从源码学习线程池的使用原理及核心思想解析

public ThreadPoolExecutor(int corePoolSize,//主线程数
						int maximumPoolSize, //最大线程数
						long keepAliveTime, //线程存活时间（除主线程外，其他的线程在没有任务执行的时候需要回收，多久后回收）
						TimeUnit unit, //存活时间的时间单位
						BlockingQueue<Runnable>workQueue//阻塞队列，我们需要执行的task都在该队列) {
	this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
	}

    public ThreadPoolExecutor(int corePoolSize,//主线程数
                              int maximumPoolSize,//最大线程数
                              long keepAliveTime,//线程存活时间（除主线程外，其他的线程在没有任务执行的时候需要回收，多久后回收）
                              TimeUnit unit,//存活时间的时间单位
                              BlockingQueue<Runnable> workQueue,//阻塞队列，我们需要执行的task都在该队列
                              ThreadFactory threadFactory,//生成thread的工厂
                              RejectedExecutionHandler handler//拒绝饱和策略，当队列满了并且线程个数达到maximunPoolSize后采取的策略) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            //对数值传递不合理及最大线程数小于主线程数的情况做异常处理
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
        //这里能看出来这三个参数不能传null或不传
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ? null :AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

    public void execute(Runnable command) {
        if (command == null)//如果要执行的任务是空的，异常
            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();//这里是啥见下文解释
        //高三位代表线程池的状态，低29位代表线程池中的线程数量 
        //如果当前线程数小于主线程数，添加线程
        if (workerCountOf(c) < corePoolSize) {//workerCountOf见下文
            if (addWorker(command, true))//addWorker才是添加线程的方法
                return;
            c = ctl.get();
        }
        //如果超过主线程数，将任务添加至workqueue 阻塞队列
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //如果线程池关闭，移除并拒绝
            if (! isRunning(recheck) && remove(command))
                reject(command);
                //否则如果当前线程池线程空，则添加一个线程
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        //阻塞队列已满，添加失败，采用拒绝策略
        else if (!addWorker(command, false))
            reject(command);
    }

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));

private static int workerCountOf(int c)  { 
	return c & CAPACITY; //CAPACITY 与(&) 上面的ctl
}

    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;
	
	//Integer类的一个常量
    @Native public static final int SIZE = 32;

    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:  //这是goto语句  下面我会写一个demo讲解这玩意
        for (;;) {//大自旋检查线程池的状态。阻塞队列是否为空等判断
            int c = ctl.get();
            int rs = runStateOf(c);//线程池运行状态

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;//线程池被关或者任务为null（无任务可执行）
			
            for (;;) {//小自旋
                int wc = workerCountOf(c);
                //如果现有线程数大于最大值，或者大于等于最大线程数（主线程数）返回false
                //意味着线程都是满的
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                    //cas添加线程  线程+1
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                //如果失败了，继续自旋外层循环判断
                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 {
        //开启一个线程，Worker实现了runnable接口
            w = new Worker(firstTask);//这里点进去看worker的run方法
            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();
                        //添加至wokers
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                //添加成功
                if (workerAdded) {
                    t.start();//启动线程，会调用我们线程的run接口，也就是我们worker的run
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

	retry:
	for (int i = 0; i < 3; i++) {
		for (int j = 3; j < 10; j++) {
			//if (j == 4) {
			// break retry; 
			//跳出外面循环 // 
		}
		if(j == 7){
		 	continue retry;
		  	//继续外面循环 
		}
		  System.out.println(i+":"+j); 
		} 
	}

        public void run() {
            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 {
        	//只要一直能获取到task，就一直会执行，不会关闭，所以线程也不会销毁，线程销毁只有当task为null
            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 {
                	//调用线程执行之前方法，这里可以重写，放一些我们自己的业务前置逻辑
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                    	//调用task的run方法，这里就是去执行我们的业务逻辑了
                        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 {
                    	//调用线程执行之后方法，这里可以重写，放一些我们自己的业务后置逻辑
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?
		//自旋获取
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);//获取线程池状态

            // Check if queue empty only if necessary.必要时检 查空，状态是否停止或者shutdown
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;//线程池状态异常或无任务返回null
            }
			//获取线程数量
            int wc = workerCountOf(c);

            // Are workers subject to culling?
            //线程数大于主线程数时，或者allowCoreThreadTimeOut参数为 true, allowCoreThreadTimeOut默认为false
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
			//超过最大线程，或者timed为true && （wc大于1个，并且任务队列为空）的时候
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                //线程数-1，并且返回null，该线程结束
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
            //线程复用的关键就在这里了↓↓↓↓↓↓↓↓↓↓↓↓
            	//如果timed是true，超过时间不阻塞，不然一直阻塞，不回收
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) : //移除并返回队列头部的元素，如果为空，超过时间返回null
                    workQueue.take();//移除并返回队列头部的元素，如果为空,一直阻塞
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

        threadPoolExecutor.allowCoreThreadTimeOut(true);

        //阻塞队列已满，添加失败，采用拒绝策略
        else if (!addWorker(command, false))
            reject(command);

    final void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }

        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) {
            throw new RejectedExecutionException("Task " + r.toString() + //就是这个错误
                                                 " rejected from " +
                                                 e.toString());
        }
    }

public class ExecJavaTemplate implements RejectedExecutionHandler { 
	@Override 
	public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
		//在这里根据自己的业务写一个暂存的逻辑，然后塞回去即可
		//比较懒的话暂存策略也不用写，不断地塞回去就行，慢就慢点，别让它报错即可
		System.out.println("进入拒绝策略"); 
		executor.execute(r); //再次调用execute
	} 
}