Ribbon源码分析之LoadBalanceAutoConfiguration

一 类图

二 LoadBalancerClient接口

1 代码

public interface ServiceInstanceChooser {
    ServiceInstance choose(String serviceId);
}
public interface LoadBalancerClient extends ServiceInstanceChooser {
    <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;
    <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;
    URI reconstructURI(ServiceInstance instance, URI original);
}

2 说明

1 ServiceInstance choose(String serviceId)：根据传入服务名serviceId,从负载均衡器中挑选一个对应服务的实例。

2 T execute(String serviceId, LoadBalancerRequest<T> 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。

3  URI reconstructURI(ServiceInstance instance, URI original)：该操作的定义中，前者ServiceInstance对象带有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:port的请求地址。

三 LoadBalanceAutoConfiguration类

1 代码

@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 loadBalancedRestTemplateInitializerDeprecated(
            final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
        return () -> restTemplateCustomizers.ifAvailable(customizers -> {
            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 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 LoadBalancedRetryFactory loadBalancedRetryFactory() {
            return new LoadBalancedRetryFactory() {};
        }
    }

    @Configuration
    @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);
            };
        }
    }
}

2 说明

LoadBalanceAutoConfiguration为实现客户端负载均衡器的自动化配置类。

从LoadBalanceAutoConfiguration类头上的注解可知，Ribbon实现的负载均衡自动化配置满足下面两个条件。

a @ConditionalOnClass(RestTemplate.class)：RestTemplate类必须存在于当前工程的环境中。

b @ConditionalOnBean(LoadBalancerClient.class)：在Spring的Bean工程中必须有LoadBalancerClient的实现Bean。

在该自动化配置类中，主要做了下面三件事：

a 创建了一个RetryLoadBalancerInterceptor，用于实现客户端发起请求时的拦截，以实现客户端负载均衡。

b 创建了一个RestTemplateCustomizer的Bean，用于给RestTemplate增加RetryLoadBalancerInterceptor拦截器。

c 维护了一个被@LoadBalanced注解修饰过的RestTemplate对象列表，并在这里进行了初始化，通过RestTemplateCustomizer的实例给需要客户端负载均衡的RestTemplate增加RetryLoadBalancerInterceptor拦截器。