@LoadBalanced注解使用的理解

在使用springcloud ribbon客户端负载均衡的时候，可以给RestTemplate bean 加一个@LoadBalanced注解，就能让这个RestTemplate在请求时拥有客户端负载均衡的能力：

@Bean
@LoadBalanced //开启负载均衡的功能
RestTemplate restTemplate() {
    return new RestTemplate();
}

@LoadBalanced的注解源码:

/**
 * 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 {
}

关于@Qulifier注解作用自行百度。

LoadBalancerAutoConfiguration是为实现客户端负载均衡器的自动装配类：

/**
 * Auto configuration for Ribbon (client side load balancing).
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Will Tran
 */
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

   @LoadBalanced
   @Autowired(required = false)
   private List<RestTemplate> restTemplates = Collections.emptyList();

   @Bean
   public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
         final List<RestTemplateCustomizer> customizers) {
      return new SmartInitializingSingleton() {
         @Override
         public void afterSingletonsInstantiated() {
            for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
               for (RestTemplateCustomizer customizer : customizers) {
                  customizer.customize(restTemplate);
               }
            }
         }
      };
   }

   @Autowired(required = false)
   private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();

   @Bean
   @ConditionalOnMissingBean
   public LoadBalancerRequestFactory loadBalancerRequestFactory(
         LoadBalancerClient loadBalancerClient) {
      return new LoadBalancerRequestFactory(loadBalancerClient, transformers);
   }

   @Configuration
   @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
   static class LoadBalancerInterceptorConfig {
      @Bean
      public LoadBalancerInterceptor ribbonInterceptor(
            LoadBalancerClient loadBalancerClient,
            LoadBalancerRequestFactory requestFactory) {
         return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
      }

      @Bean
      @ConditionalOnMissingBean
      public RestTemplateCustomizer restTemplateCustomizer(
            final LoadBalancerInterceptor loadBalancerInterceptor) {
         return new RestTemplateCustomizer() {
            @Override
            public void customize(RestTemplate restTemplate) {
               List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                     restTemplate.getInterceptors());
               list.add(loadBalancerInterceptor);
               restTemplate.setInterceptors(list);
            }
         };
      }
   }

   @Configuration
   @ConditionalOnClass(RetryTemplate.class)
   public static class RetryAutoConfiguration {
      @Bean
      public RetryTemplate retryTemplate() {
         RetryTemplate template =  new RetryTemplate();
         template.setThrowLastExceptionOnExhausted(true);
         return template;
      }

      @Bean
      @ConditionalOnMissingBean
      public LoadBalancedRetryPolicyFactory loadBalancedRetryPolicyFactory() {
         return new LoadBalancedRetryPolicyFactory.NeverRetryFactory();
      }
   }

   @Configuration
   @ConditionalOnClass(RetryTemplate.class)
   public static class RetryInterceptorAutoConfiguration {
      @Bean
      @ConditionalOnMissingBean
      public RetryLoadBalancerInterceptor ribbonInterceptor(
            LoadBalancerClient loadBalancerClient, LoadBalancerRetryProperties properties,
            LoadBalancedRetryPolicyFactory lbRetryPolicyFactory,
            LoadBalancerRequestFactory requestFactory) {
         return new RetryLoadBalancerInterceptor(loadBalancerClient, properties,
               lbRetryPolicyFactory, requestFactory);
      }

      @Bean
      @ConditionalOnMissingBean
      public RestTemplateCustomizer restTemplateCustomizer(
            final RetryLoadBalancerInterceptor loadBalancerInterceptor) {
         return new RestTemplateCustomizer() {
            @Override
            public void customize(RestTemplate restTemplate) {
               List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                     restTemplate.getInterceptors());
               list.add(loadBalancerInterceptor);
               restTemplate.setInterceptors(list);
            }
         };
      }
   }
}

从LoadBalancerAutoConfiguration类注解可以知道，负载均衡自动配置需要ConditionalOnClass(RestTemplate.class)和@ConditionalOnBean(LoadBalancerClient.class)。

这里直接解释下为什么@LoadBalanced注解能让RestTemplate在请求时拥有客户端负载均衡的能力。LoadBalancerAutoConfiguration类会产生一个SmartInitializingSingleton的Bean：

@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
      final List<RestTemplateCustomizer> customizers) {
   return new SmartInitializingSingleton() {
      @Override
      public void afterSingletonsInstantiated() {
         for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
            for (RestTemplateCustomizer customizer : customizers) {
               customizer.customize(restTemplate);
            }
         }
      }
   };
}

LoadBalancerAutoConfiguration.this.restTemplates即所有被@LoadBalanced注解的RestTemplate，这里的customizers如下：

   @Bean
   @ConditionalOnMissingBean
   public RestTemplateCustomizer restTemplateCustomizer(
         final LoadBalancerInterceptor loadBalancerInterceptor) {
      return new RestTemplateCustomizer() {
         @Override
         public void customize(RestTemplate restTemplate) {
            List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                  restTemplate.getInterceptors());
            list.add(loadBalancerInterceptor);
            restTemplate.setInterceptors(list);
         }
      };
   }
}

会给restTemplate设置一个loadBalancerInterceptor，这个LoadBalancerInterceptor如下：

@Bean
public LoadBalancerInterceptor ribbonInterceptor(
      LoadBalancerClient loadBalancerClient,
      LoadBalancerRequestFactory requestFactory) {
   return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
}

入参就有一个loadBalancerClient。当RestTemplate发起一个请求，请求就会被LoadBalancerInterceptor拦截：

@Override
public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
      final ClientHttpRequestExecution execution) throws IOException {
   final URI originalUri = request.getURI();
   String serviceName = originalUri.getHost();
   Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
   return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
}

实际的请求是由loadBalancer发起的。