一、概要
我们通过RestTemplate调用其它服务的API时，所需要的参数须在请求的URL中进行拼接，如果参数少的话或许我们还可以忍受，一旦有多个参数的话，这时拼接请求字符串就会效率低下,维护起来也比较麻烦
有没有跟优雅的方式来解决这样的问题，Feign
什么是Feign?
Feign是Netflix开发的声明式(目前由Spring在维护)、模板化的HTTP客户端， Feign可以帮助我们更快捷、优雅地调用HTTP API。
简单的来说就是一个远程调用服务的框架/工具，让开发者以更少耦合更少代码更快更兼容的方法进行远程服务调用
Feign功能
可插拔的注解支持，包括Feign注解和JAX-RS注解;
与Ribbon负载均衡器、Hystrix或Sentinel熔断器无缝集成。
支持可插拔的HTTP编码器和解码器;
支持HTTP请求和响应的压缩。
二、基本使用
创建cloud-nacos-feign项目
添加依赖
父pom文件
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="xml" cid="n22" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-dependencies</artifactId>
<version>{spring-cloud-alibaba-dependencies.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement></pre>
子pom文件
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="xml" cid="n24" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;"><dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>
<dependency>
<groupId>com.alibaba.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
</dependency></pre>
配置文件
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="" cid="n26" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">server:
port: 8090
spring:
application:
name: nacos-feign-example
cloud:
nacos:
discovery:
server-addr: 112.74.42.138:8848</pre>
开启注册发现
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="java" cid="n28" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@SpringBootApplication
@EnableFeignClients
public class NacosFeignApplication {
public static void main(String[] args) {
SpringApplication.run(NacosFeignApplication.class, args);
}
}</pre>
调用服务
调用方的
Service层
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="java" cid="n32" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@FeignClient("nacos-server-provider")
public interface FeignExampleService {
@RequestMapping(method = RequestMethod.GET, value = "/")
String getData();
}</pre>
被调用方的
Controller层
<pre spellcheck="false" class="md-fences md-end-block ty-contain-cm modeLoaded" lang="java" cid="n35" mdtype="fences" style="box-sizing: border-box; overflow: visible; font-family: Monaco, Consolas, "Andale Mono", "DejaVu Sans Mono", monospace; margin-top: 0px; margin-bottom: 20px; font-size: 0.9rem; display: block; break-inside: avoid; text-align: left; white-space: normal; background: rgb(51, 51, 51); position: relative !important; padding: 10px 10px 10px 30px; width: inherit; color: rgb(184, 191, 198); font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-style: initial; text-decoration-color: initial;">@RestController
public class FeignExampleController {
@Resource
FeignExampleService service;
@RequestMapping("/")
public String hello() {
String s = service.getData();
return s;
}
}</pre>
三、调试测试
检查后台注册
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调用服务服务接口
四、工作原理
1、@EnableFeignClients
通过@EnableFeignClients注解开启对FeignClients的扫描
2、@FeignClient
当程序启动时会扫描所有的@FeignClients的注解类注入到Spring IOC容器，当Feign接口被定义的时候
通过JDK动态代理的方式为每个接口生成具体的RequestTemplate实例,RequestTemplate中包含了所有的HTTP请求信息
3、Request
生成的Request对象最后会交给Client去处理(UrlConnection或者HttpClient,或者OkHttp),
最后Client被封装成LoadBalanceClient对象来处理,结合Ribbon负载均衡调用远程服务
五、Ribbon和Feign
Ribbon和Feign都是用于调用其他服务的，不过方式不同。
启动类使用的注解不同，Ribbon用的是@RibbonClient，Feign用的是@EnableFeignClients。
服务的指定位置不同，Ribbon是在@RibbonClient注解上声明，Feign则是在定义抽象方法的接口中使用@FeignClient声明。
调用方式不同，Ribbon需要自己构建http请求，模拟http请求然后使用RestTemplate发送给其他服务，步骤相当繁琐。Feign则是在Ribbon的基础上进行了一次改进，采用接口的方式，将需要调用的其他服务的方法定义成抽象方法即可，不需要自己构建http请求。不过要注意的是抽象方法的注解、方法签名要和提供服务的方法完全一致。
