Spring Cloud声明式调用Feign负载均衡FeignClient详解

为了深入理解Feign，下面将从源码的角度来讲解Feign。首先来看看FeignClient注解@FeignClient的源码，代码如下：

FeignClient注解被@Target(ElementType.TYPE)修饰，表示FeignClient的作用目标在接口上。@Retention(RetentionPolicy.RUNTIME)注解表明该注解会在Class字节码中存在，在运行时可以通过反射获取到。@Documented表明该注解被包含在javadoc中。

@FeignClient用于创建声明是API接口，该接口是RESTful风格的。Feign被设计成插拔式的，可注入其他组件和Feign一起使用。最典型的是如果Ribbon可用，Feign会和Ribbon相结合进行负载均衡。

在代码中，value()和name()一样，是被调用的服务的ServiceId。url()直接填写硬编码URL地址。decode404（）即404是被解码，还是抛异常。configuration（）指明FeignClient的配置类，默认的配置类为FeignClientsConfiguration类，在缺省情况下，这个类注入了默认的Decoder、Encoder和Constant等配置的bean。fallback()为配置熔断器的处理类。

@FeignClient 接口调用

在项目的启动文件加入：@EnableFeignClients 注解

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.cloud.netflix.feign.EnableFeignClients;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;

@EnableEurekaClient
@SpringBootApplication
@EnableFeignClients
public class FeignApp {

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

实例结构如下：

那么有实体类: User.java

Fengn客户端：UserFeignClient.java

控制器: MovieController.java调取第三方user接口

User.java

import java.math.BigDecimal;

public class User {

    private  Long id;

    private String username;

    private String name;

    private int age;

    private BigDecimal balance;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public BigDecimal getBalance() {
        return balance;
    }

    public void setBalance(BigDecimal balance) {
        this.balance = balance;
    }

}

UserFeign客户端

其中：@FeignClient("spring-boot-user")： spring-boot-user是eureka服务里面user项目的名称，加入此注解，能直接连接user项目接口

import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;

import com.muyang.bootmovie.entity.User;

@FeignClient("spring-boot-user")
public interface UserFeignClient {

    // 两个坑：1. @GetMapping不支持   2. @PathVariable得设置value
    @RequestMapping(value="/simple/{id}", method=RequestMethod.GET)
    public User findById(@PathVariable("id") Long id);

    @RequestMapping(value="/test", method=RequestMethod.POST)
    public User postUser(@RequestBody User user);
}

MovieController控制中心，调取UserFeign客户端

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.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
import com.muyang.bootmovie.entity.User;
import com.muyang.bootmovie.feign.UserFeignClient;

@RestController
public class MovieController {

    @Autowired
    private UserFeignClient userFeignClient;

    @GetMapping("/movie/{id}")
    public User findById(@PathVariable("id") Long id) {
        return this.userFeignClient.findById(id);
    }

    @RequestMapping(value="/test", method=RequestMethod.GET)
    public User userPost(User user)
    {
        return this.userFeignClient.postUser(user);

    }
}