• 线程池ThreadPoolExecutor


    1、线程池的处理流程(execute方法)
    当向线程池提交一个任务后,其经历的流程如下:
    1)、如果当前线程数小于核心线程数(corePoolSize),则创建新线程来执行该任务;
    2)、如果当前线程数不小于,即等于或大于核心线程数(corePoolSize),则将任务添加到阻塞队列(BlockingQueue)中;
    3)、如果阻塞队列中的任务已满,且此时线程数小于最大线程数(maximumPoolSize)时,则创建新线程来执行该任务;
    4)、执行对应的任务策略,一般是拒绝任务,抛出异常。
    
    2、任务策略:
    1)、抛出异常
        ThreadPoolExecutor.AbortPolicy()
    2)、丢弃当前的任务
        ThreadPoolExecutor.DiscardPolicy()
    3)、丢弃老的任务
        ThreadPoolExecutor.DiscardOldestPolicy()
    4)、重试添加当前的任务
        ThreadPoolExecutor.CallerRunsPolicy()
    
    3、线程池源码分析
    1)、若干变量
        
        //将工作线程数和线程池状态放在一个int类型变量中存储而设置的一个原子类型的变量
        //故在ctl中,低29位是用于表示工作线程数,高位用于表示线程池状态,如RUNNING、SHUTDOWN等。
        //故一个线程池中最多有工作线程的个数为(2^29) - 1
        private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
        //低29位
        private static final int COUNT_BITS = Integer.SIZE - 3;
        //线程池中最大的工作线程数
        private static final int CAPACITY   = (1 << COUNT_BITS) - 1;
    
        // runState is stored in the high-order bits
        //线程池状态,用高3位表示
        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
        //获取当前线程池的状态
        private static int runStateOf(int c)     { return c & ~CAPACITY; }
        //获取当前线程池中的工作线程数
        private static int workerCountOf(int c)  { return c & CAPACITY; }
        //组合当前线程池状态和工作线程数为一个int类型的变量
        private static int ctlOf(int rs, int wc) { return rs | wc; }
    
    
    2)、execute()方法
    
    
    public void execute(Runnable command) {
            //当提交的任务为null时,则抛出空指针异常
            if (command == null)
                throw new NullPointerException();
            //获取当前线程池用于记录状态和工作线程数的变量
            int c = ctl.get();
            if (workerCountOf(c) < corePoolSize) {
                //检测当前线程池中的工作线程数小于核心线程数时,则直接创建新线程,执行任务
                if (addWorker(command, true))
                    return;
                //当创建新线程失败时,需要重新获取用于记录状态和工作线程数的变量
                c = ctl.get();
            }
            if (isRunning(c) && workQueue.offer(command)) {
                 //当前线程池是运行状态,且将任务添加到阻塞队列中成功时
                //再次获取用于记录状态和工作线程数的变量
                int recheck = ctl.get();
                if (! isRunning(recheck) && remove(command))
                    //当前线程池不是运行状态,且删除成功时,使用任务策略
                    reject(command);
                else if (workerCountOf(recheck) == 0)
                    //当前工作线程数为0时,直接添加空任务
                    addWorker(null, false);
            }
            else if (!addWorker(command, false))
                //阻塞队列已满且当前工作线程数小于最大线程数时,则直接创建线程,执行任务
                //若还失败,则直接使用任务策略
                reject(command);
        }
    
    
     private boolean addWorker(Runnable firstTask, boolean core) {
            retry:
            for (;;) {
                int c = ctl.get();
                //获取当前线程池的状态
                int rs = runStateOf(c);
    
                //检测当前线程池是否处于关闭状态
                // Check if queue empty only if necessary.
                if (rs >= SHUTDOWN &&
                    ! (rs == SHUTDOWN &&
                       firstTask == null &&
                       ! workQueue.isEmpty()))
                    return false;
    
                for (;;) {
                    //获取当前线程池的工作线程数
                    int wc = workerCountOf(c);
                    //如果超过了限制,则返回false
                    if (wc >= CAPACITY ||
                        wc >= (core ? corePoolSize : maximumPoolSize))
                        return false;
                    //通过CAS增加一个工作线程
                    if (compareAndIncrementWorkerCount(c))
                        break retry;
                    //再次获取用于标记线程池状态和记录工作线程数的变量,并比对当前状态是否一直,若不是,则继续外环循环,否则继续内环循环
                    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 {
                final ReentrantLock mainLock = this.mainLock;
                //新建一个工作线程
                w = new Worker(firstTask);
                final Thread t = w.thread;
                if (t != null) {
                    mainLock.lock();
                    //加锁
                    try {
                        // Recheck while holding lock.
                        // Back out on ThreadFactory failure or if
                        // shut down before lock acquired.
                        int c = ctl.get();
                        int rs = runStateOf(c);
    
                        if (rs < SHUTDOWN ||
                            (rs == SHUTDOWN && firstTask == null)) {
                            if (t.isAlive()) // precheck that t is startable
                                throw new IllegalThreadStateException();
                            //将工作线程添加到线程集合Set中
                            workers.add(w);
                            int s = workers.size();
                            if (s > largestPoolSize)
                                largestPoolSize = s;
                            workerAdded = true;
                        }
                    } finally {
                        mainLock.unlock();
                    }
                    if (workerAdded) {
                        //工作线程开始启动,执行提交的任务
                        t.start();
                        workerStarted = true;
                    }
                }
            } finally {
                if (! workerStarted)
                    addWorkerFailed(w);
            }
            return workerStarted;
        }
    
        //工作线程的构造方法
            Worker(Runnable firstTask) {
                setState(-1); // inhibit interrupts until runWorker
                this.firstTask = firstTask;
                this.thread = getThreadFactory().newThread(this);
            }
        //线程执行体
            /** Delegates main run loop to outer runWorker  */
            public void run() {
                //调用父类的runWorker方法
                runWorker(this);
            }
    
    
        final void runWorker(Worker w) {
            Thread wt = Thread.currentThread();
            Runnable task = w.firstTask;
            w.firstTask = null;
            w.unlock(); // allow interrupts
            boolean completedAbruptly = true;
            try {
                //不断的从任务队列中获取任务,并执行
                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
                    //线程是否中断关闭
                    if ((runStateAtLeast(ctl.get(), STOP) ||
                         (Thread.interrupted() &&
                          runStateAtLeast(ctl.get(), STOP))) &&
                        !wt.isInterrupted())
                        wt.interrupt();
                    try {
                        //任务执行前的执行方法
                        beforeExecute(wt, task);
                        Throwable thrown = null;
                        try {
                            //执行任务
                            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 {
                            //任务执行或的执行方法
                            afterExecute(task, thrown);
                        }
                    } finally {
                        task = null;
                        w.completedTasks++;
                        w.unlock();
                    }
                }
                completedAbruptly = false;
            } finally {
                processWorkerExit(w, completedAbruptly);
            }
        }
    
    //调用该方法后,该线程池不会再接受新任务,当已经存在的任务执行完毕后,线程池就会关闭
    void shutdown()
    
    //调用该方法后,该线程池会尝试关闭现有的线程,直到所有的线程都关闭,线程池就会关闭
    List<Runnable> shutdownNow()
    
    4、常用的线程池
    1)、固定大小线程的线程池 newFixedThreadPool
    2)、单一线程的线程池,当线程发生异常结束时,则会另外创建一个新的线程,以保持线程池自始至终只有一个线程 newSingleThreadExecutor
    3)、无限制线程数的线程池,当空闲线程超过空闲时间时(默认1分钟),线程会被回收 newCachedThreadPool
    
    
    
    5、阻塞队列
    
     //往队列中添加元素,成功返回true,失败抛出异常
     boolean add(E e)
     
     //往队列中添加元素,成功返回true,失败返回false 
     boolean offer(E e)
     
     //往队列中添加元素,在指定的时间内若是添加不了,则返回false,否则返回true
     boolean offer(E e, long timeout, TimeUnit unit)
    
     //有阻塞的添加元素,即肯定能将元素添加到队列中,但是可能一直被阻塞
     void put(E e) throws InterruptedException
    
     //获取队列中的首元素,没有返回null
     E poll()
    
     //获取队列中的首元素,在指定的时间内若是获取不到,则返回null
     E poll(long timeout, TimeUnit unit) 
    
     //获取队列中的首元素,当队列中没有元素时,则一直阻塞,直到有元素时,才返回首元素
     E take()
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  • 原文地址:https://www.cnblogs.com/xiaoxian1369/p/5403885.html
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