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java.util.prefs
Class Preferences

java.lang.Object   
  java.util.prefs.Preferences

Direct Known Subclasses:: AbstractPreferences

public abstract class Preferences
extends Object
extends Object

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 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">

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. -->
    <!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 >

Every Preferences implementation must have an associated PreferencesFactory

sample code for java.util.prefs.PreferencesFactory

definition code for java.util.prefs.PreferencesFactory

implementation. Every J2SE 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 alternative implementation.

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 thrown.
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 is thrown.
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.

Since:: 1.4

Field Summary
`static int`	`MAX_KEY_LENGTH` Maximum length of string allowed as a key (80 characters).
`static int`	`MAX_NAME_LENGTH` Maximum length of a node name (80 characters).
`static int`	`MAX_VALUE_LENGTH` Maximum length of string allowed as a value (8192 characters).

Constructor Summary
`protected`	`Preferences ()` Sole constructor.

Method Summary
`abstract String`	`absolutePath ()` Returns this preference node's absolute path name.
`abstract void`	`addNodeChangeListener (NodeChangeListener ncl)` Registers the specified listener to receive node change events for this node.
`abstract void`	`addPreferenceChangeListener (PreferenceChangeListener pcl)` Registers the specified listener to receive preference change events for this preference node.
`abstract String []`	`childrenNames ()` Returns the names of the children of this preference node, relative to this node.
`abstract void`	`clear ()` Removes all of the preferences (key-value associations) in this preference node.
`abstract void`	`exportNode (OutputStream os)` Emits on the specified output stream an XML document representing all of the preferences contained in this node (but not its descendants).
`abstract void`	`exportSubtree (OutputStream os)` Emits an XML document representing all of the preferences contained in this node and all of its descendants.
`abstract void`	`flush ()` Forces any changes in the contents of this preference node and its descendants to the persistent store.
`abstract String`	`get (String key, String def)` Returns the value associated with the specified key in this preference node.
`abstract boolean`	`getBoolean (String key, boolean def)` Returns the boolean value represented by the string associated with the specified key in this preference node.
`abstract byte[]`	`getByteArray (String key, byte[] def)` Returns the byte array value represented by the string associated with the specified key in this preference node.
`abstract double`	`getDouble (String key, double def)` Returns the double value represented by the string associated with the specified key in this preference node.
`abstract float`	`getFloat (String key, float def)` Returns the float value represented by the string associated with the specified key in this preference node.
`abstract int`	`getInt (String key, int def)` Returns the int value represented by the string associated with the specified key in this preference node.
`abstract long`	`getLong (String key, long def)` Returns the long value represented by the string associated with the specified key in this preference node.
`static void`	`importPreferences (InputStream is)` Imports all of the preferences represented by the XML document on the specified input stream.
`abstract boolean`	`isUserNode ()` Returns `true` if this preference node is in the user preference tree, `false` if it's in the system preference tree.
`abstract String []`	`keys ()` Returns all of the keys that have an associated value in this preference node.
`abstract String`	`name ()` Returns this preference node's name, relative to its parent.
`abstract Preferences`	`node (String pathName)` Returns the named preference node in the same tree as this node, creating it and any of its ancestors if they do not already exist.
`abstract boolean`	`nodeExists (String pathName)` Returns true if the named preference node exists in the same tree as this node.
`abstract Preferences`	`parent ()` Returns the parent of this preference node, or `null` if this is the root.
`abstract void`	`put (String key, String value)` Associates the specified value with the specified key in this preference node.
`abstract void`	`putBoolean (String key, boolean value)` Associates a string representing the specified boolean value with the specified key in this preference node.
`abstract void`	`putByteArray (String key, byte[] value)` Associates a string representing the specified byte array with the specified key in this preference node.
`abstract void`	`putDouble (String key, double value)` Associates a string representing the specified double value with the specified key in this preference node.
`abstract void`	`putFloat (String key, float value)` Associates a string representing the specified float value with the specified key in this preference node.
`abstract void`	`putInt (String key, int value)` Associates a string representing the specified int value with the specified key in this preference node.
`abstract void`	`putLong (String key, long value)` Associates a string representing the specified long value with the specified key in this preference node.
`abstract void`	`remove (String key)` Removes the value associated with the specified key in this preference node, if any.
`abstract void`	`removeNode ()` Removes this preference node and all of its descendants, invalidating any preferences contained in the removed nodes.
`abstract void`	`removeNodeChangeListener (NodeChangeListener ncl)` Removes the specified `NodeChangeListener`, so it no longer receives change events.
`abstract void`	`removePreferenceChangeListener (PreferenceChangeListener pcl)` Removes the specified preference change listener, so it no longer receives preference change events.
`abstract void`	`sync ()` Ensures that future reads from this preference node and its descendants reflect any changes that were committed to the persistent store (from any VM) prior to the `sync` invocation.
`static Preferences`	`systemNodeForPackage (Class <?> c)` Returns the preference node from the system preference tree that is associated (by convention) with the specified class's package.
`static Preferences`	`systemRoot ()` Returns the root preference node for the system.
`abstract String`	`toString ()` Returns a string representation of this preferences node, as if computed by the expression:`(this.isUserNode() ? "User" : "System") + " Preference Node: " + this.absolutePath()`.
`static Preferences`	`userNodeForPackage (Class <?> c)` Returns the preference node from the calling user's preference tree that is associated (by convention) with the specified class's package.
`static Preferences`	`userRoot ()` Returns the root preference node for the calling user.

Methods inherited from class java.lang.Object
`clone , equals , finalize , getClass , hashCode , notify , notifyAll , wait , wait , wait`

Field Detail

MAX_KEY_LENGTH

public static final int MAX_KEY_LENGTH

Maximum length of string allowed as a key (80 characters).

See Also:: Constant Field Values

MAX_VALUE_LENGTH

public static final int MAX_VALUE_LENGTH

Maximum length of string allowed as a value (8192 characters).

See Also:: Constant Field Values

MAX_NAME_LENGTH

public static final int MAX_NAME_LENGTH

Maximum length of a node name (80 characters).

See Also:: Constant Field Values

Constructor Detail

Preferences

protected Preferences()

Sole constructor. (For invocation by subclass constructors, typically implicit.)

Method Detail

userNodeForPackage

public static Preferences    userNodeForPackage(Class   <?> c)

Returns the preference node from the calling user's preference tree that is associated (by convention) with the specified class's package. The convention is as follows: the absolute path name of the node is the fully qualified package name, preceded by a slash ('/'), and with each period ('.') replaced by a slash. For example the absolute path name of the node associated with the class com.acme.widget is /com/acme/widget.

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);

This idiom obviates the need for using a string to describe the preferences node and decreases the likelihood of a run-time failure. (If the class name is misspelled, it will typically result in a compile-time error.)

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.
Returns:: the user preference node associated with the package of which c is a member.
Throws:: NullPointerException - if c is null.; SecurityException - if a security manager is present and it denies RuntimePermission("preferences").
See Also:: RuntimePermission

systemNodeForPackage

public static Preferences    systemNodeForPackage(Class   <?> c)

Returns the preference node from the system preference tree that is associated (by convention) with the specified class's package. The convention is as follows: the absolute path name of the node is the fully qualified package name, preceded by a slash ('/'), and with each period ('.') replaced by a slash. For example the absolute path name of the node associated with the class com.acme.widget is /com/acme/widget.

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);

Parameters:: c - the class for whose package a system preference node is desired.
Returns:: the system preference node associated with the package of which c is a member.
Throws:: NullPointerException - if c is null.; SecurityException - if a security manager is present and it denies RuntimePermission("preferences").
See Also:: RuntimePermission

userRoot

public static Preferences    userRoot()

Returns the root preference node for the calling user.

Returns:: the root preference node for the calling user.
Throws:: SecurityException - If a security manager is present and it denies RuntimePermission("preferences").
See Also:: RuntimePermission

systemRoot

public static Preferences    systemRoot()

Returns the root preference node for the system.

Returns:: the root preference node for the system.
Throws:: SecurityException - If a security manager is present and it denies RuntimePermission("preferences").
See Also:: RuntimePermission

put

public abstract void put(String    key,
                         String    value)

Associates the specified value with the specified key in this preference node.

Parameters:: key - key with which the specified value is to be associated.; value - value to be associated with the specified key.
Throws:: NullPointerException - if key or value is null.; IllegalArgumentException - if key.length() exceeds MAX_KEY_LENGTH or if value.length exceeds MAX_VALUE_LENGTH.; IllegalStateException - if this node (or an ancestor) has been removed with the removeNode() method.

get

public abstract String    get(String    key,
                           String    def)

Returns the value associated with the specified key in this preference node. Returns the specified default if there is no value associated with the key, or the backing store is inaccessible.

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.; def - the value to be returned in the event that this preference node has no value associated with key.
Returns:: the value associated with key, or def if no value is associated with key, or the backing store is inaccessible.
Throws:: IllegalStateException - if this node (or an ancestor) has been removed with the removeNode() method.; NullPointerException - if key is null. (A null value for def is permitted.)

remove

public abstract void remove(String    key)

Removes the value associated with the specified key in this preference node, if any.

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

java.util.prefs Class Preferences

MAX_KEY_LENGTH

MAX_VALUE_LENGTH

MAX_NAME_LENGTH

Preferences

userNodeForPackage

systemNodeForPackage

userRoot

systemRoot

put

get

remove

java.util.prefs
Class Preferences