SpringCloud之Fegin

　　Fegin是一个声明似的web服务客户端，它使得编写web服务客户端变得更加容易。使用Fegin创建一个接口并对它进行注解。它具有可插拔的注解支持包括Feign注解与JAX-RS注解，Feign还支持可插拔的编码器与解码器，Spring Cloud 增加了对 Spring MVC的注解，Spring Web 默认使用了HttpMessageConverters, Spring Cloud 集成 Ribbon 和 Eureka 提供的负载均衡的HTTP客户端 Feign。

声明式REST客户端：Feign

　　创建一个maven工程eureka_feign_client，pom文件如下：

<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>1.4..RELEASE</version>
<relativePath/> <!-- lookup parent from repository -->
</parent>
<properties>
<project.build.sourceEncoding>UTF-</project.build.sourceEncoding>
<java.version>1.8</java.version>
</properties>
<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-feign</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-eureka</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</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>Brixton.SR5</version>
    <type>pom</type>
    <scope>import</scope>
</dependency>
    </dependencies>
</dependencyManagement>

　　　在应用主类中通过@EnableFeignClients注解开启Feign功能，启动文件FeignApplication.java如下：

@SpringBootApplication
@EnableDiscoveryClient
@EnableFeignClients
public class FeignApplication {

    public static void main(String[] args) {
        SpringApplication.run(FeignApplication.class, args);
    }

}

　　定义服务接口类UserClient.java，使用@FeignClient("service01")注解来绑定该接口对应service01服务

@FeignClient("service01")
public interface UserClient {

    @RequestMapping(method = RequestMethod.GET, value = "/getuser")
    public User getuserinfo();

    @RequestMapping(method = RequestMethod.GET, value = "/getuser")
    public String getuserinfostr();

    @RequestMapping(method = RequestMethod.GET, value = "/info")
    public  String  info();

}

　　在web层中调用上面定义的UserClient，具体如下

@RestController
public class UserController {

    @Autowired
    UserClient userClient;

    @RequestMapping(value = "/getuserinfo", method = RequestMethod.GET)
    public User getuserinfo() {
        return userClient.getuserinfo();
    }

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

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

}

　　其实通过Feign封装了HTTP调用服务方法，使得客户端像调用本地方法那样直接调用方法，类似Dubbo中暴露远程服务的方式，区别在于Dubbo是基于私有二进制协议，而Feign本质上还是个HTTP客户端。