See Also: Preferences Members
An instance of the class Preferences represents one node in a preference tree, which provides a mechanism to store and access configuration data in a hierarchical way. Two hierarchy trees are maintained, one for system preferences shared by all users and the other for user preferences specific to the user. Preferences hierarchy trees and data are stored in an implementation-dependent back-end.
Every node has one name and one unique absolute path following the same notational conventions as directories in a file system. The root node's name is "", and other node name strings cannot contain the slash character and cannot be empty. The root node's absolute path is "/", and all other nodes' absolute paths are constructed in the standard way: <parent's absolute path> + "/" + <node's name>. Since the set of nodes forms a tree with the root node at its base, all absolute paths start with the slash character. Every node has one relative path to each of its ancestors. The relative path doesn't start with slash: it equals the node's absolute path with leading substring removed corresponding to the ancestor's absolute path and a slash.
Modification to preferences data may be asynchronous, which means that preference update method calls may return immediately instead of blocking. The flush() and sync() methods force the back-end to synchronously perform all pending updates, but the implementation is permitted to perform the modifications on the underlying back-end data at any time between the moment the request is made and the moment the flush() or sync() method returns. Please note that if the JVM exits normally, the implementation must assure all modifications are persisted implicitly.
When invoking a method that retrieves preferences, the user must provide a default value. The default value is returned when the preferences cannot be found or the back-end is unavailable. Some other methods will throw BackingStoreException when the back-end is unavailable.
Preferences can be exported to and imported from an XML files. These documents must have an XML DOCTYPE declaration:
java Example
There must be a concrete PreferencesFactory type for every concrete Preferences type developed. Every J2SE implementation must provide a default implementation for every supported platform, and must also provide a means of replacing the default implementation. This implementation uses the system property java.util.prefs.PreferencesFactory to determine which preferences implementation to use.
The methods of this class are thread-safe. If multiple JVMs are using the same back-end concurrently, the back-end won't be corrupted, but no other behavior guarantees are made.