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自定义线程池原理(ThreadPoolExecutor)

深夜瞎琢磨 2021-09-24 阅读 75
ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 10, 3, TimeUnit.SECONDS, new LinkedBlockingQueue<>());
executor.execute(thread1);

一. 对ThreadPoolExecutor进行初始化的时候主要做了以下几件事情:

1.参数Executors.defaultThreadFactory():初始化默认线程工厂,此线程工厂实现ThreadFactory接口(里面维护了一个newThread()方法,在executor调用execute方法里面对Worker这个内部类做了初始化,然后通过调用TreadFactory的newThread()方法创建新线程并赋值给Worker里维护的Thread成员变量)。
2.对ThreadPoolExecutor中的一些new出来的成员变量进行初始化,包括:

1)、ctl(记录线程池状态(前三位)和正在工作的线程数量(后29位))

2)、mainLock(主锁,在对工作集合内容进行添加删除时会掉用主锁来锁住这个过程)

3)、workers(工作线程的容器,又叫线程池,里面维护了多个worker对象,worker实现了Runnable接口本身可以作为线程使用,worker里面维护了业务任务、一个由DefaultThreadFactory创建并赋值的线程以及一个记录每个线程任务数量的long变量)

4)、termination(mianLock.newCondition()出来的Condition对象,未具体探究)

  1. 根据构造方法里的参数值对ThreadPoolExecutor里的变量进行赋值
问题小结:
用new创建并初始化对象的步骤:
    1)给对象的实例变量(非常量)分配内存空间,默认初始化成员变量。
    2)成员变量声明时的初始化;
    3)初始化块初始化(又称为构造代码块和非静态代码块);
    4) 构造方法的初始化;

二. 执行execute() 方法里面做了下面的事:
1.当前工作线程数小于核心线程数,则向线程池(set集合)添加用于处理任务的线程。

   if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }

2.工作线程数大于核心线程数、线程池的工作状态是Running状态(线程池接受新任务并处理队列的任务),然后向队列添加任务成功,添加任务成功后再次获取线程池的状态值(以为在此期间,线程池的状态有可能发生变化),判断状态是否是Running,如果是则删除队列里的任务,执行拒绝策略,否则在Running状态下往线程池里添加一个线程用来处理任务队列里的任务。

  if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }

3.当工作线程数大于核心线程数并且向任务队列添加任务失败前提下,增加线程池的数量(corePool++),如果增加失败则执行拒绝策略。

   else if (!addWorker(command, false))
        reject(command);

addWorker()方法

   private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // 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 ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            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 {
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            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.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

主要做了两件事:一个是对线程池当前状态下处理任务的线程数量的递增(CAS操作);
一个是在线程池当前状态下,向线程池添加处理任务的线程,在添加成功的前提下,启动Worker里的线程来处理Worker里的任务(加了ReentrantLock的锁操作)。

需要注意的是这种处理结构:

Worker实现了Runnable接口,本身是一个线程(这个线程就是处理任务的线程),而具体的任务赋值给Worker内部的变量Runnable,由Worker内部维护的Thread(ThreadPoolExecutor初始化时,为这里的Thread变量赋值)来处理。

   private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

    /** Thread this worker is running in.  Null if factory fails. */
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

    /** Delegates main run loop to outer runWorker  */
    public void run() {
        runWorker(this);
    }

    // Lock methods
    //
    // The value 0 represents the unlocked state.
    // The value 1 represents the locked state.

    protected boolean isHeldExclusively() {
        return getState() != 0;
    }

    protected boolean tryAcquire(int unused) {
        if (compareAndSetState(0, 1)) {
            setExclusiveOwnerThread(Thread.currentThread());
            return true;
        }
        return false;
    }

    protected boolean tryRelease(int unused) {
        setExclusiveOwnerThread(null);
        setState(0);
        return true;
    }

    public void lock()        { acquire(1); }
    public boolean tryLock()  { return tryAcquire(1); }
    public void unlock()      { release(1); }
    public boolean isLocked() { return isHeldExclusively(); }

    void interruptIfStarted() {
        Thread t;
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {
                t.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    }
}

Worker类中的runWorker(this)方法:

  final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
         //如果前面的firstTask有值,就直接执行这个任务
        //如果没有具体的任务,就执行getTask()方法从队列中获取任务
        //这里会不断执行循环体,除非线程中断或者getTask()返回null才会跳出这个循环
        while (task != null || (task = getTask()) != null) {
            w.lock();
            // If pool is stopping, ensure thread is interrupted;
            // if not, ensure thread is not interrupted.  This
            // requires a recheck in second case to deal with
            // shutdownNow race while clearing interrupt
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    task.run();
                } catch (RuntimeException x) {
                    thrown = x; throw x;
                } catch (Error x) {
                    thrown = x; throw x;
                } catch (Throwable x) {
                    thrown = x; throw new Error(x);
                } finally {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

runWorker()方法做了响应中断的处理:

  w.unlock(); // allow interrupts
    boolean completedAbruptly = true;

其中getTask()方法(CAS操作)这里面涉及到keepAliveTime的使用,从这个方法我们可以看出先吃池是怎么让超过corePoolSize的那部分worker销毁的。

  private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        int wc = workerCountOf(c);

        // 如果设置了核心worker也会超时或者当前正在运行的worker数量超过了corePoolSize,就要根据时间判断是否要销毁线程了
        //其实就是从队列获取任务的时候要不要设置超时间时间,如果超过这个时间队列还没有任务进来,就会返回null
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            //poll(long timeout, TimeUnit unit):从BlockingQueue取出一个队首的对象,如果在指定时间内,队列一旦有数据可取,则立即返回队列中的数据。否则直到时间超时还没有数据可取,返回失败。
            // take():取走BlockingQueue里排在首位的对象,若BlockingQueue为空,阻断进入等待状态直到BlockingQueue有新的数据被加入;
                    Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

问题小结:

1.短路与(&&)的使用,若前面为false则后面不执行;短路或(||)的使用,若前面为true则后面不执行。
2.线程池的五个状态:(负数在java里-用1表示,绝对值的补码表示具体值)

Running (-1<<29 )

111 00000000000000000000000000000
接受新任务并处理队列里的任务

ShutDown (0<<29)

000 00000000000000000000000000000
不接受新任务但处理队列里的任务

Stop (1<<29)

001 00000000000000000000000000000
不接受新任务,不处理队列里的任务,中断正在处理的任务

Tidying (2<<29)

010 00000000000000000000000000000
所有任务都已结束,workCount为0,线程过渡到TIDYING状态,将会执行terminated()钩子方法

Terminated (3<<29)

011 00000000000000000000000000000
terminated()完成

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