Feign自动装配原理

spring.factories

按照以往的惯例，在研究源码的时候，我们先看一下spring.factories文件下自动装配的类FeignAutoConfiguration，其中比较重要的东西有这么几个

    @Autowired(required = false)
    private List<FeignClientSpecification> configurations = new ArrayList<>();


    @Bean
    public FeignContext feignContext() {
        FeignContext context = new FeignContext();
        context.setConfigurations(this.configurations);
        return context;
    }

    @Configuration
    @ConditionalOnClass(name = "feign.hystrix.HystrixFeign")
    protected static class HystrixFeignTargeterConfiguration {
        @Bean
        @ConditionalOnMissingBean
        public Targeter feignTargeter() {
            return new HystrixTargeter();
        }
    }

    @Configuration
    @ConditionalOnMissingClass("feign.hystrix.HystrixFeign")
    protected static class DefaultFeignTargeterConfiguration {
        @Bean
        @ConditionalOnMissingBean
        public Targeter feignTargeter() {
            return new DefaultTargeter();
        }
    }

属性configurations代表的是各个Feign客户端的配置类，这个稍后会再次提到
FeignContext这个bean看名字就知道，Feign的上下文环境，包含了所有feign客户端的配置
接下来是两个Targeter是看当前是否存在hystrix环境，接下来也会提到
除此之外这个类还包含了HttpClient相关的配置就不展开了

`@EnableFeignClients`注解解析

查看完自动装配的类，接着看@EnableFeignClients注解

进入这个注解发现，它引入了配置类 FeignClientsRegistrar，由于这个类实现了ImportBeanDefinitionRegistrar接口，所以按照我们以往的经历直接看一下registerBeanDefinitions方法吧

    public void registerBeanDefinitions(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
        registerDefaultConfiguration(metadata, registry);
         registerFeignClients(metadata, registry);
    }

这里分为了2步

注册缺省配置

    private void registerDefaultConfiguration(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {
        // 获取注解@EnableFeignClients的注解属性     
        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();
            }
            // 各种信息准备就绪，现在执行注册
            registerClientConfiguration(registry, name,
                    defaultAttrs.get("defaultConfiguration"));
        }
    }
    private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,
            Object configuration) {
        BeanDefinitionBuilder builder = BeanDefinitionBuilder
                .genericBeanDefinition(FeignClientSpecification.class);
        builder.addConstructorArgValue(name);
        builder.addConstructorArgValue(configuration);
        registry.registerBeanDefinition(
                name + "." + FeignClientSpecification.class.getSimpleName(),
                builder.getBeanDefinition());
    }

可以看到这里就是处理注册@EnableFeignClients上defaultConfiguration属性所指定的客户端的缺省配置，注意这里配置都是注册为了FeignClientSpecification类型的bean，这个类型的bean也是本文刚开始提到的被Feign上下文持有的各个Feign客户端持有的

注册各个`Feign`客户端

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) {
                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);
            }
        }
    }

这里一共分为3个步骤：

使用ClassPathScanningCandidateComponentProvider扫描所有的标注了@FeignClient注解的接口
将注解上包含的属性作为bean注册，这些属性也就是每个Feign客户端端的配置
将@Feign客户端注册


private void registerFeignClient(BeanDefinitionRegistry registry,
                                 AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
    // 1.获取标注@Feign注解的接口名称
    String className = annotationMetadata.getClassName();
   
    // 2.使用BeanDefinitionBuilder构造bean：FeignClientFactoryBean
    BeanDefinitionBuilder definition = BeanDefinitionBuilder
        .genericBeanDefinition(FeignClientFactoryBean.class);
    validate(attributes);
    
    // 3.添加FeignClientFactoryBean的各个属性
    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);
 
    // 4.设置别名
    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;
    }
 
    // 5.注册FeignClientFactoryBean
    BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
                                                           new String[] { alias });
    BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
}

上方这个方法通过了5步把各个FeignClient都注册成了bean：FeignClientFactoryBean，相信看过之前文章的同学都知道FactoryBean系列的bean是干什么的了。而Feign整合Ribbon和Hystrix的核心应该也在这个类里面了

Feign自动装配原理

spring.factories

@EnableFeignClients注解解析

注册缺省配置

注册各个Feign客户端

`@EnableFeignClients`注解解析

注册各个`Feign`客户端