《netty》netty源码分析之future和promise

future

 java.util.concurrent.Future,   Future表示一个可能还没有完成的异步任务的结果，针对这个结果可以添加Callback以便在任务执行成功或失败后作出相应的操作。

// 取消异步操作
boolean cancel(boolean mayInterruptIfRunning);
// 异步操作是否取消
boolean isCancelled();
// 异步操作是否完成，正常终止、异常、取消都是完成
boolean isDone();
// 阻塞直到取得异步操作结果
V get() throws InterruptedException, ExecutionException;
// 同上，但最长阻塞时间为timeout
V get(long timeout, TimeUnit unit)
    throws InterruptedException, ExecutionException, TimeoutException;

 

举个例子: FutureTask-->RunnableFuture--> Future

	public static void main(String[] args) throws InterruptedException, ExecutionException {
		Callable callable = new Callable(){
			 
			@Override
			public String call() throws Exception {
				try {
					Thread.sleep(1000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				return "工作做完了";
			}
		};
		FutureTask futureTask = new FutureTask(callable);
		
		new Thread(futureTask).start();
		
		//阻塞或取结果
		System.out.println("----"+futureTask.get());
	}

 

ChannelFuture

netty对future做了扩展

public interface ChannelFuture extends Future {

    /**
     * Returns a channel where the I/O operation associated with this
     * future takes place.
     */
    Channel channel();


    // 异步操作完成且正常终止
     boolean isSuccess();

    // 异步操作是否可以取消
    boolean isCancellable();

     // 异步操作失败的原因
    Throwable cause();

    // 添加一个监听者，异步操作完成时回调，类比javascript的回调函数
    Future addListener(GenericFutureListener> listener);
    Future removeListener(GenericFutureListener> listener);

    // 阻塞直到异步操作完成
    Future await() throws InterruptedException;

    // 同上，但异步操作失败时抛出异常
    Future sync() throws InterruptedException;

    // 非阻塞地返回异步结果，如果尚未完成返回null
    V getNow();
}

重点看一下

 ChannelFuture addListener(GenericFutureListener> listener);

GenericFutureListener

public interface GenericFutureListener> extends EventListener {

    /**
     * Invoked when the operation associated with the {@link Future} has been completed.
     *
     * @param future  the source {@link Future} which called this callback
     */
    void operationComplete(F future) throws Exception;
}

这个类相当于执行操作worker ,  执行完成后 回调.  对与主线程来说是异步的.

监听器对Future的扩展起到了很灵活的作用，当某个计算完毕，会触发相应的时间，得到Future的结果，因为jdk的get方法我们不知道什么时候去掉好，调早了需要等待，调晚了浪费了一段时间，还有isDone里边有2种情况，无法区分到底是正常的io完毕返回的true还是被取消之后返回的true，所有到了netty的Future里边加了一个isSuccess()方法，只有正常的io处理结束isSuccess()才返回true。

ChannelFutureListener 

/**
 * Listens to the result of a {@link ChannelFuture}.  The result of the
 * asynchronous {@link Channel} I/O operation is notified once this listener
 * is added by calling {@link ChannelFuture#addListener(GenericFutureListener)}.
 *
 * 
Return the control to the caller quickly
 *
 * {@link #operationComplete(Future)} is directly called by an I/O
 * thread.  Therefore, performing a time consuming task or a blocking operation
 * in the handler method can cause an unexpected pause during I/O.  If you need
 * to perform a blocking operation on I/O completion, try to execute the
 * operation in a different thread using a thread pool.
 */
public interface ChannelFutureListener extends GenericFutureListener {

    /**
     * A {@link ChannelFutureListener} that closes the {@link Channel} which is
     * associated with the specified {@link ChannelFuture}.
     */
    ChannelFutureListener CLOSE = new ChannelFutureListener() {
        @Override
        public void operationComplete(ChannelFuture future) {
            future.channel().close();
        }
    };

    /**
     * A {@link ChannelFutureListener} that closes the {@link Channel} when the
     * operation ended up with a failure or cancellation rather than a success.
     */
    ChannelFutureListener CLOSE_ON_FAILURE = new ChannelFutureListener() {
        @Override
        public void operationComplete(ChannelFuture future) {
            if (!future.isSuccess()) {
                future.channel().close();
            }
        }
    };

    /**
     * A {@link ChannelFutureListener} that forwards the {@link Throwable} of the {@link ChannelFuture} into the
     * {@link ChannelPipeline}. This mimics the old behavior of Netty 3.
     */
    ChannelFutureListener FIRE_EXCEPTION_ON_FAILURE = new ChannelFutureListener() {
        @Override
        public void operationComplete(ChannelFuture future) {
            if (!future.isSuccess()) {
                future.channel().pipeline().fireExceptionCaught(future.cause());
            }
        }
    };

    // Just a type alias
}

 

CompleteFuture 

    @Override
    public Future addListener(GenericFutureListener> listener) {
        if (listener == null) {
            throw new NullPointerException("listener");
        }
        DefaultPromise.notifyListener(executor(), this, listener);
        return this;
    }

 看下如何添加监听   DefaultPromise.notifyListener(executor(), this, listener);

追到了DefaultPromise 下面.

    //DefaultPromise
    private static void notifyListenerWithStackOverFlowProtection(final EventExecutor executor,
                                                                  final Future future,
                                                                  final GenericFutureListener listener) {
        if (executor.inEventLoop()) {
            final InternalThreadLocalMap threadLocals = InternalThreadLocalMap.get();
            final int stackDepth = threadLocals.futureListenerStackDepth();
            if (stackDepth < MAX_LISTENER_STACK_DEPTH) {
                threadLocals.setFutureListenerStackDepth(stackDepth + 1);
                try {
                    notifyListener0(future, listener);
                } finally {
                    threadLocals.setFutureListenerStackDepth(stackDepth);
                }
                return;
            }
        }

        safeExecute(executor, new Runnable() {
            @Override
            public void run() {
                notifyListener0(future, listener);
            }
        });
    }

看下safeExecute() 怎么执行的,  哦哦原来是一个线程池.

    private static void safeExecute(EventExecutor executor, Runnable task) {
        try {
            executor.execute(task);
        } catch (Throwable t) {
            rejectedExecutionLogger.error("Failed to submit a listener notification task. Event loop shut down?", t);
        }
    }

再看 notifyListener0(future, listener);  原来是手动调用--监听器operationComplete接口方法.

    private static void notifyListener0(Future future, GenericFutureListener l) {
        try {
            l.operationComplete(future);
        } catch (Throwable t) {
            if (logger.isWarnEnabled()) {
                logger.warn("An exception was thrown by " + l.getClass().getName() + ".operationComplete()", t);
            }
        }
    }

 

 CompleteChannelFuture << CompleteFuture<

CompleteFuture中定义了executor()

public abstract class CompleteFuture extends AbstractFuture {

    private final EventExecutor executor;

    /**
     * Creates a new instance.
     *
     * @param executor the {@link EventExecutor} associated with this future
     */
    protected CompleteFuture(EventExecutor executor) {
        this.executor = executor;
    }

    /**
     * Return the {@link EventExecutor} which is used by this {@link CompleteFuture}.
     */
    protected EventExecutor executor() {
        return executor;
    }
}

 

回调函数

https://blog.csdn.net/jiayi_yao/article/details/51046526

promise

promise类似,提供了write的异步回调.

总结:  对future接口做扩展,  通过线程池,异步执行事件监听器实现.