core threads and the queue is full ,
    // Create threads and put them in the thread pool , And run the current task .
    else if (!addWorker(command, false))
        // When the number of running threads is greater than the maximum number of threads , If it fails, it refuses the task 
        reject(command);
}

stay execute In the method , Called many times addWorker Method , Take another look at this method ：

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        // obtain clt,clt It records the status of thread pool and the number of running threads .
        int c = ctl.get();
        // Get the running state of thread pool .
        int rs = runStateOf(c);

        // Thread pool is closed , Or the current task is null
        // Or the queue is not empty , Then directly return to failure .
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            // Get the number of threads in the thread pool 
            int wc = workerCountOf(c);
            // The number of threads exceeds CAPACITY, Then return to false;
            // there core yes addWorker The second argument to the method ,
            // If true Then compare according to the number of core threads ,
            // If false Then compare according to the maximum number of threads .
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            // Try to increase the number of threads , If it works , I jump out of the first one for loop 
            if (compareAndIncrementWorkerCount(c))
                break retry;
            // If it fails to increase the number of threads , Then we can get back ctl
            c = ctl.get();
            // If the current running state is not equal to rs, Indicates that the state has been changed ,
            // Return to the first for Loop continuation 
            if (runStateOf(c) != rs)
                continue retry;
        }
    }

    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        // Create based on current task Worker object 
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // After getting the lock , Recheck thread pool status 
                int rs = runStateOf(ctl.get());

                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive())
                        throw new IllegalThreadStateException();
                    // Add the newly created thread to the thread pool 
                    workers.add(w);
                    int s = workers.size();
                    // Record the maximum number of threads in the thread pool 
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                // Start thread , Start running tasks 
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

interview

A middle-aged man sat in front of me , Said to me, ：“ Hello! , I'm the interviewer today .” I replied with a smile ：“ Hello! .” The interviewer asked me without expression ：“ Thread pool must have been used , Can you talk about the internal principle of thread pool ？” I almost laughed , Say... Confidently ……