通俗易懂的JUC源码剖析-ThreadPoolExecutor

前言

ThreadPoolExecutor相信大家都很熟悉：线程池的实现类。今天我们就来看看它内部是怎么实现的。

实现原理

先来看看它的类结构：

public class ThreadPoolExecutor extends AbstractExecutorService {
}

public abstract class AbstractExecutorService implements ExecutorService {
}

public interface ExecutorService extends Executor {
    void shutdown();
     Future submit(Callable task);
    // ...
}

public interface Executor {
     void execute(Runnable command);
}

再来看看它的关键属性：

// ctl高3位表示线程池的运行状态，低29位表示线程个数
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
// 线程个数掩码，Integer位数-3，与具体平台Integer位数有关，大部分是32-3=29
private static final int COUNT_BITS = Integer.SIZE - 3;
// 线程最大个数
private static final int CAPACITY = (1 << COUNT_BITS) - 1;
// runState is stored in the high-order bits
// 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;
// 互斥锁
private final ReentrantLock mainLock = new ReentrantLock();
// 工作线程集合
private final HashSet workers = new HashSet();
// 线程池终止条件
private final Condition termination = mainLock.newCondition();
// 线程池核心参数
// 阻塞队列
private final BlockingQueue workQueue;
// 线程工厂
private volatile ThreadFactory threadFactory;
// 拒绝策略
private volatile RejectedExecutionHandler handler;
// 线程闲置时长
private volatile long keepAliveTime;
// 核心线程数
private volatile int corePoolSize;
// 最大线程数
private volatile int maximumPoolSize;
// 默认拒绝策略：AbortPolicy抛出异常
private static final RejectedExecutionHandler defaultHandler =
    new AbortPolicy();

其中Worker是它的内部类，代表工作线程。

private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable {
}

它继承了AQS，实现了Runnable接口

再来看关键方法：
execute()

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    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);
    }
    else if (!addWorker(command, false))
        reject(command);
}

其中addWorker()方法如下：

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 (;;) {
            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) {
             t.start();
             workerStarted = true;
          }
       }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

我滴个乖乖，addWorker()这么复杂，明天再来吧。
晚安全世界！