FutureTask详解

1 基本概念

1.1 Callable与Future

Runnable封装一个异步运行的任务，可以把它想象成为一个没有参数和返回值的异步方法。Callable与Runnable类似，但是有返回值。Callable接口是一个参数化的类型，只有一个方法call。

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

类型参数是返回值的类型。例如，

Callable<Integer>表示一个最终返回Integer对象的异步计算。

Future保存异步计算的结果。可以启动一个计算，将Future对象交给某个线程，然后忘掉它。Future对象的所有者在结果计算好之后就可以获得它。
Future接口具有下面的方法：

public interface Future<V> {
	boolean cancel(boolean mayInterruptIfRunning);
	boolean isCancelled();
	boolean isDone();
	V get() throws InterruptedException, ExecutionException;
	V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException;
}

第一个get方法的调用被阻塞，知道计算完成。如果在计算完成之前，第二个get方法的调用超时，抛出一个TimeoutException异常。如果运行该计算的线程被中断，两个方法都将抛出InterruptedException。如果计算已经完成，那么get方法立即返回。

如果计算还在进行，isDone方法返回false；如果完成了，则返回true。

可以用cancel方法取消该计算。如果计算还没有开始，它被取消且不再开始。如果计算处于运行之中，那么如果mayInterrupt参数为true，它就被中断。

1.2 FutureTask

FutureTask包装器是一种非常便利的机制，同时实现了Future和Runnable接口。

类图如下：

FutureTask的状态转换过程：

 * NEW -> COMPLETING -> NORMAL
 * NEW -> COMPLETING -> EXCEPTIONAL
 * NEW -> CANCELLED
 * NEW -> INTERRUPTING -> INTERRUPTED

1.3 FutureTask的执行过程

创建一个futureTask对象task
提交task到调度器executor等待调度或者在另外一个线程中执行task

等待调度中...

如果此时currentThread调取执行结果task.get(),会有几种情况
if task 还没有被executor调度或正在执行中
    阻塞当前线程，并加入到一个阻塞链表中waitNode
else if task被其它Thread取消，并取消成功 或task处于中断状态
    throw exception
else if task执行完毕，返回执行结果，或执行存在异常，返回异常信息
    
        
如果此时有另外一个线程调用task.get()
    
执行过程同上

2 应用场景

1. Future用于异步获取执行结果或者取消任务。
2. 在高并发场景下确保任务只执行一次。

3 基本例子

Callable<Integer> myComputation = ...;
FutureTask<Integer> task = new FutureTask<Integer>(myComputation);
Thread t = new Thread(task);
t.start();
...
Integer result = task.get(); //获取结果

4 FutureTask源码分析

4.1 核心状态

 /**
 * The run state of this task, initially NEW.  The run state
 * transitions to a terminal state only in methods set,
 * setException, and cancel.  During completion, state may take on
 * transient values of COMPLETING (while outcome is being set) or
 * INTERRUPTING (only while interrupting the runner to satisfy a
 * cancel(true)). Transitions from these intermediate to final
 * states use cheaper ordered/lazy writes because values are unique
 * and cannot be further modified.
 * 
 * Possible state transitions:
 * 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;

4.2 创建FutureTask

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

4.3 获取执行结果

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);
}

4.4 执行方法

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);
    }
}

4.5 设置状态

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();
    }
}

5 高级示例

public class Memoizer<A, V> implements Computable<A, V> {
	private final ConcurrentMap<A, Future<V>> cache = new ConcurrentMap<A, Future>>();
	private final Computable<A, V> c;
	
	public Memoizer(Computable<A, V> c) {
		this.c = c;
	}
	
	public C computer(final A arg) throws InterruptedException {
		while(true) {
			Future<V> f = cache.get(arg);
			if(f == null) {
				Callable<V> eval = new Callable<V>() {
					public V call() throws InterruptedException {
						return c.compute(arg);
					}
				};
				FutureTask<V> ft = new FutureTask<V>(eval);
				f = cache.putIfAbsent(arg, ft);
				if(f == null) {
					f = ft;
					ft.run();
				}
			}
			
			try {
				return f.get();
			} catch(CancellationException e) {
				cache.remove(arg, f);
			} catch(ExecutionException e) {
				throw launderThrowable(e.getCause());
			}
		}
	}
}