Spring cloud是如何集成Feign的

feign客户端的注入源于注解@EnableFeignClients,先看下注解定义：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {

	/**
	 * Alias for the {@link #basePackages()} attribute. Allows for more concise annotation
	 * declarations e.g.: {@code @ComponentScan("org.my.pkg")} instead of
	 * {@code @ComponentScan(basePackages="org.my.pkg")}.
	 * @return the array of 'basePackages'.
	 */
	String[] value() default {};

	/**
	 * Base packages to scan for annotated components.
	 * <p>
	 * {@link #value()} is an alias for (and mutually exclusive with) this attribute.
	 * <p>
	 * Use {@link #basePackageClasses()} for a type-safe alternative to String-based
	 * package names.
	 * @return the array of 'basePackages'.
	 */
	String[] basePackages() default {};

	/**
	 * Type-safe alternative to {@link #basePackages()} for specifying the packages to
	 * scan for annotated components. The package of each class specified will be scanned.
	 * <p>
	 * Consider creating a special no-op marker class or interface in each package that
	 * serves no purpose other than being referenced by this attribute.
	 * @return the array of 'basePackageClasses'.
	 */
	Class<?>[] basePackageClasses() default {};

	/**
	 * A custom <code>@Configuration</code> for all feign clients. Can contain override
	 * <code>@Bean</code> definition for the pieces that make up the client, for instance
	 * {@link feign.codec.Decoder}, {@link feign.codec.Encoder}, {@link feign.Contract}.
	 *
	 * @see FeignClientsConfiguration for the defaults
	 * @return list of default configurations
	 */
	Class<?>[] defaultConfiguration() default {};

	/**
	 * List of classes annotated with @FeignClient. If not empty, disables classpath
	 * scanning.
	 * @return list of FeignClient classes
	 */
	Class<?>[] clients() default {};

}

可以看出他导入的是FeignClientsRegistrar，如下：

class FeignClientsRegistrar
		implements ImportBeanDefinitionRegistrar,ResourceLoaderAware, EnvironmentAware

抓到重点：ImportBeanDefinitionRegistrar，他必然实现了registerBeanDefinitions方法进行的导入：

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

可以看出他主要是registerDefaultConfiguration和registerFeignClients的实现，我们逐项来进行分析：

registerDefaultConfiguration

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;
        //这里判断了下是不是内部类
        if (metadata.hasEnclosingClass()) {
            name = "default." + metadata.getEnclosingClassName();
        } else {
            name = "default." + metadata.getClassName();
        }
        registerClientConfiguration(registry, name, defaultAttrs.get("defaultConfiguration"));
    }
}

//主要是注册了默认配置类进入Spring容器
private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,
        Object configuration) {
    BeanDefinitionBuilder builder = BeanDefinitionBuild