实践反应式编程

Posted on 2020-02-29 | In design | Visitors

响应式编程（reactive programming）是一种基于数据流（data stream）和变化传递（propagation of change）的声明式（declarative）的编程范式。

数据驱动程序执行，而不用阻塞等待数据，没有数据，什么也不发生。

The Reactive Manifesto

The Reactive Manifesto

Reactive Streams

Reactive Streams

<dependency>
  <groupId>org.reactivestreams</groupId>
  <artifactId>reactive-streams</artifactId>
  <version>1.0.2</version>
</dependency>

Publisher

package org.reactivestreams;

/**
 * A {@link Publisher} is a provider of a potentially unbounded number of sequenced elements, publishing them according to
 * the demand received from its {@link Subscriber}(s).
 * <p>
 * A {@link Publisher} can serve multiple {@link Subscriber}s subscribed {@link #subscribe(Subscriber)} dynamically
 * at various points in time.
 *
 * @param <T> the type of element signaled.
 */
public interface Publisher<T> {

    /**
     * Request {@link Publisher} to start streaming data.
     * <p>
     * This is a "factory method" and can be called multiple times, each time starting a new {@link Subscription}.
     * <p>
     * Each {@link Subscription} will work for only a single {@link Subscriber}.
     * <p>
     * A {@link Subscriber} should only subscribe once to a single {@link Publisher}.
     * <p>
     * If the {@link Publisher} rejects the subscription attempt or otherwise fails it will
     * signal the error via {@link Subscriber#onError}.
     *
     * @param s the {@link Subscriber} that will consume signals from this {@link Publisher}
     */
    public void subscribe(Subscriber<? super T> s);
}

Subscriber

package org.reactivestreams;

/**
 * Will receive call to {@link #onSubscribe(Subscription)} once after passing an instance of {@link Subscriber} to {@link Publisher#subscribe(Subscriber)}.
 * <p>
 * No further notifications will be received until {@link Subscription#request(long)} is called.
 * <p>
 * After signaling demand:
 * <ul>
 * <li>One or more invocations of {@link #onNext(Object)} up to the maximum number defined by {@link Subscription#request(long)}</li>
 * <li>Single invocation of {@link #onError(Throwable)} or {@link Subscriber#onComplete()} which signals a terminal state after which no further events will be sent.
 * </ul>
 * <p>
 * Demand can be signaled via {@link Subscription#request(long)} whenever the {@link Subscriber} instance is capable of handling more.
 *
 * @param <T> the type of element signaled.
 */
public interface Subscriber<T> {
    /**
     * Invoked after calling {@link Publisher#subscribe(Subscriber)}.
     * <p>
     * No data will start flowing until {@link Subscription#request(long)} is invoked.
     * <p>
     * It is the responsibility of this {@link Subscriber} instance to call {@link Subscription#request(long)} whenever more data is wanted.
     * <p>
     * The {@link Publisher} will send notifications only in response to {@link Subscription#request(long)}.
     * 
     * @param s
     *            {@link Subscription} that allows requesting data via {@link Subscription#request(long)}
     */
    public void onSubscribe(Subscription s);

    /**
     * Data notification sent by the {@link Publisher} in response to requests to {@link Subscription#request(long)}.
     * 
     * @param t the element signaled
     */
    public void onNext(T t);

    /**
     * Failed terminal state.
     * <p>
     * No further events will be sent even if {@link Subscription#request(long)} is invoked again.
     *
     * @param t the throwable signaled
     */
    public void onError(Throwable t);

    /**
     * Successful terminal state.
     * <p>
     * No further events will be sent even if {@link Subscription#request(long)} is invoked again.
     */
    public void onComplete();
}

Subscription

package org.reactivestreams;

/**
 * A {@link Subscription} represents a one-to-one lifecycle of a {@link Subscriber} subscribing to a {@link Publisher}.
 * <p>
 * It can only be used once by a single {@link Subscriber}.
 * <p>
 * It is used to both signal desire for data and cancel demand (and allow resource cleanup).
 *
 */
public interface Subscription {
    /**
     * No events will be sent by a {@link Publisher} until demand is signaled via this method.
     * <p>
     * It can be called however often and whenever needed—but the outstanding cumulative demand must never exceed Long.MAX_VALUE.
     * An outstanding cumulative demand of Long.MAX_VALUE may be treated by the {@link Publisher} as "effectively unbounded".
     * <p>
     * Whatever has been requested can be sent by the {@link Publisher} so only signal demand for what can be safely handled.
     * <p>
     * A {@link Publisher} can send less than is requested if the stream ends but
     * then must emit either {@link Subscriber#onError(Throwable)} or {@link Subscriber#onComplete()}.
     * 
     * @param n the strictly positive number of elements to requests to the upstream {@link Publisher}
     */
    public void request(long n);

    /**
     * Request the {@link Publisher} to stop sending data and clean up resources.
     * <p>
     * Data may still be sent to meet previously signalled demand after calling cancel.
     */
    public void cancel();
}

Processor

package org.reactivestreams;

/**
 * A Processor represents a processing stage—which is both a {@link Subscriber}
 * and a {@link Publisher} and obeys the contracts of both.
 *
 * @param <T> the type of element signaled to the {@link Subscriber}
 * @param <R> the type of element signaled by the {@link Publisher}
 */
public interface Processor<T, R> extends Subscriber<T>, Publisher<R> {
}

特点

变化传递（propagation of change）
数据流（data stream）

具备“异步非阻塞”特性和“流量控制”能力的数据流，我们称之为响应式流（Reactive Stream）。
申明式（declarative）

相对于命令式，命令式是面向过程的，声明式是面向结构的
背压（backpressure）
异步非阻塞

参考实现

相关书籍

《Java编程方法论：响应式Spring Reactor 3设计与实现》

《Java编程方法论：响应式RxJava与代码设计实战》

《Spring响应式微服务：Spring Boot 2+Spring 5+Spring Cloud实战》

《Hands-On Reactive Programming in Spring 5》《Spring响应式编程》

《Java异步编程实战》