apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/environment/PortalMetaDomainService.java/PortalMetaDomainService/PortalMetaDomainService
class PortalMetaDomainService:    PortalMetaDomainService(final PortalConfig portalConfig) {
        // high priority with data in database
        portalMetaServerProviders.add(new DatabasePortalMetaServerProvider(portalConfig));

        // System properties, OS environment, configuration file
        portalMetaServerProviders.add(new DefaultPortalMetaServerProvider());
    }