Spring Cloud之Open Feign调用流程和源码分析（一）

 Open Feign整个核心的调用过程大致如下：

 

 下面根据源码详细分析：

1. @EnableFeignClients引入FeignClientsRegistrar类实现了ImportBeanDefinitionRegistrar接口用过registerBeanDefinitions方法向spring容器中注入FeignClientSpecification类（FeignClient需要的重试策略，超时策略，日志等配置，如果某个服务没有设置，则读取默认的配置）

// 通过开启feign注解引入FeignClientsRegistrar类
@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Import({FeignClientsRegistrar.class})
public @interface EnableFeignClients {
   //.....
}

 FeignClientsRegistrar实现了ImportBeanDefinitionRegistrar类，重写registerBeanDefinitions方法可以自定义bean在spring中的注册：

//自定义注册bean到spring容器
class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {
//...
    //自定义实现注册bean
  public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
        this.registerDefaultConfiguration(metadata, registry);
        this.registerFeignClients(metadata, registry);
    }

//扫描包所有加了@FeignClient的接口类，并将其注册到spring容器中。
public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
//...

        Iterator var17 = ((Set)basePackages).iterator();

        while(var17.hasNext()) {
            String basePackage = (String)var17.next();
            Set candidateComponents = scanner.findCandidateComponents(basePackage);
            Iterator var21 = candidateComponents.iterator();

            while(var21.hasNext()) {
                BeanDefinition candidateComponent = (BeanDefinition)var21.next();
                if (candidateComponent instanceof AnnotatedBeanDefinition) {
                    AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition)candidateComponent;
                    AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
                    Assert.isTrue(annotationMetadata.isInterface(), "@FeignClient can only be specified on an interface");
                    Map attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());
                    String name = this.getClientName(attributes);
                    this.registerClientConfiguration(registry, name, attributes.get("configuration"));
                    this.registerFeignClient(registry, annotationMetadata, attributes);
                }
            }
        }
}


}

2.接着上面的registerFeignClient()方法，成feign client bean的包装类FeignClientFactoryBean，并将@FeignClient注解的属性注册到bean中。注册的FeignClientFactoryBean，是一个包装了我们需要执行的rpc服务的请求的类、url、服务名称以及回调方法等。

//生成feign client bean的包装类FeignClientFactoryBean，并将@FeignClient注解的属性注册到bean中。
 private void registerFeignClient(BeanDefinitionRegistry registry, AnnotationMetadata annotationMetadata, Map attributes) {
        String className = annotationMetadata.getClassName();
        BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(FeignClientFactoryBean.class);
        this.validate(attributes);
        definition.addPropertyValue("url", this.getUrl(attributes));
        definition.addPropertyValue("path", this.getPath(attributes));
        String name = this.getName(attributes);
        definition.addPropertyValue("name", name);
        definition.addPropertyValue("type", className);
        definition.addPropertyValue("decode404", attributes.get("decode404"));
        definition.addPropertyValue("fallback", attributes.get("fallback"));
        definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
        definition.setAutowireMode(2);
        String alias = name + "FeignClient";
        AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
        boolean primary = (Boolean)attributes.get("primary");
        beanDefinition.setPrimary(primary);
        String qualifier = this.getQualifier(attributes);
        if (StringUtils.hasText(qualifier)) {
            alias = qualifier;
        }

        BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className, new String[]{alias});
        BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
    }

FeignClientFactoryBean类定义： 

private Class type;
private String name;
private String url;
private String path;
private boolean decode404;
private ApplicationContext applicationContext;
private Class fallback;
private Class fallbackFactory;

3.FeignClientFactoryBean实现了FactoryBean