深入理解@LoadBalanced注解的实现原理与客户端负载均衡
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概述
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,先前有细嚼过但是没有做过笔记,刚好处理此类问题记录下
@LoadBalanced
/**
* 注释将RestTemplate bean标记为配置为使用LoadBalancerClient。
*/
@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}通过源码可以发现这是一个LoadBalanced标记注解并且标记了@Qualifier(基于Spring Boot的自动配置机制),我们可以溯源到LoadBalancerAutoConfiguration
LoadBalancerAutoConfiguration
/**
* 功能区的自动配置（客户端负载平衡）
*/
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {
@LoadBalanced
@Autowired(required = false)
private List<RestTemplate> restTemplates = Collections.emptyList(); //这里持有@LoadBalanced标记的RestTemplate实例
@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) {
//为restTemplate添加定制
customizer.customize(restTemplate);
}
}
});
}
// ...
/**
* 以下针对classpath存在RetryTemplate.class的情况配置，先忽略
*/
@Configuration
@ConditionalOnClass(RetryTemplate.class)
public static class RetryAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public LoadBalancedRetryFactory loadBalancedRetryFactory() {
return new LoadBalancedRetryFactory() {
};
}
}
// ...
}@LoadBalanced和@Autowried结合使用,意思就是这里注入的RestTempate Bean是所有加有@LoadBalanced注解标记的(持有@LoadBalanced标记的RestTemplate实例)
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。
关键问下自己：为什么？
- RestTemplate实例是怎么被收集的？
- 怎样通过负载均衡规则获取具体的具体的server？
继续扒看源码>
上面可以看出，会LoadBalancerAutoConfiguration类对我们加上@LoadBalanced注解的bean 添加loadBalancerInterceptor拦截器
LoadBalancerInterceptor
/**
* 功能区的自动配置（客户端负载平衡）。
*/
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));
}
}重点看intercept方法 当我们restTemplate执行请求操作时，就会被拦截器拦截进入intercept方法,而loadBalancer是LoadBalancerClient的具体实现
RibbonLoadBalancerClient
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);
}看到这里相信都遇到过类似的错误，恍然大悟
No instances available for xxxxx总结
- 1.根据serviceId 获取对应的loadBalancer
- 2.根据loadBalancer获取具体的server（这里根据负载均衡规则，获取到具体的服务实例）
- 3.创建RibbonServer
- 4.执行具体请求
这里
注意: @LoadBalanced 标记注解获取到最后通过负载均衡规则获取具体的具体的server来发起请求
案例
/**
* 服务注册中心配置
*
* @author <a href="mailto:shangzhi.ibyte@gmail.com">iByte</a>
* @since 1.0.1
*/
@Configuration
@EnableConfigurationProperties(ModuleMappingHelper.class)
public class DiscoveryConfig {
@Autowired
Environment environment;
/**
* DiscoveryHeaderHelper默认bean
* @return
*/
@Bean
public DiscoveryHeaderHelper discoveryHeaderHelper() {
DiscoveryHeaderHelper discoveryHeaderHelper = new DiscoveryHeaderHelper(environment);
DiscoveryHeaderHelper.INSTANCE = discoveryHeaderHelper;
return discoveryHeaderHelper;
}
/**
* resttemplate构建
*/
@Resource
private RestTemplateBuilder restTemplateBuilder;
/**
* resttemplate请求bean,更改系统本身的builder
* @return
*/
@Bean
@LoadBalanced
public RestTemplate restTemplate() {
RestTemplate restTemplate = restTemplateBuilder.configure(new RestTemplate());
//RestTemplate interceptors 远程调用请求增加头部信息处理
restTemplate.getInterceptors().add(new RestApiHeaderInterceptor());
//RestTemplate Set the error handler 错误处理
restTemplate.setErrorHandler(new RestResponseErrorHandler());
return restTemplate;
}
@Bean
public DiscoveryClient.DiscoveryClientOptionalArgs discoveryClientOptionalArgs() {
DiscoveryClient.DiscoveryClientOptionalArgs discoveryClientOptionalArgs = new DiscoveryClient.DiscoveryClientOptionalArgs();
discoveryClientOptionalArgs.setAdditionalFilters(Collections.singletonList(new DiscoveryHeaderClientFilter()));
discoveryClientOptionalArgs.setEventListeners(Collections.singleton(new EurekaClientEventListener()));
return discoveryClientOptionalArgs;
}
}源码地址 > DiscoveryConfig
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