Feign之源码剖析

青旅半醒 2023-06-28 04:53 56阅读 0赞

[附：SpringCloud之系列汇总跳转地址][SpringCloud]

## 什么是Feign ##

Feign是由Retrofit，JAXRS-2.0和WebSocket启发的一个java到http客户端绑定。 Feign的主要目标是将Java Http Clients变得简单。

## 写一个Feign ##

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);
    }

其配置文件如下：

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

查看FeignClient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;

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
    	@ConditionalOnMissingBean
    	public Decoder feignDecoder() {
    		return new ResponseEntityDecoder(new SpringDecoder(this.messageConverters));
    	}
    
    	@Bean
    	@ConditionalOnMissingBean
    	public Encoder feignEncoder() {
    		return new SpringEncoder(this.messageConverters);
    	}
    
    	@Bean
    	@ConditionalOnMissingBean
    	public 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，代码如下：

@Configuration
    public class FeignConfig {
    
        @Bean
        public 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"));
    		}
    	}

扫描到FeignClient后，将信息取出，以bean的形式注入到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() {
    				@Override
    				protected 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 interface
    					AnnotatedBeanDefinition 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 null
    
    		beanDefinition.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：

@Override
      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;
          }
        }
      }

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
    	@ConditionalOnMissingBean
    	public 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：

@Override
      public 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：

@Override
    	public 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>() {
                        @Override
                        public 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 selected
                        public 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>() {
                @Override
                public 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。

## 总结 ##

总的来说，Feign的源码实现的过程如下：

*  **首先通过@EnableFeignClients注解开启FeignClient**
 *  **根据Feign的规则实现接口，并加@FeignClient注解**
 *  **程序启动后，会进行包扫描，扫描所有的@FeignClient的注解的类，并将这些信息注入到ioc容器中**
 *  **当接口的方法被调用，通过jdk的代理，来生成具体的RequestTemplate**
 *  **RequestTemplate再生成Request**
 *  **Request交给Client去处理，其中Client可以是HttpUrlConnection、HttpClient，也可以是Okhttp**
 *  **最后Client被封装到LoadBalanceClient类，这个类结合Ribbon做到了负载均衡**

[附：SpringCloud之系列汇总跳转地址][SpringCloud]

[SpringCloud]: https://blog.csdn.net/qq_24313635/article/details/104061217