ThreadPoolExecutor 源码

构造方法

// 线程池5种状态
private static final int RUNNING    = -1 << 29; // 接收任务
private static final int SHUTDOWN   =  0 << 29; // 不接收新任务，执行已有
private static final int STOP       =  1 << 29; // 中断所有任务
private static final int TIDYING    =  2 << 29; // 所有线程结束，准备terminated
private static final int TERMINATED =  3 << 29; // 彻底关闭

// 工具方法：拆分ctl
private static int runStateOf(int c)     { return c & ~CAPACITY; } // 获取状态
private static int workerCountOf(int c)  { return c & CAPACITY; } // 获取当前工作线程数
private static int ctlOf(int rs, int wc) { return rs | wc; }      // 合并状态+线程数

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    // 参数合法性校验
    if (corePoolSize < 0 ||
        maximumPoolSize <= 0 ||
        maximumPoolSize < corePoolSize ||
        keepAliveTime < 0)
        throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    // 1. 核心状态控制变量 ctl = 高3位运行状态 + 低29位线程数量
    this.ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    this.workQueue = workQueue;
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.threadFactory = threadFactory;
    this.handler = handler;
    // 单位转换为纳秒
    this.keepAliveTime = unit.toNanos(keepAliveTime);
}

execute

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    // ========== 分支1：当前线程数 < 核心线程数，新建核心线程 ==========
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get(); // 添加失败，重新获取ctl
    }
    // ========== 分支2：线程池运行中，任务放入阻塞队列 ==========
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        // 双重校验：放入队列后线程池关闭了，移除任务执行拒绝策略
        if (! isRunning(recheck) && remove(command))
            reject(command);
        // 队列有任务，但没有工作线程，补一个空worker
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
        return;
    }
    // ========== 分支3：队列满了，创建非核心线程；失败执行拒绝策略 ==========
    if (!addWorker(command, false))
        reject(command);
}

addWorker

core=true：新建核心线程，上限 corePoolSize

core=false：新建普通线程，上限 maximumPoolSize

新建 Worker 后调用 thread.start()，会进入 Worker 的 run 方法

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

        // 状态校验：SHUTDOWN及以上不允许新建线程
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            // 线程数超限判断
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            // CAS增加工作线程计数，成功跳出外层循环
            if (ctl.compareAndSet(c, ctlOf(rs, wc + 1)))
                break retry;
            c = ctl.get();
            if (runStateOf(c) != rs)
                continue retry;
        }
    }

    // CAS计数成功，创建Worker
    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;
            // 上锁的原因
            // 保护非线程安全的 HashSet workers，防止并发增删错乱
            // 保证 添加 Worker + 更新最大线程峰值 largestPoolSize 两个操作原子执行
            // 和 shutdown、processWorkerExit、监控查询共用一把锁，实现 workers 读写全局互斥
            mainLock.lock();
            try {
                int rs = runStateOf(ctl.get());
                // 再次校验状态
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive())
                        throw new IllegalThreadStateException();
                    workers.add(w); // workers是HashSet存储所有工作线程
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                t.start(); // 启动线程，执行Worker.run()
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w); // 创建失败，回滚线程计数
    }
    return workerStarted;
}

Worker

private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable 
{
    final Thread thread;
    Runnable firstTask;
    volatile long completedTasks;

    Worker(Runnable firstTask) {
        setState(-1); // 初始禁止中断，run前不响应中断
        this.firstTask = firstTask;
        // 通过线程工厂创建线程，传入自身Worker作为Runnable
        this.thread = getThreadFactory().newThread(this);
    }

    // t.start() 最终执行这里
    public void run() {
        runWorker(this);
    }
}

runWorker

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // 允许中断
    boolean completedAbruptly = true;
    try {
        // 循环：有任务就执行，getTask()阻塞获取队列任务
        while (task != null || (task = getTask()) != null) {
            w.lock();
            // 线程池STOP状态，强制中断线程
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task); // 钩子方法
                Throwable thrown = null;
                try {
                    task.run(); // 执行用户任务（main里的lambda）
                } 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); // 钩子方法
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        // getTask返回null，线程退出，执行回收
        processWorkerExit(w, completedAbruptly);
    }
}