Java中Future的使用场景和解析

package test;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.FutureTask;

public class CallableFutureTest {

	public static void main(String[] args) {
		long startTime = System.currentTimeMillis();
		Callable<Integer> calculateCallable = new Callable<Integer>() {
			@Override
			public Integer call() throws Exception {
				// TODO Auto-generated method stub
				Thread.sleep(2000);//模拟耗时时间
				int result = 1+2;
				return result;
			}
		};
		FutureTask<Integer> calculateFutureTask = new FutureTask<>(calculateCallable);
		Thread t1 = new Thread(calculateFutureTask);
		t1.start();
		//现在加入Thread运行的是一个模拟远程调用耗时的服务，并且依赖他的计算结果（比如网络计算器）
		try {
			//模拟耗时任务，主线程做自己的事情，体现多线程的优势
			Thread.sleep(3000);
			int a = 3+5;
			Integer result = calculateFutureTask.get();
			System.out.println("result = "+(a+result));//模拟主线程依赖子线程的运行结果
			long endTime = System.currentTimeMillis();
			System.out.println("time = "+(endTime-startTime)+"ms");
		} catch (InterruptedException | ExecutionException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}
}

public void run() {
        if (state != NEW ||
            !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                         null, Thread.currentThread()))
            return;
        try {
            Callable<V> c = callable;//构造方法传入的calculateCallable
            if (c != null && state == NEW) {
                V result;
                boolean ran;
                try {
                    result = c.call();//最终执行的是callablede.call()方法,有返回值的
                    ran = true;
                } catch (Throwable ex) {
                    result = null;
                    ran = false;
                    setException(ex);//保存抛出的异常
                }
                if (ran)
                    set(result);//保存执行的结果
            }
        } 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);
        }
    }

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

/**
     * Removes and signals all waiting threads, invokes done(), and
     * nulls out callable.
     */
//这个方法，就是当执行完成完成的时候，唤醒get()方法挂起的线程，从而使得get()方法在阻塞
//的for循环中能够正确的获取执行完的结果
    private void finishCompletion() {
        // assert state > COMPLETING;
        for (WaitNode q; (q = waiters) != null;) {
            if (UNSAFE.compareAndSwapObject(this, waitersOffset, q, null)) {
                for (;;) {
                    Thread t = q.thread;
                    if (t != null) {
                        q.thread = null;
                        LockSupport.unpark(t);//唤醒线程
                    }
                    WaitNode next = q.next;
                    if (next == null)
                        break;
                    q.next = null; // unlink to help gc
                    q = next;
                }
                break;
            }
        }

        done();

        callable = null;        // to reduce footprint
    }

//get方法表示如果执行过程完成，就获取执行的结果，否则就将当前线程挂起
public V get() throws InterruptedException, ExecutionException {
        int s = state;
        if (s <= COMPLETING)
            s = awaitDone(false, 0L);//如果状态没完成
        return report(s);//返回执行完的结果
    }

//这里是执行的状态
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;

private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        final long deadline = timed ? System.nanoTime() + nanos : 0L;
        WaitNode q = null;
        boolean queued = false;
        for (;;) {//堵塞的方法
            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();//将线程挂起，让出cpu资源
            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);
        }
    }


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