前言

Future可以看做一个异步的计算结果的票据，类似我们排队过程中获取的号，后面根据这个号去操作。

简单使用

Future需要配合Callable接口和线程池使用

@FunctionalInterface
public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

Callable就是有返回值的Runnable。

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class Client {

  public static void main(String[] args) throws ExecutionException, InterruptedException {
    ExecutorService executorService = Executors.newFixedThreadPool(5);
    Future<Integer> future = executorService.submit(() -> {
      System.out.println("start execute");
      try {
        Thread.sleep(2_000);
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
      System.out.println("end execute");
      return 5;
    });
    executorService.shutdown();
    System.out.println(future.get());
  }

}

创建一个线程池，提交一个Callable，返回一个Future，get()方法会使当前线程阻塞，直到Callable执行结束。

原理分析

public abstract interface Future<V> {
 public abstract boolean cancel(boolean paramBoolean);
 public abstract boolean isCancelled();
 public abstract boolean isDone();
 public abstract V get() throws InterruptedException, ExecutionException;
 public abstract V get(long paramLong, TimeUnit paramTimeUnit)
 throws InterruptedException, ExecutionException, TimeoutException; 
}

Future在等待结果过程中可以取消，但不一定取消成功，因为任务可能已经完成。
java提供了一个Future的实现类FutureTask

/**
 *
 * state可能的状态转变路径如下：
 * NEW -> COMPLETING -> NORMAL
 * NEW -> COMPLETING -> EXCEPTIONAL
 * NEW -> CANCELLED
 * NEW -> INTERRUPTING -> INTERRUPTED
 */
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;

内部维护一个运行状态，刚开始为NEW，正常结束为NORMAL，异常结束为EXCEPTIONAL

/** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** The result to return or exception to throw from get() */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters;

使用一个Treiber栈来存储多个阻塞线程，可以简单看做一个线程安全且高效的栈。

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

/**
     * 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 or at timeout
     */
    private int awaitDone(boolean timed, long nanos)
        throws InterruptedException {
        long startTime = 0L;    // Special value 0L means not yet parked
        WaitNode q = null;
        boolean queued = false;
        for (;;) {
            int s = state;
            if (s > COMPLETING) {
                if (q != null)
                    q.thread = null;
                return s;
            }
            else if (s == COMPLETING)
                // We may have already promised (via isDone) that we are done
                // so never return empty-handed or throw InterruptedException
                Thread.yield();
            else if (Thread.interrupted()) {
                removeWaiter(q);
                throw new InterruptedException();
            }
            else if (q == null) {
                if (timed && nanos <= 0L)
                    return s;
                q = new WaitNode();
            }
            else if (!queued)
                queued = WAITERS.weakCompareAndSet(this, q.next = waiters, q);
            else if (timed) {
                final long parkNanos;
                if (startTime == 0L) { // first time
                    startTime = System.nanoTime();
                    if (startTime == 0L)
                        startTime = 1L;
                    parkNanos = nanos;
                } else {
                    long elapsed = System.nanoTime() - startTime;
                    if (elapsed >= nanos) {
                        removeWaiter(q);
                        return state;
                    }
                    parkNanos = nanos - elapsed;
                }
                // nanoTime may be slow; recheck before parking
                if (state < COMPLETING)
                    LockSupport.parkNanos(this, parkNanos);
            }
            else
                LockSupport.park(this);
        }
    }

调用get方法时，如果任务未完成，将当前线程加入等待队列并阻塞。使用CAS无锁技术来修改内部状态和等待队列。更多原理请参考 FutureTask源码分析。

参考

FutureTask源码分析
FutureTask中Treiber堆的实现
理解与使用Treiber Stack