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
