Spring Cloud中RestTemplate是如何通過註解與Ribbon結合的

我們在寫Spring Cloud代碼時，如何調用Spring Cloud中的其他服務呢？

只需要定義一個RestTemplate，然後在其上加一個註解@LoadBalanced即可。如下：

@Bean
@LoadBalanced
RestTemplate restTemplate() {
	return new RestTemplate();
}

我們就可以直接使用服務名稱來調用該服務的接口。

那使用了@LoadBalanced註解以後，RestTemplate是如何就可以直接使用服務名調用了呢？

要找到如何關聯的，就需要找到：

• 哪些類在方法中使用了@LoadBalanced註解。
• 這些類是如何在啓動類中被初始化的。

我們查詢LoadBalancer相關的啓動類，發現了兩個類：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.client.loadbalancer.AsyncLoadBalancerAutoConfiguration,\
org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration

而@LoadBalancer的使用，也在這兩個類中。

@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();
		}

		@Bean
		@ConditionalOnMissingBean
		public LoadBalancedBackOffPolicyFactory loadBalancedBackOffPolicyFactory() {
			return new LoadBalancedBackOffPolicyFactory.NoBackOffPolicyFactory();
		}
	}

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

		@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);
				}
			};
		}
	}
}

該類中定義了一個@LoadBalanced的List<RestTemplate>，所有@LoadBalanced修飾的RestTemplate都會注入到這個List中。

然後再初始化的過程中，調用customizer.customize(restTemplate)方法來給RestTemplate增加攔截器LoadBalancerInterceptor。

具體cutomizer的定義，在類啓動類RibbonAutoConfiguration中。

		@Bean
		public RestTemplateCustomizer restTemplateCustomizer(
				final RibbonClientHttpRequestFactory ribbonClientHttpRequestFactory) {
			return new RestTemplateCustomizer() {
				@Override
				public void customize(RestTemplate restTemplate) {
					restTemplate.setRequestFactory(ribbonClientHttpRequestFactory);
				}
			};
		}

		@Bean
		public RibbonClientHttpRequestFactory ribbonClientHttpRequestFactory() {
			return new RibbonClientHttpRequestFactory(this.springClientFactory);
		}

而LoadBalancerInterceptor用於實時攔截。具體攔截方法如下：

	@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));
	}