这个 Properties.clone() 方法将创建一个新的 Properties 对象，包括默认值。此克隆可以清空，剩下的是“默认值”：

public static Properties getDefaultProperties(final Properties properties) {
    // use a clone to not modify the supplied properties
    final Properties clone = (Properties) properties.clone();
    // since we cannot access the default properties, we simply remove all non-default
    clone.clear();
    // what remains are the default properties that we will copy
    final Properties defaultProperties = new Properties();
    for (final String property : clone.stringPropertyNames()) {
        defaultProperties.setProperty(property, clone.getProperty(property));
    }
    return defaultProperties;
}