【源码笔记】FutureTask

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 * This code is free software; you can redistribute it and/or modify it
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 * published by the Free Software Foundation.  Oracle designates this
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * accompanied this code).
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
 
package java.util.concurrent;
import java.util.concurrent.locks.LockSupport;
 
/**
 * A cancellable asynchronous computation.  This class provides a base
 * implementation of {@link Future}, with methods to start and cancel
 * a computation, query to see if the computation is complete, and
 * retrieve the result of the computation.  The result can only be
 * retrieved when the computation has completed; the {@code get}
 * methods will block if the computation has not yet completed.  Once
 * the computation has completed, the computation cannot be restarted
 * or cancelled (unless the computation is invoked using
 * {@link #runAndReset}).
 *
 * <p>A {@code FutureTask} can be used to wrap a {@link Callable} or
 * {@link Runnable} object.  Because {@code FutureTask} implements
 * {@code Runnable}, a {@code FutureTask} can be submitted to an
 * {@link Executor} for execution.
 *
 * <p>In addition to serving as a standalone class, this class provides
 * {@code protected} functionality that may be useful when creating
 * customized task classes.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The result type returned by this FutureTask's {@code get} methods
 */
public class FutureTask<V> implements RunnableFuture<V> {
    /*
     * Revision notes: This differs from previous versions of this
     * class that relied on AbstractQueuedSynchronizer, mainly to
     * avoid surprising users about retaining interrupt status during
     * cancellation races. Sync control in the current design relies
     * on a "state" field updated via CAS to track completion, along
     * with a simple Treiber stack to hold waiting threads.
     *
     * Style note: As usual, we bypass overhead of using
     * AtomicXFieldUpdaters and instead directly use Unsafe intrinsics.
     */
 
    /**
     * 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
     */
    // 表示当前task的状态
    // task的状态要及时让各线程知道，所以用volatile修饰
    private volatile int state;
    private static final int NEW          = 0; // 表示当前任务尚未执行
    private static final int COMPLETING   = 1; // 表示当前任务正在结束，尚未完全结束
    private static final int NORMAL       = 2; // 表示当前任务正常结束
    private static final int EXCEPTIONAL  = 3; // 表示当前任务执行过程中发生了异常。内部封装的callable.run()向上抛出了异常
    private static final int CANCELLED    = 4; // 表示当前任务被取消
    private static final int INTERRUPTING = 5; // 表示当前任务中断中
    private static final int INTERRUPTED  = 6; // 表示当前任务已中断
 
    /** The underlying callable; nulled out after running */
    // submit(runnable/callable) 提交的对象最后都会放到这个对象中
    // runnable使用装饰者模式伪装成Callable
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    // 正常情况下：任务正常结束，outcome保存执行结果（callable返回值）
    // 非正常情况下：callable向上抛出异常，outcome保存异常
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    // 当前任务被执行期间，保存当前执行任务的线程对象引用
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    // 因为会有很多线程去get当前任务的执行结果
    // 所以这里使用栈来保存这些线程
    private volatile WaitNode waiters;
 
    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    private V report(int s) throws ExecutionException {
        // 正常情况下，outcome保存的是callable运行结束的结果
        // 非正常情况下，保存的是callable抛出的异常
        Object x = outcome;
 
        // 条件成立：当前任务状态正常结束 --> 直接返回callable计算结果
        if (s == NORMAL)
            return (V)x;
        // 条件成立：canceled，被取消了
        if (s >= CANCELLED)
            throw new CancellationException();
        // 行到此处：说明发生了异常 --> callable实现的代码有问题
        throw new ExecutionException((Throwable)x);
    }
 
    /**
     * 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();
        // callable就是程序员自己实现的业务类
        this.callable = callable;
        // 设置当前任务状态为NEW
        this.state = NEW;       // ensure visibility of callable
    }
 
    /**
     * 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) {
        // 使用装饰者模式，将runnable转换为callable接口
        // 当前任务执行j结束时，结果可能为null也可能为传进来的值
        this.callable = Executors.callable(runnable, result);
        this.state = NEW;       // ensure visibility of callable
    }
 
    public boolean isCancelled() {
        return state >= CANCELLED;
    }
 
    public boolean isDone() {
        return state != NEW;
    }
 
    public boolean cancel(boolean mayInterruptIfRunning) {
        // 条件1：state == NEW
        //       true --> 当前任务处于运行中，或者处于任务队列中
        // 条件2：UNSAFE.compareAndSwapInt(this, stateOffset, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED))
        //       true --> 改变当前task的状态成功 -> 仅能有一个线程改变task的状态成功
        // 条件1 && 条件2
        //       true --> 当前task的状态为NEW，且改变task的状态成功
        //       false -> 1.当前task的状态不为NEW
        //                2.当前task的状态为NEW但CAS state失败（多线程竞争失败）
        //                false的话会直接返回
        if (!(state == NEW &&
              UNSAFE.compareAndSwapInt(this, stateOffset, NEW, mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
            return false;
 
        try {    // in case call to interrupt throws exception
            // mayInterruptIfRunning若为true，则说明需要给正在运行的线程发一个中断信号
            if (mayInterruptIfRunning) {
                try {
                    // 执行当前futureTask的线程
                    Thread t = runner;
                    // 有可能现在是null：当前任务在队列中还没有线程获取到它
                    if (t != null)
                        // 说明runner正在执行当前task，发送一个中断信号
                        // 如果你的程序是响应中断的，则会走中断逻辑
                        // 如果你的程序不是响应中断的，则什么也不会发生
                        t.interrupt();
                } finally { // final state
                    // 设置任务状态为：中断完成
                    UNSAFE.putOrderedInt(this, stateOffset, INTERRUPTED);
                }
            }
        } finally {
            // 唤醒所有get阻塞的线程
            finishCompletion();
        }
        return true;
    }
 
    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        // 获取当前任务的状态
        int s = state;
        // 条件成立：未执行、正在执行、已完成 --> 调用get的外部线程会被阻塞在get方法上
        if (s <= COMPLETING)
            // 等待执行完成
            s = awaitDone(false, 0L);
        return report(s);
    }
 
    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException {
        if (unit == null)
            throw new NullPointerException();
        int s = state;
        if (s <= COMPLETING &&
            (s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)
            throw new TimeoutException();
        return report(s);
    }
 
    /**
     * 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() { }
 
    /**
     * 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
     */
    protected void set(V v) {
        // 使用CAS方式将当前任务状态设置为完成中
        // 有可能会失败 --> 外部线程等不及了，直接在set执行CAS之前，将task取消了
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            // 将执行结果赋值给outcome
            outcome = v;
            // 将执行结果赋值给outcome之后，马上将当前任务的状态设置为正常结束
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }
 
    /**
     * 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
     */
    protected void setException(Throwable t) {
        // 使用CAS方式将当前任务状态设置为完成中
        // 有可能会失败 --> 外部线程等不及了，直接在set执行CAS之前，将task取消了
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            // 将异常赋值给outcome
            outcome = t;
            // 将执行结果赋值给outcome之后，马上将当前任务的状态设置为正常结束
            // 将当前任务状态修改为EXCEPTIONAL
            UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
            finishCompletion();
        }
    }
 
    // 线程执行的入口
    // --> submit(runnabke/callable)
    //  -> AbstractExecutorService    : RunnableFuture<Void> ftask = newTaskFor(task, null);
    //                                   new FutureTask<T>(runnable, value)
    //  -> FutureTask                 : this.callable = Executors.callable(runnable, result);
    // --> execute(task)
    public void run() {
        // 条件1：state != NEW
        //    true -> 说明当前task已经被执行过了，或者canceled了
        //    总之，非NEW状态的任务，线程不处理了
        // 条件2：!RUNNER.compareAndSet(this, null, Thread.currentThread()))
        //    true -> 说明当前task没有被执行，
        //            注意，这个关系是，当前是一个正在运行的线程，在访问一个FutureTask的run方法，以期望运行这个task
        //            所以，如果这个task没有线程执行它，则当前线程应该去尝试执行它。
        //            但是可能有多个线程竞争这一个task，所以使用CAS，只有一个线程能够执行成功。
        //            - 执行成功的类，代表获得了task执行的权力，所以应该继续完成run接下来的动作。
        //            - 执行失败的类，说明在多线程竞争中失败，没有执行task的权力，return。
        //    总之，NEW状态的任务，如果没有竞争到执行的权力，则也不处理了
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
 
        // 执行到此处，说明当前task的状态是NEW
        // 且，当前线程竞争到了执行task的权力
        try {
            // callable就是程序员自己写的一些执行逻辑
            Callable<V> c = callable;
            // 条件1：c != null --> 防止程序员直接提交一个null进来
            // 条件2：state == NEW --> 防止外部线程cancel当前线程
            if (c != null && state == NEW) {
                // 结果引用
                V result;
                // callable.call()代码是否执行成功
                boolean ran;
                try {
                    // 调用程序员自己实现的callable或者装饰后的runnable
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    // 说明程序员自己写的逻辑块有bug了
                    result = null;
                    ran = false;
                    setException(ex);
                }
 
                if (ran)
                    // 能执行到这里，说明当前task执行成功
                    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;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
    }
 
    /**
     * Executes the computation without setting its result, and then
     * resets this future to initial state, failing to do so if the
     * computation encounters an exception or is cancelled.  This is
     * designed for use with tasks that intrinsically execute more
     * than once.
     *
     * @return {@code true} if successfully run and reset
     */
    protected boolean runAndReset() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return false;
        boolean ran = false;
        int s = state;
        try {
            Callable<V> c = callable;
            if (c != null && s == NEW) {
                try {
                    c.call(); // don't set result
                    ran = true;
                } catch (Throwable ex) {
                    setException(ex);
                }
            }
        } 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
            s = state;
            if (s >= INTERRUPTING)
                handlePossibleCancellationInterrupt(s);
        }
        return ran && s == NEW;
    }
 
    /**
     * 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.
        if (s == INTERRUPTING)
            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();
    }
 
    /**
     * Simple linked list nodes to record waiting threads in a Treiber
     * stack.  See other classes such as Phaser and SynchronousQueue
     * for more detailed explanation.
     */
    static final class WaitNode {
        volatile Thread thread;
        volatile WaitNode next;
        WaitNode() { thread = Thread.currentThread(); }
    }
 
    /**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        // q指向waters链表的头节点
        for (WaitNode q; (q = waiters) != null;) {
            // 使用CAS把头节点置空
            // 使用CAS设置waiters为null的原因：外部线程可以使用cancel取消当前任务，也会触发finishCompletion方法
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
 
                // 行到此处，task的waiters已经被置空了
                for (;;) {
                    // q从头节点开始一直向后遍历
 
                    // 拿到q中的线程
                    Thread t = q.thread;
                    // 如果线程不为空，则把线程唤醒
                    if (t != null) {
                        // help gc
                        q.thread = null;
                        LockSupport.unpark(t);
                    }
 
                    // 如果q没有nx，则跳出循环
                    WaitNode next = q.next;
                    if (next == null)
                        break;
 
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
 
        // 空方法
        done();
 
        // 将task的callable置空
        callable = null;        // to reduce footprint
    }
 
    /**
     * 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 {
        // 0 --> 不带超时
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        // 引用当前线程 --> 封装成WaitNode对象
        WaitNode q = null;
        // 表示当前线程WaitNode对象有没有入队
        boolean queued = false;
 
        for (;;) {
            // ---------------------------------------------------------------------------------------------------------
            // Part2：线程被唤醒（可能是被正常唤醒，也可能是被其它线程使用中断唤醒）
            // ---------------------------------------------------------------------------------------------------------
            // 条件成立：说明当前线程唤醒 --> 被其它线程使用中断唤醒
            // interrupted()返回true后，会将Thread的中断标记重置回false
            if (Thread.interrupted()) {
                // 当前线程node出队
                removeWaiter(q);
                // 抛出中断异常 --> get()方法的调用处会收到中断异常，而不是得到结果
                throw new InterruptedException();
            }
 
            // 假设当前线程是被其它线程使用unpack(thread)唤醒的话，会正常自旋，走下面逻辑
 
            // 获取当前任务最新状态
            int s = state;
            // 条件成立：说明当前任务已经有结果了
            if (s > COMPLETING) {
                if (q != null)
                    // help gc
                    q.thread = null;
                // 直接返回当前状态
                return s;
            }
            // 条件成立说明当前任务接近完成
            else if (s == COMPLETING) // cannot time out yet
                // 让当前线程释放一次cpu，之后再进行下一次抢占
                Thread.yield();
 
            // ---------------------------------------------------------------------------------------------------------
            // Part1：线程在get()中判断当前线程还未结束（s <= COMPLETING）
            //        --> 线程还没有结果
            //        --> 应该park当前线程，等待别的线程唤醒
            // ---------------------------------------------------------------------------------------------------------
            // 条件成立：第一次自旋，当前线程还未创建WaitNode对象 --> 为当前线程创建WaitNode对象
            else if (q == null) {
                q = new java.util.concurrent.FutureTask.WaitNode();
            }
            // 条件成立：第二次自旋，当前线程已经创建了WaitNode对象，但是还未入队 --> 入队（头插）
            // waiters一直指向队列的头
            else if (!queued) {
                // 如果CAS失败，则说明其它线程先一步插入了WaiteNode
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            }
 
            // 第三次自旋
            // Case1.带超时
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            // Case2.不带超时
            else
                // park当前线程 -> 线程状态会变成WAITING状态 -> 相当于休眠了
                // 除非有其它线程将当前线程唤醒或者中断
                LockSupport.park(this);
        }
    }
 
    /**
     * 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.
     */
    private void removeWaiter(WaitNode node) {
        if (node != null) {
            // 将node的thread置空，后续代码就清除所有thread为null的node
            node.thread = null;
 
            retry:
            for (;;) {          // restart on removeWaiter race
                // 初始的pred为null
                // q为cur
                // s记录nx节点，初始不赋值
                for (WaitNode pred = null, q = waiters, s; q != null; q = s) {
                    // 为nx节点赋值，s在每一次遍历循环之初都指向cur的nx
                    // q在初始的时候，指向waiter（链表头节点），之后每一次遍历，q都指向s，然后s再指向q的nx
                    s = q.next;
                    if (q.thread != null)
                        // 如果q.thread不为null，则说明q是一个有效的节点 --> 挪动pred
                        pred = q;
                    // ------------------------- 行到此处，说明q.thread为null --> 需要从链表中摘除 -------------------------
 
                    // Case1.pred不为null
                    else if (pred != null) {
                        // pred指向nx
                        pred.next = s;
                        // ？？
                        if (pred.thread == null) // check for race
                            continue retry;
                    }
                    // Case2.pred为null --> waiters指向nx
                    else if (!UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                          q, s))
                        continue retry;
                }
                break;
            }
        }
    }
 
    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long stateOffset;
    private static final long runnerOffset;
    private static final long waitersOffset;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> k = FutureTask.class;
            stateOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("state"));
            runnerOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("runner"));
            waitersOffset = UNSAFE.objectFieldOffset
                (k.getDeclaredField("waiters"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
 
}