SpringCloud源码：客户端分析（一）- SpringBootApplication注解类加载流程

总结一句话

用@EnableDiscoveryClient注解客户端-启动类，配合@springbootapplication，完成两个步骤：

自动读取spring-factories文件的全限定类名内容
通过selectImport对这些类进行初始化

背景

spring.factories作用

在maven依赖：

spring-cloud-netflix-eureka-client/2.2.6.RELEASE/spring-cloud-netflix-eureka-client-2.2.6.RELEASE.jar!的 META-INF 目录下，有一个spring.factories文件：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.netflix.eureka.config.EurekaClientConfigServerAutoConfiguration,\
org.springframework.cloud.netflix.eureka.config.DiscoveryClientOptionalArgsConfiguration,\
org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration,\
org.springframework.cloud.netflix.ribbon.eureka.RibbonEurekaAutoConfiguration,\
org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration,\
org.springframework.cloud.netflix.eureka.reactive.EurekaReactiveDiscoveryClientConfiguration,\
org.springframework.cloud.netflix.eureka.loadbalancer.LoadBalancerEurekaAutoConfiguration


org.springframework.cloud.bootstrap.BootstrapConfiguration=\
org.springframework.cloud.netflix.eureka.config.EurekaConfigServerBootstrapConfiguration

@SpringBootApplication为一个组合注解，通过@EnableAutoConfiguration开启自动装配。

spring.factories文件位于JAR包的META-INF目录下。

当Spring Boot应用启动时，它会自动扫描所有的spring.factories文件，并根据里面的配置加载相应的类。

在上述例子中，当应用启动时，EurekaClientAutoConfiguration 类将会被自动注册为一个bean，并可以通过依赖注入在其他组件中使用。

@SpringBootApplication注解

@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
      @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
}

@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}

AutoConfigurationImportSelector资源类

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
      ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {


    // ....
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
       List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
             getBeanClassLoader());
       Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
             + "are using a custom packaging, make sure that file is correct.");
       return configurations;
    }
}

分析：

getCandidateConfigurations：此处是自动导入spring.factories文件的全部实现类

我们从debug可以看到，已经读出了spring.factories的配置类，包括了EurekaClientAutoConfiguration和EurekaDiscoveryClientConfiguration 两个自动装配类。

小结

EurekaClientAutoConfiguration和EurekaDiscoveryClientConfiguration在Spring Cloud Eureka中扮演着不同的角色，它们的主要区别在于功能和职责的不同。

自动化配置类	功能	职责
EurekaClientAutoConfiguration	配置EurekaClient	确保了Eureka客户端能够正确地： - 注册到Eureka服务端 - 周期性地发送心跳信息来更新服务租约 - 下线时通知Eureka服务端 - 获取服务实例列表；更侧重于Eureka客户端的基本配置和功能实现
EurekaDiscoveryClientConfiguration	配置EurekaDiscoveryClient	创建RefreshScopeRefreshedEvent事件的监听类，用于重启注册；更多地涉及到服务的自动注册、健康检查以及事件处理等方面

这两个配置类共同工作，确保了Spring Boot应用能够顺利地与Eureka服务注册中心进行交互，实现服务的注册和发现。

所以下文我们讨论客户端的4个功能（注册租约、更新租约、下线租约、获取服务列表），都是倾向于分析EurekaClient的代码。

EurekaClientAutoConfiguration自动配置类

自动化装配EurekaClient的相关bean，

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties
@ConditionalOnClass(EurekaClientConfig.class)
@ConditionalOnProperty(value = "eureka.client.enabled", matchIfMissing = true)
@ConditionalOnDiscoveryEnabled
@AutoConfigureBefore({ NoopDiscoveryClientAutoConfiguration.class,
      CommonsClientAutoConfiguration.class, ServiceRegistryAutoConfiguration.class })
@AutoConfigureAfter(name = {
      "org.springframework.cloud.netflix.eureka.config.DiscoveryClientOptionalArgsConfiguration",
      "org.springframework.cloud.autoconfigure.RefreshAutoConfiguration",
      "org.springframework.cloud.netflix.eureka.EurekaDiscoveryClientConfiguration",
      "org.springframework.cloud.client.serviceregistry.AutoServiceRegistrationAutoConfiguration" })
public class EurekaClientAutoConfiguration {


    // 【1】EurekaClientConfigBean：Eureka客户端bean，保存了环境配置信息
    @Bean
    @ConditionalOnMissingBean(value = EurekaClientConfig.class, search = SearchStrategy.CURRENT)
    public EurekaClientConfigBean eurekaClientConfigBean(ConfigurableEnvironment env) {
       return new EurekaClientConfigBean();
    }


    //【2】EurekaInstanceConfigBean：Eureka客户端实例bean，保存了实例信息
    @Bean
    @ConditionalOnMissingBean(value = EurekaInstanceConfig.class,
          search = SearchStrategy.CURRENT)
    public EurekaInstanceConfigBean eurekaInstanceConfigBean(InetUtils inetUtils,
          ManagementMetadataProvider managementMetadataProvider) {
       String hostname = getProperty("eureka.instance.hostname");
       boolean preferIpAddress = Boolean
             .parseBoolean(getProperty("eureka.instance.prefer-ip-address"));
       String ipAddress = getProperty("eureka.instance.ip-address");
       boolean isSecurePortEnabled = Boolean
             .parseBoolean(getProperty("eureka.instance.secure-port-enabled"));


       String serverContextPath = env.getProperty("server.servlet.context-path", "/");
       int serverPort = Integer.parseInt(
             env.getProperty("server.port", env.getProperty("port", "8080")));


       Integer managementPort = env.getProperty("management.server.port", Integer.class);
       String managementContextPath = env
             .getProperty("management.server.servlet.context-path");
       Integer jmxPort = env.getProperty("com.sun.management.jmxremote.port",
             Integer.class);
       EurekaInstanceConfigBean instance = new EurekaInstanceConfigBean(inetUtils);


       instance.setNonSecurePort(serverPort);
       instance.setInstanceId(getDefaultInstanceId(env));
       instance.setPreferIpAddress(preferIpAddress);
       instance.setSecurePortEnabled(isSecurePortEnabled);
       if (StringUtils.hasText(ipAddress)) {
          instance.setIpAddress(ipAddress);
       }


       if (isSecurePortEnabled) {
          instance.setSecurePort(serverPort);
       }


       if (StringUtils.hasText(hostname)) {
          instance.setHostname(hostname);
       }
       String statusPageUrlPath = getProperty("eureka.instance.status-page-url-path");
       String healthCheckUrlPath = getProperty("eureka.instance.health-check-url-path");


       // ....省略很多配置项赋值
       setupJmxPort(instance, jmxPort);
       return instance;
    }
}

分析：

构造了EurekaInstanceConfigBean
- EurekaClientConfigBean是一个配置类，会读取所有eureka.client打头的配置项，例如，eureka.client.serviceUrl.defaultZone=http://localhost:8881/eureka/,http://localhost2:8882/eureka/
构造了EurekaClientConfigBean

EurekaClientConfiguration配置类

EurekaClientAutoConfiguration 内部有一个内部配置类EurekaClientConfiguration，它会继续初始化 CloudEurekaClient 和 ApplicationInfoManager 两个bean。

@Configuration(proxyBeanMethods = false)
@ConditionalOnMissingRefreshScope
protected static class EurekaClientConfiguration {


    // 【3】
    @Bean(destroyMethod = "shutdown")
    @ConditionalOnMissingBean(value = EurekaClient.class,
          search = SearchStrategy.CURRENT)
    public EurekaClient eurekaClient(ApplicationInfoManager manager,
          EurekaClientConfig config) {
       return new CloudEurekaClient(manager, config, this.optionalArgs,
             this.context);
    }


    // 【4】
    @Bean
    @ConditionalOnMissingBean(value = ApplicationInfoManager.class,
          search = SearchStrategy.CURRENT)
    public ApplicationInfoManager eurekaApplicationInfoManager(
          EurekaInstanceConfig config) {
       InstanceInfo instanceInfo = new InstanceInfoFactory().create(config);
       return new ApplicationInfoManager(config, instanceInfo);
    }
}

小结

EurekaClientAutoConfiguration构造了EurekaClientConfigBean、EurekaInstanceConfigBean以及EurekaServiceRegistry，之后在这几个对象的基础上进一步构建ApplicationInfoManager、CloudEurekaClient等。

其中ApplicationInfoManager负责变更实例状态并发布StatusChangeEvent事件，而CloudEurekaClient继承了com.netflix.discovery.DiscoveryClient，里头包含了statusChangeListener用于响应StatusChangeEvent，最后触发的是DiscoveryClient.register方法，与远程的Eureka Server通信，同步实例状态。

另外，EurekaClientAutoConfiguration构建的EurekaClient-bean，会被EurekaDiscoveryClientConfiguration构建的EurekaDiscoveryClient，所注入使用

EurekaDiscoveryClientConfiguration自动配置类

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties
@ConditionalOnClass(EurekaClientConfig.class)
@ConditionalOnProperty(value = "eureka.client.enabled", matchIfMissing = true)
@ConditionalOnDiscoveryEnabled
@ConditionalOnBlockingDiscoveryEnabled
public class EurekaDiscoveryClientConfiguration {


    // 【5】
    @Bean
    @ConditionalOnMissingBean
    public EurekaDiscoveryClient discoveryClient(EurekaClient client,
          EurekaClientConfig clientConfig) {
       return new EurekaDiscoveryClient(client, clientConfig);
    }


    // 【6】
    @Configuration(proxyBeanMethods = false)
    @ConditionalOnProperty(value = "eureka.client.healthcheck.enabled", matchIfMissing = false)
    protected static class EurekaHealthCheckHandlerConfiguration {


        @Autowired(required = false)
        private StatusAggregator statusAggregator = new SimpleStatusAggregator();


        @Bean
        @ConditionalOnMissingBean(HealthCheckHandler.class)
        public EurekaHealthCheckHandler eurekaHealthCheckHandler() {
          return new EurekaHealthCheckHandler(this.statusAggregator);
        }
    }
}

分析：