深入理解Feign之源码解析

转载请标明出处：
https://blog.csdn.net/forezp/article/details/73480304
本文出自方志朋的博客
出自方志朋的博客

个人博客纯净版：https://www.fangzhipeng.com/springcloud/2017/08/11/sc-feign-raw.html

什么是Feign

Feign是受到Retrofit，JAXRS-2.0和WebSocket的影响，它是一个jav的到http客户端绑定的开源项目。 Feign的主要目标是将Java Http 客户端变得简单。Feign的源码地址：https://github.com/OpenFeign/feign

写一个Feign

在我之前的博文有写到如何用Feign去消费服务，文章地址：http://blog.csdn.net/forezp/article/details/69808079 。

现在来简单的实现一个Feign客户端，首先通过@FeignClient，客户端，其中value为调用其他服务的名称，FeignConfig.class为FeignClient的配置文件，代码如下：


@FeignClient(value = "service-hi",configuration = FeignConfig.class)
public interface SchedualServiceHi {@GetMapping(value = "/hi")String sayHiFromClientOne(@RequestParam(value = "name") String name);
}

其自定义配置文件如下，当然也可以不写配置文件，用默认的即可：

@Configuration
public class FeignConfig {@Beanpublic Retryer feignRetryer() {return new Retryer.Default(100, SECONDS.toMillis(1), 5);}}

查看FeignClient注解的源码，其代码如下：


@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FeignClient {@AliasFor("name")
String value() default "";@AliasFor("value")
String name() default "";@AliasFor("value")
String name() default "";
String url() default "";
boolean decode404() default false;Class<?>[] configuration() default {};
Class<?> fallback() default void.class;Class<?> fallbackFactory() default void.class;
}String path() default "";boolean primary() default true;

FeignClient注解被@Target(ElementType.TYPE)修饰，表示FeignClient注解的作用目标在接口上；
@Retention(RetentionPolicy.RUNTIME)，注解会在class字节码文件中存在，在运行时可以通过反射获取到；@Documented表示该注解将被包含在javadoc中。

feign 用于声明具有该接口的REST客户端的接口的注释应该是创建（例如用于自动连接到另一个组件。如果功能区可用，那将是
用于负载平衡后端请求，并且可以配置负载平衡器
使用与伪装客户端相同名称（即值）@RibbonClient 。

其中value()和name()一样，是被调用的 service的名称。
url(),直接填写硬编码的url,decode404()即404是否被解码，还是抛异常；configuration()，标明FeignClient的配置类，默认的配置类为FeignClientsConfiguration类，可以覆盖Decoder、Encoder和Contract等信息，进行自定义配置。fallback(),填写熔断器的信息类。

FeignClient的配置

默认的配置类为FeignClientsConfiguration，这个类在spring-cloud-netflix-core的jar包下，打开这个类，可以发现它是一个配置类，注入了很多的相关配置的bean，包括feignRetryer、FeignLoggerFactory、FormattingConversionService等,其中还包括了Decoder、Encoder、Contract，如果这三个bean在没有注入的情况下，会自动注入默认的配置。

Decoder feignDecoder: ResponseEntityDecoder(这是对SpringDecoder的封装)
Encoder feignEncoder: SpringEncoder
Logger feignLogger: Slf4jLogger
Contract feignContract: SpringMvcContract
Feign.Builder feignBuilder: HystrixFeign.Builder

代码如下：

@Configuration
public class FeignClientsConfiguration {...//省略代码@Bean@ConditionalOnMissingBeanpublic Decoder feignDecoder() {return new ResponseEntityDecoder(new SpringDecoder(this.messageConverters));}@Bean@ConditionalOnMissingBeanpublic Encoder feignEncoder() {return new SpringEncoder(this.messageConverters);}@Bean@ConditionalOnMissingBeanpublic Contract feignContract(ConversionService feignConversionService) {return new SpringMvcContract(this.parameterProcessors, feignConversionService);}...//省略代码
}

重写配置：

你可以重写FeignClientsConfiguration中的bean，从而达到自定义配置的目的，比如FeignClientsConfiguration的默认重试次数为Retryer.NEVER_RETRY，即不重试，那么希望做到重写，写个配置文件，注入feignRetryer的bean,代码如下：

@Configuration
public class FeignConfig {@Beanpublic Retryer feignRetryer() {return new Retryer.Default(100, SECONDS.toMillis(1), 5);}}

在上述代码更改了该FeignClient的重试次数，重试间隔为100ms，最大重试时间为1s,重试次数为5次。

Feign的工作原理

feign是一个伪客户端，即它不做任何的请求处理。Feign通过处理注解生成request，从而实现简化HTTP API开发的目的，即开发人员可以使用注解的方式定制request api模板，在发送http request请求之前，feign通过处理注解的方式替换掉request模板中的参数，这种实现方式显得更为直接、可理解。

通过包扫描注入FeignClient的bean，该源码在FeignClientsRegistrar类：
首先在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。代码如下：

private void registerDefaultConfiguration(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {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();}registerClientConfiguration(registry, name,defaultAttrs.get("defaultConfiguration"));}}

程序启动后通过包扫描，当类有@FeignClient注解，将注解的信息取出，连同类名一起取出，赋给BeanDefinitionBuilder，然后根据BeanDefinitionBuilder得到beanDefinition，最后beanDefinition式注入到ioc容器中，源码如下：

public void registerFeignClients(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {ClassPathScanningCandidateComponentProvider scanner = getScanner();scanner.setResourceLoader(this.resourceLoader);Set<String> basePackages;Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(FeignClient.class);final Class<?>[] clients = attrs == null ? null: (Class<?>[]) attrs.get("clients");if (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) {Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);for (BeanDefinition candidateComponent : candidateComponents) {if (candidateComponent instanceof AnnotatedBeanDefinition) {// verify annotated class is an interfaceAnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;AnnotationMetadata annotationMetadata = 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);registerClientConfiguration(registry, name,attributes.get("configuration"));registerFeignClient(registry, annotationMetadata, attributes);}}}}private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {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 });BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);}

注入bean之后，通过jdk的代理，当请求Feign Client的方法时会被拦截，代码在ReflectiveFeign类，代码如下：

 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;}

在SynchronousMethodHandler类进行拦截处理，当被FeignClient的方法被拦截会根据参数生成RequestTemplate对象，该对象就是http请求的模板，代码如下：

 @Overridepublic 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;}}}

其中有个executeAndDecode()方法，该方法是通RequestTemplate生成Request请求对象，然后根据用client获取response。

  Object executeAndDecode(RequestTemplate template) throws Throwable {Request request = targetRequest(template);...//省略代码response = client.execute(request, options);...//省略代码}

Client组件

其中Client组件是一个非常重要的组件，Feign最终发送request请求以及接收response响应，都是由Client组件完成的，其中Client的实现类，只要有Client.Default，该类由HttpURLConnnection实现网络请求，另外还支持HttpClient、Okhttp.

首先来看以下在FeignRibbonClient的自动配置类，FeignRibbonClientAutoConfiguration ，主要在工程启动的时候注入一些bean,其代码如下：

@ConditionalOnClass({ ILoadBalancer.class, Feign.class })
@Configuration
@AutoConfigureBefore(FeignAutoConfiguration.class)
public class FeignRibbonClientAutoConfiguration {@Bean@ConditionalOnMissingBeanpublic Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,SpringClientFactory clientFactory) {return new LoadBalancerFeignClient(new Client.Default(null, null),cachingFactory, clientFactory);}}

在缺失配置feignClient的情况下，会自动注入new Client.Default(),跟踪Client.Default()源码，它使用的网络请求框架为HttpURLConnection，代码如下：

  @Overridepublic Response execute(Request request, Options options) throws IOException {HttpURLConnection connection = convertAndSend(request, options);return convertResponse(connection).toBuilder().request(request).build();}

怎么在feign中使用HttpClient，查看FeignRibbonClientAutoConfiguration的源码

@ConditionalOnClass({ ILoadBalancer.class, Feign.class })
@Configuration
@AutoConfigureBefore(FeignAutoConfiguration.class)
public class FeignRibbonClientAutoConfiguration {
...//省略代码@Configuration@ConditionalOnClass(ApacheHttpClient.class)@ConditionalOnProperty(value = "feign.httpclient.enabled", matchIfMissing = true)protected static class HttpClientFeignLoadBalancedConfiguration {@Autowired(required = false)private HttpClient httpClient;@Bean@ConditionalOnMissingBean(Client.class)public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,SpringClientFactory clientFactory) {ApacheHttpClient delegate;if (this.httpClient != null) {delegate = new ApacheHttpClient(this.httpClient);}else {delegate = new ApacheHttpClient();}return new LoadBalancerFeignClient(delegate, cachingFactory, clientFactory);}}...//省略代码
}

从代码@ConditionalOnClass(ApacheHttpClient.class)注解可知道，只需要在pom文件加上HttpClient的classpath就行了，另外需要在配置文件上加上feign.httpclient.enabled为true，从 @ConditionalOnProperty注解可知，这个可以不写，在默认的情况下就为true.

在pom文件加上：


<dependency><groupId>com.netflix.feign</groupId><artifactId>feign-httpclient</artifactId><version>RELEASE</version>
</dependency>

同理，如果想要feign使用Okhttp，则只需要在pom文件上加上feign-okhttp的依赖：

<dependency><groupId>com.netflix.feign</groupId><artifactId>feign-okhttp</artifactId><version>RELEASE</version>
</dependency>

feign的负载均衡是怎么样实现的呢？

通过上述的FeignRibbonClientAutoConfiguration类配置Client的类型(httpurlconnection，okhttp和httpclient)时候，可知最终向容器注入的是LoadBalancerFeignClient，即负载均衡客户端。现在来看下LoadBalancerFeignClient的代码：

	@Overridepublic Response execute(Request request, Request.Options options) throws IOException {try {URI asUri = URI.create(request.url());String clientName = asUri.getHost();URI uriWithoutHost = cleanUrl(request.url(), clientName);FeignLoadBalancer.RibbonRequest ribbonRequest = new FeignLoadBalancer.RibbonRequest(this.delegate, request, uriWithoutHost);IClientConfig requestConfig = getClientConfig(options, clientName);return lbClient(clientName).executeWithLoadBalancer(ribbonRequest,requestConfig).toResponse();}catch (ClientException e) {IOException io = findIOException(e);if (io != null) {throw io;}throw new RuntimeException(e);}}

其中有个executeWithLoadBalancer()方法，即通过负载均衡的方式请求。

  public T executeWithLoadBalancer(final S request, final IClientConfig requestConfig) throws ClientException {RequestSpecificRetryHandler handler = getRequestSpecificRetryHandler(request, requestConfig);LoadBalancerCommand<T> command = LoadBalancerCommand.<T>builder().withLoadBalancerContext(this).withRetryHandler(handler).withLoadBalancerURI(request.getUri()).build();try {return command.submit(new ServerOperation<T>() {@Overridepublic Observable<T> call(Server server) {URI finalUri = reconstructURIWithServer(server, request.getUri());S requestForServer = (S) request.replaceUri(finalUri);try {return Observable.just(AbstractLoadBalancerAwareClient.this.execute(requestForServer, requestConfig));} catch (Exception e) {return Observable.error(e);}}}).toBlocking().single();} catch (Exception e) {Throwable t = e.getCause();if (t instanceof ClientException) {throw (ClientException) t;} else {throw new ClientException(e);}}}

其中服务在submit()方法上，点击submit进入具体的方法,这个方法是LoadBalancerCommand的方法：

     Observable<T> o = (server == null ? selectServer() : Observable.just(server)).concatMap(new Func1<Server, Observable<T>>() {@Override// Called for each server being selectedpublic Observable<T> call(Server server) {context.setServer(server);}}

上述代码中有个selectServe()，该方法是选择服务的进行负载均衡的方法，代码如下：

    private Observable<Server> selectServer() {return Observable.create(new OnSubscribe<Server>() {@Overridepublic void call(Subscriber<? super Server> next) {try {Server server = loadBalancerContext.getServerFromLoadBalancer(loadBalancerURI, loadBalancerKey);   next.onNext(server);next.onCompleted();} catch (Exception e) {next.onError(e);}}});}

最终负载均衡交给loadBalancerContext来处理，即之前讲述的Ribbon，在这里不再重复。