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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 data.

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">
 

 <?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. -->
 <!ATTLIST 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 -->
 <!ATTLIST node
 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 >
 <!ATTLIST entry
 key   CDATA #REQUIRED
 value CDATA #REQUIRED >
 

Implementation note: In Sun's JRE, the PreferencesFactory implementation is located as follows:

1. 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 thrown.

2. If a PreferencesFactory implementation class file has been installed in a jar file that is visible to the java.lang.ClassLoader.getSystemClassLoader 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 is thrown.

3. 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.

The XML document will have the following DOCTYPE declaration:

 <!DOCTYPE preferences SYSTEM "http://java.sun.com/dtd/preferences.dtd">
 

This method is an exception to the general rule that the results of concurrently executing multiple methods in this class yields results equivalent to some serial execution.

Events are only guaranteed for changes made within the same JVM as the registered listener, though some implementations may generate events for changes made outside this JVM. Events may be generated before the changes have become permanent. Events are not generated when indirect descendants of this node are added or removed; a caller desiring such events must register with each descendant.

Few guarantees can be made regarding node creation. Because nodes are created implicitly upon access, it may not be feasible for an implementation to determine whether a child node existed in the backing store prior to access (for example, because the backing store is unreachable or cached information is out of date). Under these circumstances, implementations are neither required to generate node change events nor prohibited from doing so.
Parameters:
  ncl - The NodeChangeListener to add.
throws:
  NullPointerException - if ncl is null.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
See Also:   Preferences.removeNodeChangeListener(NodeChangeListener)
See Also:   Preferences.addPreferenceChangeListener(PreferenceChangeListener)

Events are only guaranteed for changes made within the same JVM as the registered listener, though some implementations may generate events for changes made outside this JVM. Events may be generated before the changes have been made persistent. Events are not generated when preferences are modified in descendants of this node; a caller desiring such events must register with each descendant.
Parameters:
  pcl - The preference change listener to add.
throws:
  NullPointerException - if pcl is null.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
See Also:   Preferences.removePreferenceChangeListener(PreferenceChangeListener)
See Also:   Preferences.addNodeChangeListener(NodeChangeListener)

If this implementation supports stored defaults, and this node in the preferences hierarchy contains any such defaults, the stored defaults will be "exposed" by this call, in the sense that they will be returned by succeeding calls to get.
throws:
  BackingStoreException - if this operation cannot be completeddue to a failure in the backing store, or inability to communicate with it.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
See Also:   Preferences.removeNode()

The XML document will have the following DOCTYPE declaration:

 <!DOCTYPE preferences SYSTEM "http://java.sun.com/dtd/preferences.dtd">
 

This method is an exception to the general rule that the results of concurrently executing multiple methods in this class yields results equivalent to some serial execution. If the preferences at this node are modified concurrently with an invocation of this method, the exported preferences comprise a "fuzzy snapshot" of the preferences contained in the node; some of the concurrent modifications may be reflected in the exported data while others may not.
Parameters:
  os - the output stream on which to emit the XML document.
throws:
  IOException - if writing to the specified output streamresults in an IOException.
throws:
  BackingStoreException - if preference data cannot be read frombacking store.
See Also:   Preferences.importPreferences(InputStream)
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.

The XML document will have the following DOCTYPE declaration:

 <!DOCTYPE preferences SYSTEM "http://java.sun.com/dtd/preferences.dtd">
 

This method is an exception to the general rule that the results of concurrently executing multiple methods in this class yields results equivalent to some serial execution. If the preferences or nodes in the subtree rooted at this node are modified concurrently with an invocation of this method, the exported preferences comprise a "fuzzy snapshot" of the subtree; some of the concurrent modifications may be reflected in the exported data while others may not.
Parameters:
  os - the output stream on which to emit the XML document.
throws:
  IOException - if writing to the specified output streamresults in an IOException.
throws:
  BackingStoreException - if preference data cannot be read frombacking store.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
See Also:   Preferences.importPreferences(InputStream)
See Also:   Preferences.exportNode(OutputStream)

Implementations are free to flush changes into the persistent store at any time. They do not need to wait for this method to be called.

When a flush occurs on a newly created node, it is made persistent, as are any ancestors (and descendants) that have yet to be made persistent. Note however that any preference value changes in ancestors are not guaranteed to be made persistent.

If this method is invoked on a node that has been removed with the Preferences.removeNode() method, flushSpi() is invoked on this node, but not on others.
throws:
  BackingStoreException - if this operation cannot be completeddue to a failure in the backing store, or inability to communicate with it.
See Also:   Preferences.sync()

Some implementations may store default values in their backing stores. If there is no value associated with the specified key but there is such a stored default, it is returned in preference to the specified default.
Parameters:
  key - key whose associated value is to be returned.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with key. the value associated with key, or defif no value is associated with key, or the backingstore is inaccessible.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null. (A null value for def is permitted.)

Returns the specified default if there is no value associated with the key, the backing store is inaccessible, or if the associated value is something other than "true" or "false", ignoring case.

If the implementation supports stored defaults and such a default exists and is accessible, it is used in preference to the specified default, unless the stored default is something other than "true" or "false", ignoring case, in which case the specified default is used.
Parameters:
  key - key whose associated value is to be returned as a boolean.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with keyor the associated value cannot be interpreted as a boolean,or the backing store is inaccessible. the boolean value represented by the string associated withkey in this preference node, or def if theassociated value does not exist or cannot be interpreted asa boolean.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null.
See Also:   Preferences.get(String,String)
See Also:   Preferences.putBoolean(String,boolean)

Returns the specified default if there is no value associated with the key, the backing store is inaccessible, or if the associated value is not a valid Base64 encoded byte array (as defined above).

If the implementation supports stored defaults and such a default exists and is accessible, it is used in preference to the specified default, unless the stored default is not a valid Base64 encoded byte array (as defined above), in which case the specified default is used.
Parameters:
  key - key whose associated value is to be returned as a byte array.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with keyor the associated value cannot be interpreted as a byte array,or the backing store is inaccessible. the byte array value represented by the string associated withkey in this preference node, or def if theassociated value does not exist or cannot be interpreted asa byte array.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null. (A null value for def is permitted.)
See Also:   Preferences.get(String,String)
See Also:   Preferences.putByteArray(String,byte[])

If the implementation supports stored defaults and such a default exists, is accessible, and could be converted to a double with Double.parseDouble, this double is returned in preference to the specified default.
Parameters:
  key - key whose associated value is to be returned as a double.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with keyor the associated value cannot be interpreted as a double,or the backing store is inaccessible. the double value represented by the string associated withkey in this preference node, or def if theassociated value does not exist or cannot be interpreted asa double.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null.
See Also:   Preferences.putDouble(String,double)
See Also:   Preferences.get(String,String)

If the implementation supports stored defaults and such a default exists, is accessible, and could be converted to a float with Float.parseFloat, this float is returned in preference to the specified default.
Parameters:
  key - key whose associated value is to be returned as a float.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with keyor the associated value cannot be interpreted as a float,or the backing store is inaccessible. the float value represented by the string associated withkey in this preference node, or def if theassociated value does not exist or cannot be interpreted asa float.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null.
See Also:   Preferences.putFloat(String,float)
See Also:   Preferences.get(String,String)

If the implementation supports stored defaults and such a default exists, is accessible, and could be converted to an int with Integer.parseInt, this int is returned in preference to the specified default.
Parameters:
  key - key whose associated value is to be returned as an int.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with keyor the associated value cannot be interpreted as an int,or the backing store is inaccessible. the int value represented by the string associated withkey in this preference node, or def if theassociated value does not exist or cannot be interpreted asan int.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null.
See Also:   Preferences.putInt(String,int)
See Also:   Preferences.get(String,String)

If the implementation supports stored defaults and such a default exists, is accessible, and could be converted to a long with Long.parseLong, this long is returned in preference to the specified default.
Parameters:
  key - key whose associated value is to be returned as a long.
Parameters:
  def - the value to be returned in the event that thispreference node has no value associated with keyor the associated value cannot be interpreted as a long,or the backing store is inaccessible. the long value represented by the string associated withkey in this preference node, or def if theassociated value does not exist or cannot be interpreted asa long.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
throws:
  NullPointerException - if key is null.
See Also:   Preferences.putLong(String,long)
See Also:   Preferences.get(String,String)

The XML document must have the following DOCTYPE declaration:

 <!DOCTYPE preferences SYSTEM "http://java.sun.com/dtd/preferences.dtd">
 

This method is an exception to the general rule that the results of concurrently executing multiple methods in this class yields results equivalent to some serial execution. The method behaves as if implemented on top of the other public methods in this class, notably Preferences.node(String) and Preferences.put(String,String) .
Parameters:
  is - the input stream from which to read the XML document.
throws:
  IOException - if reading from the specified input streamresults in an IOException.
throws:
  InvalidPreferencesFormatException - Data on input stream does notconstitute a valid XML document with the mandated document type.
throws:
  SecurityException - If a security manager is present andit denies RuntimePermission("preferences").
See Also:   RuntimePermission

If the implementation supports stored defaults and there are any such defaults at this node that have not been overridden, by explicit preferences, the defaults are returned in the array in addition to any explicit preferences. an array of the keys that have an associated value in thispreference node.
throws:
  BackingStoreException - if this operation cannot be completeddue to a failure in the backing store, or inability to communicate with it.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.

If the returned node did not exist prior to this call, this node and any ancestors that were created by this call are not guaranteed to become permanent until the flush method is called on the returned node (or one of its ancestors or descendants).
Parameters:
  pathName - the path name of the preference node to return. the specified preference node.
throws:
  IllegalArgumentException - if the path name is invalid (i.e.,it contains multiple consecutive slash characters, or endswith a slash character and is more than one character long).
throws:
  NullPointerException - if path name is null.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.
See Also:   Preferences.flush()

If this node (or an ancestor) has already been removed with the Preferences.removeNode() method, it is legal to invoke this method, but only with the path name ""; the invocation will return false. Thus, the idiom p.nodeExists("") may be used to test whether p has been removed.
Parameters:
  pathName - the path name of the node whose existenceis to be checked. true if the specified node exists.
throws:
  BackingStoreException - if this operation cannot be completeddue to a failure in the backing store, or inability to communicate with it.
throws:
  IllegalArgumentException - if the path name is invalid (i.e.,it contains multiple consecutive slash characters, or endswith a slash character and is more than one character long).
throws:
  NullPointerException - if path name is null.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method andpathName is not the empty string ("").

If this implementation supports stored defaults, and there is such a default for the specified preference, the stored default will be "exposed" by this call, in the sense that it will be returned by a succeeding call to get.
Parameters:
  key - key whose mapping is to be removed from the preference node.
throws:
  NullPointerException - if key is null.
throws:
  IllegalStateException - if this node (or an ancestor) has beenremoved with the Preferences.removeNode() method.

The removal is not guaranteed to be persistent until the flush method is called on this node (or an ancestor).

If this implementation supports stored defaults, removing a node exposes any stored defaults at or below this node. Thus, a subsequent call to nodeExists on this node's path name may return true, and a subsequent call to node on this path name may return a (different) Preferences instance representing a non-empty collection of preferences and/or children.
throws:
  BackingStoreException - if this operation cannot be completeddue to a failure in the backing store, or inability to communicate with it.
throws:
  IllegalStateException - if this node (or an ancestor) has alreadybeen removed with the Preferences.removeNode() method.
throws:
  UnsupportedOperationException - if this method is invoked on the root node.
See Also:   Preferences.flush()

This convention does not apply to the unnamed package, whose associated preference node is <unnamed>. This node is not intended for long term use, but for convenience in the early development of programs that do not yet belong to a package, and for "throwaway" programs. Valuable data should not be stored at this node as it is shared by all programs that use it.

A class Foo wishing to access preferences pertaining to its package can obtain a preference node as follows:

 static Preferences prefs = Preferences.systemNodeForPackage(Foo.class);
 

Invoking this method will result in the creation of the returned node and its ancestors if they do not already exist. If the returned node did not exist prior to this call, this node and any ancestors that were created by this call are not guaranteed to become permanent until the flush method is called on the returned node (or one of its ancestors or descendants).
Parameters:
  c - the class for whose package a system preference node is desired. the system preference node associated with the package of whichc is a member.
throws:
  NullPointerException - if c is null.
throws:
  SecurityException - if a security manager is present andit denies RuntimePermission("preferences").
See Also:   RuntimePermission

This convention does not apply to the unnamed package, whose associated preference node is <unnamed>. This node is not intended for long term use, but for convenience in the early development of programs that do not yet belong to a package, and for "throwaway" programs. Valuable data should not be stored at this node as it is shared by all programs that use it.

A class Foo wishing to access preferences pertaining to its package can obtain a preference node as follows:

 static Preferences prefs = Preferences.userNodeForPackage(Foo.class);
 

Invoking this method will result in the creation of the returned node and its ancestors if they do not already exist. If the returned node did not exist prior to this call, this node and any ancestors that were created by this call are not guaranteed to become permanent until the flush method is called on the returned node (or one of its ancestors or descendants).
Parameters:
  c - the class for whose package a user preference node is desired. the user preference node associated with the package of whichc is a member.
throws:
  NullPointerException - if c is null.
throws:
  SecurityException - if a security manager is present andit denies RuntimePermission("preferences").
See Also:   RuntimePermission