Android Handler机制12之Callable、Future和FutureTask

public  interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

/**
 * A task that returns a result and may throw an exception.
 * Implementors define a single method with no arguments called
 * {@code call}.
 *
 * <p>The {@code Callable} interface is similar to {@link
 * java.lang.Runnable}, in that both are designed for classes whose
 * instances are potentially executed by another thread.  A
 * {@code Runnable}, however, does not return a result and cannot
 * throw a checked exception.
 *
 * <p>The {@link Executors} class contains utility methods to
 * convert from other common forms to {@code Callable} classes.
 *
 * @see Executor
 * @since 1.5
 * @author Doug Lea
 * @param <V> the result type of method {@code call}
 */
@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
     //
    V call() throws Exception;
}

public interface Future<V> {
    boolean cancel(boolean mayInterruptIfRunning);
    boolean isCancelled();
    boolean isDone();
    V get() throws InterruptedException, ExecutionException;
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

public class FutureTask<V> implements RunnableFuture<V>

/**
 * 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();
}

/**
 * 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
 */

    /**
     * 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(); }
    }

    /*
     * 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
     */
    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 */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

    /**
     * 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
    }

    /**
     * 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
    }

    /**
     * Returns a {@link Callable} object that, when
     * called, runs the given task and returns the given result.  This
     * can be useful when applying methods requiring a
     * {@code Callable} to an otherwise resultless action.
     * @param task the task to run
     * @param result the result to return
     * @param <T> the type of the result
     * @return a callable object
     * @throws NullPointerException if task null
     */
    public static <T> Callable<T> callable(Runnable task, T result) {
        if (task == null)
            throw new NullPointerException();
        return new RunnableAdapter<T>(task, result);
    }

    /**
     * A callable that runs given task and returns given result.
     */
    private static final class RunnableAdapter<T> implements Callable<T> {
        private final Runnable task;
        private final T result;
        RunnableAdapter(Runnable task, T result) {
            this.task = task;
            this.result = result;
        }
        public T call() {
            task.run();
            return result;
        }
    }

    /**
     * 判断任务的状态是否是初始化的状态
     * 判断执行任务的线程对象runner是否为null，为空就将当前执行线程赋值给runner属性
     *  不为空说明应有线程准备执行这个任务了
     */
    public void run() {
        if (state != NEW ||
            !U.compareAndSwapObject(this, RUNNER, null, Thread.currentThread()))
            return;
        try {
           // 任务状态时NEW，并且callable不为空，则执行任务
           // 如果认为被cancel了，callable会被置空
            Callable<V> c = callable;
            if (c != null && state == NEW) {
                
                V result;  // 结果的变量
                
                boolean ran;  // 执行完毕的变量
                try {
                    // 执行任务并返回结果
                    result = c.call();
                    ran = true;
                } catch (Throwable ex) {
                    // 执行异常
                    result = null;
                    ran = false;
                    setException(ex);
                }
                if (ran)
                    //任务执行完毕就设置结果
                    set(result);
            }
        } finally {
            // runner must be non-null until state is settled to
            // prevent concurrent calls to run()
            // 将执行任务的执行线程清空 
            runner = null;
            // state must be re-read after nulling runner to prevent
            // leaked interrupts
            int s = state;
            if (s >= INTERRUPTING)
                //判断线程的状态
                handlePossibleCancellationInterrupt(s);
        }
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
    private static final long STATE;
    private static final long RUNNER;
    private static final long WAITERS;
    static {
        try {
            STATE = U.objectFieldOffset
                (FutureTask.class.getDeclaredField("state"));
            RUNNER = U.objectFieldOffset
                (FutureTask.class.getDeclaredField("runner"));
            WAITERS = U.objectFieldOffset
                (FutureTask.class.getDeclaredField("waiters"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }

        // Reduce the risk of rare disastrous classloading in first call to
        // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
        Class<?> ensureLoaded = LockSupport.class;
    }

    /**
     * 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) {
         // state状态 NEW-> COMPLETING
        if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = t;
             // // COMPLETING -> EXCEPTIONAL 到达稳定状态
            U.putOrderedInt(this, STATE, EXCEPTIONAL); // final state
            // 一些 结束工作
            finishCompletion();
        }
    }

    /**
     * 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) {
        // state 状态  NEW->COMPLETING
        if (U.compareAndSwapInt(this, STATE, NEW, COMPLETING)) {
            outcome = v;
            // COMPLETING -> NORMAL 到达稳定状态
            U.putOrderedInt(this, STATE, NORMAL); // final state
            // 一些结束工作
            finishCompletion();
        }
    }

    /**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
    private void finishCompletion() {
        // assert state > COMPLETING;
        // 遍历等待节点
        for (WaitNode q; (q = waiters) != null;) {
            if (U.compareAndSwapObject(this, WAITERS, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        // 唤醒等待线程
                        LockSupport.unpark(t);
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }
        // 这里可以自定义实现任务完成后要做的事情(在子类重写done()方法)
        done();
        // 清空callable
        callable = null;        // to reduce footprint
    }

public native void unpark(Thread jthread); 
public native void park(boolean isAbsolute, long time); 

    /**
     * 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.
        // 如果当前正在中断过程中，自等待，等中断完成
        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();
    }

    /**
     * @throws CancellationException {@inheritDoc}
     */
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        // state 小于 COMPLETING 则说明任务仍然在执行，且没有被取消
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

    /**
     * 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 or at timeout
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        // The code below is very delicate, to achieve these goals:
        // - call nanoTime exactly once for each call to park
        // - if nanos <= 0L, return promptly without allocation or nanoTime
        // - if nanos == Long.MIN_VALUE, don't underflow
        // - if nanos == Long.MAX_VALUE, and nanoTime is non-monotonic
        //   and we suffer a spurious wakeup, we will do no worse than
        //   to park-spin for a while
        // 起始时间
        long startTime = 0L;    // Special value 0L means not yet parked
        // 当前等待线程的节点
        WaitNode q = null;
         // 是否将节点放在了等待列表中
        boolean queued = false;
        // 通过死循环来实现线程阻塞等待
        for (;;) {
            int s = state;
            if (s > COMPLETING) {
                 // 任务可能已经完成或者被取消了
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING)
                // We may have already promised (via isDone) that we are done
                // so never return empty-handed or throw InterruptedException
                // 任务线程可能被阻塞了，让出cpu
                Thread.yield();
            else if (Thread.interrupted()) {
                // 线程中断则移除等待线程并抛出异常
                removeWaiter(q);
                throw new InterruptedException();
            }
            else if (q == null) {
                // 等待节点为空，则初始化新节点并关联当前线程
                if (timed && nanos <= 0L)
                   // 如果需要等待，并且等待时间小于0表示立即，则直接返回
                    return s;
                // 如果不需要等待，或者需要等待但是等待时间大于0。
                q = new WaitNode();
            }
            else if (!queued)
                // 等待线程入队，因为如果入队成功则queued=true
                queued = U.compareAndSwapObject(this, WAITERS,
                                                q.next = waiters, q);
            else if (timed) {
                //如果有超时设置
                final long parkNanos;
                if (startTime == 0L) { // first time
                    startTime = System.nanoTime();
                    if (startTime == 0L)
                        startTime = 1L;
                    parkNanos = nanos;
                } else {
                    long elapsed = System.nanoTime() - startTime;
                    if (elapsed >= nanos) {
                        // 已经超时，则移除等待节点
                        removeWaiter(q);
                        return state;
                    }
                    parkNanos = nanos - elapsed;
                }
                // nanoTime may be slow; recheck before parking
                if (state < COMPLETING)
                    // 任务还在执行，且没有被取消，所以继续等待
                    LockSupport.parkNanos(this, parkNanos);
            }
            else
                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 域置空
            node.thread = null;
            /**
             * 下面过程中会将node从等待队列中移除，以thread为null为依据
             * 如果过程中发生了竞争，重试
             */
            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 (!U.compareAndSwapObject(this, WAITERS, q, s))
                        continue retry;
                }
                break;
            }
        }
    }

    /**
     * Returns result or throws exception for completed task.
     *
     * @param s completed state value
     */
    @SuppressWarnings("unchecked")
    private V report(int s) throws ExecutionException {
        Object x = outcome;
         // 如果任务正常执行完成，返回任务执行结果
        if (s == NORMAL)
            return (V)x;

        // 如果任务被取消，抛出异常
        if (s >= CANCELLED)
            throw new CancellationException();
        
        // 其他状态 抛出执行异常 ExecutionException 
        throw new ExecutionException((Throwable)x);
    }

    /**
     * @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);
    }

    public boolean cancel(boolean mayInterruptIfRunning) {
        // 如果任务状态不是初始化状态，则取消任务
         //如果此时任务已经执行了，并且可能执行完成，但是状态改变还没有来得及修改，也就是在run()方法中的set()方法还没来得及调用
         //   继续判断任务的当前状态时否为NEW，因为此时执行任务线程可能再度获得处理了，任务状态可能已发生改变
        if (!(state == NEW &&
              U.compareAndSwapInt(this, STATE, NEW,
                  mayInterruptIfRunning ? INTERRUPTING : CANCELLED)))
                return false;

        // 如果任务状态依然是NEW，也就是执行线程没有改变任务的状态，
        // 则让执行线程中断(在这个过程中执行线程可能会改变任务的状态)
        try {    // in case call to interrupt throws exception
            if (mayInterruptIfRunning) {
                try {
                    Thread t = runner;
                    if (t != null)
                        t.interrupt();
                } finally { // final state
                    // 将任务状态设置为中断
                    U.putOrderedInt(this, STATE, INTERRUPTED);
                }
            }
        } finally {
            // 处理任务完成的结果
            finishCompletion();
        }
        return true;
    }

    /**
     * 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 ||
            !U.compareAndSwapObject(this, RUNNER, 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;
    }