A node in a hierarchical collection of preference data. This class
allows applications to store and retrieve user and system
preference and configuration data. This data is stored
persistently in an implementation-dependent backing store. Typical
implementations include flat files, OS-specific registries,
directory servers and SQL databases. The user of this class needn't
be concerned with details of the backing store.
There are two separate trees of preference nodes, one for user
preferences and one for system preferences. Each user has a separate user
preference tree, and all users in a given system share the same system
preference tree. The precise description of "user" and "system" will vary
from implementation to implementation. Typical information stored in the
user preference tree might include font choice, color choice, or preferred
window location and size for a particular application. Typical information
stored in the system preference tree might include installation
configuration data for an application.
Nodes in a preference tree are named in a similar fashion to
directories in a hierarchical file system. Every node in a preference
tree has a node name (which is not necessarily unique),
a unique absolute path name, and a path name relative to each
ancestor including itself.
The root node has a node name of the empty string (""). Every other
node has an arbitrary node name, specified at the time it is created. The
only restrictions on this name are that it cannot be the empty string, and
it cannot contain the slash character ('/').
The root node has an absolute path name of "/". Children of
the root node have absolute path names of "/" <node
name>. All other nodes have absolute path names of <parent's
absolute path name> "/" <node name>.
Note that all absolute path names begin with the slash character.
A node n's path name relative to its ancestor a
is simply the string that must be appended to a's absolute path name
in order to form n's absolute path name, with the initial slash
character (if present) removed. Note that:
No relative path names begin with the slash character.
Every node's path name relative to itself is the empty string.
Every node's path name relative to its parent is its node name (except
for the root node, which does not have a parent).
Every node's path name relative to the root is its absolute path name
with the initial slash character removed.
Note finally that:
No path name contains multiple consecutive slash characters.
No path name with the exception of the root's absolute path name
ends in the slash character.
Any string that conforms to these two rules is a valid path name.
All of the methods that modify preferences data are permitted to operate
asynchronously; they may return immediately, and changes will eventually
propagate to the persistent backing store with an implementation-dependent
delay. The flush method may be used to synchronously force
updates to the backing store. Normal termination of the Java Virtual
Machine will not result in the loss of pending updates -- an explicit
flush invocation is not required upon termination to ensure
that pending updates are made persistent.
All of the methods that read preferences from a Preferences
object require the invoker to provide a default value. The default value is
returned if no value has been previously set or if the backing store is
unavailable. The intent is to allow applications to operate, albeit
with slightly degraded functionality, even if the backing store becomes
unavailable. Several methods, like flush, have semantics that
prevent them from operating if the backing store is unavailable. Ordinary
applications should have no need to invoke any of these methods, which can
be identified by the fact that they are declared to throw BackingStoreException.
The methods in this class may be invoked concurrently by multiple threads
in a single JVM without the need for external synchronization, and the
results will be equivalent to some serial execution. If this class is used
concurrently by multiple JVMs that store their preference data in
the same backing store, the data store will not be corrupted, but no
other guarantees are made concerning the consistency of the preference
This class contains an export/import facility, allowing preferences
to be "exported" to an XML document, and XML documents representing
preferences to be "imported" back into the system. This facility
may be used to back up all or part of a preference tree, and
subsequently restore from the backup.
The XML document has the following DOCTYPE declaration:
<!DOCTYPE preferences SYSTEM "http://java.sun.com/dtd/preferences.dtd">
Note that the system URI (http://java.sun.com/dtd/preferences.dtd) is
not accessed when exporting or importing preferences; it merely
serves as a string to uniquely identify the DTD, which is:
<?xml version="1.0" encoding="UTF-8"?>
<!-- DTD for a Preferences tree. -->
<!-- The preferences element is at the root of an XML document
representing a Preferences tree. -->
<!ELEMENT preferences (root)>
<!-- The preferences element contains an optional version attribute,
which specifies version of DTD. -->
<!ATTLIST preferences EXTERNAL_XML_VERSION CDATA "0.0" >
<!-- The root element has a map representing the root's preferences
(if any), and one node for each child of the root (if any). -->
<!ELEMENT root (map, node*) >
<!-- Additionally, the root contains a type attribute, which
specifies whether it's the system or user root. -->
type (system|user) #REQUIRED >
<!-- Each node has a map representing its preferences (if any),
and one node for each child (if any). -->
<!ELEMENT node (map, node*) >
<!-- Additionally, each node has a name attribute -->
name CDATA #REQUIRED >
<!-- A map represents the preferences stored at a node (if any). -->
<!ELEMENT map (entry*) >
<!-- An entry represents a single preference, which is simply
a key-value pair. -->
<!ELEMENT entry EMPTY >
key CDATA #REQUIRED
value CDATA #REQUIRED >
Every Preferences implementation must have an associated PreferencesFactory implementation. Every Java(TM) SE implementation must provide
some means of specifying which PreferencesFactory implementation
is used to generate the root preferences nodes. This allows the
administrator to replace the default preferences implementation with an
Implementation note: In Sun's JRE, the PreferencesFactory
implementation is located as follows:
If the system property
java.util.prefs.PreferencesFactory is defined, then it is
taken to be the fully-qualified name of a class implementing the
PreferencesFactory interface. The class is loaded and
instantiated; if this process fails then an unspecified error is
If a PreferencesFactory implementation class file
has been installed in a jar file that is visible to the
system class loader,
and that jar file contains a provider-configuration file named
java.util.prefs.PreferencesFactory in the resource
directory META-INF/services, then the first class name
specified in that file is taken. If more than one such jar file is
provided, the first one found will be used. The class is loaded
and instantiated; if this process fails then an unspecified error
Finally, if neither the above-mentioned system property nor
an extension jar file is provided, then the system-wide default
PreferencesFactory implementation for the underlying
platform is loaded and instantiated.