启用EurekaServer

@SpringBootApplication
	@EnableEurekaServer
	public class EurekaApplication {	
		public static void main(String[] args) {
			SpringApplication.run(EurekaApplication.class, args);
		}
	}

@EnableEurekaServer 源码：

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

将配置类 EurekaServerMarkerConfiguration 加入spring 容器。

EurekaServerMarkerConfiguration 的源码

/**
 *
 * 激活 EurekaServerAutoConfiguration 的开关
 */
@Configuration
public class EurekaServerMarkerConfiguration {

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

	class Marker {
	}
}

我们看到只是实例化了一个空类，没有任何实现，从注释中可以看到 EurekaServerMarkerConfiguration 是一个激活 EurekaServerAutoConfiguration 的开关。我们在项目源码的 META-INF/spring.factories中看到

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
  org.springframework.cloud.netflix.eureka.server.EurekaServerAutoConfiguration

真正的配置信息在 EurekaServerAutoConfiguration 中，我们看到 @ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class) 只有存在 Marker 实例的时候，才会继续加载配置项，这也就要求必须有 @EnableEurekaServer 注解，才能正常的启动。

@Configuration
@Import(EurekaServerInitializerConfiguration.class)
@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)
@EnableConfigurationProperties({ EurekaDashboardProperties.class,
		InstanceRegistryProperties.class })
@PropertySource("classpath:/eureka/server.properties")
public class EurekaServerAutoConfiguration extends WebMvcConfigurerAdapter {

}

简单的梳理一下 @EnableEurekaServer 的流程为：

import扫描到条件判断加载具体配置信息EnableEurekaServerEurekaServerMarkerConfigurationEurekaServerAutoConfigurationConditionalOnBeanEurekaServerAutoConfiguration

EurekaServerAutoConfiguration 源码

在源码中我们可以看到一个很重要的注解 @Import(EurekaServerInitializerConfiguration.class) ，该注解是真正的的启动了服务，下面我们就看一下它都有哪些操作

@Configuration
@Import(EurekaServerInitializerConfiguration.class)
@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class)
@EnableConfigurationProperties({ EurekaDashboardProperties.class,
		InstanceRegistryProperties.class })
@PropertySource("classpath:/eureka/server.properties")
public class EurekaServerAutoConfiguration extends WebMvcConfigurerAdapter {

}

1、@Configuration 表明这是一个配置类
2、@Import(EurekaServerInitializerConfiguration.class) 导入启动EurekaServer的bean
3、@ConditionalOnBean(EurekaServerMarkerConfiguration.Marker.class) 这个是表示只有在spring容器
里面含有Market这个实例的时候，才会加载当前这个Bean（EurekaServerAutoConfiguration ），这个就是控制
是否开启EurekaServer的关键，

4、@EnableConfigurationProperties({ EurekaDashboardProperties.class, InstanceRegistryProperties.class })
加载配置
5、@PropertySource(“classpath:/eureka/server.properties”) 加载配置文件。

EurekaServerInitializerConfiguration 实现了SmartLifecycle.start方法，在spring 初始化的时候，会被调用

@Configuration
@CommonsLog
public class EurekaServerInitializerConfiguration
      implements ServletContextAware, SmartLifecycle, Ordered {
 
   @Autowired
   private EurekaServerConfig eurekaServerConfig;
 
   private ServletContext servletContext;
 
   @Autowired
   private ApplicationContext applicationContext;
 
   @Autowired
   private EurekaServerBootstrap eurekaServerBootstrap;
 
   private boolean running;
 
   private int order = 1;
 
   @Override
   public void setServletContext(ServletContext servletContext) {
      this.servletContext = servletContext;
   }
 
   @Override
   public void start() {
      // 启动一个线程
      new Thread(new Runnable() {
         @Override
         public void run() {
            try {
               //初始化EurekaServer，同时启动Eureka Server
               eurekaServerBootstrap.contextInitialized(EurekaServerInitializerConfiguration.this.servletContext);
               log.info("Started Eureka Server");
                // 发布EurekaServer的注册事件
               publish(new EurekaRegistryAvailableEvent(getEurekaServerConfig()));
                // 设置启动的状态为true
               EurekaServerInitializerConfiguration.this.running = true;
                // 发布Eureka Start 事件，另外还有其他事件，我们可以根据需要监听这些事件，然后做一些特定的业务需求
               publish(new EurekaServerStartedEvent(getEurekaServerConfig()));
            }
            catch (Exception ex) {
              
               log.error("Could not initialize Eureka servlet context", ex);
            }
         }
      }).start();
   }
 
   private EurekaServerConfig getEurekaServerConfig() {
      return this.eurekaServerConfig;
   }
 
   private void publish(ApplicationEvent event) {
      this.applicationContext.publishEvent(event);
   }
 
   @Override
   public void stop() {
      this.running = false;
      eurekaServerBootstrap.contextDestroyed(this.servletContext);
   }
 
  .................省略...............
}

eurekaServerBootstrap是spring cloud定义的类，其代码完全拷贝了Eureka启动类的实现。 这里看到在Eureka启动后，会向外发送EurekaRegistryAvailableEvent 和 EurekaServerStartedEvent 事件，我们可以根据需要订阅这两个事件，做具体的处理操作。

接着我们一下 EurekaServerBootstrap 做了哪些具体的操作，从源码中可以看到方法 contextInitialized(ServletContext context) 进行了环境和上下文的初始化工作，下面我们分别看一下：

EurekaServerBootstrap > contextInitialized

public void contextInitialized(ServletContext context) {
		try {
            //初始化执行环境
			initEurekaEnvironment();
            //初始化上下文
			initEurekaServerContext();

			context.setAttribute(EurekaServerContext.class.getName(), this.serverContext);
		}
		catch (Throwable e) {
			log.error("Cannot bootstrap eureka server :", e);
			throw new RuntimeException("Cannot bootstrap eureka server :", e);
		}
	}

EurekaServerBootstrap > contextInitialized > initEurekaEnvironment

protected void initEurekaEnvironment() throws Exception {
		log.info("Setting the eureka configuration..");

        //设置数据中心
		String dataCenter = ConfigurationManager.getConfigInstance()
				.getString(EUREKA_DATACENTER);
		if (dataCenter == null) {
			log.info(
					"Eureka data center value eureka.datacenter is not set, defaulting to default");
			ConfigurationManager.getConfigInstance()
					.setProperty(ARCHAIUS_DEPLOYMENT_DATACENTER, DEFAULT);
		}
		else {
			ConfigurationManager.getConfigInstance()
					.setProperty(ARCHAIUS_DEPLOYMENT_DATACENTER, dataCenter);
		}
        //设置 Eureka 环境
		String environment = ConfigurationManager.getConfigInstance()
				.getString(EUREKA_ENVIRONMENT);
		if (environment == null) {
			ConfigurationManager.getConfigInstance()
					.setProperty(ARCHAIUS_DEPLOYMENT_ENVIRONMENT, TEST);
			log.info(
					"Eureka environment value eureka.environment is not set, defaulting to test");
		}
		else {
			ConfigurationManager.getConfigInstance()
					.setProperty(ARCHAIUS_DEPLOYMENT_ENVIRONMENT, environment);
		}
	}

EurekaServerBootstrap > contextInitialized > initEurekaEnvironment

protected void initEurekaServerContext() throws Exception {
		// For backward compatibility
		JsonXStream.getInstance().registerConverter(new V1AwareInstanceInfoConverter(),
				XStream.PRIORITY_VERY_HIGH);
		XmlXStream.getInstance().registerConverter(new V1AwareInstanceInfoConverter(),
				XStream.PRIORITY_VERY_HIGH);

		if (isAws(this.applicationInfoManager.getInfo())) {
			this.awsBinder = new AwsBinderDelegate(this.eurekaServerConfig,
					this.eurekaClientConfig, this.registry, this.applicationInfoManager);
			this.awsBinder.start();
		}

		EurekaServerContextHolder.initialize(this.serverContext);

		log.info("Initialized server context");

		// 从邻近的eureka节点复制注册表
		int registryCount = this.registry.syncUp();
		this.registry.openForTraffic(this.applicationInfoManager, registryCount);

		// 注册所有监控统计信息
		EurekaMonitors.registerAllStats();
	}

至此，EurekaServer启动完毕, EurekaServerBootstrap是完全复制了原生EurekaBootstrap的代码， 因为原生的Eureka
是在servlet应用，但是spring cloud的应用是运行在内嵌的Tomcat等WEB服务器里面的，所以就使用EurekaServerBootstrap
来做替换，最终使Eureka能够在spring boot中使用。