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这篇文章将为大家详细讲解有关Spring Cloud Ribbon原理的示例分析，小编觉得挺实用的，因此分享给大家做个参考，希望大家阅读完这篇文章后可以有所收获。
简介
Spring Cloud Ribbon 是一个基于Http和TCP的客服端负载均衡工具，它是基于Netflix Ribbon实现的。它不像服务注册中心、配置中心、API网关那样独立部署，但是它几乎存在于每个微服务的基础设施中。包括前面的提供的声明式服务调用也是基于该Ribbon实现的。理解Ribbon对于我们使用Spring Cloud来讲非常的重要，因为负载均衡是对系统的高可用、网络压力的缓解和处理能力扩容的重要手段之一。在上节的例子中，我们采用了声明式的方式来实现负载均衡。实际上，内部调用维护了一个RestTemplate对象，该对象会使用Ribbon的自动化配置，同时通过@LoadBalanced开启客户端负载均衡。其实RestTemplate是Spring自己提供的对象，不是新的内容。读者不知道RestTemplate可以查看相关的文档。
现象
前两天碰到一个ribbon相关的问题，觉得值得记录一下。表象是对外的接口返回内部异常，这个是封装的统
一错误信息，Spring的异常处理器catch到未捕获异常统一返回的信息。因此到日志平台查看实际的异常:
org.springframework.web.client.HttpClientErrorException: 404 null
这里介绍一下背景，出现问题的开放网关，做点事情说白了就是转发对应的请求给后端的服务。这里用到了ribbon去做服务负载均衡、eureka负责服务发现。
这里出现404，首先看了下请求的url以及对应的参数，都没有发现问题，对应的后端服务也没有收到请求。这就比较诡异了，开始怀疑是ribbon或者Eureka的缓存导致请求到了错误的ip或端口，但由于日志中打印的是Eureka的serviceId而不是实际的ip:port，因此先加了个日志:
@Slf4j
public class CustomHttpRequestInterceptor implements ClientHttpRequestInterceptor {
@Override
public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) throws IOException {
log.info("Request , url:{},method:{}.", request.getURI(), request.getMethod());
return execution.execute(request, body);
}
}这里是通过给RestTemplate添加拦截器的方式，但要注意，ribbon也是通过给RestTemplate添加拦截器实现的解析serviceId到实际的ip:port，因此需要注意下优先级添加到ribbon的 LoadBalancerInterceptor 之后，我这里是通过Spring的初始化完成事件的回调中添加的，另外也添加了另一条日志，在catch到这个异常的时候，利用Eureka的 DiscoveryClient#getInstances 获取到当前的实例信息。
之后在测试环境中复现了这个问题，看了下日志，eurek中缓存的实例信息是对的，但是实际调用的确实另外一个服务的地址，从而导致了接口404。
源码解析
从上述的信息中可以知道，问题出在ribbon中，具体的原因后面会说，这里先讲一下Spring Cloud Ribbon的初始化流程。
@Configuration
@ConditionalOnClass({ IClient.class, RestTemplate.class, AsyncRestTemplate.class, Ribbon.class})
@RibbonClients
@AutoConfigureAfter(name = "org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration")
@AutoConfigureBefore({LoadBalancerAutoConfiguration.class, AsyncLoadBalancerAutoConfiguration.class})
@EnableConfigurationProperties({RibbonEagerLoadProperties.class, ServerIntrospectorProperties.class})
public class RibbonAutoConfiguration {
}注意这个注解 @RibbonClients , 如果想要覆盖Spring Cloud提供的默认Ribbon配置就可以使用这个注解，最终的解析类是:
public class RibbonClientConfigurationRegistrar implements ImportBeanDefinitionRegistrar {
@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
BeanDefinitionRegistry registry) {
Map attrs = metadata.getAnnotationAttributes(
RibbonClients.class.getName(), true);
if (attrs != null && attrs.containsKey("value")) {
AnnotationAttributes[] clients = (AnnotationAttributes[]) attrs.get("value");
for (AnnotationAttributes client : clients) {
registerClientConfiguration(registry, getClientName(client),
client.get("configuration"));
}
}
if (attrs != null && attrs.containsKey("defaultConfiguration")) {
String name;
if (metadata.hasEnclosingClass()) {
name = "default." + metadata.getEnclosingClassName();
} else {
name = "default." + metadata.getClassName();
}
registerClientConfiguration(registry, name,
attrs.get("defaultConfiguration"));
}
Map client = metadata.getAnnotationAttributes(
RibbonClient.class.getName(), true);
String name = getClientName(client);
if (name != null) {
registerClientConfiguration(registry, name, client.get("configuration"));
}
}
private String getClientName(Map client) {
if (client == null) {
return null;
}
String value = (String) client.get("value");
if (!StringUtils.hasText(value)) {
value = (String) client.get("name");
}
if (StringUtils.hasText(value)) {
return value;
}
throw new IllegalStateException(
"Either 'name' or 'value' must be provided in @RibbonClient");
}
private void registerClientConfiguration(BeanDefinitionRegistry registry,
Object name, Object configuration) {
BeanDefinitionBuilder builder = BeanDefinitionBuilder
.genericBeanDefinition(RibbonClientSpecification.class);
builder.addConstructorArgValue(name);
builder.addConstructorArgValue(configuration);
registry.registerBeanDefinition(name + ".RibbonClientSpecification",
builder.getBeanDefinition());
}
} atrrs包含defaultConfiguration，因此会注册RibbonClientSpecification类型的bean,注意名称以 default. 开头，类型是RibbonAutoConfiguration,注意上面说的RibbonAutoConfiguration被@RibbonClients修饰。
然后再回到上面的源码:
public class RibbonAutoConfiguration {
//上文中会解析被@RibbonClients注解修饰的类，然后注册类型为RibbonClientSpecification的bean。
//主要有两个: RibbonAutoConfiguration、RibbonEurekaAutoConfiguration
@Autowired(required = false)
private List configurations = new ArrayList<>();
@Bean
public SpringClientFactory springClientFactory() {
//初始化SpringClientFactory,并将上面的配置注入进去，这段很重要。
SpringClientFactory factory = new SpringClientFactory();
factory.setConfigurations(this.configurations);
return factory;
}
//其他的都是提供一些默认的bean配置
@Bean
@ConditionalOnMissingBean(LoadBalancerClient.class)
public LoadBalancerClient loadBalancerClient() {
return new RibbonLoadBalancerClient(springClientFactory());
}
@Bean
@ConditionalOnClass(name = "org.springframework.retry.support.RetryTemplate")
@ConditionalOnMissingBean
public LoadBalancedRetryPolicyFactory loadBalancedRetryPolicyFactory(SpringClientFactory clientFactory) {
return new RibbonLoadBalancedRetryPolicyFactory(clientFactory);
}
@Bean
@ConditionalOnMissingClass(value = "org.springframework.retry.support.RetryTemplate")
@ConditionalOnMissingBean
public LoadBalancedRetryPolicyFactory neverRetryPolicyFactory() {
return new LoadBalancedRetryPolicyFactory.NeverRetryFactory();
}
@Bean
@ConditionalOnClass(name = "org.springframework.retry.support.RetryTemplate")
@ConditionalOnMissingBean
public LoadBalancedBackOffPolicyFactory loadBalancedBackoffPolicyFactory() {
return new LoadBalancedBackOffPolicyFactory.NoBackOffPolicyFactory();
}
@Bean
@ConditionalOnClass(name = "org.springframework.retry.support.RetryTemplate")
@ConditionalOnMissingBean
public LoadBalancedRetryListenerFactory loadBalancedRetryListenerFactory() {
return new LoadBalancedRetryListenerFactory.DefaultRetryListenerFactory();
}
@Bean
@ConditionalOnMissingBean
public PropertiesFactory propertiesFactory() {
return new PropertiesFactory();
}
@Bean
@ConditionalOnProperty(value = "ribbon.eager-load.enabled", matchIfMissing = false)
public RibbonApplicationContextInitializer ribbonApplicationContextInitializer() {
return new RibbonApplicationContextInitializer(springClientFactory(),
ribbonEagerLoadProperties.getClients());
}
@Configuration
@ConditionalOnClass(HttpRequest.class)
@ConditionalOnRibbonRestClient
protected static class RibbonClientConfig {
@Autowired
private SpringClientFactory springClientFactory;
@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);
}
}
//TODO: support for autoconfiguring restemplate to use apache http client or okhttp
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnRibbonRestClientCondition.class)
@interface ConditionalOnRibbonRestClient { }
private static class OnRibbonRestClientCondition extends AnyNestedCondition {
public OnRibbonRestClientCondition() {
super(ConfigurationPhase.REGISTER_BEAN);
}
@Deprecated //remove in Edgware"
@ConditionalOnProperty("ribbon.http.client.enabled")
static class ZuulProperty {}
@ConditionalOnProperty("ribbon.restclient.enabled")
static class RibbonProperty {}
}
} 注意这里的SpringClientFactory, ribbon默认情况下，每个eureka的serviceId(服务)，都会分配自己独立的Spring的上下文,即ApplicationContext, 然后这个上下文中包含了必要的一些bean,比如: ILoadBalancer 、 ServerListFilter 等。而Spring Cloud默认是使用RestTemplate封装了ribbon的调用，核心是通过一个拦截器:
@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(
final LoadBalancerInterceptor loadBalancerInterceptor) {
return new RestTemplateCustomizer() {
@Override
public void customize(RestTemplate restTemplate) {
List list = new ArrayList<>(
restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
}
};
} 因此核心是通过这个拦截器实现的负载均衡:
public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
private LoadBalancerClient loadBalancer;
private LoadBalancerRequestFactory requestFactory;
@Override
public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
final ClientHttpRequestExecution execution) throws IOException {
final URI originalUri = request.getURI(); //这里传入的url是解析之前的，即http://serviceId/服务地址的形式
String serviceName = originalUri.getHost(); //解析拿到对应的serviceId
Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
}
}然后将请求转发给LoadBalancerClient:
public class RibbonLoadBalancerClient implements LoadBalancerClient {
@Override
public T execute(String serviceId, LoadBalancerRequest request) throws IOException {
ILoadBalancer loadBalancer = getLoadBalancer(serviceId); //获取对应的LoadBalancer
Server server = getServer(loadBalancer); //获取服务器，这里会执行对应的分流策略，比如轮训
//、随机等
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);
}
} 而这里的LoadBalancer是通过上文中提到的SpringClientFactory获取到的，这里会初始化一个新的Spring上下文，然后将Ribbon默认的配置类，比如说: RibbonAutoConfiguration 、 RibbonEurekaAutoConfiguration 等添加进去, 然后将当前spring的上下文设置为parent，再调用refresh方法进行初始化。
