前言
Springcloud中的服务间调用是通过Feign进行调用的，在调用方服务中，我们需要定义一些带有@FeignClient注解的接口类。并且在启动类上加上@EnableFeignClients注解。
程序启动的时候，会检查是否有@EnableFeignClients注解，如果有该注解，则开启包扫描，扫描带有@FeignClient注解的接口。
这里结合之前遇到的一个问题来和大家一起学习下@EnableFeignClients启动过程。
问题描述
之前搭建一个简单demo的时候，启动之后总是报错
Field client1Feign in com.aiqinhai.client2.controller.Testrequired a bean of type’com.aiqinhai.client2.feignclient.Client1Feign’ that could not be found.Action:Consider defining a bean of type ‘com.aiqinhai.client2.feignclient.Client1Feign’ in your configuration.Process finished with exit code 1
排查了一顿之后，发现启动上的@EnableFeignClients注解，没有指定basePackages包扫描路径。
而且Client1Feign接口不是和启动类在同一目录下，所以启动的时候就会报上述错误。
后来在@EnableFeignClients指定了扫描包路径就解决了。
@SpringBootApplication@EnableDiscoveryClient@EnableFeignClients(basePackages = {"com.aiqinhai.client2.feignclient"})public class Client2Application { public static void main(String[] args) { SpringApplication.run(Client2Application.class, args); }}
问题确实解决了，但是我们还是需要了解服务启动的时候，@EnableFeignClients注解干了哪些事。
刨根@EnableFeignClients
想了解清楚这个注解的作用，最好的方式就是看看注解的源码。点进去之后，可以看到
/** * 扫描注解@FeignClient标注的接口 */@Retention(RetentionPolicy.RUNTIME)@Target({ElementType.TYPE})@Documented@Import(FeignClientsRegistrar.class)public @interface EnableFeignClients { //同basePackages String[] value() default {}; //扫描的package. String[] basePackages() default {}; //feigin client全局配置，默认配置在FeignClientsConfiguration类中 Class<?>[] defaultConfiguration() default {}; //@FeignClient注解的接口，如果指定了该属性，则关闭扫描。 Class<?>[] clients() default {};}
可以看到，注解中import了FeignClientsRegistrar类，我们进去看看，可以看到包扫描的逻辑就是在FeignClientsRegistrar中实现的，会扫描所有的.class文件，过滤出@FeignClient标注的接口，然后通过BeanDefinitionBuilder生成FeignClientFactoryBean对象，注入到IOC容器中。
具体代码如下
class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar,ResourceLoaderAware, EnvironmentAware {//包扫描方法入口@Overridepublic void registerBeanDefinitions(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {//注册默认配置bean到iocregisterDefaultConfiguration(metadata, registry);//注册@FeignClients标注的接口bean,为其生成动态代理registerFeignClients(metadata, registry);}private void registerDefaultConfiguration(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {//获取@EnableFeignClients注解属性Map<String, Object> defaultAttrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName(), true);if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {String name;if (metadata.hasEnclosingClass()) {name = "default." + metadata.getEnclosingClassName();}else {name = "default." + metadata.getClassName();}//注册@EnableFeignClients defaultConfiguration 类中定义的bean到iocregisterClientConfiguration(registry, name,defaultAttrs.get("defaultConfiguration"));}}public void registerFeignClients(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {//spring 扫描工具类ClassPathScanningCandidateComponentProvider scanner = getScanner();scanner.setResourceLoader(this.resourceLoader); //待扫描的包Set<String> basePackages; //@EnableFeignClients注解属性Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());//@FeignClient注解过滤器，只扫描@FeignClient注解标注的接口AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(FeignClient.class);//解析@EnableFeignClient 属性clientsfinal Class<?>[] clients = attrs == null ? null: (Class<?>[]) attrs.get("clients");//如果@EnableFeignClient没有指定clients,则开启包扫描，否则关闭扫描，使用clientsif (clients == null || clients.length == 0) {scanner.addIncludeFilter(annotationTypeFilter);basePackages = getBasePackages(metadata);}else {final Set<String> clientClasses = new HashSet<>();basePackages = new HashSet<>();for (Class<?> clazz : clients) {basePackages.add(ClassUtils.getPackageName(clazz));clientClasses.add(clazz.getCanonicalName());}AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {@Overrideprotected boolean match(ClassMetadata metadata) {String cleaned = metadata.getClassName().replaceAll("\\$", ".");return clientClasses.contains(cleaned);}};scanner.addIncludeFilter(new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));}for (String basePackage : basePackages) { //从类路径下的所有.class文件中扫描@FeignClient注解的接口Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);for (BeanDefinition candidateComponent : candidateComponents) {if (candidateComponent instanceof AnnotatedBeanDefinition) {// verify annotated class is an interfaceAnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;//获取注解AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();//@FeignClient注解必须应用在接口上，否则抛出异常。Assert.isTrue(annotationMetadata.isInterface(),"@FeignClient can only be specified on an interface"); //获取FeignClient注解属性Map<String, Object> attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName()); //服务名称String name = getClientName(attributes);//注册configuration中指定的beanregisterClientConfiguration(registry, name,attributes.get("configuration"));//生成FeignClient bean,并注册到iocregisterFeignClient(registry, annotationMetadata, attributes);}}}} //生成bean,注入到IOC容器private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {// feignclient类名称String className = annotationMetadata.getClassName();BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(FeignClientFactoryBean.class);validate(attributes);definition.addPropertyValue("url", getUrl(attributes));definition.addPropertyValue("path", getPath(attributes));String name = 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(AbstractBeanDefinition.AUTOWIRE_BY_TYPE); String alias = name + "FeignClient";AbstractBeanDefinition beanDefinition = definition.getBeanDefinition(); boolean primary = (Boolean)attributes.get("primary"); // has a default, won't be nullbeanDefinition.setPrimary(primary);String qualifier = getQualifier(attributes);if (StringUtils.hasText(qualifier)) {alias = qualifier;}BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,new String[] { alias });//注册到iocBeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);}}
@FeignClient 类，注入找不到类原因：
@FeignClinet 和 @EnableFeignClients 不是同一个包。
可能因为springboot和springcloud的版本不一致造成的。
以上为个人经验，希望能给大家一个参考，也希望大家多多支持。
以前我是个爱仰望天空的人，苍蓝的天空总是给我求生的勇气，
