多线程编程 —— 线程池源码解析

之前写过一篇线程池，可以大致了解关于线程池的概念以及运行过程。 Java 线程池

线程池常量

// 标记线程池昨天，默认RUNNING
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 线程个数
private static final int COUNT_BITS = Integer.SIZE - 3;
// 线程最大个数
private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

// 线程池状态
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;

ThreadPoolExecutor#execute

execute（Runnable command） 方法是提交任务到线程池执行的。

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    // 获取当前线程池的状态+线程个数遍历组合值
    int c = ctl.get();
    // 当前线程池小于核心线程池，开启新线程执行
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 如果线程池处于RUNNING，将任务添加到队列中
    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);
    }
    // 如果队列满了，创建非核心线程执行任务，如果失败，执行拒绝策略
    else if (!addWorker(command, false))
        reject(command);
}

这块代码的逻辑就是线程池如何处理新任务的逻辑。

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

        // 检测队列是否为空
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;
        // 循环CAS 增加线程数
        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) {
                // 调用 w.run，实际调用 runWorker方法
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

addWorker() 方法中参数 core 表明当前任务是绑定核心线程还是非核心线程。

非核心线程池回收

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);

        // Are workers subject to culling?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
        // 判断当前活跃线程数是否大于最大线程数 || 允许超时 || 上次超时获取到的线程为null，接着判断 如果 wc >1 或队列为空，则将线程数减1，并且返回一个null的task
        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

当 timed = true 时，会执行 workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) 方法。而 timed 的判断是在 boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;，allowCoreThreadTimeOut 默认 false，所以看 wc > corePoolSize 条件了，wc 是活跃线程数，表达式的意思是判断活跃线程数>核心线程数。

如果当前活跃线程数大于核心线程数，给定 keepAliveTime 时间等待获取队列任务，如果获取不到，返回null；否则直接从队列中获取任务。

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task,在这里调用 getTask 方法，获取任务，如果获取不到任务，则执行 processWorkerExit 方法处理线程退出
        while (task != null || (task = getTask()) != null) {
            w.lock();
            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 {
        // 当获取不到任务的时候，执行
        processWorkerExit(w, completedAbruptly);
    }
}

// 当线程获取不到 tasks，就调用 processWorkerExit 方法，处理线程退出
private void processWorkerExit(Worker w, boolean completedAbruptly) {
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
        decrementWorkerCount();

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks;
        // 执行remove操作，线程回收
        workers.remove(w);
    } finally {
        mainLock.unlock();
    }

    tryTerminate();

    int c = ctl.get();
    if (runStateLessThan(c, STOP)) {
        if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            if (workerCountOf(c) >= min)
                return; // replacement not needed
        }
        addWorker(null, false);
    }
}

对于线程池中的线程，是没有核心线程和非核心线程的区别，只有当前工作线程。线程池对于非核心线程的回收，只是判断当前工作线程是否大于核心线程。所以，当工作线程大于核心线程数之后，所以的线程都是 poll，也就是说，所以线程都可能被回收。

总结：在 runWorker 执行任务时，会循环从队列中获取任务（getTask），当活跃线程大于核心线程时，就会设置过期超时时间获取队列任务，否则会直接获取队列任务。当获取不到任务时，就会执行线程退出任务，这时候，就会把多余的线程删除。

参考

• 线程池源码分析
• 线程池非核心线程回收