Java线程池源码分析

@FunctionalInterface
public interface Runnable {
    public abstract void run();
}

@FunctionalInterface
public interface Callable<V> {
    V call() throws Exception;
}

public interface Future<V> {
   // 取消任务执行，mayInterruptIfRunning参数为true时将中断正在执行任务的线程，否则正在执行的任务将继续执行
    boolean cancel(boolean mayInterruptIfRunning);

   // 是否任务在执行完成前被取消
    boolean isCancelled();

   // 任务是否完成，不管任务正常结束、抛出异常还是被取消都认为任务完成
    boolean isDone();

   // 等待任务完成并获得结果
   // 计算被取消时抛出CalcellationException
   // 计算抛出异常时抛出ExecutionException
   // 当前线程等待时被中断抛出InteruptedException
    V get() throws InterruptedException, ExecutionException;

  // 超时版本的get，如果等待超时抛出TimeoutException
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

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

void shutdownAndAwaitTermination(ExecutorService pool) {
    pool.shutdown(); // Disable new tasks from being submitted
    try {
        // Wait a while for existing tasks to terminate
        if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
            pool.shutdownNow(); // Cancel currently executing tasks
            // Wait a while for tasks to respond to being cancelled
            if (!pool.awaitTermination(60, TimeUnit.SECONDS))
                System.err.println("Pool did not terminate");
        }
    } catch (InterruptedException ie) {
        // (Re-)Cancel if current thread also interrupted
        pool.shutdownNow();
        // Preserve interrupt status
        Thread.currentThread().interrupt();
    }
}

public interface RunnableFuture<V> extends Runnable, Future<V> {
    void run();
}

public abstract class AbstractExecutorService implements ExecutorService {

   // 将runnable包装成FutureTask
    protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {
        return new FutureTask<T>(runnable, value);
    }

  // 将Callable包装成FutureTask
    protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {
        return new FutureTask<T>(callable);
    }

   // 包装成FutureTask，并交给底层execute方法执行
    public Future<?> submit(Runnable task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<Void> ftask = newTaskFor(task, null);
        execute(ftask);
        return ftask;
    }

    public <T> Future<T> submit(Runnable task, T result) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task, result);
        execute(ftask);
        return ftask;
    }

    public <T> Future<T> submit(Callable<T> task) {
        if (task == null) throw new NullPointerException();
        RunnableFuture<T> ftask = newTaskFor(task);
        execute(ftask);
        return ftask;
    }

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

// runState is stored in the high-order bits
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;

// Packing and unpacking ctl
private static int runStateOf(int c)     { return c & ~CAPACITY; }
private static int workerCountOf(int c)  { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }

/*
 * Bit field accessors that don't require unpacking ctl.
 * These depend on the bit layout and on workerCount being never negative.
 */

private static boolean runStateLessThan(int c, int s) {
    return c < s;
}

private static boolean runStateAtLeast(int c, int s) {
    return c >= s;
}

private static boolean isRunning(int c) {
    return c < SHUTDOWN;
}

private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{

    /** Thread this worker is running in.  Null if factory fails. */
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

    /** Delegates main run loop to outer runWorker  */
    public void run() {
        runWorker(this);
    }

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
   // 如果当前线程数少于核心线程数，直接添加一个worker来执行任务，将当前任务作为它的第一个任务
   // addWorker调用会原子的检查runState和workerCount，避免错误添加新的线程
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
   // 如果线程池处于RUNNING状态，将这个任务添加到任务队列workQueue中
    if (isRunning(c) && workQueue.offer(command)) {
       // double-check
        int recheck = ctl.get();
       // 如果线程不处于RUNNING状态，移除已经入队的任务，并执行拒绝策略
        if (! isRunning(recheck) && remove(command))
            reject(command);
       // 如果线程池还是RUNNING状态，并且线程数为0，那么开启新的线程
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
   // 如果队列满了，尝试创建新的线程，如果已经达到最大线程数，执行拒绝策略
    else if (!addWorker(command, false))
        reject(command);
}

private void addWorkerFailed(Worker w) {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
       // 移除创建的worker
        if (w != null)
            workers.remove(w);
       // 减低worker计数
        decrementWorkerCount();
       // 如果当前worker阻碍线程池终止，重新检查
        tryTerminate();
    } finally {
        mainLock.unlock();
    }
}

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
      // 处理第一个任务
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
          // 循环获取任务
        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
              // 如果线程状态大于等于STOP，需要中断线程
              // 否则清理线程中断状态并且检查此时线程池状态是否大于等于STOP，如果是，则中断线程
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                  // 执行前操作
                beforeExecute(wt, task);
                try {
                    task.run();
                    afterExecute(task, null);
                } catch (Throwable ex) {
                      // 执行后操作
                    afterExecute(task, ex);
                    throw ex;
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
          // 执行线程关闭
          // 1. 说明getTask返回null，队列中没有任务需要执行了，执行关闭
          // 2. completdAbruptly
        processWorkerExit(w, completedAbruptly);
    }
}

private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

    for (;;) {
        int c = ctl.get();

        // Check if queue empty only if necessary.
          // 1. 线程池处于SHUTDOWN状态，并且workQueue为空
          // 2. 线程池状态>= STOP
        if (runStateAtLeast(c, SHUTDOWN)
            && (runStateAtLeast(c, STOP) || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        int wc = workerCountOf(c);

        // Are workers subject to culling?
          // 允许核心线程数内的线程回收，或者当前线程数超过了核心线程数，那么有可能发生超时关闭
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

          // 如果线程数量超过最大线程数量，比如开发者调用setMaximumPoolSize调小了线程池，那么多余的worker就需要被关闭
          // 如果允许超时，并且获取任务发生超时，则关闭线程
          // 在队列不为空时，不允许关闭最后一个线程
        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) {
              // 如果此 worker 发生了中断，采取的方案是重试
            // 解释下为什么会发生中断，这个读者要去看 setMaximumPoolSize 方法，中断空闲的worker
            timedOut = false;
        }
    }
}

public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>());
}

public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>());
}