FutureTask源码解析

FutureTask是Future的实现类，它实现了RunnableFuture接口，而RunnableFutrue又继承了Runnable和Future接口。

FutureTask可以拆分为Future和Task，Future对应Future接口的相关功能，Task对应Runnabl接口相关功能。因此，可以把FutureTask当做是一个任务，一个简单的Runnable对象。

类图

FutureTask的类图如下所示：

FutureTask实现了RunnableFuture接口

public class FutureTask<V> implements RunnableFuture<V> 

RunnableFuture接口只是继承了Runnable和Future接口，如下：

/**
 * A {@link Future} that is {@link Runnable}. Successful execution of
 * the {@code run} method causes completion of the {@code Future}
 * and allows access to its results.
 * @see FutureTask
 * @see Executor
 * @since 1.6
 * @author Doug Lea
 * @param <V> The result type returned by this Future's {@code get} method
 */
public interface RunnableFuture<V> extends Runnable, Future<V> {
    /**
     * Sets this Future to the result of its computation
     * unless it has been cancelled.
     */
    void run();
}

主要属性

/**
     * The run state of this task, initially NEW.  The run state
     * transitions to a terminal state only in methods set,
     * setException, and cancel.  During completion, state may take on
     * transient values of COMPLETING (while outcome is being set) or
     * INTERRUPTING (only while interrupting the runner to satisfy a
     * cancel(true)). Transitions from these intermediate to final
     * states use cheaper ordered/lazy writes because values are unique
     * and cannot be further modified.
     *
     * Possible state transitions:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    // 当前任务的状态，初始化为NEW，状态只会在set，setException和cancel方法中被设置
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

    /** The underlying callable; nulled out after running */
	// 运行的任务，完成之后被设置为null
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    // 包装返回结果或者异常，没有被volatile修饰，状态保护读写安全
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    // 运行的线程
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    // 单链表，是一个线程的栈的结构
    private volatile WaitNode waiters;

主要方法

有参构造函数-指定运行任务

/**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Callable}.
     *
     * @param  callable the callable task
     * @throws NullPointerException if the callable is null
     */
    public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

任务不能为null，状态设置为NEW

有参构造函数-指定任务和成功返回结果

传入要执行的任务和任务执行成功后的结果。将Runnable对象封装为Callable对象。任务执行成功，或者成功结束后，返回传入的result，如果不需要返回结果的话，可以直接传入null

/**
     * Creates a {@code FutureTask} that will, upon running, execute the
     * given {@code Runnable}, and arrange that {@code get} will return the
     * given result on successful completion.
     *
     * @param runnable the runnable task
     * @param result the result to return on successful completion. If
     * you don't need a particular result, consider using
     * constructions of the form:
     * {@code Future<?> f = new FutureTask<Void>(runnable, null)}
     * @throws NullPointerException if the runnable is null
     */
    public FutureTask(Runnable runnable, V result) {
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }

isCanceled

任务是否取消。状态如果大于等于CANCELLED，就表示已经取消

public boolean isCancelled() {
        return state >= CANCELLED;
    }

isDone

任务状态不为NEW，就表示已经完成

public boolean isDone() {
        return state != NEW;
    }

cancel

取消当前正在执行的任务。

如果当前任务状态不为NEW，或者设置状态INTERRUPTING（mayInterruptIfRunning = true）或CANCELLED（mayInterruptIfRunning=false）失败，返回false

如果mayInterruptIfRunning设置为true，那么就中断当前线程，在finally中会设置线程的状态为INTERRUPTED

public boolean cancel(boolean mayInterruptIfRunning) {
        if (!(state == NEW &&
              UNSAFE.compareAndSwapInt(this, stateOffset, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
                }
            }
        } finally {
            finishCompletion();
        }
        return true;
    }

finishCompletion

/**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
    // 移除并激活所有的等待线程，调用done方法，并设置callable为false
    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        // 激活当前线程
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }

        done();

        callable = null;        // to reduce footprint
    }

done

done方法在FutureTask中是一个空实现，使用FutureTask的时候可以重写这个方法

/**
     * Protected method invoked when this task transitions to state
     * {@code isDone} (whether normally or via cancellation). The
     * default implementation does nothing.  Subclasses may override
     * this method to invoke completion callbacks or perform
     * bookkeeping. Note that you can query status inside the
     * implementation of this method to determine whether this task
     * has been cancelled.
     */
    protected void done() { }

get

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        // 如果当前任务还没有完成，等待.没有超时时间，如果任务一直不完成，一直等待
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
    	// 
        return report(s);
    }

awaitDone

等待任务完成或者放弃或者中断或者超时。

/**
     * Awaits completion or aborts on interrupt or timeout.
     *
     * @param timed true if use timed waits
     * @param nanos time to wait, if timed
     * @return state upon completion
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            // 如果当前线程已经被中断了，从waiter中移除，并抛出中断异常,如果q为null的话，方法直接返回
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }

            int s = state;
            // 再次检查，此时任务可能已经完成
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            // 如果任务状态为COMPLETING，可以参考上面的任务状态解释，让出cpu
            // 表示任务已经结束但是任务执行线程还没来得及给outcome赋值
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                // q == null，说明是来等待任务完成的，需要新建一个节点
                q = new WaitNode();
            else if (!queued)
                // 节点新建完成后，在下一个循环将原等待队列挂在到当前节点之后
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {    // 如果设置了超时
                nanos = deadline - System.nanoTime();
                // 如果已经超时
                if (nanos <= 0L) {
                    // 从等待队列中移除
                    removeWaiter(q);
                    // 返回当前状态
                    return state;
                }
                // 阻塞当前线程nanos纳秒
                LockSupport.parkNanos(this, nanos);
            }
            else
                // 无限期阻塞当前线程
                LockSupport.park(this);
        }
    }

removeWaiter

/**
     * Tries to unlink a timed-out or interrupted wait node to avoid
     * accumulating garbage.  Internal nodes are simply unspliced
     * without CAS since it is harmless if they are traversed anyway
     * by releasers.  To avoid effects of unsplicing from already
     * removed nodes, the list is retraversed in case of an apparent
     * race.  This is slow when there are a lot of nodes, but we don't
     * expect lists to be long enough to outweigh higher-overhead
     * schemes.
     */
    // 从等待队列中删除超时或者已经中断的节点，便于垃圾回收
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            node.thread = null;
            retry:
            for (;;) {          // restart on removeWaiter race
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    s = q.next;
                    if (q.thread != null)
                        pred = q;
                    else if (pred != null) {
                        pred.next = s;
                        if (pred.thread == null) // check for race
                            continue retry;
                    }
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }

report

/**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
	// 根据状态从outcome中获得结果
    private V report(int s) throws ExecutionException {
        Object x = outcome;
        // 如果状态为normal，说明任务正常执行完成
        if (s == NORMAL)
            return (V)x;
        // 如果状态值大于等于CANCELLED，说明任务被取消了
        if (s >= CANCELLED)
            throw new CancellationException();
        // 其他状态，直接抛异常
        throw new ExecutionException((Throwable)x);
    }

set

/**
     * Sets the result of this future to the given value unless
     * this future has already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon successful completion of the computation.
     *
     * @param v the value
     */
	// FutureTask的任务只会set一次，因为状态不可能从中间状态或者终结状态变为NEW
    // 将结果set给outcome之后，任务的状态才会变为NORMAL，COMPLETING只是一个中间状态
    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }

setException

/**
     * Causes this future to report an {@link ExecutionException}
     * with the given throwable as its cause, unless this future has
     * already been set or has been cancelled.
     *
     * <p>This method is invoked internally by the {@link #run} method
     * upon failure of the computation.
     *
     * @param t the cause of failure
     */
	// 设置任务的异常信息。也只会设置一次。
    // 也只有在设置完outcome之后，任务的状态才会变为EXCEPTIONAL
    protected void setException(Throwable t) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = t;
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }

run

public void run() {
        // 如果状态不是NEW，说明任务已经完成或者被取消
        // 状态为NEW，但是设置runner为当前线程失败，返回
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    // 出现异常，设置异常
                    setException(ex);
                }
                // 任务运行完毕，设置结果
                if (ran)
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            // 如果线程被中断了，处理中断
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }

handlePossibleCancellationInterrupt

/**
     * Ensures that any interrupt from a possible cancel(true) is only
     * delivered to a task while in run or runAndReset.
     */
    private void handlePossibleCancellationInterrupt(int s) {
        // It is possible for our interrupter to stall before getting a
        // chance to interrupt us.  Let's spin-wait patiently.
        // 如果当前状态是INTERRUPTING
        if (s == INTERRUPTING)
            // 一直自旋，直到state为INTERRUPTING为止，让出CPU
            while (state == INTERRUPTING)
                Thread.yield(); // wait out pending interrupt

        // assert state == INTERRUPTED;

        // We want to clear any interrupt we may have received from
        // cancel(true).  However, it is permissible to use interrupts
        // as an independent mechanism for a task to communicate with
        // its caller, and there is no way to clear only the
        // cancellation interrupt.
        //
        // Thread.interrupted();
    }

run方法重点做了以下几件事：

1. 将runner属性设置成当前正在执行run方法的线程
2. 调用callable成员变量的call方法来执行任务
3. 设置执行结果outcome, 如果执行成功, 则outcome保存的就是执行结果；如果执行过程中发生了异常, 则outcome中保存的就是异常，设置结果之前，先将state状态设为中间态
4. 对outcome的赋值完成后，设置state状态为终止态(NORMAL或者EXCEPTIONAL)
5. 唤醒栈中所有等待的线程
6. 善后清理(waiters, callable，runner设为null)
7. 检查是否有遗漏的中断，如果有，等待中断状态完成。