一、实例

1、配置feign

添加依赖

在maven的pom中添加feign

   <dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-feign</artifactId></dependency>

配置启用

在Application启动类中添加@EnableFeignClients注解。

@EnableEurekaClient
@EnableFeignClients
public class Application {public static void main(String[] args) {SpringApplication.run(Application.class, args);}}

2、新建feign

@FeignClient(name = "SERVICE-NAME", url = "${***}")
public interface TestFeign {
}

其中：
name：微服务的名称，一定要以eureka后台配置的保持一致。
url：可以手动指定feign的调用地址
fallback：标记容错后执行的类

在feign中定义接口的方式与正常接口并无差异，需注意参数名称等保持一致。如：

 @RequestMapping(value = "/user/message", method = RequestMethod.POST)JSONObject sendMessage(@RequestParam("userId") String userId, @RequestParam("content") String content);

3、调用feign

在ServiceImpl中注入feign接口，正常使用即可。

 @AutowiredTestFeign testFeign;

二、原理

如果不使用springcloud的相关组件，调用服务需要要走http，配置请求head、body，然后才能发起请求。获得响应体后，还需解析等操作，十分繁琐。

那为什么feign的使用如此简单轻量呢？

1、如何启用

启动配置上检查是否有@EnableFeignClients注解，如果有该注解，则开启包扫描，扫描被@FeignClient注解接口。扫描出该注解后，通过beanDefinition注入到IOC容器中，方便后续被调用使用。

在FeignClientsRegistrar中，registerFeignClients()完成了注册feign的操作。

public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {ClassPathScanningCandidateComponentProvider scanner = this.getScanner();scanner.setResourceLoader(this.resourceLoader);Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());......//遍历该项目所需调用的服务
Iterator var17 = ((Set)basePackages).iterator();while(var17.hasNext()) {String basePackage = (String)var17.next();Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);Iterator var21 = candidateComponents.iterator();while(var21.hasNext()) {BeanDefinition candidateComponent = (BeanDefinition)var21.next();if (candidateComponent instanceof AnnotatedBeanDefinition) {AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition)candidateComponent;//获取feign中的详细信息AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();Map<String, Object> attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());String name = this.getClientName(attributes);//注册配置信息this.registerClientConfiguration(registry, name, attributes.get("configuration"));//注册feign客户端this.registerFeignClient(registry, annotationMetadata, attributes);}}}}

注册feign客户端，包括使用注解时配置的所有信息。

private void registerFeignClient(BeanDefinitionRegistry registry, AnnotationMetadata annotationMetadata, Map<String, Object> 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")).booleanValue();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);}

2、如何发起请求

ReflectiveFeign内部使用了jdk的动态代理为目标接口生成了一个动态代理类，这里会生成一个InvocationHandler(jdk动态代理原理)统一的方法拦截器，同时为接口的每个方法生成一个SynchronousMethodHandler拦截器，并解析方法上的元数据，生成一个http请求模板。

在SynchronousMethodHandler类中生成RequestTemplate发起请求。

 public Object invoke(Object[] argv) throws Throwable {RequestTemplate template = buildTemplateFromArgs.create(argv);Retryer retryer = this.retryer.clone();while (true) {try {return executeAndDecode(template);} catch (RetryableException e) {retryer.continueOrPropagate(e);if (logLevel != Logger.Level.NONE) {logger.logRetry(metadata.configKey(), logLevel);}continue;}}}

 Object executeAndDecode(RequestTemplate template) throws Throwable {Request request = targetRequest(template);Response response;long start = System.nanoTime();try {response = client.execute(request, options);// ensure the request is set. TODO: remove in Feign 10response.toBuilder().request(request).build();} catch (IOException e) {if (logLevel != Logger.Level.NONE) {logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));}throw errorExecuting(request, e);}}

发送http请求

 @Overridepublic Response execute(Request request, Options options) throws IOException {HttpURLConnection connection = convertAndSend(request, options);return convertResponse(connection).toBuilder().request(request).build();}HttpURLConnection convertAndSend(Request request, Options options) throws IOException {final HttpURLConnectionconnection =(HttpURLConnection) new URL(request.url()).openConnection();if (connection instanceof HttpsURLConnection) {HttpsURLConnection sslCon = (HttpsURLConnection) connection;if (sslContextFactory != null) {sslCon.setSSLSocketFactory(sslContextFactory);}if (hostnameVerifier != null) {sslCon.setHostnameVerifier(hostnameVerifier);}}connection.setConnectTimeout(options.connectTimeoutMillis());connection.setReadTimeout(options.readTimeoutMillis());connection.setAllowUserInteraction(false);connection.setInstanceFollowRedirects(true);connection.setRequestMethod(request.method());Collection<String> contentEncodingValues = request.headers().get(CONTENT_ENCODING);booleangzipEncodedRequest =contentEncodingValues != null && contentEncodingValues.contains(ENCODING_GZIP);booleandeflateEncodedRequest =contentEncodingValues != null && contentEncodingValues.contains(ENCODING_DEFLATE);boolean hasAcceptHeader = false;Integer contentLength = null;for (String field : request.headers().keySet()) {if (field.equalsIgnoreCase("Accept")) {hasAcceptHeader = true;}for (String value : request.headers().get(field)) {if (field.equals(CONTENT_LENGTH)) {if (!gzipEncodedRequest && !deflateEncodedRequest) {contentLength = Integer.valueOf(value);connection.addRequestProperty(field, value);}} else {connection.addRequestProperty(field, value);}}}// Some servers choke on the default accept string.if (!hasAcceptHeader) {connection.addRequestProperty("Accept", "*/*");}if (request.body() != null) {if (contentLength != null) {connection.setFixedLengthStreamingMode(contentLength);} else {connection.setChunkedStreamingMode(8196);}connection.setDoOutput(true);OutputStream out = connection.getOutputStream();if (gzipEncodedRequest) {out = new GZIPOutputStream(out);} else if (deflateEncodedRequest) {out = new DeflaterOutputStream(out);}try {out.write(request.body());} finally {try {out.close();} catch (IOException suppressed) { // NOPMD}}}return connection;}

三、小结

1、约定大于配置

2、新瓶装旧酒

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