apollo-client/src/main/java/com/ctrip/framework/apollo/spring/config/PropertySourcesProcessor.java/PropertySourcesProcessor/ensureBootstrapPropertyPrecedence
class PropertySourcesProcessor:  private void ensureBootstrapPropertyPrecedence(ConfigurableEnvironment environment) {
    MutablePropertySources propertySources = environment.getPropertySources();

    PropertySource<?> bootstrapPropertySource = propertySources
        .get(PropertySourcesConstants.APOLLO_BOOTSTRAP_PROPERTY_SOURCE_NAME);

    // not exists or already in the first place
    if (bootstrapPropertySource == null || propertySources.precedenceOf(bootstrapPropertySource) == 0) {
      return;
    }

    propertySources.remove(PropertySourcesConstants.APOLLO_BOOTSTRAP_PROPERTY_SOURCE_NAME);
    propertySources.addFirst(bootstrapPropertySource);
  }