由于历史原因，原有业务系统的大部分接口都是通过SpringBoot RESTFul暴露服务的且能过Nginx做网关。微服务架构近几年也非常的火，各个公司开始将业务服务

 

 在使用Ribbon进行服务消费的时候，我们用到了RestTemplate，但是熟悉Spring的人是否产生过这样的疑问：RestTemplate不是Spring自己就有的吗？跟Ribbon的客户端负载均衡又有什么关系呢？带着这些疑问，我们就来看RestTemplate和Ribbon是如何联系起来并实现客户端负载均衡的。

首先，对于消费者示例：我们是如何实现客户端负载均衡的？仔细观察一下代码之前的代码，我们可以发现在消费者的例子中，可能就是这个注解@LoadBalanced是之前没有接触过的，并且从命名上来看也与负载均衡相关。我们不妨根据这个线索来看看源码实现的机制。

从@LoadBalanced注解源码的注释中，我们可以知道该注解用来给RestTemplate标记，以使用负载均衡的客户端（LoadBalancerClient）来配置它。

package org.springframework.cloud.client.loadbalancer;

import org.springframework.beans.factory.annotation.Qualifier;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * 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 {
}

通过搜索LoadBalancerClient，我们可以发现这是Spring Cloud中定义的一个接口：

package org.springframework.cloud.client.loadbalancer;

import org.springframework.cloud.client.ServiceInstance;

import java.io.IOException;
import java.net.URI;

/**
 * Represents a client side load balancer
 * @author Spencer Gibb
 */
public interface LoadBalancerClient extends ServiceInstanceChooser {

	/**
	 * execute request using a ServiceInstance from the LoadBalancer for the specified
	 * service
	 * @param serviceId the service id to look up the LoadBalancer
	 * @param request allows implementations to execute pre and post actions such as
	 * incrementing metrics
	 * @return the result of the LoadBalancerRequest callback on the selected
	 * ServiceInstance
	 */
	<T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;

	/**
	 * execute request using a ServiceInstance from the LoadBalancer for the specified
	 * service
	 * @param serviceId the service id to look up the LoadBalancer
	 * @param serviceInstance the service to execute the request to
	 * @param request allows implementations to execute pre and post actions such as
	 * incrementing metrics
	 * @return the result of the LoadBalancerRequest callback on the selected
	 * ServiceInstance
	 */
	<T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;

	/**
	 * Create a proper URI with a real host and port for systems to utilize.
	 * Some systems use a URI with the logical serivce name as the host,
	 * such as http://myservice/path/to/service.  This will replace the
	 * service name with the host:port from the ServiceInstance.
	 * @param instance
	 * @param original a URI with the host as a logical service name
	 * @return a reconstructed URI
	 */
	URI reconstructURI(ServiceInstance instance, URI original);
}

 该接口中继承了ServiceInstanceChooser接口，我们可以通过定义的抽象方法来了解到客户端负载均衡器中应具备的几种能力：

• ServiceInstance choose(String serviceId)：根据传入的服务名serviceId，从负载均衡器中挑选一个对应服务的实例。
• T execute(String serviceId, LoadBalancerRequest request) throws IOException：使用从负载均衡器中挑选出的服务实例来执行请求内容。
• URI reconstructURI(ServiceInstance instance, URI original)：为系统构建一个合适的“host:port”形式的URI。在分布式系统中，我们使用逻辑上的服务名称作为host来构建URI（替代服务实例的“host:port”形式）进行请求，比如：http://myservice/path/to/service。在该操作的定义中，前者ServiceInstance对象是带有host和port的具体服务实例，而后者URI对象则是使用逻辑服务名定义为host的URI，而返回的URI内容则是通过ServiceInstance的服务实例详情拼接出的具体“host:post”形式的请求地址。

顺着LoadBalancerClient接口的所属包org.springframework.cloud.client.loadbalancer，我们对其内容进行整理，可以得出如下图的关系：

 从类的命名上我们初步判断LoadBalancerAutoConfiguration为实现客户端负载均衡器的自动化配置类。通过查看源码，我们可以验证这一点假设：

@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
		@ConditionalOnMissingBean
		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();
		}

		@Bean
		@ConditionalOnMissingBean
		public LoadBalancedRetryListenerFactory loadBalancedRetryListenerFactory() {
			return new LoadBalancedRetryListenerFactory.DefaultRetryListenerFactory();
		}
	}

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

		@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类头上的注解可以知道Ribbon实现的负载均衡自动化配置需要满足下面两个条件：

• @ConditionalOnClass(RestTemplate.class)：RestTemplate类必须存在于当前工程的环境中。
• @ConditionalOnBean(LoadBalancerClient.class)：在Spring的Bean工程中有必须有LoadBalancerClient的实现Bean。

在该自动化配置类中，主要做了下面三件事：

• 创建了一个LoadBalancerInterceptor的Bean，用于实现对客户端发起请求时进行拦截，以实现客户端负载均衡。
• 创建了一个RestTemplateCustomizer的Bean，用于给RestTemplate增加LoadBalancerInterceptor拦截器。
• 维护了一个被@LoadBalanced注解修饰的RestTemplate对象列表，并在这里进行初始化，通过调用RestTemplateCustomizer的实例来给需要客户端负载均衡的RestTemplate增加LoadBalancerInterceptor拦截器。

接下来，我们看看LoadBalancerInterceptor拦截器是如何将一个普通的RestTemplate变成客户端负载均衡的：

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {

	private LoadBalancerClient loadBalancer;
	private LoadBalancerRequestFactory requestFactory;

	public LoadBalancerInterceptor(LoadBalancerClient loadBalancer, LoadBalancerRequestFactory requestFactory) {
		this.loadBalancer = loadBalancer;
		this.requestFactory = requestFactory;
	}

	public LoadBalancerInterceptor(LoadBalancerClient loadBalancer) {
		// for backwards compatibility
		this(loadBalancer, new LoadBalancerRequestFactory(loadBalancer));
	}

	@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));
	}
}

通过源码以及之前的自动化配置类，我们可以看到在拦截器中注入了LoadBalancerClient的实现。当一个被@LoadBalanced注解修饰的RestTemplate对象向外发起HTTP请求时，会被LoadBalancerInterceptor类的intercept函数所拦截。由于我们在使用RestTemplate时候采用了服务名作为host，所以直接从HttpRequest的URI对象中通过getHost()就可以拿到服务名，然后调用execute函数去根据服务名来选择实例并发起实际的请求。

分析到这里，LoadBalancerClient还只是一个抽象的负载均衡器接口，所以我们还需要找到它的具体实现类来进一步分析。通过查看ribbon的源码，我们可以很容易的在org.springframework.cloud.netflix.ribbon包下找到对应的实现类：RibbonLoadBalancerClient。

	@Override
	public <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException {
		Server server = null;
		if(serviceInstance instanceof RibbonServer) {
			server = ((RibbonServer)serviceInstance).getServer();
		}
		if (server == null) {
			throw new IllegalStateException("No instances available for " + serviceId);
		}

		RibbonLoadBalancerContext context = this.clientFactory
				.getLoadBalancerContext(serviceId);
		RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

		try {
			T returnVal = request.apply(serviceInstance);
			statsRecorder.recordStats(returnVal);
			return returnVal;
		}
		// catch IOException and rethrow so RestTemplate behaves correctly
		catch (IOException ex) {
			statsRecorder.recordStats(ex);
			throw ex;
		}
		catch (Exception ex) {
			statsRecorder.recordStats(ex);
			ReflectionUtils.rethrowRuntimeException(ex);
		}
		return null;
	}

可以看到，在execute函数的实现中，首先从服务列表中获取RibbonServer，然后根据服务ID获取RibbonLoadBalanceContext上下文对象，最后创建一个Ribbon的统计记录对象，代码如下：

	public static class RibbonServer implements ServiceInstance {
		private final String serviceId;
		private final Server server;
		private final boolean secure;
		private Map<String, String> metadata;

		public RibbonServer(String serviceId, Server server) {
			this(serviceId, server, false, Collections.<String, String> emptyMap());
		}

		public RibbonServer(String serviceId, Server server, boolean secure,
				Map<String, String> metadata) {
			this.serviceId = serviceId;
			this.server = server;
			this.secure = secure;
			this.metadata = metadata;
		}

		@Override
		public String getServiceId() {
			return this.serviceId;
		}

		@Override
		public String getHost() {
			return this.server.getHost();
		}

		@Override
		public int getPort() {
			return this.server.getPort();
		}

		@Override
		public boolean isSecure() {
			return this.secure;
		}

		@Override
		public URI getUri() {
			return DefaultServiceInstance.getUri(this);
		}

		@Override
		public Map<String, String> getMetadata() {
			return this.metadata;
		}

		public Server getServer() {
			return this.server;
		}

		@Override
		public String toString() {
			final StringBuffer sb = new StringBuffer("RibbonServer{");
			sb.append("serviceId='").append(serviceId).append('\'');
			sb.append(", server=").append(server);
			sb.append(", secure=").append(secure);
			sb.append(", metadata=").append(metadata);
			sb.append('}');
			return sb.toString();
		}
	}