DiscoveryClient和Ribbon

@EnableDiscoveryClient

• EnableDiscoveryClient自动导入了AutoServiceRegistrationConfiguration和EurekaDiscoveryClientConfiguration.
• AutoServiceRegistrationConfiguration是用来初始化配置的：AutoServiceRegistrationProperties
• EurekaDiscoveryClientConfiguration这个是通过springfactories动态导入的
• EurekaDiscoveryClientConfiguration初始化了一个EurekaDiscoveryClientConfiguration.Marker对象
• EurekaDiscoveryClientConfiguration.Marker是用来激活EurekaClientAutoConfiguration的
• EurekaClientAutoConfiguration中初始化了EurekaDiscoveryClient,EurekaDiscoveryClient为DiscoveryClient的子类
• DiscoveryClient提供了以下方法

    /**
     * Get all ServiceInstances associated with a particular serviceId
     * @param serviceId the serviceId to query
     * @return a List of ServiceInstance
     */
    List<ServiceInstance> getInstances(String serviceId);
  
    /**
     * @return all known service ids
     */
    List<String> getServices();
  

• DiscoveryClient的使用方法

@Autowired
private DiscoveryClient discoveryClient;

public Object getResult(String api) {
RestTemplate restTemplate = new RestTemplate();
List<ServiceInstance> instances =
discoveryClient.getInstances("serviceName");
if (instances.size()==0) return null;
String serviceUri = String.format("%s/%s",
instances.get(0).getUri().toString(),
api);
ResponseEntity< Object > restExchange =
restTemplate.exchange(
serviceUri,
HttpMethod.GET,
null, Object.class);
return restExchange.getBody();
}

DiscoveryClient会返回所有的可用服务方，由你自主选择使用哪一个进行调用。

@LoadBalanced && Ribbon

• @LoadBalanced的本质是一种@Qualifier，源码及解释如下:

 /**
 * Annotation to mark a RestTemplate bean to be configured to use a LoadBalancerClient
 * @author Spencer Gibb
 */
@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}

/**
 * This annotation may be used on a field or parameter as a qualifier for
 * candidate beans when autowiring. It may also be used to annotate other
 * custom annotations that can then in turn be used as qualifiers.
 *
 * @author Mark Fisher
 * @author Juergen Hoeller
 * @since 2.5
 * @see Autowired
 */
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.TYPE, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Qualifier {

    String value() default "";

}

• 被@LoadBalanced标注的RestTemplate对象会被AsyncLoadBalancerAutoConfiguration和LoadBalancerAutoConfiguration加工，加工方式为添加ribbon的拦截器，ribbon拦截器会拦截发出的请求，将请求中的servicename替换为url，随机替换算法由ribbon使用DiscoveryClient实现