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  • 本章是《Spring Cloud源码分析》系列文章的第二篇,我们从注册中心Eureka开始这段历程;

Spring Cloud源码下载

启动类上的注解

package com.bolingcavalry.springclouddeepeureka;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.netflix.eureka.server.EnableEurekaServer;

@SpringBootApplication
@EnableEurekaServer
public class SpringclouddeepeurekaApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringclouddeepeurekaApplication.class, args);
    }
}
  • 上面这段代码与一般的SpringBoot启动类不同之处在于多了个注解@EnableEurekaServer,今天的源码分析都是围绕这个类开展的;

  • 看看此注解的源码:
    
    /**
    * Annotation to activate Eureka Server related configuration {@link EurekaServerAutoConfiguration}
    *
    * @author Dave Syer
    * @author Biju Kunjummen
    *
    */

@Target(ElementType.TYPE)br/>@Retention(RetentionPolicy.RUNTIME)
@Documented
br/>@Import(EurekaServerMarkerConfiguration.class)
public @interface EnableEurekaServer {

}


- 上述代码中,注解**@Import(EurekaServerMarkerConfiguration.class)**表示,EurekaServerMarkerConfiguration这个类也会被作为bean做实例化;
另外请注意注释中的内容:**注解EnableEurekaServer 用来激活Eureka Server相关的配置:EurekaServerAutoConfiguration**,记下这是EurekaServerAutoConfiguration第一次出现在我们面前;

- 接下来去看被实例化了的EurekaServerMarkerConfiguration的源码:

```java
/**
 * Responsible for adding in a marker bean to activate
 * {@link EurekaServerAutoConfiguration}
 *
 * @author Biju Kunjummen
 */
@Configuration
public class EurekaServerMarkerConfiguration {

    @Bean
    public Marker eurekaServerMarkerBean() {
        return new Marker();
    }

    class Marker {
    }
}
  • 如上所示,简单到只有个一内部类EurekaServerMarkerConfiguration.Marker,我的猜测是:有的bean会通过注解ConditionalOnBean作为自己是否实例化的条件,而条件对应的bean就是EurekaServerMarkerConfiguration.Marker

  • 请注意注释中的内容:注解EurekaServerMarkerConfiguration 用来响应激活EurekaServerAutoConfiguration,这是EurekaServerAutoConfiguration第二次出现在我们面前;

  • 根据前面两次注释的提示,EurekaServerAutoConfiguration类是必须要看了,打开这个类,先看注解:
@Configuration
@Import(EurekaServerInitializerConfiguration.class)
@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)
@EnableConfigurationProperties({ EurekaDashboardProperties.class,
        InstanceRegistryProperties.class })
@PropertySource("classpath:/eureka/server.properties")
  • 符合之前的猜测,通过@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)注解,保证了EurekaServerAutoConfiguration类会被实例化后注册到spring容器中,也就是说这里面的配置都生效了;
  • 接下来就是spring容器对bean进行实例化和初始化了,重点需要关注的是EurekaServerInitializerConfiguration、EurekaServerContext、EurekaServerBootstrap这三个类;
  • EurekaServerAutoConfiguration中的@Bean注解会实例化EurekaServerContext、EurekaServerBootstrap,这两个实例已经不是SpringCloud工程的内容了,它们都来自com.netflix.eureka,它们接手了真正的EurekaServer的启动逻辑:
@Bean
public EurekaServerContext eurekaServerContext(ServerCodecs serverCodecs,
        PeerAwareInstanceRegistry registry, PeerEurekaNodes peerEurekaNodes) {
    return new DefaultEurekaServerContext(this.eurekaServerConfig, serverCodecs,
            registry, peerEurekaNodes, this.applicationInfoManager);
}

@Bean
public EurekaServerBootstrap eurekaServerBootstrap(PeerAwareInstanceRegistry registry,
        EurekaServerContext serverContext) {
    return new EurekaServerBootstrap(this.applicationInfoManager,
            this.eurekaClientConfig, this.eurekaServerConfig, registry,
            serverContext);
}
  • EurekaServerInitializerConfiguration这个类出现在EurekaServerAutoConfiguration的注解中,通过@Import注解被实例化,由于实现了Lifecycle接口,因此会被spring容器回调start方法:
@Override
public void start() {
    new Thread(new Runnable() {
        @Override
        public void run() {
            try {
                //TODO: is this class even needed now?
                eurekaServerBootstrap.contextInitialized(EurekaServerInitializerConfiguration.this.servletContext);
                log.info("Started Eureka Server");

                //发送广播,将EurekaServer的配置信息广播给全部订阅了该类型消息的监听
                publish(new EurekaRegistryAvailableEvent(getEurekaServerConfig()));
                EurekaServerInitializerConfiguration.this.running = true;
                //发送广播,将EurekaServer的配置信息广播给全部订阅了该类型消息的监听
                publish(new EurekaServerStartedEvent(getEurekaServerConfig()));
            }
            catch (Exception ex) {
                // Help!
                log.error("Could not initialize Eureka servlet context", ex);
            }
        }
    }).start();
}
  • 如上所示,EurekaServerInitializerConfiguration初始化的时候,除了主动调用bootstrap的初始化方法,还通过广播将eureka的配置信息发出去;

  • eureka的配置信息EurekaServerConfig来自何处呢?EurekaServerAutoConfiguration的内部类EurekaServerConfigBeanConfiguration 负责生成这些配置信息,实例类型为EurekaServerConfigBean:
@Configuration
protected static class EurekaServerConfigBeanConfiguration {
    @Bean
    @ConditionalOnMissingBean
    public EurekaServerConfig eurekaServerConfig(EurekaClientConfig clientConfig) {
        EurekaServerConfigBean server = new EurekaServerConfigBean();
        if (clientConfig.shouldRegisterWithEureka()) {
            // Set a sensible default if we are supposed to replicate
            server.setRegistrySyncRetries(5);
        }
        return server;
    }
}
  • 至此,我们对EnableEurekaServer注解有了更深入的了解,虽然创建注册中心所需代码很少,但是背后隐藏着复杂的初始化服务,感谢大师们杰出的设计,封装了复杂逻辑,让业务测可以轻量级完成这些操作;

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