Feign声明式服务调用

1. Feign声明式服务调用简介

Feign是一个声明式的Web Service客户端，它使得编写Web Serivce客户端变得更加简单。我们只需要使用Feign来创建一个接口并用注解来配置它既可完成。
它具备可插拔的注解支持，包括Feign注解和JAX-RS注解。Feign也支持可插拔的编码器和解码器。
Spring Cloud为Feign增加了对Spring MVC注解的支持，还整合了Ribbon和Eureka来提供均衡负载的HTTP客户端实现。

前面Ribbon调用服务提供者，通过restTemplate调用，缺点是，多个地方调用，同一个请求要写多次，不方便统一维护。
这时候使用Feign，就直接把请求统一搞一个service作为FeignClient，然后其他调用Controller需要用到的，直接注入service，直接调用service方法即可；
同时Feign整合了Ribbon和Eureka，所以要配置负载均衡的话，直接配置Ribbon即可，无其他特殊地方；当然Fiegn也整合了服务容错保护，断路器Hystrix。

2. Feign应用

2.1) 公共模块项目microservice-common的pom.xml增加feign依赖

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

2.2) 公共模块项目microservice-common中新建service，定义了FeignClient，同时指定了调用的服务名称MICROSERVICE-STUDEN

package com.andrew.service;

import java.util.List;
import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import com.andrew.entity.Student;

@FeignClient(value = "MICROSERVICE-STUDENT")
public interface StudentClientService {
    @GetMapping(value = "/student/get/{id}")
    public Student get(@PathVariable("id") Integer id);
    @GetMapping(value = "/student/list")
    public List<Student> list();
    @PostMapping(value = "/student/save")
    public boolean save(Student student);
    @GetMapping(value = "/student/delete/{id}")
    public boolean delete(@PathVariable("id") Integer id);
}

2.3) 重新打包microservice-common

修改microservice-common
首先maven clean
然后maven install

2.4) 服务消费者项目microservice-student-consumer-feign-80搭建

new -> Maven Module
    -> create a simple project
Module Name:microservice-student-consumer-feign-80
Parent Project:microservice
Working set:SpringCloud
    -> Artifact
Group Id:com.andrew.springcloud
Artifact Id:microservice-student-consumer-feign-80
Version:0.0.1-SNAPSHOT
Packaging:jar

代码与microservice-student-consumer-80相同

2.5) 服务消费者项目microservice-student-consumer-feign-80的pom.xml增加Feign依赖

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

2.6) 服务消费者项目microservice-student-consumer-feign-80修改启动类为StudentConsumerFeignApplication_80.java同时增加注解@EnableFeignClients

package com.andrew;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.cloud.netflix.feign.EnableFeignClients;

@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class, HibernateJpaAutoConfiguration.class })
@EnableEurekaClient
@EnableFeignClients
public class StudentConsumerFeignApplication_80 {
    public static void main(String[] args) {
        SpringApplication.run(StudentConsumerFeignApplication_80.class, args);
    }
}

2.7) 服务消费者项目microservice-student-consumer-feign-80修改StudentConsumerFeignController.java，把restTemplate去掉，改成注入service，调用service方法来实现服务的调用

package com.andrew.controller;

import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import com.andrew.entity.Student;
import com.andrew.service.StudentClientService;
 
@RestController
@RequestMapping("/student")
public class StudentConsumerFeignController {
    @Autowired
    private StudentClientService studentClientService;
    @PostMapping(value="/save")
    public boolean save(Student student){
        return studentClientService.save(student);
    }
    @GetMapping(value="/list")
    public List<Student> list(){
        return studentClientService.list();
    }
    @GetMapping(value="/get/{id}")
    public Student get(@PathVariable("id") Integer id){
        return studentClientService.get(id);
    }
    @GetMapping(value="/delete/{id}")
    public boolean delete(@PathVariable("id") Integer id){
        return studentClientService.delete(id);
    }
}

启动microservice-eureka-server-2001
启动microservice-eureka-server-2002
启动microservice-eureka-server-2003
启动microservice-student-provider-1001
启动microservice-student-provider-1002
启动microservice-student-provider-1003

先测试服务提供者：
http://localhost:1001/student/list
http://localhost:1002/student/list
http://localhost:1003/student/list

再测试下 eureka：
http://eureka2001.andrew.com:2001/
http://eureka2002.andrew.com:2002/
http://eureka2003.andrew.com:2003/

然后再启动服务消费者
microservice-student-consumer-feign-80
http://localhost/student/list
list----------------------------------1001 - 1003随机调用
[{"id":1,"name":"zhangsan","grade":"11"},{"id":2,"name":"lisi","grade":"12"},{"id":3,"name":"wangwu","grade":"13"},{"id":4,"name":"student04","grade":"14"},{"id":5,"name":null,"grade":null},{"id":6,"name":"student06","grade":"16"}]