项目中经常有些任务需要异步(提交到线程池中)去执行，而主线程往往需要知道异步执行产生的结果，这时我们要怎么做呢？用runnable是无法实现的，我们需要用callable看下面的代码：

import java.util.concurrent.Callable; import java.util.concurrent.ExecutionException; import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors; import java.util.concurrent.Future; public class AddTask implements Callable<Integer> { private int a,b; public AddTask(int a, int b) { this.a = a; this.b = b; } @Override public Integer call throws Exception { Integer result = a + b; return result; } public static void main(String[] args) throws InterruptedException, ExecutionException { ExecutorService executor = Executors.newSingleThreadExecutor; //JDK目前为止返回的都是FutureTask的实例 Future<Integer> future = executor.submit(new AddTask(1, 2)); Integer result = future.get;// 只有当future的状态是已完成时(future.isDone = true),get方法才会返回 } }

虽然可以实现获取异步执行结果的需求，但是我们发现这个Future其实很不好用，因为它没有提供通知的机制，也就是说我们不知道future什么时候完成（如果我们需要轮询isDone()来判断的话感觉就没有用这个的必要了）。看下java.util.concurrent.future.Future 的接口方法：

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; }

由此可见JDK的Future机制其实并不好用，如果能给这个future加个监听器，让它在完成时通知监听器的话就比较好用了，就像下面这个IFuture：

package future; import java.util.concurrent.CancellationException; import java.util.concurrent.Future; import java.util.concurrent.TimeUnit; /** * The result of an asynchronous operation. * * @author lixiaohui * @param <V> 执行结果的类型参数 */ public interface IFuture<V> extends Future<V> { boolean isSuccess; // 是否成功 V getNow; //立即返回结果(不管Future是否处于完成状态) Throwable cause; //若执行失败时的原因 boolean isCancellable; //是否可以取消 IFuture<V> await throws InterruptedException; //等待future的完成 boolean await(long timeoutMillis) throws InterruptedException; // 超时等待future的完成 boolean await(long timeout, TimeUnit timeunit) throws InterruptedException; IFuture<V> awaitUninterruptibly; //等待future的完成，不响应中断 boolean awaitUninterruptibly(long timeoutMillis);//超时等待future的完成，不响应中断 boolean awaitUninterruptibly(long timeout, TimeUnit timeunit); IFuture<V> addListener(IFutureListener<V> l); //当future完成时，会通知这些加进来的监听器 IFuture<V> removeListener(IFutureListener<V> l); }

接下来就一起来实现这个IFuture，在这之前要说明下Object.wait,Object.notifyAll方法，因为整个Future实现的原���的核心就是这两个方法.看看JDK里面的解释:

public class Object { /** * Causes the current thread to wait until another thread invokes the * {@link java.lang.Object#notify} method or the * {@link java.lang.Object#notifyAll} method for this object. * In other words, this method behaves exactly as if it simply * performs the call {@code wait(0)}. * 调用该方法后，当前线程会释放对象监视器锁，并让出CPU使用权。直到别的线程调用notify/notifyAll */ public final void wait throws InterruptedException { wait(0); } /** * Wakes up all threads that are waiting on this object's monitor. A * thread waits on an object's monitor by calling one of the * {@code wait} methods. * <p> * The awakened threads will not be able to proceed until the current * thread relinquishes the lock on this object. The awakened threads * will compete in the usual manner with any other threads that might * be actively competing to synchronize on this object; for example, * the awakened threads enjoy no reliable privilege or disadvantage in * being the next thread to lock this object. */ public final native void notifyAll; }

知道这个后，我们要自己实现Future也就有了思路，当线程调用了IFuture.await等一系列的方法时，如果Future还未完成，那么就调用future.wait 方法使线程进入WAITING状态。而当别的线程设置Future为完成状态（注意这里的完成状态包括正常结束和异常结束）时，就需要调用future.notifyAll方法来唤醒之前因为调用过wait方法而处于WAITING状态的那些线程。完整的实现如下(代码应该没有很难理解的地方，我是参考netty的Future机制的。有兴趣的可以去看看netty的源码)：

package future; import java.util.Collection; import java.util.concurrent.CancellationException; import java.util.concurrent.CopyOnWriteArrayList; import java.util.concurrent.ExecutionException; import java.util.concurrent.TimeUnit; import java.util.concurrent.TimeoutException; /** * <pre> * 正常结束时, 若执行的结果不为null, 则result为执行结果; 若执行结果为null, 则result = {@link AbstractFuture#SUCCESS_SIGNAL} * 异常结束时, result为 {@link CauseHolder} 的实例;若是被取消而导致的异常结束, 则result为 {@link CancellationException} 的实例, 否则为其它异常的实例 * 以下情况会使异步操作由未完成状态转至已完成状态, 也就是在以下情况发生时调用notifyAll方法: * <ul> * <li>异步操作被取消时(cancel方法)</li> * <li>异步操作正常结束时(setSuccess方法)</li> * <li>异步操作异常结束时(setFailure方法)</li> * </ul> * </pre> * * @author lixiaohui * * @param <V> * 异步执行结果的类型 */ public class AbstractFuture<V> implements IFuture<V> { protected volatile Object result; // 需要保证其可见性 /** * 监听器集 */ protected Collection<IFutureListener<V>> listeners = new CopyOnWriteArrayList<IFutureListener<V>>; /** * 当任务正常执行结果为null时, 即客户端调用{@link AbstractFuture#setSuccess(null)}时, * result引用该对象 */ private static final SuccessSignal SUCCESS_SIGNAL = new SuccessSignal; @Override public boolean cancel(boolean mayInterruptIfRunning) { if (isDone) { // 已完成了不能取消 return false; } synchronized (this) { if (isDone) { // double check return false; } result = new CauseHolder(new CancellationException); notifyAll; // isDone = true, 通知等待在该对象的wait的线程 } notifyListeners; // 通知监听器该异步操作已完成 return true; } @Override public boolean isCancellable { return result == null; } @Override public boolean isCancelled { return result != null && result instanceof CauseHolder && ((CauseHolder) result).cause instanceof CancellationException; } @Override public boolean isDone { return result != null; } @Override public V get throws InterruptedException, ExecutionException { await; // 等待执行结果 Throwable cause = cause; if (cause == null) { // 没有发生异常，异步操作正常结束 return getNow; } if (cause instanceof CancellationException) { // 异步操作被取消了 throw (CancellationException) cause; } throw new ExecutionException(cause); // 其他异常 } @Override public V get(long timeout, TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException { if (await(timeout, unit)) {// 超时等待执行结果 Throwable cause = cause; if (cause == null) {// 没有发生异常，异步操作正常结束 return getNow; } if (cause instanceof CancellationException) {// 异步操作被取消了 throw (CancellationException) cause; } throw new ExecutionException(cause);// 其他异常 } // 时间到了异步操作还没有结束, 抛出超时异常 throw new TimeoutException; } @Override public boolean isSuccess { return result == null ? false : !(result instanceof CauseHolder); } @SuppressWarnings("unchecked") @Override public V getNow { return (V) (result == SUCCESS_SIGNAL ? null : result); } @Override public Throwable cause { if (result != null && result instanceof CauseHolder) { return ((CauseHolder) result).cause; } return null; } @Override public IFuture<V> addListener(IFutureListener<V> listener) { if (listener == null) { throw new NullPointerException("listener"); } if (isDone) { // 若已完成直接通知该监听器 notifyListener(listener); return this; } synchronized (this) { if (!isDone) { listeners.add(listener); return this; } } notifyListener(listener); return this; } @Override public IFuture<V> removeListener(IFutureListener<V> listener) { if (listener == null) { throw new NullPointerException("listener"); } if (!isDone) { listeners.remove(listener); } return this; } @Override public IFuture<V> await throws InterruptedException { return await0(true); } private IFuture<V> await0(boolean interruptable) throws InterruptedException { if (!isDone) { // 若已完成就直接返回了 // 若允许终端且被中断了则抛出中断异常 if (interruptable && Thread.interrupted) { throw new InterruptedException("thread " + Thread.currentThread.getName + " has been interrupted."); } boolean interrupted = false; synchronized (this) { while (!isDone) { try { wait; // 释放锁进入waiting状态，等待其它线程调用本对象的notify/notifyAll方法 } catch (InterruptedException e) { if (interruptable) { throw e; } else { interrupted = true; } } } } if (interrupted) { // 为什么这里要设中断标志位？因为从wait方法返回后, 中断标志是被clear了的, // 这里重新设置以便让其它代码知道这里被中断了。 Thread.currentThread.interrupt; } } return this; } @Override public boolean await(long timeoutMillis) throws InterruptedException { return await0(TimeUnit.MILLISECONDS.toNanos(timeoutMillis), true); } @Override public boolean await(long timeout, TimeUnit unit) throws InterruptedException { return await0(unit.toNanos(timeout), true); } private boolean await0(long timeoutNanos, boolean interruptable) throws InterruptedException { if (isDone) { return true; } if (timeoutNanos <= 0) { return isDone; } if (interruptable && Thread.interrupted) { throw new InterruptedException(toString); } long startTime = timeoutNanos <= 0 ? 0 : System.nanoTime; long waitTime = timeoutNanos; boolean interrupted = false; try { synchronized (this) { if (isDone) { return true; } if (waitTime <= 0) { return isDone; } for (;;) { try { wait(waitTime / 1000000, (int) (waitTime % 1000000)); } catch (InterruptedException e) { if (interruptable) { throw e; } else { interrupted = true; } } if (isDone) { return true; } else { waitTime = timeoutNanos - (System.nanoTime - startTime); if (waitTime <= 0) { return isDone; } } } } } finally { if (interrupted) { Thread.currentThread.interrupt; } } } @Override public IFuture<V> awaitUninterruptibly { try { return await0(false); } catch (InterruptedException e) { // 这里若抛异常了就无法处理了 throw new java.lang.InternalError; } } @Override public boolean awaitUninterruptibly(long timeoutMillis) { try { return await0(TimeUnit.MILLISECONDS.toNanos(timeoutMillis), false); } catch (InterruptedException e) { throw new java.lang.InternalError; } } @Override public boolean awaitUninterruptibly(long timeout, TimeUnit unit) { try { return await0(unit.toNanos(timeout), false); } catch (InterruptedException e) { throw new java.lang.InternalError; } } protected IFuture<V> setFailure(Throwable cause) { if (setFailure0(cause)) { notifyListeners; return this; } throw new IllegalStateException("complete already: " + this); } private boolean setFailure0(Throwable cause) { if (isDone) { return false; } synchronized (this) { if (isDone) { return false; } result = new CauseHolder(cause); notifyAll; } return true; } protected IFuture<V> setSuccess(Object result) { if (setSuccess0(result)) { // 设置成功后通知监听器 notifyListeners; return this; } throw new IllegalStateException("complete already: " + this); } private boolean setSuccess0(Object result) { if (isDone) { return false; } synchronized (this) { if (isDone) { return false; } if (result == null) { // 异步操作正常执行完毕的结果是null this.result = SUCCESS_SIGNAL; } else { this.result = result; } notifyAll; } return true; } private void notifyListeners { for (IFutureListener<V> l : listeners) { notifyListener(l); } } private void notifyListener(IFutureListener<V> l) { try { l.operationCompleted(this); } catch (Exception e) { e.printStackTrace; } } private static class SuccessSignal { } private static final class CauseHolder { final Throwable cause; CauseHolder(Throwable cause) { this.cause = cause; } } }

那么要怎么使用这个呢，有了上面的骨架实现，我们就可以定制各种各样的异步结果了。下面模拟一下一个延时的任务：

package future.test; import future.IFuture; import future.IFutureListener; /** * 延时加法 * @author lixiaohui * */ public class DelayAdder { public static void main(String[] args) { new DelayAdder.add(3 * 1000, 1, 2).addListener(new IFutureListener<Integer> { @Override public void operationCompleted(IFuture<Integer> future) throws Exception { System.out.println(future.getNow); } }); } /** * 延迟加 * @param delay 延时时长 milliseconds * @param a 加数 * @param b 加数 * @return 异步结果 */ public DelayAdditionFuture add(long delay, int a, int b) { DelayAdditionFuture future = new DelayAdditionFuture; new Thread(new DelayAdditionTask(delay, a, b, future)).start; return future; } private class DelayAdditionTask implements Runnable { private long delay; private int a, b; private DelayAdditionFuture future; public DelayAdditionTask(long delay, int a, int b, DelayAdditionFuture future) { super; this.delay = delay; this.a = a; this.b = b; this.future = future; } @Override public void run { try { Thread.sleep(delay); Integer i = a + b; // TODO 这里设置future为完成状态(正常执行完毕) future.setSuccess(i); } catch (InterruptedException e) { // TODO 这里设置future为完成状态(异常执行完毕) future.setFailure(e.getCause); } } } } package future.test; import future.AbstractFuture; import future.IFuture; //只是把两个方法对外暴露 public class DelayAdditionFuture extends AbstractFuture<Integer> { @Override public IFuture<Integer> setSuccess(Object result) { return super.setSuccess(result); } @Override public IFuture<Integer> setFailure(Throwable cause) { return super.setFailure(cause); } }

可以看到客户端不用主动去询问future是否完成，而是future完成时自动回调operationcompleted方法，客户端只需在回调里实现逻辑即可。

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