Java线程池ThreadPoolExecutor

为什么使用线程池

如果每次使用线程都创建，每次创建和销毁的开销会很大，线程池主要用来解决线程生命周期开销问题和资源不足问题，也消除了线程创建所带来的延迟。

线程池分析

先看下构造函数，其他构造函数都是调用此构造函数

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {

}

参数分析
corePoolSize核心线程池的大小。
maximumPoolSize最大允许的线程池大小。
keepAliveTime，unit当线程池数大于corePoolSize时，当一个线程空闲时间超过此时间，将会结束。
workQueue阻塞队列，用于存储还没有执行的任务。
threadFactory用来创建线程。
handler拒绝处理策略，当线程池和队列都满了，有一下几种策略

ThreadPoolExecutor.AbortPolicy:丢弃任务并抛出RejectedExecutionException异常。
ThreadPoolExecutor.DiscardPolicy：也是丢弃任务，但是不抛出异常。
ThreadPoolExecutor.DiscardOldestPolicy：丢弃队列最前面的任务，然后重新尝试执行任务（重复此过程）
ThreadPoolExecutor.CallerRunsPolicy：由调用线程处理该任务

当向线程池提交任务时，将会执行execute()方法。

 public void execute(Runnable command) {
 //如果为null，抛出空指针异常
        if (command == null)
            throw new NullPointerException();

        int c = ctl.get();
        //当运行的线程小于核心池大小时
        if (workerCountOf(c) < corePoolSize) {
            //添加任务，失败返回false
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //线程池在运行态，尝试加入线程池
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //如果线程池不在运行态，将任务移除
            if (! isRunning(recheck) && remove(command))
            //移除任务失败，执行拒绝策略
                reject(command);
                //检查一下当前线程池的缓存队列数量是否为0
            else if (workerCountOf(recheck) == 0)
            //是否需要新加一个线程
                addWorker(null, false);
        }
        //再次添加任务，如果失败，就是线程池满了
        else if (!addWorker(command, false))
            reject(command);
    }

再来看看addWorker()方法，添加任务，并执行

 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 (;;) {
            //获取线程数
                int wc = workerCountOf(c);
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                    //线程池未过限,将线程池计数器扩增1，跳出标签
                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();
                            //向运行线程池内加上该线程
                        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)
            //如果任务启动失败，回滚前面的操作中断线程池或者中断当前线程
                addWorkerFailed(w);
        }
        return workerStarted;
    }

接下来看执行线程的方法runWorker()

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
        //循环获取任务和子任务
            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 {
                //扩展方法，提供子类实现
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                    //当前线程运行
                        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);
        }
    }

线程池的主要方法就学习到这。

其他参考

http://blog.youkuaiyun.com/zclzhangcl/article/details/50772434

http://blog.youkuaiyun.com/u013142781/article/details/51387749