SpringCloud之Feign：REST客户端

在Spring Cloud集群中，各个角色的通信基于REST服务，在调用服务时，需要使用REST客户端，常用，除了使用Spring自带的RestTemplate，也可使用另一个REST客户端：Feign。

使用Feign时，可以使用自带注解或第三方注解来修饰接口，使得接口具有访问Web Service的能力。
Feign还支持插件式的编码器和解码器，对请求和响应进行不同的封装和解析。
Spring Cloud将Feign集成到Netflix项目中，与Eureka、Ribbon集成时，Feign就具有负载均衡的功能。
Feign在Spring Boot Web项目的使用例子可参考：《Spring Boot 2 发布与调用REST服务》

下面例子为在Spring Cloud的使用。
开发工具：IntelliJ IDEA 2019.2.3

一、服务器端

1、创建项目

IDEA中创建一个新的SpringBoot项目，名称为“spring-feign-server”，SpringBoot版本选择2.1.10，在选择Dependencies(依赖)的界面勾选Spring Cloud Discovery ->
Eureka Server，创建完成后的pom.xml配置文件自动添加SpringCloud最新稳定版本依赖，当前为Greenwich.SR3。
pom.xml完整内容如下：

<?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 https://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>2.1.10.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>spring-feign-server</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>spring-feign-server</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
        <spring-cloud.version>Greenwich.SR3</spring-cloud.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
        </dependency>

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

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>${spring-cloud.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

2、修改配置application.yml

修改端口号为8761；取消将自己信息注册到Eureka服务器，不从Eureka服务器抓取注册信息。

server:
  port: 8761
eureka:
  client:
    register-with-eureka: false
    fetch-registry: false

3、修改启动类代码SpringFeignServerApplication.java

增加注解@EnableEurekaServer

package com.example.springfeignserver;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@SpringBootApplication
@EnableEurekaServer
public class SpringFeignServerApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringFeignServerApplication.class, args);
    }
}

二、服务提供者

1、创建项目

IDEA中创建一个新的SpringBoot项目，除了名称为“spring-feign-provider”，其它步骤和上面创建服务器端一样。

2、修改配置application.yml

spring:
  application:
    name: spring-feign-provider
eureka:
  instance:
    hostname: localhost
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/

3、添加一个实体类User.java

package com.example.springfeignprovider;

public class User {
    String name;
    Integer age;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
}

4、修改启动类代码SpringFeignProviderApplication.java

增加注解@EnableEurekaClient和@RestController；
让类在启动时读取控制台输入，决定使用哪个端口启动服务器；
增加2个测试用的控制器方法。

package com.example.springfeignprovider;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import javax.servlet.http.HttpServletRequest;
import java.util.Scanner;

@SpringBootApplication
@EnableEurekaClient
@RestController
public class SpringFeignProviderApplication {

    public static void main(String[] args) {
        //SpringApplication.run(SpringFeignProviderApplication.class, args);
        Scanner scan = new Scanner(System.in);
        String port = scan.nextLine();
        new SpringApplicationBuilder(SpringFeignProviderApplication.class).properties("server.port=" + port).run(args);
    }

    @RequestMapping("/hello")
    public String hello(HttpServletRequest request) {
        return "hello world." + request.getServerPort();
    }

    @RequestMapping(value="/user/{name}", produces = MediaType.APPLICATION_JSON_VALUE)
    public User user(@PathVariable String name) {
        User u = new User();
        u.setName(name);
        u.setAge(30);
        return u;
    }
}

三、服务调用者

1、创建项目
IDEA中创建一个新的SpringBoot项目，名称为“spring-feign-invoker”，SpringBoot版本选择2.1.10，在选择Dependencies(依赖)的界面勾选Spring Cloud Discovery -> Eureka Server，Spring Cloud Routing -> OpenFeign。
pom.xml完整内容如下：

<?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 https://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>2.1.10.RELEASE</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example</groupId>
    <artifactId>spring-feign-invoker</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>spring-feign-invoker</name>
    <description>Demo project for Spring Boot</description>

    <properties>
        <java.version>1.8</java.version>
        <spring-cloud.version>Greenwich.SR3</spring-cloud.version>
    </properties>

    <dependencies>
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-eureka-server</artifactId>
        </dependency>

        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-openfeign</artifactId>
        </dependency>

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

    <dependencyManagement>
        <dependencies>
            <dependency>
                <groupId>org.springframework.cloud</groupId>
                <artifactId>spring-cloud-dependencies</artifactId>
                <version>${spring-cloud.version}</version>
                <type>pom</type>
                <scope>import</scope>
            </dependency>
        </dependencies>
    </dependencyManagement>

    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>

</project>

2、修改配置application.yml

server:
  port: 9000
spring:
  application:
    name: spring-feign-invoker
eureka:
  instance:
    hostname: localhost
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/

3、添加一个实体类User.java

package com.example.springfeigninvoker;

public class User {
    String name;
    Integer age;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }
}

4、添加一个客户端接口UserClient.java

package com.example.springfeigninvoker;

import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

//声明调用的服务名称
@FeignClient("spring-feign-provider")
public interface UserClient {
    @RequestMapping(method = RequestMethod.GET, value = "/hello")
    String hello();

    @RequestMapping(method = RequestMethod.GET, value = "/user/{name}")
    User getUser(@PathVariable("name") String name);
}

5、修改启动类代码CloudInvokerApplication.java

增加注解@EnableEurekaClient和@EnableFeignClients。

package com.example.springfeigninvoker;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.cloud.openfeign.EnableFeignClients;

@SpringBootApplication
@EnableEurekaClient
@EnableFeignClients
public class SpringFeignInvokerApplication {
    public static void main(String[] args) {
        SpringApplication.run(SpringFeignInvokerApplication.class, args);
    }
}

6、添加控制器 InvokerController.java 

package com.example.springfeigninvoker;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.MediaType;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;

@RestController
@Configuration
public class InvokerController {
    @Autowired
    private UserClient userClient;

    @RequestMapping(value = "/invokeHello", method = RequestMethod.GET)
    public String invokeHello(){
        return userClient.hello();
    }

    @RequestMapping(value = "/invokeUser", method = RequestMethod.GET, produces = MediaType.APPLICATION_JSON_VALUE)
    public String invokeUser(){
        return userClient.getUser("小明").getAge().toString();
    }
}

四、测试

1、启动服务器端。
浏览器访问http://localhost:8761/，正常
2、启动两个服务提供者，在控制台中分别输入8080和8081启动。
（1）浏览器访问 http://localhost:8080/user/小明
输出：{"name":"小明","age":30}
（2）浏览器访问 http://localhost:8081/user/小强
输出：{"name":"小强","age":30}
3、启动服务调用者。
（1）浏览器访问http://localhost:9000/invokeHello，页面显示在下面8080和8081端口之间切换：
hello world.8080
hello world.8081
（2）浏览器访问http://localhost:9000/invokeUser，输出：
30