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【一起学源码-微服务】Nexflix Eureka 源码五：EurekaClient启动要经历哪些艰难险阻？

前言

在源码分析三、四都有提及到EurekaClient启动的一些过程。因为EurekaServer在集群模式下自己本身就是一个client，所以之前初始化eurekaServerContext就有涉及到eurekaClient的初始化。

我们也看了EurekaClient（DiscoveryClient）初始化的过程，繁杂的启动过程让人眼花缭乱，这篇文章就专门来唠唠里面经历的一些艰难险阻。这也会是后面client注册的一个前置文章。

如若转载请标明来源：一枝花算不算浪漫

从ExampleEurekaClient开始说起

在第一讲我们就说过，eureka项目有一个examples模块的，现在看一下其中的EurekaClientExample对象：

public class ExampleEurekaClient {

    private static ApplicationInfoManager applicationInfoManager;
    private static EurekaClient eurekaClient;

    private static synchronized ApplicationInfoManager initializeApplicationInfoManager(EurekaInstanceConfig instanceConfig) {
        if (applicationInfoManager == null) {
            InstanceInfo instanceInfo = new EurekaConfigBasedInstanceInfoProvider(instanceConfig).get();
            applicationInfoManager = new ApplicationInfoManager(instanceConfig, instanceInfo);
        }

        return applicationInfoManager;
    }

    private static synchronized EurekaClient initializeEurekaClient(ApplicationInfoManager applicationInfoManager, EurekaClientConfig clientConfig) {
        if (eurekaClient == null) {
            eurekaClient = new DiscoveryClient(applicationInfoManager, clientConfig);
        }

        return eurekaClient;
    }


    public void sendRequestToServiceUsingEureka(EurekaClient eurekaClient) {
        // initialize the client
        // this is the vip address for the example service to talk to as defined in conf/sample-eureka-service.properties
        String vipAddress = "sampleservice.mydomain.net";

        InstanceInfo nextServerInfo = null;
        try {
            nextServerInfo = eurekaClient.getNextServerFromEureka(vipAddress, false);
        } catch (Exception e) {
            System.err.println("Cannot get an instance of example service to talk to from eureka");
            System.exit(-1);
        }

        System.out.println("Found an instance of example service to talk to from eureka: "
                + nextServerInfo.getVIPAddress() + ":" + nextServerInfo.getPort());

        System.out.println("healthCheckUrl: " + nextServerInfo.getHealthCheckUrl());
        System.out.println("override: " + nextServerInfo.getOverriddenStatus());

        Socket s = new Socket();
        int serverPort = nextServerInfo.getPort();
        try {
            s.connect(new InetSocketAddress(nextServerInfo.getHostName(), serverPort));
        } catch (IOException e) {
            System.err.println("Could not connect to the server :"
                    + nextServerInfo.getHostName() + " at port " + serverPort);
        } catch (Exception e) {
            System.err.println("Could not connect to the server :"
                    + nextServerInfo.getHostName() + " at port " + serverPort + "due to Exception " + e);
        }
        try {
            String request = "FOO " + new Date();
            System.out.println("Connected to server. Sending a sample request: " + request);

            PrintStream out = new PrintStream(s.getOutputStream());
            out.println(request);

            System.out.println("Waiting for server response..");
            BufferedReader rd = new BufferedReader(new InputStreamReader(s.getInputStream()));
            String str = rd.readLine();
            if (str != null) {
                System.out.println("Received response from server: " + str);
                System.out.println("Exiting the client. Demo over..");
            }
            rd.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) throws Exception{
        injectEurekaConfiguration();
        ExampleEurekaClient sampleClient = new ExampleEurekaClient();

        // MyDataCenterInstanceConfig 就是加载eureka-client.properties配置信息,形成一个服务实例的配置EurekaInstanceConfig
        // 基于EurekaClient配置，构造了一个服务实例（InstanceInfo）
        // 基于eureka client配置和服务实例，构造了一个服务实例管理器（ApplicationInfoManager）
        // 读取eureka-client.properties配置文件，形成了一个eureka client的配置，接口对外提供eureka client的配置项的读取
        // 基于eureka client的配置，和服务实例管理器，来构造了一个EurekaClient（DiscoveryClient
        // ），保存了一些配置，处理服务的注册和注册表的抓取，启动了几个线程池，启动了网络通信组件，启动了一些调度任务，注册了监控项
        ApplicationInfoManager applicationInfoManager = initializeApplicationInfoManager(new MyDataCenterInstanceConfig());
        EurekaClient client = initializeEurekaClient(applicationInfoManager, new DefaultEurekaClientConfig());

        // use the client
        sampleClient.sendRequestToServiceUsingEureka(client);


        // shutdown the client
        eurekaClient.shutdown();
    }

    /**
     * This will be read by server internal discovery client. We need to salience it.
     */
    private static void injectEurekaConfiguration() throws UnknownHostException {
        String myHostName = InetAddress.getLocalHost().getHostName();
        String myServiceUrl = "http://" + myHostName + ":8080/v2/";

        System.setProperty("eureka.region", "default");
        System.setProperty("eureka.name", "eureka");
        System.setProperty("eureka.vipAddress", "eureka.mydomain.net");
        System.setProperty("eureka.port", "8080");
        System.setProperty("eureka.preferSameZone", "false");
        System.setProperty("eureka.shouldUseDns", "false");
        System.setProperty("eureka.shouldFetchRegistry", "true");
        System.setProperty("eureka.serviceUrl.defaultZone", myServiceUrl);
        System.setProperty("eureka.serviceUrl.default.defaultZone", myServiceUrl);
        System.setProperty("eureka.awsAccessId", "fake_aws_access_id");
        System.setProperty("eureka.awsSecretKey", "fake_aws_secret_key");
        System.setProperty("eureka.numberRegistrySyncRetries", "0");
    }
}

这里我们从main函数开始看起：

注入eureka配置信息： injectEurekaConfiguration();
读取eureka-client.properties配置文件，形成一个服务器实例的配置，基于接口对外提供实例的配置项读取
这里就是涉及到我么你之前讲解的 DynamicPropertyFactory 和 ConfigurationManager ,这里可以查看 new MyDataCenterInstanceConfig() 然后一步步往后跟。
基于服务实例的配置，构造了一个服务实例（InstanceInfo）。 initializeApplicationInfoManager 中会构建InstanceInfo信息
基于服务实例的配置和服务实例，初始化服务实例管理器（ApplicationInfoManager）
基于eureka client配置，和服务实例管理器，来构造了一个EurekaClient（DiscoveryClient），保存了一些配置，处理服务的注册和注册表的抓取，启动了几个线程池，启动了网络通信组件，启动了一些调度任务，注册了监控项
具体可查看 EurekaClient client = initializeEurekaClient(applicationInfoManager, new DefaultEurekaClientConfig());