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java基础——线程池源码分析

程序员文章站 2022-03-22 14:04:03
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序言

我们知道,线程池帮我们重复管理线程,避免创建大量的线程增加开销。
合理的使用线程池能够带来3个很明显的好处:
1.降低资源消耗:通过重用已经创建的线程来降低线程创建和销毁的消耗
2.提高响应速度:任务到达时不需要等待线程创建就可以立即执行。
3.提高线程的可管理性:线程池可以统一管理、分配、调优和监控。
java源生的线程池,实现于ThreadPoolExecutor类,这也是我们今天讨论的重点

ThreadPoolExecutor类

Jdk使用ThreadPoolExecutor类来创建线程池,我们来看看它的构造方法。

/**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @param maximumPoolSize the maximum number of threads to allow in the
     *        pool
     * @param keepAliveTime when the number of threads is greater than
     *        the core, this is the maximum time that excess idle threads
     *        will wait for new tasks before terminating.
     * @param unit the time unit for the {@code keepAliveTime} argument
     * @param workQueue the queue to use for holding tasks before they are
     *        executed.  This queue will hold only the {@code Runnable}
     *        tasks submitted by the {@code execute} method.
     * @param threadFactory the factory to use when the executor
     *        creates a new thread
     * @param handler the handler to use when execution is blocked
     *        because the thread bounds and queue capacities are reached
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }
  • int corePoolSize, //核心线程的数量

  • int maximumPoolSize, //最大线程数量

  • long keepAliveTime, //超出核心线程数量以外的线程空闲时,线程存活的时间

  • TimeUnit unit, //存活时间的单位

  • BlockingQueue<Runnable> workQueue, //保存待执行任务的队列

  • ThreadFactory threadFactory, //创建新线程使用的工厂

  • RejectedExecutionHandler handler // 当任务无法执行时的处理器(线程拒绝策略)

核心类变量

ctl变量

ThreadPoolExecutor中有一个控制状态的属性叫ctl,它是一个AtomicInteger类型的变量,它一个int值可以储存两个概念的信息:

  • workerCount:表明当前池中有效的线程数,通过workerCountOf方法获得,workerCount上限是(2^29)-1。(最后存放在ctl的低29bit)

  • runState:表明当前线程池的状态,通过workerCountOf方法获得,最后存放在ctl的高3bit中,他们是整个线程池的运行生命周期,有如下取值,分别的含义是:

  1. RUNNING:可以新加线程,同时可以处理queue中的线程。

  2. SHUTDOWN:不增加新线程,但是处理queue中的线程。

  3. STOP 不增加新线程,同时不处理queue中的线程。

  4. TIDYING 所有的线程都终止了(queue中),同时workerCount为0,那么此时进入TIDYING

  5. TERMINATED terminated()方法结束,变为TERMINATED

    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;

    // Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }

COUNT_BITS=32(integer的size)-3=29,于是五种状态左移29位分别是:

  • RUNNING: 11100000000000000000000000000000

  • SHUTDOWN: 00000000000000000000000000000000

  • STOP: 00100000000000000000000000000000

  • TIDYING: 01000000000000000000000000000000

  • TERMINATED:01100000000000000000000000000000

而ThreadPoolExecutor是通过runStateOf和workerCountOf获得者两个概念的值的。

runStateOf和workerCountOf方法是如何剥离出ctl变量的两个有效值呢?这其中我们可以看到CAPACITY是实现一个字段存两个值的最重要的字段。

CAPACITY变量

CAPACITY=(1 << COUNT_BITS) – 1 转成二进制为:000 11111111111111111111111111111,他是线程池理论上可以允许的最大的线程数。
所以很明显,它的重点在于,其高3bit为0,低29bit为1;
这样,workderCountOf方法中,CAPACITY和ctl进行&运算时,它能获得高3位都是0,低29位和ctl低29位相同的值,这个值就是workerCount
同理,runStateOf方法,CAPACITY的取反和ctl进行&操作,获得高3位和ctl高三位相等,低29位都为0的值,这个值就是runState

workQueue

/**
     * The queue used for holding tasks and handing off to worker
     * threads.  We do not require that workQueue.poll() returning
     * null necessarily means that workQueue.isEmpty(), so rely
     * solely on isEmpty to see if the queue is empty (which we must
     * do for example when deciding whether to transition from
     * SHUTDOWN to TIDYING).  This accommodates special-purpose
     * queues such as DelayQueues for which poll() is allowed to
     * return null even if it may later return non-null when delays
     * expire.
     */
    private final BlockingQueue<Runnable> workQueue;

一个BlockingQueue<Runnable>队列,本身的结构可以保证访问的线程安全(这里不展开了)。这是一个排队等待队列。当我们线程池里线程达到corePoolSize的时候,一些需要等待执行的线程就放在这个队列里等待。

workers

/**
     * Set containing all worker threads in pool. Accessed only when
     * holding mainLock.
     */
    private final HashSet<Worker> workers = new HashSet<Worker>();

一个HashSet<Worker>的集合。线程池里所有可以立即执行的线程都放在这个集合里。这也是我们直观理解的线程的池子

核心内部类

Worker

Worker类是线程池中具化一个线程的对象,是线程池的核心,我们来看看源码:

/**
     * Class Worker mainly maintains interrupt control state for
     * threads running tasks, along with other minor bookkeeping.
     * This class opportunistically extends AbstractQueuedSynchronizer
     * to simplify acquiring and releasing a lock surrounding each
     * task execution.  This protects against interrupts that are
     * intended to wake up a worker thread waiting for a task from
     * instead interrupting a task being run.  We implement a simple
     * non-reentrant mutual exclusion lock rather than use
     * ReentrantLock because we do not want worker tasks to be able to
     * reacquire the lock when they invoke pool control methods like
     * setCorePoolSize.  Additionally, to suppress interrupts until
     * the thread actually starts running tasks, we initialize lock
     * state to a negative value, and clear it upon start (in
     * runWorker).
     */
    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) {
            //设置AQS的同步状态private volatile int state,是一个计数器,大于0代表锁已经被获取
            // 在调用runWorker()前,禁止interrupt中断,在interruptIfStarted()方法中会判断 getState()>=0
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);//根据当前worker创建一个线程对象
            //当前worker本身就是一个runnable任务,也就是不会用参数的firstTask创建线程,而是调用当前worker.run()时调用firstTask.run()
            //后面在addworker中,我们会启动worker对象中组合的Thread,而我们的执行逻辑runWorker方法是在worker的run方法中被调用。
            //为什么执行thread的run方法会调用worker的run方法呢,原因就是在这里进行了注入,将worker本身this注入到了thread中
        }
        /** Delegates main run loop to outer runWorker  */
        public void run() {
            runWorker(this);
        }//runWorker()是ThreadPoolExecutor的方法

        // Lock methods
        //
        // The value 0 represents the unlocked state. 0代表“没被锁定”状态
        // The value 1 represents the locked state. 1代表“锁定”状态
        protected boolean isHeldExclusively() {
            return getState() != 0;
        }
        /**
         * 尝试获取锁
         * 重写AQS的tryAcquire(),AQS本来就是让子类来实现的
         */
        protected boolean tryAcquire(int unused) {
            //尝试一次将state从0设置为1,即“锁定”状态,但由于每次都是state 0->1,而不是+1,那么说明不可重入
            //且state==-1时也不会获取到锁
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }
        /**
         * 尝试释放锁
         * 不是state-1,而是置为0
         */
        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(); }
        /**
         * 中断(如果运行)
         * shutdownNow时会循环对worker线程执行
         * 且不需要获取worker锁,即使在worker运行时也可以中断
         */
        void interruptIfStarted() {
            Thread t;
            //如果state>=0、t!=null、且t没有被中断
            //new Worker()时state==-1,说明不能中断
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }

我们看worker类时,会发现最重要的几个部分在于它里面定义了一个Thread thread和Runnable firstTask。看到这里,我们可能会比较奇怪,我们只是要一个可以执行的线程,这里放一个Thread和一个Runnable的变量做什么呢?
其实之所以Worker自己实现Runnable,并创建Thread,在firstTask外包一层,是因为要通过Worker负责控制中断,而firstTask这个工作任务只是负责执行业务,worker的run方法调用了runWorker方法,在这里面,worker里的firstTask的run方法被执行。稍后我们会聚焦这个执行任务的runWorker方法。

核心方法

好了,基本上我们将线程池的几个主角,ctl,workQueue,workers,Worker简单介绍了一遍,现在,我们来看看线程池是怎么玩的。

execute方法

这是线程池实现类外露供给外部实现提交线程任务command的核心方法,对于无需了解线程池内部的使用者来说,这个方法就是把某个任务交给线程池,正常情况下,这个任务会在未来某个时刻被执行,实现和注释如下:

    /**
     * Executes the given task sometime in the future.  The task
     * may execute in a new thread or in an existing pooled thread.
     * * 在未来的某个时刻执行给定的任务。这个任务用一个新线程执行,或者用一个线程池中已经存在的线程执行
     *
     * If the task cannot be submitted for execution, either because this
     * executor has been shutdown or because its capacity has been reached,
     * the task is handled by the current {@code RejectedExecutionHandler}.
     * 如果任务无法被提交执行,要么是因为这个Executor已经被shutdown关闭,要么是已经达到其容量上限,任务会被当前的RejectedExecutionHandler处理
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     *         {@code RejectedExecutionHandler}, if the task
     *         cannot be accepted for execution
     * @throws NullPointerException if {@code command} is null
     */
    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.
         * 如果运行的线程少于corePoolSize,尝试开启一个新线程去运行command,command作为这个线程的第一个任务
         *
         * 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.
         *  如果任务成功放入队列,我们仍需要一个双重校验去确认是否应该新建一个线程(因为可能存在有些线程在我们上次检查后死了)
         *  或者 从我们进入这个方法后,pool被关闭了
         *  所以我们需要再次检查state,如果线程池停止了需要回滚入队列,如果池中没有线程了,新开启 一个线程
         *
         * 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.
         * 如果无法将任务入队列(可能队列满了),需要新开区一个线程(自己:往maxPoolSize发展)
        * 如果失败了,说明线程池shutdown 或者 饱和了,所以我们拒绝任务
         */
        int c = ctl.get();
        // 1、如果当前线程数少于corePoolSize(可能是由于addWorker()操作已经包含对线程池状态的判断,如此处没加,而入workQueue前加了)
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;

            /**
             * 没有成功addWorker(),再次获取c(凡是需要再次用ctl做判断时,都会再次调用ctl.get())
             * 失败的原因可能是:
             * 1、线程池已经shutdown,shutdown的线程池不再接收新任务
             * 2、workerCountOf(c) < corePoolSize 判断后,由于并发,别的线程先创建了worker线程,导致workerCount>=corePoolSize
             */
            c = ctl.get();
        }
        /**
         * 2、如果线程池RUNNING状态,且入队列成功
         */
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();

            /**
             * 再次校验放入workerQueue中的任务是否能被执行
             * 1、如果线程池不是运行状态了,应该拒绝添加新任务,从workQueue中删除任务
             * 2、如果线程池是运行状态,或者从workQueue中删除任务失败(刚好有一个线程执行完毕,并消耗了这个任务),
             * 确保还有线程执行任务(只要有一个就够了)
             */
            //如果再次校验过程中,线程池不是RUNNING状态,并且remove(command)--workQueue.remove()成功,拒绝当前command
            if (! isRunning(recheck) && remove(command))
                reject(command);

            //如果当前worker数量为0,通过addWorker(null, false)创建一个线程,其任务为null
            //为什么只检查运行的worker数量是不是0呢?? 为什么不和corePoolSize比较呢??
            //只保证有一个worker线程可以从queue中获取任务执行就行了??
            //因为只要还有活动的worker线程,就可以消费workerQueue中的任务
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);//第一个参数为null,说明只为新建一个worker线程,没有指定firstTask
                                       ////第二个参数为true代表占用corePoolSize,false占用maxPoolSize
        }
        /**
         * 3、如果线程池不是running状态 或者 无法入队列
         *   尝试开启新线程,扩容至maxPoolSize,如果addWork(command, false)失败了,拒绝当前command
         */
        else if (!addWorker(command, false))
            reject(command);
    }

我们可以简单归纳如下(注:图来源见水印,谢谢大神的归纳):
java基础——线程池源码分析

addWorker

在execute方法中,我们看到核心的逻辑是由addWorker方法来实现的,当我们将一个任务提交给线程池,线程池会如何处理,就是主要由这个方法加以规范:

java基础——线程池源码分析

该方法有两个参数:

  1. firstTask: worker线程的初始任务,可以为空

  2. core: true:将corePoolSize作为上限,false:将maximumPoolSize作为上限

排列组合,addWorker方法有4种传参的方式:

1、addWorker(command, true)
2、addWorker(command, false)
3、addWorker(null, false)
4、addWorker(null, true)

在execute方法中就使用了前3种,结合这个核心方法进行以下分析

第一个:线程数小于corePoolSize时,放一个需要处理的task进Workers Set。如果Workers Set长度超过corePoolSize,就返回false
第二个:当队列被放满时,就尝试将这个新来的task直接放入Workers Set,而此时Workers Set的长度限制是maximumPoolSize。如果线程池也满了的话就返回false
第三个:放入一个空的task进workers Set,长度限制是maximumPoolSize。这样一个task为空的worker在线程执行的时候会去任务队列里拿任务,这样就相当于创建了一个新的线程,只是没有马上分配任务
第四个:这个方法就是放一个null的task进Workers Set,而且是在小于corePoolSize时,如果此时Set中的数量已经达到corePoolSize那就返回false,什么也不干。实际使用中是在prestartAllCoreThreads()方法,这个方法用来为线程池预先启动corePoolSize个worker等待从workQueue中获取任务执行
    /**
     * Checks if a new worker can be added with respect to current
     * pool state and the given bound (either core or maximum). If so,
     * the worker count is adjusted accordingly, and, if possible, a
     * new worker is created and started, running firstTask as its
     * first task. This method returns false if the pool is stopped or
     * eligible to shut down. It also returns false if the thread
     * factory fails to create a thread when asked.  If the thread
     * creation fails, either due to the thread factory returning
     * null, or due to an exception (typically OutOfMemoryError in
     * Thread.start()), we roll back cleanly.
     * 检查根据当前线程池的状态和给定的边界(core or maximum)是否可以创建一个新的worker
     * 如果是这样的话,worker的数量做相应的调整,如果可能的话,创建一个新的worker并启动,参数中的firstTask作为worker的第一个任务
     * 如果方法返回false,可能因为pool已经关闭或者调用过了shutdown
     * 如果线程工厂创建线程失败,也会失败,返回false
     * 如果线程创建失败,要么是因为线程工厂返回null,要么是发生了OutOfMemoryError
     *
     * @param firstTask the task the new thread should run first (or
     * null if none). Workers are created with an initial first task
     * (in method execute()) to bypass queuing when there are fewer
     * than corePoolSize threads (in which case we always start one),
     * or when the queue is full (in which case we must bypass queue).
     * Initially idle threads are usually created via
     * prestartCoreThread or to replace other dying workers.
     *
     * @param core if true use corePoolSize as bound, else
     * maximumPoolSize. (A boolean indicator is used here rather than a
     * value to ensure reads of fresh values after checking other pool
     * state).
     * @return true if successful
     */
    private boolean addWorker(Runnable firstTask, boolean core) {
        //外层循环,负责判断线程池状态
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            /**
             * 线程池的state越小越是运行状态,runnbale=-1,shutdown=0,stop=1,tidying=2,terminated=3
             * 1、如果线程池state已经至少是shutdown状态了
             * 2、并且以下3个条件任意一个是false
             *   rs == SHUTDOWN         (隐含:rs>=SHUTDOWN)false情况: 线程池状态已经超过shutdown,
             *                               可能是stop、tidying、terminated其中一个,即线程池已经终止
             *   firstTask == null      (隐含:rs==SHUTDOWN)false情况: firstTask不为空,rs==SHUTDOWN 且 firstTask不为空,
             *                               return false,场景是在线程池已经shutdown后,还要添加新的任务,拒绝
             *   ! workQueue.isEmpty()  (隐含:rs==SHUTDOWN,firstTask==null)false情况: workQueue为空,
             *                               当firstTask为空时是为了创建一个没有任务的线程,再从workQueue中获取任务,
             *                               如果workQueue已经为空,那么就没有添加新worker线程的必要了
             * return false,即无法addWorker()
             */
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;
            //内层循环,负责worker数量+1
            for (;;) {
                int wc = workerCountOf(c);
                //入参core在这里起作用,表示加入的worker是加入corePool还是非corepool,换句话说,受到哪个size的约束
                //如果worker数量>线程池最大上限CAPACITY(即使用int低29位可以容纳的最大值)
                //或者( worker数量>corePoolSize 或  worker数量>maximumPoolSize ),即已经超过了给定的边界,不添加worker
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //CAS尝试增加线程数,,如果成功加了wc,那么break跳出检查
                //如果失败,证明有竞争,那么重新到retry。
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                //如果不成功,重新获取状态继续检查
                c = ctl.get();  // Re-read ctl
                //如果状态不等于之前获取的state,跳出内层循环,继续去外层循环判断
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
                // else CAS失败时因为workerCount改变了,继续内层循环尝试CAS对worker数量+1
            }
        }
         //worker数量+1成功的后续操作
         // 添加到workers Set集合,并启动worker线程
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            //新建worker//构造方法做了三件事//1、设置worker这个AQS锁的同步状态state=-1
            w = new Worker(firstTask);  //2、将firstTask设置给worker的成员变量firstTask
                                        //3、使用worker自身这个runnable,调用ThreadFactory创建一个线程,并设置给worker的成员变量thread
            final Thread t = w.thread;
            if (t != null) {
                //获取重入锁,并且锁上
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    int rs = runStateOf(ctl.get());
                     // rs!=SHUTDOWN ||firstTask!=null
                     // 如果线程池在运行running<shutdown 或者
                     // 线程池已经shutdown,且firstTask==null(可能是workQueue中仍有未执行完成的任务,创建没有初始任务的worker线程执行)
                     // worker数量-1的操作在addWorkerFailed()
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // // precheck that t is startable   线程已经启动,抛非法线程状态异常
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        //设置最大的池大小largestPoolSize,workerAdded设置为true
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {//如果往HashSet中添加worker成功,启动线程
                    //通过t.start()方法正式执行线程。在这里一个线程才算是真正的执行起来了。
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            //如果启动线程失败
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

同样的,我们可以归纳一下:
java基础——线程池源码分析

runWorker方法

在addWorker方法中,我们将一个新增进去的worker所组合的线程属性thread启动了,但我们知道,在worker的构造方法中,它将自己本身注入到了thread的target属性里,所以绕了一圈,线程启动后,调用的还是worker的run方法,而在这里面,runWorker定义了线程执行的逻辑:

/**
     * Main worker run loop.  Repeatedly gets tasks from queue and
     * executes them, while coping with a number of issues:
     *
     * 1. We may start out with an initial task, in which case we
     * don't need to get the first one. Otherwise, as long as pool is
     * running, we get tasks from getTask. If it returns null then the
     * worker exits due to changed pool state or configuration
     * parameters.  Other exits result from exception throws in
     * external code, in which case completedAbruptly holds, which
     * usually leads processWorkerExit to replace this thread.
     * 我们可能使用一个初始化任务开始,即firstTask为null
     * 然后只要线程池在运行,我们就从getTask()获取任务
     * 如果getTask()返回null,则worker由于改变了线程池状态或参数配置而退出
     * 其它退出因为外部代码抛异常了,这会使得completedAbruptly为true,这会导致在processWorkerExit()方法中替换当前线程
     *
     * 2. Before running any task, the lock is acquired to prevent
     * other pool interrupts while the task is executing, and then we
     * ensure that unless pool is stopping, this thread does not have
     * its interrupt set.
     * 在任何任务执行之前,都需要对worker加锁去防止在任务运行时,其它的线程池中断操作
     * clearInterruptsForTaskRun保证除非线程池正在stoping,线程不会被设置中断标示
     *
     * 3. Each task run is preceded by a call to beforeExecute, which
     * might throw an exception, in which case we cause thread to die
     * (breaking loop with completedAbruptly true) without processing
     * the task.
     * 每个任务执行前会调用beforeExecute(),其中可能抛出一个异常,这种情况下会导致线程die(跳出循环,且completedAbruptly==true),没有执行任务
     * 因为beforeExecute()的异常没有cache住,会上抛,跳出循环
     *
     * 4. Assuming beforeExecute completes normally, we run the task,
     * gathering any of its thrown exceptions to send to afterExecute.
     * We separately handle RuntimeException, Error (both of which the
     * specs guarantee that we trap) and arbitrary Throwables.
     * Because we cannot rethrow Throwables within Runnable.run, we
     * wrap them within Errors on the way out (to the thread's
     * UncaughtExceptionHandler).  Any thrown exception also
     * conservatively causes thread to die.
     *
     * 5. After task.run completes, we call afterExecute, which may
     * also throw an exception, which will also cause thread to
     * die. According to JLS Sec 14.20, this exception is the one that
     * will be in effect even if task.run throws.
     *
     * The net effect of the exception mechanics is that afterExecute
     * and the thread's UncaughtExceptionHandler have as accurate
     * information as we can provide about any problems encountered by
     * user code.
     *
     * @param w the worker
     */
    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        //标识线程是不是异常终止的
        boolean completedAbruptly = true;
        try {
            //task不为null情况是初始化worker时,如果task为null,则去队列中取线程--->getTask()
            //可以看到,只要getTask方法被调用且返回null,那么worker必定被销毁,而确定一个线程是否应该被销毁的逻辑,在getTask方法中
            while (task != null || (task = getTask()) != null) {
                w.lock();
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    wt.interrupt();
                try {
                    //线程开始执行之前执行此方法,可以实现Worker未执行退出,本类中未实现
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        task.run();//runWorker方法最本质的存在意义,就是调用task的run方法
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        //线程执行后执行,可以实现标识Worker异常中断的功能,本类中未实现
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;//运行过的task标null
                    w.completedTasks++;
                    w.unlock();
                }
            }
            //标识线程不是异常终止的,是因为不满足while条件,*销毁的
            completedAbruptly = false;
        } finally {
            //处理worker退出的逻辑
            processWorkerExit(w, completedAbruptly);
        }
    }

我们归纳:
java基础——线程池源码分析

getTask方法

runWorker方法中的getTask()方法是线程处理完一个任务后,从队列中获取新任务的实现,也是处理判断一个线程是否应该被销毁的逻辑所在:

    /**
     * Performs blocking or timed wait for a task, depending on
     * current configuration settings, or returns null if this worker
     * must exit because of any of:  以下情况会返回null
     * 1. There are more than maximumPoolSize workers (due to
     *    a call to setMaximumPoolSize).
     *    超过了maximumPoolSize设置的线程数量(因为调用了setMaximumPoolSize())
     * 2. The pool is stopped.
     *    线程池被stop
     * 3. The pool is shutdown and the queue is empty.
     *    线程池被shutdown,并且workQueue空了
     * 4. This worker timed out waiting for a task, and timed-out
     *    workers are subject to termination (that is,
     *    {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
     *    both before and after the timed wait.
     *    线程等待任务超时
     *
     * @return task, or null if the worker must exit, in which case
     *         workerCount is decremented
     *         返回null表示这个worker要结束了,这种情况下workerCount-1
     */
    private Runnable getTask() {
        // timedOut 主要是判断后面的poll是否要超时
        boolean timedOut = false; // Did the last poll() time out?

        /**
         * 用于判断线程池状态
         */
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            /**
             * 对线程池状态的判断,两种情况会workerCount-1,并且返回null
             * 线程池状态为shutdown,且workQueue为空(反映了shutdown状态的线程池还是要执行workQueue中剩余的任务的)
             * 线程池状态为>=stop()(只有TIDYING和TERMINATED会大于stop)(shutdownNow()会导致变成STOP)(此时不用考虑workQueue的情况)
             */
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();//循环的CAS减少worker数量,直到成功
                return null;
            }

            int wc = workerCountOf(c);

            // Are workers subject to culling?

            //allowCoreThreadTimeOut字段,表示是否允许核心线程超过闲置时间后被摧毁,默认为false
            //我们前面说过,如果getTask方法返回null,那么这个worker只有被销毁一途
            //于是这个timed有3种情况
            //(1)当没有超过核心线程,且默认allowCoreThreadTimeOut为false时
            //          timed值为false,除非目前线程数大于最大值,否则下面的if始终进不去,该方法不可能返回null,worker也就不可能被销毁
            //(2)当超过核心线程数,且默认allowCoreThreadTimeOut为false时//timed值为true,
            //(3)如果allowCoreThreadTimeOut为true,则timed始终为true
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            //wc > maximumPoolSize则必销毁,因为wc>1也肯定满足
            //wc <= maximumPoolSize,如果(timed && timedOut) = true 一般情况下也意味着worker要被销毁,因为超时一般是由阻塞队列为空造成的
            
            //一般情况是这样,那不一般的情况呢?阻塞队列没有为空,但是因为一些原因,还是超时了,这时候取决于wc > 1,它为真就销毁,为假就不销毁。
            // 也就是说,如果阻塞队列还有任务,但是wc=1,线程池里只剩下自己这个线程了,那么就不能销毁,这个if不满足,我们的代码继续往下走
            
            //当核心线程数<线程数<最大线程数,或者允许核心线程超时时,
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                //如果timed为true那么使用poll取线程。否则使用take()
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    //workQueue.poll():如果在keepAliveTime时间内,阻塞队列还是没有任务,返回null
                    workQueue.take();
                    //workQueue.take():如果阻塞队列为空,当前线程会被挂起等待;当队列中有任务加入时,线程被唤醒,take方法返回任务
                //如果正常返回,那么返回取到的task。
                if (r != null)
                    return r;
                //否则,设为超时,重新执行循环,
                timedOut = true;
            } catch (InterruptedException retry) {
            //在阻塞从workQueue中获取任务时,可以被interrupt()中断,代码中捕获了InterruptedException,重置timedOut为初始值false,再次执行第1步中的判断,满足就继续获取任务,不满足return null,会进入worker退出的流程
                timedOut = false;
            }
        }

归纳:

java基础——线程池源码分析

processWorkerExit方法

在runWorker方法中,我们看到当不满足while条件后,线程池会执行退出线程的操作,这个操作,就封装在processWorkerExit方法中。

/**
 * Performs cleanup and bookkeeping for a dying worker. Called
 * only from worker threads. Unless completedAbruptly is set,
 * assumes that workerCount has already been adjusted to account
 * for exit.  This method removes thread from worker set, and
 * possibly terminates the pool or replaces the worker if either
 * it exited due to user task exception or if fewer than
 * corePoolSize workers are running or queue is non-empty but
 * there are no workers.
 *
 * @param w the worker
 * @param completedAbruptly if the worker died due to user exception
 */
private void processWorkerExit(Worker w, boolean completedAbruptly) {
    //参数:
        //worker:                      要结束的worker
        //completedAbruptly: 是否突然完成(是否因为异常退出)
        
    /**
     * 1、worker数量-1
     * 如果是突然终止,说明是task执行时异常情况导致,即run()方法执行时发生了异常,那么正在工作的worker线程数量需要-1
     * 如果不是突然终止,说明是worker线程没有task可执行了,不用-1,因为已经在getTask()方法中-1了
     */
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted 代码和注释正好相反啊
        decrementWorkerCount();
 
    /**
     * 2、从Workers Set中移除worker
     */
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks; //把worker的完成任务数加到线程池的完成任务数
        workers.remove(w); //从HashSet<Worker>中移除
    } finally {
        mainLock.unlock();
    }
 
    /**
     * 3、在对线程池有负效益的操作时,都需要“尝试终止”线程池
     * 主要是判断线程池是否满足终止的状态
     * 如果状态满足,但线程池还有线程,尝试对其发出中断响应,使其能进入退出流程
     * 没有线程了,更新状态为tidying->terminated
     */
    tryTerminate();
 
    /**
     * 4、是否需要增加worker线程
     * 线程池状态是running 或 shutdown
     * 如果当前线程是突然终止的,addWorker()
     * 如果当前线程不是突然终止的,但当前线程数量 < 要维护的线程数量,addWorker()
     * 故如果调用线程池shutdown(),直到workQueue为空前,线程池都会维持corePoolSize个线程,然后再逐渐销毁这corePoolSize个线程
     */
    int c = ctl.get();
    //如果状态是running、shutdown,即tryTerminate()没有成功终止线程池,尝试再添加一个worker
    if (runStateLessThan(c, STOP)) {
        //不是突然完成的,即没有task任务可以获取而完成的,计算min,并根据当前worker数量判断是否需要addWorker()
        if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize; //allowCoreThreadTimeOut默认为false,即min默认为corePoolSize
             
            //如果min为0,即不需要维持核心线程数量,且workQueue不为空,至少保持一个线程
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
             
            //如果线程数量大于最少数量,直接返回,否则下面至少要addWorker一个
            if (workerCountOf(c) >= min)
                return; // replacement not needed
        }
         
        //添加一个没有firstTask的worker
        //只要worker是completedAbruptly突然终止的,或者线程数量小于要维护的数量,就新添一个worker线程,即使是shutdown状态
        addWorker(null, false);
    }
}

总而言之:如果线程池还没有完全终止,就仍需要保持一定数量的线程。

线程池状态是running 或 shutdown的情况下:

A、如果当前线程是突然终止的,addWorker()
B、如果当前线程不是突然终止的,但当前线程数量 < 要维护的线程数量,addWorker()
故如果调用线程池shutdown(),直到workQueue为空前,线程池都会维持corePoolSize个线程,然后再逐渐销毁这corePoolSize个线程

submit方法

前面我们讲过execute方法,其作用是将一个任务提交给线程池,以期在未来的某个时间点被执行。
submit方法在作用上,和execute方法是一样的,将某个任务提交给线程池,让线程池调度线程去执行它。
那么它和execute方法有什么区别呢?我们来看看submit方法的源码:
submit方法的实现在ThreadPoolExecutor的父类AbstractExecutorService类中,有三种重载方法:

    /**
     * 提交一个 Runnable 任务用于执行,并返回一个表示该任务的 Future。该Future的get方法在成功完成时将会返回null。
     * submit 参数: task - 要提交的任务 返回:表示任务等待完成的 Future
     * @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;
    }

    /**
     * 提交一个Runnable 任务用于执行,并返回一个表示该任务的 Future。该 Future 的 get 方法在成功完成时将会返回给定的结果。
     * submit 参数: task - 要提交的任务 result - 完成任务时要求返回的结果 
     * 返回: 表示任务等待完成的 Future
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    /**
     * 提交一个Callable的任务用于执行,返回一个表示任务的未决结果的 Future。该 Future 的 get 
方法在成功完成时将会返回该任务的结果。 
     * 如果想立即阻塞任务的等待,则可以使用 result = 
exec.submit(aCallable).get(); 形式的构造。
     * 参数: task - 要提交的任务 返回: 表示任务等待完成的Future
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     */
    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

源码很简单,submit方法,将任务task封装成FutureTask(newTaskFor方法中就是new了一个FutureTask),然后调用execute。所以submit方法和execute的所有区别,都在这FutureTask所带来的差异化实现上

总而言之,submit方法将一个任务task用future模式封装成FutureTask对象,提交给线程执行,并将这个FutureTask对象返回,以供主线程该任务被线程池执行之后得到执行结果

注意,获得执行结果的方法FutureTask.get(),会阻塞执行该方法的线程。

未完待续

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