Eureka初始化配置源码解读(一)

Eureka初始化入口-EurekaBootStrap

1.首先找到eureka-core项目下com.netflix.eureka包下的EurekaBootStrap类下的contextInitialized方法.

@Override
    public void contextInitialized(ServletContextEvent event) {
        try {
	    //初始化Eureka环境
            initEurekaEnvironment();
            //初始化配置
            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);
        }
    }

进入初始化环境initEurekaEnvironment

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

        String dataCenter = ConfigurationManager.getConfigInstance().getString(EUREKA_DATACENTER);
        if (dataCenter == null) {
            logger.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);
        }
        String environment = ConfigurationManager.getConfigInstance().getString(EUREKA_ENVIRONMENT);
        if (environment == null) {
            ConfigurationManager.getConfigInstance().setProperty(ARCHAIUS_DEPLOYMENT_ENVIRONMENT, TEST);
            logger.info("Eureka environment value eureka.environment is not set, defaulting to test");
        }
    }

上面的代码主要精华的点是在dataCenter的获取,如下所示:

 String dataCenter = ConfigurationManager.getConfigInstance().getString(EUREKA_DATACENTER);

紧接着往里跟进

 public static AbstractConfiguration getConfigInstance() {
        if (instance == null) {
            synchronized (ConfigurationManager.class) {
                if (instance == null) {
                    instance = getConfigInstance(Boolean.getBoolean(DynamicPropertyFactory.DISABLE_DEFAULT_CONFIG));
                }
            }
        }
        return instance;
    }

发现这里使用了双重检查的单例模式,以解决多线程情况下获取实例不安全的问题.

具体精华解读如下:
实例instance是加了volatile 以保证线程之间的可见性

static volatile AbstractConfiguration instance = null;

1.假设A线程先进入则获取到instance为null,紧接着获取到ConfigurationManager 锁.
2.紧接着判断instance是否为空,为空,创建instance实例.
3.释放锁.
4.由于加入了volatile会出现刷新主内存

5.B线程同时进入,获取的instance也为null,紧接着获取
ConfigurationManager锁,由于A线程已经获取了锁,所以此时获取不到锁,进行等待.
6.A线程释放锁,进入锁中,进行二次判断instance,但由于加入了volatile关键字,线程一在创建instance的时候,会将变量刷回主内存,并且重新通知B线程,此时instance不为空,直接返回instance.