Spring Cloud （十二）、Feign的继承特性

背景：

       通过继承特性来帮助我们减少复制的操作，进一步来减少代码量。

主要分为三个步骤：创建hello-service-api、重构hello-service、重构fegin-consume。

一、创建hello-service-api

1、创建一个基础的Maven工程，命名为hello-service-api，可参考idea中创建maven项目。

2、编写pom.xml，因为要用到Spring MVC的注解，所以要引入spring-boot-starter-web依赖：

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.6.RELEASE</version>
    <relativePath/> <!-- lookup parent from repository -->
  </parent>
  <groupId>com.example</groupId>
  <artifactId>hello-service-api</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>hello-service-api Maven Webapp</name>
  <!-- FIXME change it to the project's website -->
  <url>http://www.example.com</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.7</maven.compiler.source>
    <maven.compiler.target>1.7</maven.compiler.target>
  </properties>

  <dependencies>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>4.11</version>
      <scope>test</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
  </dependencies>

  <build>
    <finalName>hello-service-api</finalName>
    <pluginManagement><!-- lock down plugins versions to avoid using Maven defaults (may be moved to parent pom) -->
      <plugins>
        <plugin>
          <artifactId>maven-clean-plugin</artifactId>
          <version>3.1.0</version>
        </plugin>
        <!-- see http://maven.apache.org/ref/current/maven-core/default-bindings.html#Plugin_bindings_for_war_packaging -->
        <plugin>
          <artifactId>maven-resources-plugin</artifactId>
          <version>3.0.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-compiler-plugin</artifactId>
          <version>3.8.0</version>
        </plugin>
        <plugin>
          <artifactId>maven-surefire-plugin</artifactId>
          <version>2.22.1</version>
        </plugin>
        <plugin>
          <artifactId>maven-war-plugin</artifactId>
          <version>3.2.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-install-plugin</artifactId>
          <version>2.5.2</version>
        </plugin>
        <plugin>
          <artifactId>maven-deploy-plugin</artifactId>
          <version>2.8.2</version>
        </plugin>
      </plugins>
    </pluginManagement>
  </build>
</project>

3、将参数绑定中需要使用的User对象复制过来，保存到com.example.entity.User;

4、创建一个com.example.service.HelloService接口，内容如下：

package com.example.service;

import com.example.entity.User;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestHeader;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestParam;

/**
 * @author HDN
 */
@RequestMapping("/refactor")
public interface HelloService {
    @RequestMapping("/hello4")
    String hello(@RequestParam("name") String name);

    @RequestMapping("/hello5")
    User hello(@RequestHeader("name") String name, @RequestHeader("age") Integer age);

    @RequestMapping("/hello6")
    String hello(@RequestBody User user);
}

5、在hello-service-api工程的Ternimal中执行mvn install命令，将项目安装到本地仓库，共hello-service和feign-consume来重构。

出现BUILD SUCCESS表明安装成功。

二、重构服务提供者hello-service，以Spring Cloud （十一）、Feign参数绑定为基础。

1、hello-service工程中，在pom.xml的dependency节点中，新增对hello-service-api的依赖：

<dependency>
   <groupId>com.example</groupId>
   <artifactId>hello-service-api</artifactId>
   <version>1.0-SNAPSHOT</version>
</dependency>

2、创建RefactorHelloController类继承hello-service-api中定义的HelloService接口，实现接口，内容如下：

@RestController
public class RefactorHelloController implements HelloService {

    @Override
    public String hello(String s) {
        return "hello "+s;
    }

    @Override
    public User hello(String s, Integer integer) {
        return new User(s,integer);
    }

    @Override
    public String hello(User user) {
        return "hello "+user.getName()+", "+user.getAge();
    }
}

三、重构服务消费者fegin-consume，以Spring Cloud （十一）、Feign参数绑定为基础。

1、fegin-consume工程中，在pom.xml的dependency节点中，新增对hello-service-api的依赖：

<dependency>
   <groupId>com.example</groupId>
   <artifactId>hello-service-api</artifactId>
   <version>1.0-SNAPSHOT</version>
</dependency>

2、创建RefactorHelloService接口，并继承hello-service-api包中的HelloService接口，添加@FeignClient注解来绑定服务：

package com.example.feginconsume.Service;

import com.example.service.HelloService;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.stereotype.Service;

@FeignClient("hello-service")
@Service
public interface RefactorHelloService extends HelloService {
}

3、在ConsumerController中，注入RefactorHelloService的实例，并新增一个请求/feign-consumer3来触发对RefactorHelloService的实例的调用。

 @Autowired
    RefactorHelloService refactorHelloService;

    @RequestMapping("/feign-consumer3")
    public String helloConsumer3(){
        StringBuilder sb = new StringBuilder();
        sb.append(refactorHelloService.hello("HDN")).append("\n");
        sb.append(refactorHelloService.hello("HDN",20)).append("\n");
        sb.append(refactorHelloService.hello(new com.example.entity.User("hdn",22))).append("\n");
        return sb.toString();
    }

    @RequestMapping("/feign-requestParam3")
    public String getRequestParam3(@RequestParam String name){
        return refactorHelloService.hello(name);
    }

    @RequestMapping("/feign-requestHeader3")
    public com.example.entity.User getRequestHeader3(@RequestHeader String name, @RequestHeader Integer age){
        return refactorHelloService.hello(name,age);
    }

    @RequestMapping("/feign-requestBody3")
    public String getRequestBody3(@RequestBody com.example.entity.User user){
        return refactorHelloService.hello(user);
    }

 4、测试验证

启动注册中心（eureka-service），没有的话参考：Spring Cloud （一）、搭建服务注册中心 ，hello-service、fegin-consume。

在启动fegin-consume的时候会发现报错：The bean 'hello-service.FeignClientSpecification', defined in null, could not be registered. A bean with that name has already been defined in null and overriding is disabled。这是因为在fegin-consume项目中我们使用了两个@FeignClient("hello-service")导致的，这时我们将HelloService中的@FeignClient("hello-service")注释掉,在注释掉ConsumerController中其他多余的代码，最终ConsumerController只有一下代码：

 @Autowired
    RefactorHelloService refactorHelloService;

    @RequestMapping("/feign-consumer3")
    public String helloConsumer3(){
        StringBuilder sb = new StringBuilder();
        sb.append(refactorHelloService.hello("HDN")).append("\n");
        sb.append(refactorHelloService.hello("HDN",20)).append("\n");
        sb.append(refactorHelloService.hello(new com.example.entity.User("hdn",22))).append("\n");
        return sb.toString();
    }

    @RequestMapping("/feign-requestParam3")
    public String getRequestParam3(@RequestParam String name){
        return refactorHelloService.hello(name);
    }

    @RequestMapping("/feign-requestHeader3")
    public com.example.entity.User getRequestHeader3(@RequestHeader String name, @RequestHeader Integer age){
        return refactorHelloService.hello(name,age);
    }

    @RequestMapping("/feign-requestBody3")
    public String getRequestBody3(@RequestBody com.example.entity.User user){
        return refactorHelloService.hello(user);
    }

重新启动fegin-consume。

然后我们访问http://localhost:9001/feign-consumer3，可获得如下输出：

访问http://localhost:9001/feign-requestParam3?name=hhddnn，可获得如下输出：

访问http://localhost:9001/feign-requestHeader3，可获得如下输出：

访问http://localhost:9001/feign-requestBody3，可获得如下输出：

到此，继承特性就成功演示！！！

继承特性的优点：可以实现接口的共享。

继承特性额缺点：会出现牵一发而动全身的后果。