WebFlux基础之响应式编程

　　上篇文章，我们简单的了解了WebFlux的一些基础与背景，并通过示例来写了一个demo。我们知道WebFlux是响应式的web框架，其特点之一就是可以通过函数式编程方式配置route。另外究竟什么是响应式编程呢？这篇文章我们就简单探讨一下

一、Java8中的函数式编程

　　百科中这样定义函数式编程：

　　函数式编程是种编程方式，它将电脑运算视为函数的计算。函数编程语言最重要的基础是λ演算（lambda calculus），而且λ演算的函数可以接受函数当作输入（参数）和输出（返回值)。那么在Java8里怎么样来实现它呢？

示例一

在这里我先自己写一个例子

定义接口：

package com.bdqn.lyrk.basic.java;

/**
 * 函数式接口
 *
 * @author chen.nie
 * @date 2018/7/18
 **/
@FunctionalInterface
public interface OperateNumberFunctions {

    void operate(Integer number);

    default void print() {
        
    }
}

　　　　 

　　在定义的接口上添加@FunctionalInterface表明其是函数式接口，这个注解用于检测函数式接口规范，定义函数式接口时该接口内必须有且只有一个抽象的方法。

定义类：

package com.bdqn.lyrk.basic.java;

import java.util.Optional;
import java.util.function.Predicate;

/**
 * 定义函数式编程类
 */
public class NumberFunctions {

    private Integer number;

    private NumberFunctions() {
    }

    private static NumberFunctions numberFunctions = new NumberFunctions();

    public static NumberFunctions of(Integer number) {
        numberFunctions.number = number;
        return numberFunctions;
    }

    public NumberFunctions add(Integer number) {
        numberFunctions.number += number;
        return numberFunctions;
    }

    public NumberFunctions subtraction(Integer number) {
        numberFunctions.number -= number;
        return numberFunctions;
    }

    public Optional<NumberFunctions> filter(Predicate<Integer> predicate) {
        if (predicate.test(this.number)) return Optional.of(numberFunctions);
        return Optional.ofNullable(new NumberFunctions());

    }

    public void operate(OperateNumberFunctions functions) {
        functions.operate(this.number);
    }
}

　　在这里定义类进行简单的运算与过滤条件。那么在Main方法里可以这么写：

package com.bdqn.lyrk.basic.java;

public class Main {

    public static void main(String[] args) {
        NumberFunctions.of(10).add(30).subtraction(2).filter(number -> number>20).get().operate(System.out::println);
    }
}

　　那么输出结果为38

示例二

　　在Java8里有一个类叫Stream。Stream是数据流的意思，这个类略微有点像Reactor中Flux，它提供了类似于操作符的功能，我们来看一个例子：

Main方法

package com.bdqn.lyrk.basic.java;

import java.util.stream.Stream;

import static java.util.stream.Collectors.toList;

public class Main {

    public static void main(String[] args) {
        /*
            在这里先将Stream里的内容做乘2的操作
            然后在进行倒序排序
            紧接着过滤出是4的倍数的数字
            然后转换成集合在打印
         */
        Stream.of(15, 26, 34, 455, 5, 6).map(number -> number * 2).sorted((num1, num2) -> num2 - num1).filter(integer -> integer % 4 == 0).collect(toList()).forEach(System.out::println);
    }
}

　　运行得到的结果：

68
52
12

关于::操作符

　　该操作符是lambda表达式的更特殊写法，使用此操作符可以简化函数式接口的实现，这个方法至少满足以下特定条件：

　　1）方法返回值与函数式接口相同

　　2）方法参数与函数式接口相同

　　举例说明

package java.util.function;

/**
 * Represents a supplier of results.
 *
 * <p>There is no requirement that a new or distinct result be returned each
 * time the supplier is invoked.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #get()}.
 *
 * @param <T> the type of results supplied by this supplier
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}

　　java中Runnable接口:

@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();
}

　　java中的Predicate接口:

package java.util.function;

import java.util.Objects;

/**
 * Represents a predicate (boolean-valued function) of one argument.
 *
 * <p>This is a <a href="package-summary.html">functional interface</a>
 * whose functional method is {@link #test(Object)}.
 *
 * @param <T> the type of the input to the predicate
 *
 * @since 1.8
 */
@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param <T> the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

那么上述的接口分别可以使用如下写法，注意实现该接口的方法特点

package com.bdqn.lyrk.basic.java;

import java.util.function.Predicate;
import java.util.function.Supplier;

public class Main {
    private static int i;

    public static void main(String[] args) {

        /*
            创建对象的方式
         */
        Supplier<Object> supplier = Object::new;

        /*
            调用方法的方式(无参数)
         */
        Runnable runnable = Main::add;

        /*
            调用方法的方式(有参数)
         */
        Predicate<String> predicate = Main::filter;
    }

    public static void add() {
        i++;
        System.out.println("test" + i);
    }

    public static boolean filter(String test) {
        return test != null;
    }
}

我们可以看到使用函数式编程借助于lambda表达式，使得代码更简洁清爽 

二、Java中的响应式编程

　　关于响应式编程，百度百科是这么定义的：

　　简称RP（Reactive Programming）

　　响应式编程是一种面向数据流和变化传播的编程范式。这意味着可以在编程语言中很方便地表达静态或动态的数据流，而相关的计算模型会自动将变化的值通过数据流进行传播。

　　在这里有两个关键词：数据流与变化传播。下面我们来通过代码来演示下响应式编程是怎么回事

　Java8及以前版本

　　最典型的示例就是，JDK提供的观察者模式类Observer与Observalbe：

package com.hzgj.lyrk.demo;

import java.util.Observable;

public class ObserverDemo extends Observable {

    public static void main(String[] args) {
        ObserverDemo observable = new ObserverDemo();
        observable.addObserver((o, arg) -> {
            System.out.println("发生变化");
        });
        observable.addObserver((o, arg) -> {
            System.out.println("收到被观察者通知，准备改变");
        });
        observable.setChanged();
        observable.notifyObservers();
    }
}

　　在上述代码示例中观察者并没有及时执行，而是在接受到被观察者发送信号的时候才有了“响应”。其中setChanged()与notifyObservers方法就对应响应式编程中定义的关键词--变化与传播。还有一个典型的示例就是Swing中的事件机制，有兴趣的朋友可以下去查阅相关资料，在这里就不再进行阐述。

　Java9及其后版本

　 从java9开始，Observer与Observable已经被标记为过时的类了，取而代之的是Flow类。Flow才是真正意义上的响应式编程类，因为观察者Observer与Observable虽然能够响应，但是在数据流的体现并不是特别突出。Flow这个类，我们可以先看一下：

　　

public final class Flow {

    private Flow() {} // uninstantiable

    /**
     * A producer of items (and related control messages) received by
     * Subscribers.  Each current {@link Subscriber} receives the same
     * items (via method {@code onNext}) in the same order, unless
     * drops or errors are encountered. If a Publisher encounters an
     * error that does not allow items to be issued to a Subscriber,
     * that Subscriber receives {@code onError}, and then receives no
     * further messages.  Otherwise, when it is known that no further
     * messages will be issued to it, a subscriber receives {@code
     * onComplete}.  Publishers ensure that Subscriber method
     * invocations for each subscription are strictly ordered in <a
     * href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
     * order.
     *
     * <p>Publishers may vary in policy about whether drops (failures
     * to issue an item because of resource limitations) are treated
     * as unrecoverable errors.  Publishers may also vary about
     * whether Subscribers receive items that were produced or
     * available before they subscribed.
     *
     * @param <T> the published item type
     */
    @FunctionalInterface
    public static interface Publisher<T> {
        /**
         * Adds the given Subscriber if possible.  If already
         * subscribed, or the attempt to subscribe fails due to policy
         * violations or errors, the Subscriber's {@code onError}
         * method is invoked with an {@link IllegalStateException}.
         * Otherwise, the Subscriber's {@code onSubscribe} method is
         * invoked with a new {@link Subscription}.  Subscribers may
         * enable receiving items by invoking the {@code request}
         * method of this Subscription, and may unsubscribe by
         * invoking its {@code cancel} method.
         *
         * @param subscriber the subscriber
         * @throws NullPointerException if subscriber is null
         */
        public void subscribe(Subscriber<? super T> subscriber);
    }

    /**
     * A receiver of messages.  The methods in this interface are
     * invoked in strict sequential order for each {@link
     * Subscription}.
     *
     * @param <T> the subscribed item type
     */
    public static interface Subscriber<T> {
        /**
         * Method invoked prior to invoking any other Subscriber
         * methods for the given Subscription. If this method throws
         * an exception, resulting behavior is not guaranteed, but may
         * cause the Subscription not to be established or to be cancelled.
         *
         * <p>Typically, implementations of this method invoke {@code
         * subscription.request} to enable receiving items.
         *
         * @param subscription a new subscription
         */
        public void onSubscribe(Subscription subscription);

        /**
         * Method invoked with a Subscription's next item.  If this
         * method throws an exception, resulting behavior is not
         * guaranteed, but may cause the Subscription to be cancelled.
         *
         * @param item the item
         */
        public void onNext(T item);

        /**
         * Method invoked upon an unrecoverable error encountered by a
         * Publisher or Subscription, after which no other Subscriber
         * methods are invoked by the Subscription.  If this method
         * itself throws an exception, resulting behavior is
         * undefined.
         *
         * @param throwable the exception
         */
        public void onError(Throwable throwable);

        /**
         * Method invoked when it is known that no additional
         * Subscriber method invocations will occur for a Subscription
         * that is not already terminated by error, after which no
         * other Subscriber methods are invoked by the Subscription.
         * If this method throws an exception, resulting behavior is
         * undefined.
         */
        public void onComplete();
    }

    /**
     * Message control linking a {@link Publisher} and {@link
     * Subscriber}.  Subscribers receive items only when requested,
     * and may cancel at any time. The methods in this interface are
     * intended to be invoked only by their Subscribers; usages in
     * other contexts have undefined effects.
     */
    public static interface Subscription {
        /**
         * Adds the given number {@code n} of items to the current
         * unfulfilled demand for this subscription.  If {@code n} is
         * less than or equal to zero, the Subscriber will receive an
         * {@code onError} signal with an {@link
         * IllegalArgumentException} argument.  Otherwise, the
         * Subscriber will receive up to {@code n} additional {@code
         * onNext} invocations (or fewer if terminated).
         *
         * @param n the increment of demand; a value of {@code
         * Long.MAX_VALUE} may be considered as effectively unbounded
         */
        public void request(long n);

        /**
         * Causes the Subscriber to (eventually) stop receiving
         * messages.  Implementation is best-effort -- additional
         * messages may be received after invoking this method.
         * A cancelled subscription need not ever receive an
         * {@code onComplete} or {@code onError} signal.
         */
        public void cancel();
    }

    /**
     * A component that acts as both a Subscriber and Publisher.
     *
     * @param <T> the subscribed item type
     * @param <R> the published item type
     */
    public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
    }

    static final int DEFAULT_BUFFER_SIZE = 256;

    /**
     * Returns a default value for Publisher or Subscriber buffering,
     * that may be used in the absence of other constraints.
     *
     * @implNote
     * The current value returned is 256.
     *
     * @return the buffer size value
     */
    public static int defaultBufferSize() {
        return DEFAULT_BUFFER_SIZE;
    }

}

　　Flow这个类里定义最基本的Publisher与Subscribe，该模式就是发布订阅模式。我们来看一下代码示例：

package com.hzgj.lyrk.demo;

import java.util.concurrent.Flow;

public class Main {

    public static void main(String[] args) {
        Flow.Publisher<String> publisher = subscriber -> {
            subscriber.onNext("1"); // 1
            subscriber.onNext("2");
            subscriber.onError(new RuntimeException("出错")); // 2
            //  subscriber.onComplete();
        };
        publisher.subscribe(new Flow.Subscriber<>() {
            @Override
            public void onSubscribe(Flow.Subscription subscription) {
                subscription.cancel();
            }

            @Override
            public void onNext(String item) {
                System.out.println(item);
            }

            @Override
            public void onError(Throwable throwable) {
                System.out.println("出错了");
            }

            @Override
            public void onComplete() {
                System.out.println("publish complete");
            }
        });
    }
}

 　　代码1 是一种数据流的体现，在Publisher中每次调用onNext的时候，在中都会在Subscribe的onNext方法进行消费

　　 代码2 同样是发送错误信号，等待订阅者进行消费

　　 运行结果：

1
2
出错了

　　在上述代码中我们可以发现：Publisher在没有被订阅的时候，是不会触发任何行为的。每次调用Publisher的onNext方法的时候都像是在发信号，订阅者收到信号时执行相关内容，这就是典型的响应式编程的案例。不过java9提供的这个功能对异步的支持不太好，也不够强大。因此才会出现Reactor与RxJava等响应式框架