Eureka启动，原生启动与SpringCloudEureka启动异同

我们先看看作为原生的EurekaServer启动的过程，作为一个Servlet应用，他的启动入口就是他的主要ServletContextListener类（这里是EurekaBootStrap）的contextInitialized方法
EurekaServerBootstrap

@Override
public void contextInitialized(ServletContextEvent event) {
try {
initEurekaEnvironment();
initEurekaServerContext();

    ServletContext sc = event.getServletContext();sc.setAttribute(EurekaServerContext.class.getName(), serverContext);
} catch (Throwable e) {logger.error("Cannot bootstrap eureka server :", e);throw new RuntimeException("Cannot bootstrap eureka server :", e);
}

}

可以看出主要做了两件事，initEurekaEnvironment()与initEurekaServerContext()
对于initEurekaEnvironment()只是初始化一些必要的环境变量，由于Eureka配置基于Spring的配置中间件Archaius，这些环境变量都是针对这个配置中间件使用的。
initEurekaServerContext()是我们重点需要关心的，它初始化了EurekaServer需要的所有组件：

protected void initEurekaServerContext() throws Exception {
EurekaServerConfig eurekaServerConfig = new DefaultEurekaServerConfig();

//设置json与xml序列化工具
JsonXStream.getInstance().registerConverter(new V1AwareInstanceInfoConverter(), XStream.PRIORITY_VERY_HIGH);
XmlXStream.getInstance().registerConverter(new V1AwareInstanceInfoConverter(), XStream.PRIORITY_VERY_HIGH);
logger.info("Initializing the eureka client...");
logger.info(eurekaServerConfig.getJsonCodecName());
ServerCodecs serverCodecs = new DefaultServerCodecs(eurekaServerConfig);ApplicationInfoManager applicationInfoManager = null;//初始化EurekaClient，EurekaClient用来与其他EurekaServer进行交互
//有可能通过guice初始化Eureka，这时eurekaClient和ApplicationInfoManager通过依赖注入先被初始化
if (eurekaClient == null) {EurekaInstanceConfig instanceConfig = isCloud(ConfigurationManager.getDeploymentContext())? new CloudInstanceConfig(): new MyDataCenterInstanceConfig();applicationInfoManager = new ApplicationInfoManager(instanceConfig, new EurekaConfigBasedInstanceInfoProvider(instanceConfig).get());EurekaClientConfig eurekaClientConfig = new DefaultEurekaClientConfig();eurekaClient = new DiscoveryClient(applicationInfoManager, eurekaClientConfig);
} else {applicationInfoManager = eurekaClient.getApplicationInfoManager();
}//初始化PeerAwareInstanceRegistry, 这个类里面的方法就是与集群内其他EurekaServer实例保持业务同步的机制
PeerAwareInstanceRegistry registry;
if (isAws(applicationInfoManager.getInfo())) {registry = new AwsInstanceRegistry(eurekaServerConfig,eurekaClient.getEurekaClientConfig(),serverCodecs,eurekaClient);awsBinder = new AwsBinderDelegate(eurekaServerConfig, eurekaClient.getEurekaClientConfig(), registry, applicationInfoManager);awsBinder.start();
} else {registry = new PeerAwareInstanceRegistryImpl(eurekaServerConfig,eurekaClient.getEurekaClientConfig(),serverCodecs,eurekaClient);
}//初始化PeerEurekaNodes，里面有定时维护Eureka集群的业务逻辑
PeerEurekaNodes peerEurekaNodes = getPeerEurekaNodes(registry,eurekaServerConfig,eurekaClient.getEurekaClientConfig(),serverCodecs,applicationInfoManager
);//初始化EurekaServer上下文
serverContext = new DefaultEurekaServerContext(eurekaServerConfig,serverCodecs,registry,peerEurekaNodes,applicationInfoManager
);EurekaServerContextHolder.initialize(serverContext);serverContext.initialize();
logger.info("Initialized server context");//从其他节点中读取注册信息，并开放服务注册
// Copy registry from neighboring eureka node
int registryCount = registry.syncUp();
registry.openForTraffic(applicationInfoManager, registryCount);// Register all monitoring statistics.
EurekaMonitors.registerAllStats();

}

总结下来，总共如下几点：

1.初始化并设置序列化反序列化工具
2.初始化通信客户端EurekaClient
3.初始化集群通信类PeerAwareInstanceRegistry与PeerEurekaNodes
4.初始化EurekaServer上下文serverContext
5.从其他节点中读取注册信息，并开放服务注册

然后，由于原生的EurekaServer利用Jersey框架初始化restApi，这里还有：
6.载入Jersey，初始化Restful服务api

对于胶水代码，实现了大致同样的但是略微有些区别的功能：
一、 Server 有哪些功能:

提供服务注册功能。
消费者可以获取服务列表。
服务可以续约。
Server 集群之间的数据共享

二、 提供服务注册功能

我们使用 EurekaServer 的时候，需要在启动类上加入 @EnableEurekaServer注解，这个注解肯定就是我们研究的入口。

EnableEurekaServer 类：

/**

• Annotation to activate Eureka Server related configuration.
• {@link EurekaServerAutoConfiguration}
• @author Dave Syer
• @author Biju Kunjummen

*/

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

}

EurekaServerAutoConfiguration

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

EurekaServerInitializerConfiguration 由于实现了SmartLifecycle, 初始化启动start()方法； 额外新创建一个线程初始化initEurekaServerContext() , 发布事件等工作

@Configuration
public class EurekaServerInitializerConfiguration
implements ServletContextAware, SmartLifecycle, Ordered {
@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”);

				publish(new EurekaRegistryAvailableEvent(getEurekaServerConfig()));EurekaServerInitializerConfiguration.this.running = true;publish(new EurekaServerStartedEvent(getEurekaServerConfig()));}catch (Exception ex) {// Help!log.error("Could not initialize Eureka servlet context", ex);}}}).start();
}

}

EurekaServerAutoConfiguration 中注入EurekaServerContext

@Bean
public EurekaServerContext eurekaServerContext(ServerCodecs serverCodecs,PeerAwareInstanceRegistry registry, PeerEurekaNodes peerEurekaNodes) {return new DefaultEurekaServerContext(this.eurekaServerConfig, serverCodecs,registry, peerEurekaNodes, this.applicationInfoManager);
}

在初始化化DefaultEurekaServerContext 类的时候，会初始化initialize（）方法，是因为在此方法上面@PostConstruct注解. peerEurekaNodes.start() 中。使用单定时任务线程池，维护eureka节点

@PostConstruct
@Override
public void initialize() {logger.info("Initializing ...");peerEurekaNodes.start();try {registry.init(peerEurekaNodes);} catch (Exception e) {throw new RuntimeException(e);}logger.info("Initialized");
}