首先我们来看看future和promise接口整体设计
最顶层的future是jdk的，第二个是netty自定义的future，两个同名，继承关系
看看jdk的future接口
public interface Future<V> {
// 取消任务
boolean cancel(boolean mayInterruptIfRunning);
// 任务是否取消
boolean isCancelled();
// 任务是否完成
boolean isDone();
// 阻塞的获取执行的结果
V get() throws InterruptedException, ExecutionException;
// 在一定时间内-超时-阻塞的获取执行的结果
V get(long timeout, TimeUnit unit)
throws InterruptedException, ExecutionException, TimeoutException;
}
瞅瞅netty的future接口
public interface Future<V> extends java.util.concurrent.Future<V> {
// 是否成功
boolean isSuccess();
// 是否取消
boolean isCancellable();
Throwable cause();
// 添加listener进行回调
Future<V> addListener(GenericFutureListener<? extends Future<? super V>> listener);
Future<V> addListeners(GenericFutureListener<? extends Future<? super V>>... listeners);
Future<V> removeListener(GenericFutureListener<? extends Future<? super V>> listener);
Future<V> removeListeners(GenericFutureListener<? extends Future<? super V>>... listeners);
// 阻塞的等待任务执行，如果失败则抛出失败原因的异常
Future<V> sync() throws InterruptedException;
// 不响应中断等待异常
Future<V> syncUninterruptibly();
// 阻塞等待任务执行，失败不抛异常
Future<V> await() throws InterruptedException;
Future<V> awaitUninterruptibly();
boolean await(long timeout, TimeUnit unit) throws InterruptedException;
boolean await(long timeoutMillis) throws InterruptedException;
boolean awaitUninterruptibly(long timeout, TimeUnit unit);
boolean awaitUninterruptibly(long timeoutMillis);
// 马上获取到任务的结果，不阻塞，而jdk的future是阻塞的
V getNow();
// 取消任务执行，如果取消成功，任务会因为 CancellationException 异常而导致失败
// 也就是 isSuccess()==false，同时上面的 cause() 方法返回 CancellationException 的实例。
// mayInterruptIfRunning 说的是：是否对正在执行该任务的线程进行中断(这样才能停止该任务的执行)，
// 似乎 Netty 中 Future 接口的各个实现类，都没有使用这个参数
@Override
boolean cancel(boolean mayInterruptIfRunning);
}
netty的future在jdk的基础上扩展了它需要的方法，sync和await的区别我们放到下面看实现类的时候说
同时我们也可以看到，这个future接口跟io操作是无关的
接下来我们看看ChannelFuture接口，接口注释上写的很清楚，我们来看看
* The result of an asynchronous {@link Channel} I/O operation.
* <p>
* All I/O operations in Netty are asynchronous. It means any I/O calls will
* return immediately with no guarantee that the requested I/O operation has
* been completed at the end of the call. Instead, you will be returned with
* a {@link ChannelFuture} instance which gives you the information about the
* result or status of the I/O operation.
* <p>
* A {@link ChannelFuture} is either <em>uncompleted</em> or <em>completed</em>.
* When an I/O operation begins, a new future object is created. The new future
* is uncompleted initially - it is neither succeeded, failed, nor cancelled
* because the I/O operation is not finished yet. If the I/O operation is
* finished either successfully, with failure, or by cancellation, the future is
* marked as completed with more specific information, such as the cause of the
* failure. Please note that even failure and cancellation belong to the
* completed state.
所有io操作都是异步的，一个io操作的调用会立即返回一个带有结果或者状态的io实例。
io操作要么是未完成的，要么是完成的。当它开始时，future会被创建，一开始是未完成的，未完成的时候没有成功、失败或者取消状态
当它是完成的时候，可以是失败或者取消的，失败或者取消原因会被附加到future上。
* <pre> * +---------------------------+ * | Completed successfully | * +---------------------------+ * +----> isDone() = true | * +--------------------------+ | | isSuccess() = true | * | Uncompleted | | +===========================+ * +--------------------------+ | | Completed with failure | * | isDone() = false | | +---------------------------+ * | isSuccess() = false |----+----> isDone() = true | * | isCancelled() = false | | | cause() = non-null | * | cause() = null | | +===========================+ * +--------------------------+ | | Completed by cancellation | * | +---------------------------+ * +----> isDone() = true | * | isCancelled() = true | * +---------------------------+ * </pre>
上面那个图很清楚了，在两种过程的时候会有什么状态，我们看看接口
public interface ChannelFuture extends Future<Void> {
// 返回future关联的channel
Channel channel();
// 重写下面几个方法，修改返回值为channelfuture
@Override
ChannelFuture addListener(GenericFutureListener<? extends Future<? super Void>> listener);
@Override
ChannelFuture addListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);
@Override
ChannelFuture removeListener(GenericFutureListener<? extends Future<? super Void>> listener);
@Override
ChannelFuture removeListeners(GenericFutureListener<? extends Future<? super Void>>... listeners);
@Override
ChannelFuture sync() throws InterruptedException;
@Override
ChannelFuture syncUninterruptibly();
@Override
ChannelFuture await() throws InterruptedException;
@Override
ChannelFuture awaitUninterruptibly();
/**
* Returns {@code true} if this {@link ChannelFuture} is a void future and so not allow to call any of the
* following methods:
* <ul>
* <li>{@link #addListener(GenericFutureListener)}</li>
* <li>{@link #addListeners(GenericFutureListener[])}</li>
* <li>{@link #await()}</li>
* <li>{@link #await(long, TimeUnit)} ()}</li>
* <li>{@link #await(long)} ()}</li>
* <li>{@link #awaitUninterruptibly()}</li>
* <li>{@link #sync()}</li>
* <li>{@link #syncUninterruptibly()}</li>
* </ul>
标记该future是void的，使不能使用上面的方法
*/
boolean isVoid();
}