概述

閱讀此文，可以知曉 feign 使用、上下文隔離及源碼閱讀。源碼涉及兩方面：feign如何初始化及如何調用。

使用

配置

• pom.xm 配置 jar 包
• 啟動類添加注解 @EnableFeignClients
• feign 接口編寫

pom.xml

增加以下兩個 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 接口編寫

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 調用。

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

測試

測試如下：
http://localhost:9060/hello/feign/helloword

降級處理

官方文檔

**注意：**以下

測試接口：
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

代碼

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());}
}

測試 如下

Feign如何初始化及調用源碼閱讀

初始化

初始化關鍵要明白以下兩點：

• FeignClientsRegistrar ImportBeanDefinitionRegistrar 如何注入 @FeignClient
• 如何動態代理生成對象 FeignClientFactoryBean

關鍵切入點

org.springframework.cloud.openfeign.EnableFeignClients

斷點關鍵處如下：

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

關鍵：feign.ReflectiveFeign#newInstance，動態代理

調用

關鍵地方如下：

feign.ReflectiveFeign.FeignInvocationHandler#invoke

feign的上下文隔離機制

理解 feign 的上下文隔離機制，有助于我們對 feign 的使用有更深的理解。

源碼

關鍵斷點設置如下：

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

幾個關鍵點

org.springframework.cloud.openfeign.FeignAutoConfiguration

重要源碼，feign 實現上下方隔離 的原理在以下代碼中。

protected AnnotationConfigApplicationContext createContext(String name) {AnnotationConfigApplicationContext context;if (this.parent != null) {DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();......省略一些代碼.......// 走這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));// 實例化此 spring 容器,servlet 類型 spring 容器作為父容器context.refresh();return context;
}

由上述代碼可知，有 feign 的項目，啟動時長會更長的，因為每個 provider feign 都會生成一個 spring 容器。spring 容器初始化是比較耗時的。

結束

Feign 使用及源碼閱讀至此結束，如有問題，歡迎評論區留言。