Spring Cloud专题之二：OpenFeign 远程调用服务

欢迎查看上一篇博客：SpringCloud专题之一：Eureka。
OpenFeign是一种声明式的webservice客户端调用框架。你只需要声明接口和一些简单的注解，就能像使用普通的Bean一样调用远程服务,Ribbon 和 OpenFeign 都可以实现服务调用和实现负载均衡.OpenFeign 也内置了Ribbon.
OpenFeign是在feign的基础上做了一些修改封装,增加了对Spring Mvc注解的支持.
OpenFiegn注解讲解一般我们会使用@GetMapping和@PostMapping两种方式来调用Rest服务。
使用@RequestParam和@RequestBody来获取参数
@RequestBody只能用在Post请求中，并且一个Post请求只能有一个@RequestBody，@RequestBody的参数可以包含复杂类型。
@RequestParam可以用在Post和Get请求中，但是要注意，@RequestParam的参数只能是基本类型或者是Enum，或者是List和Map（List和Map里只能是基本类型），所以@RequestParam可以和@RequestBody一起使用。
如果是Get请求，但是有时复合类型怎么办呢？比如我们想传递一个User对象，User对象里面只有普通的两个String属性，就可以使用@SpringQueryMap 。@SpringQueryMap的参数是鞥你是普通的POJO，不能是复合类型，否则解析不了，如果必须使用复合类型，那么使用@RequestBody吧。
多个FeignClient使用同一个name的问题在eureka的客户端添加一个类：
/** * @className: Hello1Controller * @description: 测试多个feign使用相同的name的问题 * @author: charon * @create: 2021-06-06 09:35 */@RestControllerpublic class Hello1Controller {/*** 日志记录类*/private final Logger logger = LoggerFactory.getLogger(getClass());@Value("${server.port}")private String host;@Value("${spring.application.name}")private String instanceName;@RequestMapping("/sayHello1")public String sayHello1(@RequestParam("name") String name){logger.info("你好,服务名：{},端口为：{},接收到的参数为：{}",instanceName,host,name);return "你好,服务名："+instanceName+"，端口为："+host+",接收到的参数为："+name;}}eureka消费者端的controller里添加sayHello1(String name)方法：
/** * @className: CustomerController * @description: * @author: charon * @create: 2021-05-19 22:56 */@RestControllerpublic class CustomerController {@Autowiredprivate CustomerSerivce serivce;@RequestMapping("/sayHello")public String invokeSayHello(){return serivce.invokeSayHello();}@RequestMapping("/sayHello1")public String invokeSayHello1(String name){return serivce.invokeSayHello1(name);}}接口及实现类:
/** * @className: CustomerSerivce * @description: * @author: charon * @create: 2021-05-19 22:56 */public interface CustomerSerivce {String invokeSayHello();String invokeSayHello1(String name);}@Servicepublic class CustomerServiceImpl implements CustomerSerivce {@Autowiredprivate CustomerFeign feign;@Autowiredprivate Customer1Feign feign1;@Overridepublic String invokeSayHello() {return feign.sayHello();}@Overridepublic String invokeSayHello1(String name) {return feign1.sayHello1(name);}}feignClient：
/** * @className: CustomerFeign * @description: @FeignClient使用的value参数，表示从HELLO-SERVER这个服务中调用服务 * @author: charon * @create: 2021-05-19 23:01 */@FeignClient("HELLO-SERVER")public interface CustomerFeign {/*** 要求：*返回值要对应，方法名随意，参数值要对应*方法上添加SpringMVC的注解* @return*/@RequestMapping("/sayHello")String sayHello();}/** * @className: Customer1Feign * @description: 测试多个feign使用相同的name的问题 * @author: charon * @create: 2021-06-06 09:42 */@FeignClient("HELLO-SERVER")public interface Customer1Feign {/*** 要求：*必须要指定RequestParam属性的value值，同时RequestMethod的method也需要指定*方法上添加SpringMVC的注解* @return*/@RequestMapping(value = "https://tazarkount.com/sayHello1",method = RequestMethod.GET)String sayHello1(@RequestParam("name") String name);}

文章插图
如上图所示，运行时候就会报错。原因是两个FeignClient使用了同一个value，对于同一个service-id只能使用一个配置类，如果有多个@FeignClient注解使用了相同的name属性，则注解的configuration参数会被覆盖。至于谁覆盖谁要看Spring容器初始化Bean的顺序。
改动：

# 设置为true，表示后发现的bean会覆盖之前相同名称的beanspring.main.allow-bean-definition-overriding=true

源码解读openfeign的自动配置@EnableFeignClients开启openfeign
首先，我们从@EnableFeignClients这个注解开始了解。

@Retention(RetentionPolicy.RUNTIME)@Target(ElementType.TYPE)@Documented@Import(FeignClientsRegistrar.class)public @interface EnableFeignClients {}

【Spring Cloud专题之二：OpenFeign 远程调用服务】这个注解导入了一个类FeignClientsRegistrar，这个类实现了ImportBeanDefinitionRegistrar接口，该接口用于向Bean容器中注册添加BeanDefinition 。
跟进FeignClientsRegistrar的registerBeanDefinitions方法，看看它注册了哪些东西。

public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {// 注册默认配置，会读取@EnableFeignClients接口的属性，如果存在自定义配置类那么就会被注册到容器中registerDefaultConfiguration(metadata, registry);// 注册FeignClient接口的Bean，会扫描所有注解了@FeignClient的接口，然后像spring本地Bean一样地注册到容器中 。registerFeignClients(metadata, registry);}

下面重点看看，registerFeignClients方法，这个方法的核心逻辑就是扫描类路径，获取BeanDefinition，然后遍历进行注册。

public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {LinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet<>();Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());final Class<?>[] clients = attrs == null ? null: (Class<?>[]) attrs.get("clients");if (clients == null || clients.length == 0) {ClassPathScanningCandidateComponentProvider scanner = getScanner();scanner.setResourceLoader(this.resourceLoader);scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));// 扫描所有路径，默认情况下扫描启动类下的路径Set<String> basePackages = getBasePackages(metadata);for (String basePackage : basePackages) {// 将所有 @FeignClient 的接口的BeanDefinition拿到candidateComponents.addAll(scanner.findCandidateComponents(basePackage));}}else {for (Class<?> clazz : clients) {candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));}}// 遍历扫描到的FeignClient的Beanfor (BeanDefinition candidateComponent : candidateComponents) {if (candidateComponent instanceof AnnotatedBeanDefinition) {// verify annotated class is an interfaceAnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;AnnotationMetadata annotationMetadata = https://tazarkount.com/read/beanDefinition.getMetadata();Assert.isTrue(annotationMetadata.isInterface(),"@FeignClient can only be specified on an interface");Map<String, Object> attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());String name = getClientName(attributes);// 注册FeignClient的配置registerClientConfiguration(registry, name,attributes.get("configuration"));// 注册FeignClientregisterFeignClient(registry, annotationMetadata, attributes);}}}

下面来看看注册FeignClient的方法：

private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {String className = annotationMetadata.getClassName();Class clazz = ClassUtils.resolveClassName(className, null);ConfigurableBeanFactory beanFactory = registry instanceof ConfigurableBeanFactory? (ConfigurableBeanFactory) registry : null;String contextId = getContextId(beanFactory, attributes);String name = getName(attributes);FeignClientFactoryBean factoryBean = new FeignClientFactoryBean();factoryBean.setBeanFactory(beanFactory);factoryBean.setName(name);factoryBean.setContextId(contextId);factoryBean.setType(clazz);// 使用FactoryBean，将Bean的具体生成过程收拢到FeignClientFactoryBean之中BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(clazz, () -> {factoryBean.setUrl(getUrl(beanFactory, attributes));factoryBean.setPath(getPath(beanFactory, attributes));factoryBean.setDecode404(Boolean.parseBoolean(String.valueOf(attributes.get("decode404"))));Object fallback = attributes.get("fallback");if (fallback != null) {factoryBean.setFallback(fallback instanceof Class? (Class<?>) fallback: ClassUtils.resolveClassName(fallback.toString(), null));}Object fallbackFactory = attributes.get("fallbackFactory");if (fallbackFactory != null) {factoryBean.setFallbackFactory(fallbackFactory instanceof Class? (Class<?>) fallbackFactory: ClassUtils.resolveClassName(fallbackFactory.toString(),null));}return factoryBean.getObject();});definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);definition.setLazyInit(true);validate(attributes);AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();beanDefinition.setAttribute(FactoryBean.OBJECT_TYPE_ATTRIBUTE, className);beanDefinition.setAttribute("feignClientsRegistrarFactoryBean", factoryBean);// has a default, won't be nullboolean primary = (Boolean) attributes.get("primary");beanDefinition.setPrimary(primary);String[] qualifiers = getQualifiers(attributes);if (ObjectUtils.isEmpty(qualifiers)) {qualifiers = new String[] { contextId + "FeignClient" };}BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className, qualifiers);// 将这个使用了 @FeignClient 的接口的工厂Bean的 BeanDefinition 注册到Spring容器中BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);}

这里值得注意的是genericBeanDefinition方法最终生成的其实是FeignClientFactoryBean，而registerBeanDefinition方法注册进容器的也是FeignClientFactoryBean 。而FeignClientFactoryBean是FactoryBean的实现类。FactoryBean接口是spring开放出来的，用于自定义Bean的生成过程。也就是说，spring将会通过调用FeignClientFactoryBean的getObject来获取@FeignClient注解的接口对应的Bean对象。
openfeign生成并调用客户端动态代理对象从FeignClientFactoryBean的getObject()方法开始，看看代理对象的生成。getObject()方法调用了一个getTarget()方法，该方法做了一些预处理。获取了一个上下文以及Feign的构造器，没有URL的情况下拼接了一个。

@Overridepublic Object getObject() {return getTarget();}<T> T getTarget() {// 获取上下文，FeignContext是在FeignAutoConfiguration被解析的时候成为Bean.FeignContext context = beanFactory != null ? beanFactory.getBean(FeignContext.class): applicationContext.getBean(FeignContext.class);// feign用于构建代理对象，builder将会构建feignFeign.Builder builder = feign(context);if (!StringUtils.hasText(url)) {// 然后在没有url 的情况下是按照服务名进行处理，拼接url 属性为http://服务名称 。// 如果有URL会按照URL的方式进行处理，并且如果URL没有加http:// 会在这里加上，也就是URL可以只写域名加端口if (!name.startsWith("http")) {url = "http://" + name;}else {url = name;}url += cleanPath();// HardCodedTarget 对象，实际上就是一个记录的功能，记录了接口类型，服务名称，地址信息return (T) loadBalance(builder, context,new HardCodedTarget<>(type, name, url));}if (StringUtils.hasText(url) && !url.startsWith("http")) {url = "http://" + url;}String url = this.url + cleanPath();Client client = getOptional(context, Client.class);if (client != null) {if (client instanceof LoadBalancerFeignClient) {client = ((LoadBalancerFeignClient) client).getDelegate();}if (client instanceof FeignBlockingLoadBalancerClient) {client = ((FeignBlockingLoadBalancerClient) client).getDelegate();}if (client instanceof RetryableFeignBlockingLoadBalancerClient) {client = ((RetryableFeignBlockingLoadBalancerClient) client).getDelegate();}builder.client(client);}Targeter targeter = get(context, Targeter.class);return (T) targeter.target(this, builder, context,new HardCodedTarget<>(type, name, url));}

用org.springframework.cloud.openfeign.FeignClientFactoryBean#loadBalance 方法：

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,HardCodedTarget<T> target) {// 获取执行HTTP请求的client对象Client client = getOptional(context, Client.class);if (client != null) {builder.client(client);// 获取Target对象，默认为HystrixTargeterTargeter targeter = get(context, Targeter.class);// 创建代理对象return targeter.target(this, builder, context, target);}}

跟进HystrixTargeter的target方法：

@Overridepublic <T> T target(FeignClientFactoryBean factory, Feign.Builder feign,FeignContext context, Target.HardCodedTarget<T> target) {if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {return feign.target(target);}feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;String name = StringUtils.isEmpty(factory.getContextId()) ? factory.getName(): factory.getContextId();SetterFactory setterFactory = getOptional(name, context, SetterFactory.class);if (setterFactory != null) {builder.setterFactory(setterFactory);}Class<?> fallback = factory.getFallback();if (fallback != void.class) {return targetWithFallback(name, context, target, builder, fallback);}Class<?> fallbackFactory = factory.getFallbackFactory();if (fallbackFactory != void.class) {return targetWithFallbackFactory(name, context, target, builder,fallbackFactory);}return feign.target(target);}

HystrixTargeter的target方法里，最后调用了feign.target(target);方法，feign实现了构造代理对象的过程，所以这里将会回调feign的构造过程方法，在feign的target方法中，将会构造出一个Feign对象，并返回对象。

public <T> T target(Target<T> target) {return build().newInstance(target);}public Feign build() {// ...SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,logLevel, decode404, closeAfterDecode, propagationPolicy, forceDecoding);ParseHandlersByName handlersByName =new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,errorDecoder, synchronousMethodHandlerFactory);return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);}

跟进ReflectiveFeign#newInstance方法，主要是通过JDK的动态代理构建代理对象：

public <T> T newInstance(Target<T> target) {Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();for (Method method : target.type().getMethods()) {if (method.getDeclaringClass() == Object.class) {continue;} else if (Util.isDefault(method)) {DefaultMethodHandler handler = new DefaultMethodHandler(method);defaultMethodHandlers.add(handler);methodToHandler.put(method, handler);} else {methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));}}InvocationHandler handler = factory.create(target, methodToHandler);T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),new Class<?>[] {target.type()}, handler);for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {defaultMethodHandler.bindTo(proxy);}return proxy;}

代理对象的构建主要由3个内容组成：

构建Method到MethodHandler的映射关系，后面调用代理对象的时候将会根据Method找到MethodHandler，然后调用MethodHandler的invoke方法，而MethodHandler包含发起HTTP请求的实现。
jdk动态代理需要提供InvocationHandler 。而InvocationHandler将由InvocationHandlerFactory的create方法实现。
通过Proxy.newProxyInstance方法，生成proxy对象。

调用proxy对象发起HTTP请求我们都知道,JDK的动态代理将会调用FeignInvocationHandler(ReflectiveFeign的静态内部类)的invoke方法.

@Overridepublic Object invoke(Object proxy, Method method, Object[] args) throws Throwable {// equals、toString、hashcode方法特殊处理return dispatch.get(method).invoke(args);}

在前面构建代理对象的时候,构建了Method到MethodHandler的映射关系.所以在这里就是根据method来获取到MethodHandler,在调用invoke方法的.
进入到invoke方法里,MethodHandler接口的默认实现类为SynchronousMethodHandler:

@Overridepublic Object invoke(Object[] argv) throws Throwable {// 根据上面创建对象过程中解析出来的RequestTemplate克隆一个RequestTemplateRequestTemplate template = buildTemplateFromArgs.create(argv);Options options = findOptions(argv);Retryer retryer = this.retryer.clone();while (true) {try {// executeAndDecode将会负责发起http请求return executeAndDecode(template, options);} catch (RetryableException e) {try {retryer.continueOrPropagate(e);} catch (RetryableException th) {// ...}continue;}}}

Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {// 对FeignInteceptor 拦截器做处理,并将信息封装到feign.Request 类中Request request = targetRequest(template);if (logLevel != Logger.Level.NONE) {logger.logRequest(metadata.configKey(), logLevel, request);}Response response;long start = System.nanoTime();try {// 执行HTTP请求response = client.execute(request, options);// ensure the request is set. TODO: remove in Feign 12response = response.toBuilder().request(request).requestTemplate(template).build();} catch (IOException e) {if (logLevel != Logger.Level.NONE) {logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));}throw errorExecuting(request, e);}long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);// 解码结果if (decoder != null)return decoder.decode(response, metadata.returnType());CompletableFuture<Object> resultFuture = new CompletableFuture<>();asyncResponseHandler.handleResponse(resultFuture, metadata.configKey(), response,metadata.returnType(),elapsedTime);try {if (!resultFuture.isDone())throw new IllegalStateException("Response handling not done");return resultFuture.join();} catch (CompletionException e) {Throwable cause = e.getCause();if (cause != null)throw cause;throw e;}}

总结
openFeign生成@FeignClient注解的接口的代理对象是从FeignClientFactoryBean的getObject方法开始的，生成proxy对象主要由ReflectiveFeign对象来实现。动态代理方法由jdk原生的动态代理支持。
调用proxy对象，其实就是发起http请求，请求结果将被解码并返回。
所以，正如Feign本身的意义一样，http远程调用被伪装成了本地调用一样简单的代理对象，对于使用者来说就是调用本地接口一样简单
参考文章：
https://zhuanlan.zhihu.com/p/133378040
https://blog.csdn.net/manzhizhen/article/details/110013311
https://www.cnblogs.com/qlqwjy/p/14568086.html
本文版权归Charon和博客园共有，原创文章，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。