SpringCloud Feign

时间 2019-12-24

标签 springcloud feign 繁體版

原文原文链接

Feign是受到Retrofit，JAXRS-2.0和WebSocket的影响，它是一个java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/feignjava

写一个Feign 简单的实现一个Feign客户端，首先经过@FeignClient，客户端，其中value为调用其余服务的名称，FeignConfig.class为FeignClient的配置文件，代码以下：git

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

其自定义配置文件以下，固然也能够不写配置文件，用默认的便可：github

@Configuration
public class FeignConfig {

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

}

查看FeignClient注解的源码，其代码以下：spring

@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注解的做用目标在接口上； @Retention(RetentionPolicy.RUNTIME)，注解会在class字节码文件中存在，在运行时能够经过反射获取到；@Documented表示该注解将被包含在javadoc中。后端

feign 用于声明具备该接口的REST客户端的接口的注释应该是建立（例如用于自动链接到另外一个组件。若是功能区可用，那将是用于负载平衡后端请求，而且能够配置负载平衡器使用与假装客户端相同名称（即值）@RibbonClient 。api

其中value()和name()同样，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，仍是抛异常；configuration()，标明FeignClient的配置类，默认的配置类为FeignClientsConfiguration类，能够覆盖Decoder、Encoder和Contract等信息，进行自定义配置。fallback(),填写熔断器的信息类。网络

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，能够发现它是一个配置类，注入了不少的相关配置的bean，包括feignRetryer、FeignLoggerFactory、FormattingConversionService等,其中还包括了Decoder、Encoder、Contract，若是这三个bean在没有注入的状况下，会自动注入默认的配置。app

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

@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：你能够重写FeignClientsConfiguration中的bean，从而达到自定义配置的目的，好比FeignClientsConfiguration的默认重试次数为Retryer.NEVER_RETRY，即不重试，那么但愿作到重写，写个配置文件，注入feignRetryer的bean,代码以下：框架

@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的工做原理

feign是一个伪客户端，即它不作任何的请求处理。Feign经过处理注解生成request，从而实现简化HTTP API开发的目的，即开发人员可使用注解的方式定制request api模板，在发送http request请求以前，feign经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。

经过包扫描注入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，若是有该注解，则开启包扫描，扫描被@FeignClient注解接口。代码以下：

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"));
        }
    }

程序启动后经过包扫描，当类有@FeignClient注解，将注解的信息取出，连同类名一块儿取出，赋给BeanDefinitionBuilder，而后根据BeanDefinitionBuilder获得beanDefinition，最后beanDefinition式注入到ioc容器中，源码以下：

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以后，经过jdk的代理，当请求Feign Client的方法时会被拦截，代码在ReflectiveFeign类，代码以下：

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，代码以下：

@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，而后根据用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组件是一个很是重要的组件，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.

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

@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的状况下，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，代码以下：

@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，只须要在pom文件加上HttpClient的classpath就好了，另外须要在配置文件上加上feign.httpclient.enabled为true，从 @ConditionalOnProperty注解可知，这个能够不写，在默认的状况下就为true.

在pom文件加上：

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

同理，若是想要feign使用Okhttp，则只须要在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？经过上述的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。如今来看下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，即经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，代码以下：

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，即以前讲述的Ribbon，在这里再也不重复。

总结

总到来讲，Feign的源码实现的过程以下：

首先经过@EnableFeignCleints注解开启FeignCleint 根据Feign的规则实现接口，并加@FeignCleint注解程序启动后，会进行包扫描，扫描全部的@ FeignCleint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，经过jdk的代理，来生成具体的RequesTemplate RequesTemplate在生成Request Request交给Client去处理，其中Client能够是HttpUrlConnection、HttpClient也能够是Okhttp 最后Client被封装到LoadBalanceClient类，这个类结合类Ribbon作到了负载均衡。