Java并发编程 - Promise在FutureTask源码的应用场景 - 《Java学习笔记》

如何启动FutureTask线程
针对FutureTask的创建过程分析问题

如何启动FutureTask线程

public Future<Object> create(String startTime) {
     FutureTask<Object> futureTask = new FutureTask(() -> {
           // todo：处理相关异步任务
      return object;
   });
   // 使用线程池等方式调用futureTask
   ExecutorService executor = Executors.newSingleThreadExecutor();
   executor.submit(futureTask);
   // Thread thread = new Thread(futureTask);
   // thread.start();
   return futureTask;
}

public<T> Object invokeServiceMethod(Class<T> clazz, String methodName, int seconds, Object... params) {
        Class<?>[] paramTypes = new Class<?>[params.length];
        for (int i=0; i < params.length; i++){
            paramTypes[i] = params[i].getClass();
        }
        FutureTask<Object> future = new FutureTask<Object>(new Callable<Object>(){
            @Override
            public Object call() throws Exception {
                Method method = clazz.getDeclaredMethod(methodName, paramTypes);
                method.setAccessible(true);
                T bean = SpringBeanUtils.getBean(clazz);
                return method.invoke(bean, params);
            }
        });
        ExecutorService executor = Executors.newSingleThreadExecutor();
        executor.execute(future);
        try {
            // 设定获取值的阻塞等待时间
            Object result = future.get(seconds, TimeUnit.SECONDS);
            return result;
        } catch (InterruptedException | ExecutionException | TimeoutException e) {
            LogUtil.error("数据访问异常");
            LogUtil.error(e.getMessage(), e);
            return Boolean.FALSE;
        } finally{
            future.cancel(true);
            executor.shutdown();
        }
    }

针对FutureTask的创建过程分析问题

问题一： FutureTask 的构造函数，如何通过new来创建自带返回值的FutureTask对象

（1）FutureTask的构造函数源码

    /**
     * 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
    }

FutureTask的构造函数传递的参数类型是Callable，Callable与Runnable相似，但是Callable可以抛出异常。
同时，Callable中的call()方法相比Runnable中的run()方法, 前者有返回值，后者没有。
(2) Callable接口和Runnable区别
Callable与Runnable相似，但是Callable可以抛出异常。
Callable中的call()方法有返回值，Runnable中的run()方法没有返回值。

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

(3) Runnable接口

@FunctionalInterface
public interface Runnable {
    public abstract void run();
}

(4) FutureTask如何使用callable

    // 成员变量callable
    private Callable<V> callable;
    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);
        }
    }

FutureTask启动run方法，会通过callable.call()获取到返回值result，然后将返回值result通过set()方法进行存储。

(5) set()方法和get()方法
set()方法中会把传入的callable的返回值，在线程安全的前提下，赋值给FutureTask的成员变量outcome。这个过程也代表了，启动Future会通过Callable来获取到一个结果，并将这个结果放到成员变量outcome,等待get()方法的获取。

    protected void set(V v) {
        if (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {
            outcome = v;
            UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final state
            finishCompletion();
        }
    }
    public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);
        return report(s);
    }

问题二：FutureTask类型的对象futureTask，为何可以传入到Thread对象构造方法中执行

FutureTask实现了RunnableFuture接口，RunnableFuture分别继承了Runnable接口和Future接口。

问题三：为何FutureTask可以当做Future类型的返回值

FutureTask实现了RunnableFuture接口，RunnableFuture分别继承了Runnable接口和Future接口。

问题四：FutureTask对象执行get()方法时，是如何获取返回值的

调用get()方法时，如果state完成计算状态（ s>COMPLETING ）,会调用 report() 方法获取结果。
report()方法变量x赋值了outcome，outcome是前面在set()方法中callable对象的返回值，最后report()返回x值。

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

问题五：FutureTask对象执行get()方法时，为什么会阻塞

调用get()方法时，如果state还在计算状态(s <= COMPLETING)，就会调用awaitDone()方法进行等待。
如果s=COMPLETING状态，就会调用Thread.yield()使线程进入就绪状态，等待CPU的调度。
如果s>COMPLETING状态，说明当前线程完成，可以通过 return 方式执行结束。
如果s<COMPLETING状态，会通过 LockSupport.park(this) 将线程进入阻塞等待状态。

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