Question

Java in a Nutshell

异步I / O的回调样式基于 CompletionHandler ，定义了两个方法completed()和failed()，它们将在回调时被回调操作成功或失败。如果要在异步中立即通知事件，则此样式很有用 I / O-例如，如果飞行中有大量I / O操作，但是失败任何单一操作都不一定是致命的。

来自http://www.deadcoderising.com/java8-writing-asynchronous-code-with-completablefuture/

除了实施Future界面之外，还CompletableFuture   实现CompletionStage接口。

CompletionStage是一个承诺。它承诺计算   最终将完成。

CompletionStage的伟大之处在于它提供了广阔的空间   选择允许您附加回调的方法   在完成时执行。

这样我们就可以以非阻塞的方式构建系统。

CompletionHandler和CompletableFuture都可用于指定回调处理程序。

他们的关系和差异是什么？

CompletionHandler和CompletableFuture两种不同的Java异步编程方法吗？

或者它们是否一起使用？

感谢。

Answer 1

CompletionHandler<V, A>是NIO异步通道的完成界面。

它是在Java 7中引入的，几年前Java 8引入了lambda表达式并将它们转换为功能接口（使用单个抽象方法的接口），因此它有两个方法，completed(V result, A attachment)和{{1} }，而不是（现在）更舒适的单一方法。

failed(Throwable ex, A attachment)是CompletableFuture<T>接口的实现。

它是在Java 8中引入的，因此构建future的静态方法及其构建continuation的实例方法使用功能接口，您可以使用它们轻松使用lambda表达式。

您可以包装每个NIO异步调用以使用CompletionStage<T>：

CompletableFuture<T>

示例用法（仅用于说明;没有错误处理，不处理资源）：

import java.lang.*;
import java.net.*;
import java.nio.*;
import java.nio.channels.*;
import java.util.*;
import java.util.concurrent.*;

public class AsynchronousCompletionHandler<T> implements CompletionHandler<T, CompletableFuture<T>> {
    public void completed(T result, CompletableFuture<T> attachment) {
        attachment.complete(result);
    }

    public void failed(Throwable ex, CompletableFuture<T> attachment) {
        attachment.completeExceptionally(ex);
    }

    private static final ConcurrentHashMap<Class<?>, AsynchronousCompletionHandler<?>> cache = new ConcurrentHashMap<>();

    static <T> AsynchronousCompletionHandler<T> getInstance(Class<T> clazz) {
        @SuppressWarnings("unchecked")
        AsynchronousCompletionHandler<T> handler = (AsynchronousCompletionHandler<T>)cache.computeIfAbsent(clazz, c -> new AsynchronousCompletionHandler<T>());
        return handler;
    }

    //
    // AsynchronousByteChannel
    public static CompletableFuture<Integer> readAsync(AsynchronousByteChannel channel, ByteBuffer dst) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.read(dst, completableFuture, getInstance(Integer.class));
        return completableFuture;
    }

    public static CompletableFuture<Integer> writeAsync(AsynchronousByteChannel channel, ByteBuffer src) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.write(src, completableFuture, getInstance(Integer.class));
        return completableFuture;
    }

    //
    // AsynchronousFileChannel
    public static CompletableFuture<FileLock> lockAsync(AsynchronousFileChannel channel) {
        CompletableFuture<FileLock> completableFuture = new CompletableFuture<>();
        channel.lock(completableFuture, getInstance(FileLock.class));
        return completableFuture;
    }

    public static CompletableFuture<FileLock> lockAsync(AsynchronousFileChannel channel, long position, long size, boolean shared) {
        CompletableFuture<FileLock> completableFuture = new CompletableFuture<>();
        channel.lock(position, size, shared, completableFuture, getInstance(FileLock.class));
        return completableFuture;
    }

    public static CompletableFuture<Integer> readAsync(AsynchronousFileChannel channel, ByteBuffer dst, long position) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.read(dst, position, completableFuture, getInstance(Integer.class));
        return completableFuture;
    }

    public static CompletableFuture<Integer> writeAsync(AsynchronousFileChannel channel, ByteBuffer src, long position) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.write(src, position, completableFuture, getInstance(Integer.class));
        return completableFuture;
    }

    //
    // AsynchronousServerSocketChannel
    public static CompletableFuture<AsynchronousSocketChannel> acceptAsync(AsynchronousServerSocketChannel channel) {
        CompletableFuture<AsynchronousSocketChannel> completableFuture = new CompletableFuture<>();
        channel.accept(completableFuture, getInstance(AsynchronousSocketChannel.class));
        return completableFuture;
    }

    //
    // AsynchronousSocketChannel
    public static CompletableFuture<Void> connectAsync(AsynchronousSocketChannel channel, SocketAddress remote) {
        CompletableFuture<Void> completableFuture = new CompletableFuture<>();
        channel.connect(remote, completableFuture, getInstance(Void.class));
        return completableFuture;
    }

    public static CompletableFuture<Long> readAsync(AsynchronousSocketChannel channel, ByteBuffer[] dsts, int offset, int length, long timeout, TimeUnit unit) {
        CompletableFuture<Long> completableFuture = new CompletableFuture<>();
        channel.read(dsts, offset, length, timeout, unit, completableFuture, getInstance(Long.class));
        return completableFuture;
    }

    public static CompletableFuture<Integer> readAsync(AsynchronousSocketChannel channel, ByteBuffer dst) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.read(dst, completableFuture, getInstance(Integer.class));
        return completableFuture;
    } 

    public static CompletableFuture<Integer> readAsync(AsynchronousSocketChannel channel, ByteBuffer dst, long timeout, TimeUnit unit) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.read(dst, timeout, unit, completableFuture, getInstance(Integer.class));
        return completableFuture;
    }

    public static CompletableFuture<Long> writeAsync(AsynchronousSocketChannel channel, ByteBuffer[] srcs, int offset, int length, long timeout, TimeUnit unit) {
        CompletableFuture<Long> completableFuture = new CompletableFuture<>();
        channel.write(srcs, offset, length, timeout, unit, completableFuture, getInstance(Long.class));
        return completableFuture;
    }

    public static CompletableFuture<Integer> writeAsync(AsynchronousSocketChannel channel, ByteBuffer src) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.write(src, completableFuture, getInstance(Integer.class));
        return completableFuture;
    } 

    public static CompletableFuture<Integer> writeAsync(AsynchronousSocketChannel channel, ByteBuffer src, long timeout, TimeUnit unit) {
        CompletableFuture<Integer> completableFuture = new CompletableFuture<>();
        channel.write(src, timeout, unit, completableFuture, getInstance(Integer.class));
        return completableFuture;
    }
}

Answer 2

如果你看一下CompletableFuture（自Java 8以来），你会发现它拥有大量的功能，不仅仅是回调。具有链接，组合和其他有趣的功能。

将其与CompletionHandler（自Java 7以来）进行比较，差异应该是显而易见的。

没有什么可以阻止您同时使用这两种方式，甚至可能根据您使用的API类型进行操作，但如果您有机会使用CompletableFuture，那么您真的不会这样做。需要加强CompletionHandler。

“CompletionHandler”和“CompletableFuture”是Java中异步编程的两种不同方法吗？

2 个答案: