Spring Cloud Eureka（五）：Eureka Server 启动流程分析

清疚 2022-05-07 00:57 241阅读 0赞

### 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

![spring-cloud-netflix-eureka-server.png][]

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

EnableEurekaServer

EurekaServerMarkerConfiguration

EurekaServerAutoConfiguration

ConditionalOnBean

EurekaServerAutoConfiguration

### 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

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。

[spring-cloud-netflix-eureka-server.png]: http://om6hv2adi.bkt.clouddn.com/spring-cloud-netflix-eureka-server.png