Feign automatic assembly principle

spring.factories

According to the past practice, when researching the source code, we first look at spring.factoriesthe automatically assembled classes under the file FeignAutoConfiguration, among which there are several important things.

	@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();
		}
	}

1. The attribute configurationsrepresents the configuration class of each Feign client, which will be mentioned again later
2. FeignContextAs you can see from the name of this bean, Feign's context environment contains the configuration of all feign clients.
3. The next two Targeterare to see if there is currently a hystrix environment, which will also be mentioned next
4. In addition, this class also contains HttpClient related configuration and will not be expanded.

@EnableFeignClientsAnnotation parsing

After viewing the autowired classes, then look at the @EnableFeignClientsannotations

Entering this annotation, it is found that it introduces a configuration class FeignClientsRegistrar. Since this class implements the ImportBeanDefinitionRegistrarinterface, let's take a look at the registerBeanDefinitionsmethod directly according to our past experience.

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

There are 2 steps here

Register default configuration

    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());
	}

You can see that this is @EnableFeignClientsthe default configuration for processing the client specified by the defaultConfiguration attribute on the registration. Note that the configuration here is registered as FeignClientSpecificationa type of bean. This type of bean is also the Feign client held by the Feign context mentioned at the beginning of this article. held by the end

Register each Feignclient

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);
            }
        }
    }

There are 3 steps here:

1. Use to ClassPathScanningCandidateComponentProviderscan all @FeignClientannotated interfaces
2. Register the properties contained on the annotation as beans, which are the configuration of each Feign client
3. register the @Feignclient

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);
}

     The above method has passed 5 steps to register each FeignClient as a bean: FeignClientFactoryBean, I believe that students who have read the previous article know what FactoryBeanthe series of beans do. The core of Feign's integration of Ribbon and Hystrix should also be in this class

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