如何从 java.util.Properties 获取默认值

Question

A Properties object contains a map of key-value pairs and additionally a "defaults" Properties field that is protected .

One can iterate over the keys via keySet() to get the property keys of the current Properties , and also via stringPropertyNames() to get all distinct keys including the keys within the "defaults" Properties .

I would like to write a method that, given a Properties instance, returns these "defaults", including the keys and values.

Iterating the stringPropertyNames() and skipping values that are included in keySet() is not sufficient, since the entries in the "defaults" may be hidden by entries in the current Properties . Accessing the protected "defaults" field via reflection will show a warning output and may not work in future java versions.

Answer 1

The Properties.clone() method will create a new Properties object including the defaults. This clone can be emptied and what's left are the "defaults":

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;
}