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Observable:

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