apollo-core/src/main/java/com/ctrip/framework/foundation/internals/DefaultProviderManager.java/DefaultProviderManager/DefaultProviderManager
class DefaultProviderManager:  public DefaultProviderManager() {
    // Load per-application configuration, like app id, from classpath://META-INF/app.properties
    Provider applicationProvider = new DefaultApplicationProvider();
    applicationProvider.initialize();
    register(applicationProvider);

    // Load network parameters
    Provider networkProvider = new DefaultNetworkProvider();
    networkProvider.initialize();
    register(networkProvider);

    // Load environment (fat, fws, uat, prod ...) and dc, from /opt/settings/server.properties, JVM property and/or OS
    // environment variables.
    Provider serverProvider = new DefaultServerProvider();
    serverProvider.initialize();
    register(serverProvider);
  }