概述

阅读此文，可以知晓 feign 使用、上下文隔离及源码阅读。源码涉及两方面：feign如何初始化及如何调用。

使用

配置

• pom.xm 配置 jar 包
• 启动类添加注解 @EnableFeignClients
• feign 接口编写

pom.xml

增加以下两个 jar

<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency><dependency><groupId>io.github.openfeign</groupId><artifactId>feign-httpclient</artifactId>
</dependency>

feign 接口编写

package com.fun.ms.feign;import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;@FeignClient("nacos-provider")
public interface OpenFeignProviderControllerFeignClient {@GetMapping("/hello/{id}")String echo(@PathVariable String id);
}

controller

controller 中使用 feign 调用。

@Autowired
private OpenFeignProviderControllerFeignClient feignClient;@GetMapping("/hello/feign/{id}")
public String echoFeign(@PathVariable String id) {return feignClient.echo(id);
}

测试

测试如下：
http://localhost:9060/hello/feign/helloword

降级处理

官方文档

**注意：**以下

测试接口：
http://localhost:9060/hello/feign/helloword

pom.xml

<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>
<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-netflix-hystrix</artifactId><version>2.2.10.RELEASE</version>
</dependency>

application.yml

feign:circuitbreaker:enabled: true

代码

feign 接口

@FeignClient(value = "nacos-provider", fallback = OpenFeignProviderControllerFeignClientFallback.class)
public interface OpenFeignProviderControllerFeignClient {@GetMapping("/hello/{id}")String echo(@PathVariable String id);
}

fallback

@Component
public class OpenFeignProviderControllerFeignClientFallback implements OpenFeignProviderControllerFeignClient {@Overridepublic String echo(String id) {return "Fallback receive args: id=" + id + ",date:"+ new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(new Date());}
}

测试 如下

Feign如何初始化及调用源码阅读

初始化

初始化关键要明白以下两点：

• FeignClientsRegistrar ImportBeanDefinitionRegistrar 如何注入 @FeignClient
• 如何动态代理生成对象 FeignClientFactoryBean

关键切入点

org.springframework.cloud.openfeign.EnableFeignClients

断点关键处如下：

org.springframework.cloud.openfeign.FeignClientsRegistrar#registerBeanDefinitions
org.springframework.cloud.openfeign.FeignClientsRegistrar#registerFeignClient
org.springframework.context.annotation.ClassPathScanningCandidateComponentProvider#scanCandidateComponents
org.springframework.cloud.openfeign.FeignClientsRegistrar#registerOptionsBeanDefinition
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#obtainFromSupplier
org.springframework.cloud.openfeign.FeignClientFactoryBean#getObject
org.springframework.cloud.openfeign.FeignClientFactoryBean#loadBalance
org.springframework.cloud.openfeign.FeignClientFactoryBean#get
org.springframework.cloud.openfeign.DefaultTargeter#target
feign.ReflectiveFeign#newInstance
feign.InvocationHandlerFactory.Default#create

关键：feign.ReflectiveFeign#newInstance，动态代理

调用

关键地方如下：

feign.ReflectiveFeign.FeignInvocationHandler#invoke

feign的上下文隔离机制

理解 feign 的上下文隔离机制，有助于我们对 feign 的使用有更深的理解。

源码

关键断点设置如下：

org.springframework.cloud.openfeign.FeignClientFactoryBean#getTarget
org.springframework.cloud.openfeign.FeignAutoConfiguration#feignContext
org.springframework.cloud.openfeign.FeignClientFactoryBean#feign
org.springframework.cloud.openfeign.FeignClientFactoryBean#get
org.springframework.cloud.context.named.NamedContextFactory#getContext
org.springframework.cloud.context.named.NamedContextFactory#createContext

几个关键点

org.springframework.cloud.openfeign.FeignAutoConfiguration

重要源码，feign 实现上下方隔离 的原理在以下代码中。

protected AnnotationConfigApplicationContext createContext(String name) {AnnotationConfigApplicationContext context;if (this.parent != null) {DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();......省略一些代码.......// 走这context = new AnnotationConfigApplicationContext(beanFactory);context.setClassLoader(this.parent.getClassLoader());}else {context = new AnnotationConfigApplicationContext();}if (this.configurations.containsKey(name)) {for (Class<?> configuration : this.configurations.get(name).getConfiguration()) {context.register(configuration);}}for (Map.Entry<String, C> entry : this.configurations.entrySet()) {if (entry.getKey().startsWith("default.")) {for (Class<?> configuration : entry.getValue().getConfiguration()) {context.register(configuration);}}}context.register(PropertyPlaceholderAutoConfiguration.class, this.defaultConfigType);context.getEnvironment().getPropertySources().addFirst(new MapPropertySource(this.propertySourceName,Collections.<String, Object>singletonMap(this.propertyName, name)));if (this.parent != null) {// Uses Environment from parent as well as beanscontext.setParent(this.parent);}context.setDisplayName(generateDisplayName(name));// 实例化此 spring 容器,servlet 类型 spring 容器作为父容器context.refresh();return context;
}

由上述代码可知，有 feign 的项目，启动时长会更长的，因为每个 provider feign 都会生成一个 spring 容器。spring 容器初始化是比较耗时的。

结束

Feign 使用及源码阅读至此结束，如有问题，欢迎评论区留言。