使用

 @Configuration
 public class CustomConfiguration {
     @Bean
     @LoadBalanced // 开启负载均衡能力
     public RestTemplate restTemplate() {
         return new RestTemplate();
     }
 }

可看到使用Ribbon，非常简单，只需将@LoadBalanced注解加在RestTemplate的Bean上，就可以实现负载均衡。

@LoadBalanced

从该注解的源码上的注释，可看到LoadBalancerClient类来配置的

/**
 * Annotation to mark a RestTemplate or WebClient 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 {

}

查看LoadBalancerClient源码，如下：

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

    /**
     * Executes 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.
     * @param <T> type of the response
     * @throws IOException in case of IO issues.
     * @return The result of the LoadBalancerRequest callback on the selected
     * ServiceInstance.
     */
    <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;

    /**
     * Executes 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.
     * @param <T> type of the response
     * @throws IOException in case of IO issues.
     * @return The result of the LoadBalancerRequest callback on the selected
     * ServiceInstance.
     */
    <T> T execute(String serviceId, ServiceInstance serviceInstance,
            LoadBalancerRequest<T> request) throws IOException;

    /**
     * Creates a proper URI with a real host and port for systems to utilize. Some systems
     * use a URI with the logical service 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 service instance to reconstruct the URI
     * @param original A URI with the host as a logical service name.
     * @return A reconstructed URI.
     */
    URI reconstructURI(ServiceInstance instance, URI original);

}

 * Implemented by classes which use a load balancer to choose a server to send a request
 * to.
 *
 * @author Ryan Baxter
 */
public interface ServiceInstanceChooser {

    /**
     * Chooses a ServiceInstance from the LoadBalancer for the specified service.
     * @param serviceId The service ID to look up the LoadBalancer.
     * @return A ServiceInstance that matches the serviceId.
     */
    ServiceInstance choose(String serviceId);

}

ServiceInstanceChooser看类名便可知用来帮我们从同一个服务的多个服务实例中根据负载均衡策略选择出所需的服务实例。

choose：根据传入的服务名ServiceId，从负载均衡器中选择一个对应服务的实例。

execute：使用从负载均衡器中挑选出的服务实例来执行请求内容。

reconstructURI：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。

通过搜索@LoadBalanced使用地方发现，只有org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration使用到了@LoadBalanced

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

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

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

    @Bean
    public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
            final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
        return () -> restTemplateCustomizers.ifAvailable(customizers -> {
            for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
                for (RestTemplateCustomizer customizer : customizers) {
                    customizer.customize(restTemplate);
                }
            }
        });
    }

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

    @Configuration(proxyBeanMethods = false)
    @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 restTemplate -> {
                List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                        restTemplate.getInterceptors());
                list.add(loadBalancerInterceptor);
                restTemplate.setInterceptors(list);
            };
        }

    }

    /**
     * Auto configuration for retry mechanism.
     */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass(RetryTemplate.class)
    public static class RetryAutoConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public LoadBalancedRetryFactory loadBalancedRetryFactory() {
            return new LoadBalancedRetryFactory() {
            };
        }

    }

    /**
     * Auto configuration for retry intercepting mechanism.
     */
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnClass(RetryTemplate.class)
    public static class RetryInterceptorAutoConfiguration {

        @Bean
        @ConditionalOnMissingBean
        public RetryLoadBalancerInterceptor ribbonInterceptor(
                LoadBalancerClient loadBalancerClient,
                LoadBalancerRetryProperties properties,
                LoadBalancerRequestFactory requestFactory,
                LoadBalancedRetryFactory loadBalancedRetryFactory) {
            return new RetryLoadBalancerInterceptor(loadBalancerClient, properties,
                    requestFactory, loadBalancedRetryFactory);
        }

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

    }

}

这段自动装配的代码的含义不难理解，就是利用了RestTempllate的拦截器，使用RestTemplateCustomizer对所有标注了@LoadBalanced的RestTemplate Bean添加了一个LoadBalancerInterceptor拦截器，而这个拦截器的作用就是对请求的URI进行转换获取到具体应该请求哪个服务实例ServiceInstance。

@LoadBalanced @Autowired(required = false)

这两个注解在一起的意思：只注入@LoadBalanced注解的Bean ，带有@LoadBalanced注解的RestTemplate会在原有的拦截器基础上加上LoadBalancerInterceptor拦截器

@Bean
    public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
            final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
        return () -> restTemplateCustomizers.ifAvailable(customizers -> {
            for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
                for (RestTemplateCustomizer customizer : customizers) {
                    customizer.customize(restTemplate);
                }
            }
        });
    }

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

可看到RestTemplate是可以设置拦截器。

org.springframework.cloud.client.loadbalancer.LoadBalancerInterceptor实现如下：

/**
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Ryan Baxter
 * @author William Tran
 */
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,
                this.requestFactory.createRequest(request, body, execution));
    }

}

可看出：将LoadBalancerClient以及LoadBalancerRequestFactory创建的请求封装到LoadBalancerInterceptor中，并在intercept()方法中，使用LoadBalancerClient执行请求。

RestTemplate调用过程

RestTemplate.getForObject/getForEntity... --> RestTemplate.excute --> RestTemplate.doExecute --> org.springframework.http.client.ClientHttpRequestFactory#createRequest

--> org.springframework.http.client.AbstractClientHttpRequest#execute

--> org.springframework.http.client.AbstractClientHttpRequest#executeInternal

--> org.springframework.http.client.InterceptingClientHttpRequest#executeInternal

--> org.springframework.http.client.InterceptingClientHttpRequest.InterceptingRequestExecution#execute

--> org.springframework.cloud.client.loadbalancer.LoadBalancerInterceptor#intercept

--> org.springframework.cloud.client.loadbalancer.LoadBalancerClient#execute(java.lang.String, org.springframework.cloud.client.loadbalancer.LoadBalancerRequest)

--> org.springframework.cloud.netflix.ribbon.RibbonLoadBalancerClient#execute(java.lang.String, org.springframework.cloud.client.loadbalancer.LoadBalancerRequest)

整个流程最终可看到执行拦截器的execute方法，最终调用负载均衡的execute方法

最终调用org.springframework.cloud.client.loadbalancer.LoadBalancerInterceptor#intercept

@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,
                this.requestFactory.createRequest(request, body, execution));
    }

org.springframework.cloud.netflix.ribbon.RibbonLoadBalancerClient#execute(java.lang.String, org.springframework.cloud.client.loadbalancer.LoadBalancerRequest, java.lang.Object)

public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint)
            throws IOException {
        ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
        Server server = getServer(loadBalancer, hint);
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }
        RibbonServer ribbonServer = new RibbonServer(serviceId, server,
                isSecure(server, serviceId),
                serverIntrospector(serviceId).getMetadata(server));

        return execute(serviceId, ribbonServer, request);
    }

此处是RibbonLoadBalancerClient的execute方法，首先获取负载均衡器(getLoadBalancer)，然后通过负载均衡器获取服务，看下getServer()方法实现：

protected Server getServer(ILoadBalancer loadBalancer, Object hint) {
        if (loadBalancer == null) {
            return null;
        }
        // Use 'default' on a null hint, or just pass it on?
        return loadBalancer.chooseServer(hint != null ? hint : "default");
    }

  public Server chooseServer(Object key) {
        if (counter == null) {
            counter = createCounter();
        }
        counter.increment();
        if (rule == null) {
            return null;
        } else {
            try {
                return rule.choose(key);
            } catch (Exception e) {
                logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", name, key, e);
                return null;
            }
        }
    }

可看到最终是调用策略器的choose方法，选择服务器。

参考资料：

Spring Cloud微服务实战