Future的使用和原理

最新推荐文章于 2024-04-16 07:56:17 发布

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本文详细介绍了Java中FutureTask的使用方法及原理。通过示例代码展示了如何创建FutureTask并利用线程进行异步任务处理，同时深入解析了FutureTask的内部机制，包括其构造函数、状态管理、结果获取等关键环节。

java中有几种实现异步的方式(FutureTask/ListenableFuture/CompletableFuture)
这篇介绍的是FutureTask

class Worker {
    public String name;
    Worker(String name) {
        this.name = name;
    }
}

/**
 * @author dong
 */
public class FutureTaskTest {
    public static FutureTask<Worker> addWorker(String name) {
        FutureTask<Worker> futureTask = new FutureTask<>(() -> {
            System.out.println(name + " do something");
            Thread.sleep(1000);
            return new Worker(name);
        });
        new Thread(futureTask).start();
        return futureTask;
    }

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        long start = System.currentTimeMillis();
        List<FutureTask<Worker>> listTask = new ArrayList<>();
        listTask.add(addWorker("东1"));
        listTask.add(addWorker("东2"));
        listTask.add(addWorker("东3"));
        for(FutureTask<Worker> futureTask : listTask){
            Worker worker = futureTask.get();
            System.out.println("返回值：" + worker.name);
        }
        long end = System.currentTimeMillis();
        System.out.println("耗时：" + (end - start));
    }
}

上面就是简单的使用FutureTask的异步调用
每个任务创建一个FutureTask，然后使用get去等待结果
说一下原理：

public FutureTask(Callable<V> callable) {
        if (callable == null)
            throw new NullPointerException();
        this.callable = callable;
        this.state = NEW;       // ensure visibility of callable
    }

这个是FutureTask的构造函数，接收的是一个Callable，和Runnable的区别在于一个有返回值，一个没有

@FunctionalInterface
public interface Callable<V> {
    V call() throws Exception;
}

构造完FutureTask之后就利用Thread创建一个子线程去处理任务
然后看一下FutureTask的继承关系
在这里插入图片描述
发现其实实现了Runnable接口，所以子线程会去执行run方法

public void run() {
        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;
                    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);
        }
    }

主要就是执行Callable的call方法（实际上就是你要执行的任务方法）
然后会得到一个结果result，最后传入set方法

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

这里主要就是把结果v赋值给outcome（成员变量），因为这个结果将来会在get的时候才被取出来

public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

如果沒有完成任务，就等待

private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING) // cannot time out yet
                Thread.yield();
            else if (q == null)
                q = new WaitNode();
            else if (!queued)
                queued = UNSAFE.compareAndSwapObject(this, waitersOffset,
                                                     q.next = waiters, q);
            else if (timed) {
                nanos = deadline - System.nanoTime();
                if (nanos <= 0L) {
                    removeWaiter(q);
                    return state;
                }
                LockSupport.parkNanos(this, nanos);
            }
            else
                LockSupport.park(this);
        }
    }

如果已经完成任务

private V report(int s) throws ExecutionException {
        Object x = outcome;
        if (s == NORMAL)
            return (V)x;
        if (s >= CANCELLED)
            throw new CancellationException();
        throw new ExecutionException((Throwable)x);
    }