Feign - 源码分析

例子此次先不写了。。。直接源码走起。部分设计跟Ribbon同样，这边就不在累述，建议先看Ribbon系列。
依然从spring.factories提及。注意到这里有这几个类：FeignAutoConfiguration、FeignRibbonClientAutoConfiguration。

启动

FeignAutoConfiguration

加载FeignContext，这里会赋值FeignClientSpecification的集合，
后面说FeignClientSpecification类怎么来的，其实跟Ribbon用的是同一个方法。
FeignContext的defaultConfigType是FeignClientsConfiguration.class，这个和Ribboon用法同样，后面会加载。

@Autowired(required = false)
private List<FeignClientSpecification> configurations = new ArrayList<>();
@Bean
public FeignContext feignContext() {
    FeignContext context = new FeignContext();
    context.setConfigurations(this.configurations);
    return context;
}

另外一个加载的是HystrixTargeter，虽然是HystrixTargeter，若是Feign.Builder不是feign.hystrix.HystrixFeign.Builder，后面调用的仍是Feign.Builder的target方法。

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(name = "feign.hystrix.HystrixFeign")
protected static class HystrixFeignTargeterConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public Targeter feignTargeter() {
        return new HystrixTargeter();
    }

}

FeignRibbonClientAutoConfiguration

加载CachingSpringLoadBalancerFactory，这个类会注入SpringClientFactory，看过了Ribbon，是否是就知道了他要作什么。没错，他会建立一个ILoadBalancer，用于负载均衡。

@Bean
@Primary
@ConditionalOnMissingBean
@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
public CachingSpringLoadBalancerFactory cachingLBClientFactory(
        SpringClientFactory factory) {
    return new CachingSpringLoadBalancerFactory(factory);
}

FeignRibbonClientAutoConfiguration还import了DefaultFeignLoadBalancedConfiguration类，在这个类，会建立一个LoadBalancerFeignClient，后面须要的Client就是他了。

@Bean
@ConditionalOnMissingBean
public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,
        SpringClientFactory clientFactory) {
    return new LoadBalancerFeignClient(new Client.Default(null, null), cachingFactory,
            clientFactory);
}

@EnableFeignClients

咱们使用feign的时候，都会使用这个注解，注解里Import了FeignClientsRegistrar这个类，FeignClientsRegistrar实现了ImportBeanDefinitionRegistrar接口，调用registerBeanDefinitions方法。这个方法作了两件事，一件事是注册FeignClientSpecification类，后面会注入到上面的FeignContext中，这个用法跟Ribbon - 几种自定义负载均衡策略提的同样。另一件事就是建立FactoryBean。

@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
        BeanDefinitionRegistry registry) {
    // 注册FeignClientSpecification类
    registerDefaultConfiguration(metadata, registry);
    // 经过注解的信息注册FeignClientFactoryBean类，
    //这个是FactoryBean，因此容器fresh的时候会调用他的getObject方法
    registerFeignClients(metadata, registry);
}

这个方法就会扫描EnableFeignClients和FeignClient的注解，而后注册FeignClientFactoryBean类型的bean，这个是FactoryBean，因此容器fresh的时候会调用他的getObject方法。

public void registerFeignClients(AnnotationMetadata metadata,
        BeanDefinitionRegistry registry) {

    LinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet<>();
    // 扫描EnableFeignClients注解
    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");
    // 若是EnableFeignClients没有配置clients信息，扫描FeignClient注解
    if (clients == null || clients.length == 0) {
        ClassPathScanningCandidateComponentProvider scanner = getScanner();
        scanner.setResourceLoader(this.resourceLoader);
        // 扫描FeignClient注解
        scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));
        Set<String> basePackages = getBasePackages(metadata);
        for (String basePackage : basePackages) {
            candidateComponents.addAll(scanner.findCandidateComponents(basePackage));
        }
    }
    else {
        // 若是配置clients信息，就忽略FeignClient注解，直接读取clients信息
        for (Class<?> clazz : clients) {
            candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));
        }
    }

    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"));
            // 注册FeignClientFactoryBean
            registerFeignClient(registry, annotationMetadata, attributes);
        }
    }
}

FeignClientFactoryBean#getObject

getObject会调用getTarget方法，他会建立一个Feign.Builder，建立的时候会传FeignContext，这个FeignContext的加载在上面已经提过了。而后调用loadBalance方法返回代理对象，这个代理就是后面远程调用用的。

<T> T getTarget() {
    FeignContext context = applicationContext.getBean(FeignContext.class);
    Feign.Builder builder = feign(context);

    if (!StringUtils.hasText(url)) {
        if (!name.startsWith("http")) {
            url = "http://" + name;
        }
        else {
            url = name;
        }
        url += cleanPath();
        return (T) loadBalance(builder, context,
                new HardCodedTarget<>(type, name, url));
    }
    // 直接ip的形式，代码略
}

FeignClientFactoryBean#feign

建立一个Feign.Builder，FeignContext#getInstance，这个类继承了NamedContextFactory，流程和ribbon的SpringClientFactory#getContext同样。上面提到FeignContext的defaultConfigType是FeignClientsConfiguration.class，因此还会加载OptionalDecoder解码、SpringEncoder编码、SpringMvcContract，解析MVC注解、NEVER_RETRY重试、DefaultFeignLoggerFactory日志、FeignClientConfigurer、Feign.builder()。

protected Feign.Builder feign(FeignContext context) {
    FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);
    Logger logger = loggerFactory.create(type);

    // @formatter:off
    Feign.Builder builder = get(context, Feign.Builder.class)
            // required values
            .logger(logger)
            .encoder(get(context, Encoder.class))
            .decoder(get(context, Decoder.class))
            .contract(get(context, Contract.class));
    // @formatter:on
    // 整合配置Feign.Builder
    configureFeign(context, builder);

    return builder;
}

FeignClientFactoryBean#loadBalance

这里获取Client和Targeter类，Client就是上面的LoadBalancerFeignClient，他是全部共享的。Targeter是HystrixTargeter，他是serviceId私有的。上面已经提了他最终是调用Feign.Builder#target。

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
        HardCodedTarget<T> target) {
    Client client = getOptional(context, Client.class);
    if (client != null) {
        builder.client(client);
        Targeter targeter = get(context, Targeter.class);
        return targeter.target(this, builder, context, target);
    }
    // 异常略
}

Feign.Builder#target

build方法是构建一个ReflectiveFeign对象，newInstance是生成代理对象。

public <T> T target(Target<T> target) {
  return build().newInstance(target);
}

build方法中，ReflectiveFeign建立的时候传了ParseHandlersByName,InvocationHandlerFactory,SynchronousMethodHandler三个对象。

public Feign build() {
  // 其余略
  return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
}

代理对象生成，生成后，后面对代理对象的访问，都会调用FeignInvocationHandler#invoke方法。

public <T> T newInstance(Target<T> target) {
    // ParseHandlersByName是经过Contrac用来解析接口定义。
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    // 给每一个方法建立一个SynchronousMethodHandler对象
    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)));
      }
    }
    // 建立一个InvocationHandler对象，这里是FeignInvocationHandler
    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;
}

总结

启动的时候，为每一个接口，生成代理对象。

方法调用

FeignInvocationHandler#invoke

public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    // 其余略
    //dispatch.get(method)获取的是SynchronousMethodHandler
    return dispatch.get(method).invoke(args);
}

SynchronousMethodHandler#invoke

调用LoadBalancerFeignClient#execute，LoadBalancerFeignClient加载上面已经讲过了。

public Object invoke(Object[] argv) throws Throwable {
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    // 其余略
    return executeAndDecode(template, options);
   // 其余略
}

Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {
    // 其余略
    response = client.execute(request, options);
    // 其余略
}

LoadBalancerFeignClient#execute

@Override
public Response execute(Request request, Request.Options options) throws IOException {
    try {
        // 获取Uri
        URI asUri = URI.create(request.url());
        // 获取serviceId
        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);
        // 获取FeignLoadBalancer，经过FeignLoadBalancer调用请求
        return lbClient(clientName)
                .executeWithLoadBalancer(ribbonRequest, requestConfig).toResponse();
    }
    catch (ClientException e) {
        IOException io = findIOException(e);
        if (io != null) {
            throw io;
        }
        throw new RuntimeException(e);
    }
}

lbClientFactory是CachingSpringLoadBalancerFactory，因此调用CachingSpringLoadBalancerFactory#create的create方法。
CachingSpringLoadBalancerFactory有个熟悉的SpringClientFactory对象，他负责获取ILoadBalancer、IClientConfig、ServerIntrospector，而后经过这三个构建一个FeignLoadBalancer对象。既然获得了ILoadBalancer，那后续负载均衡的部分就再也不继续了，参考Ribbon - 负载均衡流程。

private FeignLoadBalancer lbClient(String clientName) {
    return this.lbClientFactory.create(clientName);
}

public FeignLoadBalancer create(String clientName) {
    FeignLoadBalancer client = this.cache.get(clientName);
    if (client != null) {
        return client;
    }
    IClientConfig config = this.factory.getClientConfig(clientName);
    ILoadBalancer lb = this.factory.getLoadBalancer(clientName);
    ServerIntrospector serverIntrospector = this.factory.getInstance(clientName,
            ServerIntrospector.class);
    client = this.loadBalancedRetryFactory != null
            ? new RetryableFeignLoadBalancer(lb, config, serverIntrospector,
                    this.loadBalancedRetryFactory)
            : new FeignLoadBalancer(lb, config, serverIntrospector);
    this.cache.put(clientName, client);
    return client;
}

总结

主要是经过代理对象，而后调用Ribbon进行负载均衡。咱们这边只讲述了主流程，怎么构建HTTP请求，怎么处理返回结果，都略过了。。。。。