Java并发编程 - Callable、Future和FutureTask的达成

发布时间：2021-12-05 13:12:10 所属栏目：教程来源：互联网

导读：启动线程执行任务，如果需要在任务执行完毕之后得到任务执行结果，可以使用从Java 1.5开始提供的Callable和Future 下面就分析一下Callable、Future以及FutureTask的具体实现及使用方法源码分析基于JDK 1.7 一、Callable 与 Runnable java.lang.Runnable是一

            V result;
            boolean ran;
            try {
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                setException(ex);
            }
            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;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}
1、任务执行状态不是NEW，直接返回；将runner属性从null->当前线程不成功，直接返回
2、调用call()方法，调用成功，使用set()设置返回值
3、调用过程发生异常，使用setException()保存异常

set() 和 setException()

protected void set(V v) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = v;
        UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
        finishCompletion();
    }
}

protected void setException(Throwable t) {
    if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
        outcome = t;
        UNSAFE.putOrderedInt(this, stateOffset, EXCEPTIONAL); // final state
        finishCompletion();
    }
}
set()和setException()的实现基本一样，都是先将任务运行状态从NEW->COMPLETING，分别设置返回值或异常给outcome，再将状态分别置为NORMAL和EXCEPTIONAL，最后调用finishCompletion()依次唤醒等待获取结果的阻塞线程

finishCompletion()实现

/**
* Removes and signals all waiting threads, invokes done(), and nulls out callable.
*/
private void finishCompletion() {
    // assert state > COMPLETING;
    for (WaitNode q; (q = waiters) != null;) {
        //将成员变量waiters置为null
        if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
            //循环唤醒WaitNode中的等待线程
            for (;;) {
                Thread t = q.thread;
                if (t != null) {
                    q.thread = null;
                    LockSupport.unpark(t);
                }
                WaitNode next = q.next;
                if (next == null)
                    break;
                q.next = null; // unlink to help gc
                q = next;
            }
            break;
        }
    }

    //由子类实现的方法
    done();

    callable = null;        // to reduce footprint
}
1、执行FutureTask类的get方法时，会把主线程封装成WaitNode节点并保存在waiters链表中
2、FutureTask任务执行完成后，通过UNSAFE设置waiters的值为null，并通过LockSupport.unpark方法依次唤醒等待获取结果的线程

FutureTask.get()的实现
get()方法有两个实现，一个是一直等待获取结果，直到任务执行完；一个是等待指定时间，超时后任务还未完成会上抛TimeoutException

public V get() throws InterruptedException, ExecutionException {
    int s = state;

    if (s <= COMPLETING)
        s = awaitDone(false, 0L);

    return report(s);
}

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);
}
内部通过awaitDone()对主线程进行阻塞，具体实现如下：

/**
* 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; //截止时间
    WaitNode q = null;

（编辑：宿州站长网）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!

2/4

首页

尾页