FutureTask 源码分析

FutureTask是JDK中Future模式的标准实现，它同时实现了Runnable和Future两个接口，提供了可取消的异步计算，并且可以利用开始和取消计算的方法、查询计算是否完成的方法和获取计算结果的方法。

public class FutureTask<V> implements RunnableFuture<V> {    /** 所有的方法全部委托sync */    private final Sync sync;    public FutureTask(Callable<V> callable) {        if (callable == null)            throw new NullPointerException();        sync = new Sync(callable);    }    public FutureTask(Runnable runnable, V result) {        sync = new Sync(Executors.callable(runnable, result));    }    public boolean isCancelled() {        return sync.innerIsCancelled();    }    public boolean isDone() {        return sync.innerIsDone();    }    public boolean cancel(boolean mayInterruptIfRunning) {        return sync.innerCancel(mayInterruptIfRunning);    }    public V get() throws InterruptedException, ExecutionException {        return sync.innerGet();    }    public V get(long timeout, TimeUnit unit)        throws InterruptedException, ExecutionException, TimeoutException {        return sync.innerGet(unit.toNanos(timeout));    }    protected void done() { }    protected void set(V v) {        sync.innerSet(v);    }    protected void setException(Throwable t) {        sync.innerSetException(t);    }    public void run() {        sync.innerRun();    }    protected boolean runAndReset() {        return sync.innerRunAndReset();    }    private final class Sync extends AbstractQueuedSynchronizer {        private static final long serialVersionUID = -7828117401763700385L;        /** State value representing that task is ready to run */        /** 代表起始状态 */        private static final int READY     = 0;        /** State value representing that task is running */        /** 代表正在运行中状态 */        private static final int RUNNING   = 1;        /** State value representing that task ran */        /** 代表运行完成的状态 */        private static final int RAN       = 2;        /** State value representing that task was cancelled */        /** 代表被取消的状态 */        private static final int CANCELLED = 4;        /** The underlying callable */        private final Callable<V> callable;        /** The result to return from get() */        private V result;        /** The exception to throw from get() */        private Throwable exception;        /**         * The thread running task. When nulled after set/cancel, this         * indicates that the results are accessible.  Must be         * volatile, to ensure visibility upon completion.         */        private volatile Thread runner;        Sync(Callable<V> callable) {            this.callable = callable;        }        /**        *  判断是否完成或者是否取消        *  传入0或者1 都返回0 说明任务没有完成 也没有取消        */        private boolean ranOrCancelled(int state) {            return (state & (RAN | CANCELLED)) != 0;        }        /**         * AbstractQueuedSynchronizer的模板方法          * 返回1可以获取锁 返回-1说明获取锁失败         * 调用innerIsDone 返回TRUE 说明任务已经执行完毕         * 返回FALSE 说明任务没有执行完毕         */        protected int tryAcquireShared(int ignore) {            return innerIsDone() ? 1 : -1;        }        /**         * 释放锁 将执行当前任务的线程设置为null         */        protected boolean tryReleaseShared(int ignore) {            runner = null;            return true;        }        //判断任务是否被取消        boolean innerIsCancelled() {            return getState() == CANCELLED;        }        //判断任务是否完成(取消也算完成)        boolean innerIsDone() {            return ranOrCancelled(getState()) && runner == null;        }        //获取结果        V innerGet() throws InterruptedException, ExecutionException {            //首先调用AbstractQueuedSynchronizer的方法，这个方法会调用子类方法tryAcquireShared 上面有讲            //如果当前任务已经完成，那么当前线程可以向下运行，否则把当前线程加入队列阻塞.            acquireSharedInterruptibly(0);            //判断状态 如果取消了就抛CancellationException异常.            if (getState() == CANCELLED)                throw new CancellationException();            //如果任务执行过程中出现异常，这里包装一下抛出ExecutionException.            if (exception != null)                throw new ExecutionException(exception);            return result;        }        //获取结果        V innerGet(long nanosTimeout) throws InterruptedException, ExecutionException, TimeoutException {            //调用AbstractQueuedSynchronizer里的方法            // return tryAcquireShared(arg) >= 0 ||doAcquireSharedNanos(arg, nanosTimeout);            // 首先tryAcquireShared调用它获取锁，也就是看任务完事没，如果任务完事了就返回TRUE，那么执行逻辑同上。            // 如果获取不到锁，那么就阻塞当前线程给定的时间，如果时间到了再次任务还没完成则抛出异常。            if (!tryAcquireSharedNanos(0, nanosTimeout))                throw new TimeoutException();            if (getState() == CANCELLED)                throw new CancellationException();            if (exception != null)                throw new ExecutionException(exception);            return result;        }        void innerSet(V v) {            for (;;) {                int s = getState();                if (s == RAN)                    return;                if (s == CANCELLED) {                    // aggressively release to set runner to null,                    // in case we are racing with a cancel request                    // that will try to interrupt runner                    releaseShared(0);                    return;                }                //正常完成 设置状态为RAN                if (compareAndSetState(s, RAN)) {                    result = v;                    releaseShared(0);                    done(); //通知子类                    return;                }            }        }        void innerSetException(Throwable t) {            for (;;) {                int s = getState();                if (s == RAN)                    return;                if (s == CANCELLED) {                    // aggressively release to set runner to null,                    // in case we are racing with a cancel request                    // that will try to interrupt runner                    releaseShared(0);                    return;                }                //设置异常                if (compareAndSetState(s, RAN)) {                    exception = t;                    releaseShared(0);                    done();//通知子类                    return;                }            }        }        //取消任务        boolean innerCancel(boolean mayInterruptIfRunning) {            for (;;) {                int s = getState();                //如果任务已经结束，则返回FALSE                if (ranOrCancelled(s))                    return false;                //设置任务的状态为CANCELLED                if (compareAndSetState(s, CANCELLED))                    break;            }            //如果参数mayInterruptIfRunning=TRUE,那么设置线程的终端状态            if (mayInterruptIfRunning) {                Thread r = runner;                if (r != null)                    r.interrupt();            }            //释放锁            releaseShared(0);            //调用子类方法，通知状态改变            done();            return true;        }        void innerRun() {            //如果任务不是初始状态则直接结束            if (!compareAndSetState(READY, RUNNING))                return;            runner = Thread.currentThread();            if (getState() == RUNNING) { // recheck after setting thread                V result;                try {                    result = callable.call();                } catch (Throwable ex) {                    //我们写的任务方法里如果出现异常则调用setException                    setException(ex);                    return;                }                //设置结果                set(result);            } else {                //释放锁                releaseShared(0); // cancel            }        }        boolean innerRunAndReset() {            if (!compareAndSetState(READY, RUNNING))                return false;            try {                runner = Thread.currentThread();                if (getState() == RUNNING)                    callable.call(); // don't set result                runner = null;                return compareAndSetState(RUNNING, READY);            } catch (Throwable ex) {                setException(ex);                return false;            }        }    }}