Back to index...

	/*
	* Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.util.prefs;

	import java.util.*;
	import java.io.*;
	import java.security.AccessController;
	import java.security.PrivilegedAction;
	// These imports needed only as a workaround for a JavaDoc bug
	import java.lang.Integer;
	import java.lang.Long;
	import java.lang.Float;
	import java.lang.Double;

	/**
	* This class provides a skeletal implementation of the {@link Preferences}
	* class, greatly easing the task of implementing it.
	*
	* <p><strong>This class is for <tt>Preferences</tt> implementers only.
	* Normal users of the <tt>Preferences</tt> facility should have no need to
	* consult this documentation. The {@link Preferences} documentation
	* should suffice.</strong>
	*
	* <p>Implementors must override the nine abstract service-provider interface
	* (SPI) methods: {@link #getSpi(String)}, {@link #putSpi(String,String)},
	* {@link #removeSpi(String)}, {@link #childSpi(String)}, {@link
	* #removeNodeSpi()}, {@link #keysSpi()}, {@link #childrenNamesSpi()}, {@link
	* #syncSpi()} and {@link #flushSpi()}. All of the concrete methods specify
	* precisely how they are implemented atop these SPI methods. The implementor
	* may, at his discretion, override one or more of the concrete methods if the
	* default implementation is unsatisfactory for any reason, such as
	* performance.
	*
	* <p>The SPI methods fall into three groups concerning exception
	* behavior. The <tt>getSpi</tt> method should never throw exceptions, but it
	* doesn't really matter, as any exception thrown by this method will be
	* intercepted by {@link #get(String,String)}, which will return the specified
	* default value to the caller. The <tt>removeNodeSpi, keysSpi,
	* childrenNamesSpi, syncSpi</tt> and <tt>flushSpi</tt> methods are specified
	* to throw {@link BackingStoreException}, and the implementation is required
	* to throw this checked exception if it is unable to perform the operation.
	* The exception propagates outward, causing the corresponding API method
	* to fail.
	*
	* <p>The remaining SPI methods {@link #putSpi(String,String)}, {@link
	* #removeSpi(String)} and {@link #childSpi(String)} have more complicated
	* exception behavior. They are not specified to throw
	* <tt>BackingStoreException</tt>, as they can generally obey their contracts
	* even if the backing store is unavailable. This is true because they return
	* no information and their effects are not required to become permanent until
	* a subsequent call to {@link Preferences#flush()} or
	* {@link Preferences#sync()}. Generally speaking, these SPI methods should not
	* throw exceptions. In some implementations, there may be circumstances
	* under which these calls cannot even enqueue the requested operation for
	* later processing. Even under these circumstances it is generally better to
	* simply ignore the invocation and return, rather than throwing an
	* exception. Under these circumstances, however, all subsequent invocations
	* of <tt>flush()</tt> and <tt>sync</tt> should return <tt>false</tt>, as
	* returning <tt>true</tt> would imply that all previous operations had
	* successfully been made permanent.
	*
	* <p>There is one circumstance under which <tt>putSpi, removeSpi and
	* childSpi</tt> <i>should</i> throw an exception: if the caller lacks
	* sufficient privileges on the underlying operating system to perform the
	* requested operation. This will, for instance, occur on most systems
	* if a non-privileged user attempts to modify system preferences.
	* (The required privileges will vary from implementation to
	* implementation. On some implementations, they are the right to modify the
	* contents of some directory in the file system; on others they are the right
	* to modify contents of some key in a registry.) Under any of these
	* circumstances, it would generally be undesirable to let the program
	* continue executing as if these operations would become permanent at a later
	* time. While implementations are not required to throw an exception under
	* these circumstances, they are encouraged to do so. A {@link
	* SecurityException} would be appropriate.
	*
	* <p>Most of the SPI methods require the implementation to read or write
	* information at a preferences node. The implementor should beware of the
	* fact that another VM may have concurrently deleted this node from the
	* backing store. It is the implementation's responsibility to recreate the
	* node if it has been deleted.
	*
	* <p>Implementation note: In Sun's default <tt>Preferences</tt>
	* implementations, the user's identity is inherited from the underlying
	* operating system and does not change for the lifetime of the virtual
	* machine. It is recognized that server-side <tt>Preferences</tt>
	* implementations may have the user identity change from request to request,
	* implicitly passed to <tt>Preferences</tt> methods via the use of a
	* static {@link ThreadLocal} instance. Authors of such implementations are
	* <i>strongly</i> encouraged to determine the user at the time preferences
	* are accessed (for example by the {@link #get(String,String)} or {@link
	* #put(String,String)} method) rather than permanently associating a user
	* with each <tt>Preferences</tt> instance. The latter behavior conflicts
	* with normal <tt>Preferences</tt> usage and would lead to great confusion.
	*
	* @author Josh Bloch
	* @see Preferences
	* @since 1.4
	*/
	public abstract class AbstractPreferences extends Preferences {
	/**
	* Our name relative to parent.
	*/
	private final String name;

	/**
	* Our absolute path name.
	*/
	private final String absolutePath;

	/**
	* Our parent node.
	*/
	final AbstractPreferences parent;

	/**
	* Our root node.
	*/
	private final AbstractPreferences root; // Relative to this node

	/**
	* This field should be <tt>true</tt> if this node did not exist in the
	* backing store prior to the creation of this object. The field
	* is initialized to false, but may be set to true by a subclass
	* constructor (and should not be modified thereafter). This field
	* indicates whether a node change event should be fired when
	* creation is complete.
	*/
	protected boolean newNode = false;

	/**
	* All known unremoved children of this node. (This "cache" is consulted
	* prior to calling childSpi() or getChild().
	*/
	private Map<String, AbstractPreferences> kidCache = new HashMap<>();

	/**
	* This field is used to keep track of whether or not this node has
	* been removed. Once it's set to true, it will never be reset to false.
	*/
	private boolean removed = false;

	/**
	* Registered preference change listeners.
	*/
	private PreferenceChangeListener[] prefListeners =
	new PreferenceChangeListener[0];

	/**
	* Registered node change listeners.
	*/
	private NodeChangeListener[] nodeListeners = new NodeChangeListener[0];

	/**
	* An object whose monitor is used to lock this node. This object
	* is used in preference to the node itself to reduce the likelihood of
	* intentional or unintentional denial of service due to a locked node.
	* To avoid deadlock, a node is <i>never</i> locked by a thread that
	* holds a lock on a descendant of that node.
	*/
	protected final Object lock = new Object();

	/**
	* Creates a preference node with the specified parent and the specified
	* name relative to its parent.
	*
	* @param parent the parent of this preference node, or null if this
	* is the root.
	* @param name the name of this preference node, relative to its parent,
	* or <tt>""</tt> if this is the root.
	* @throws IllegalArgumentException if <tt>name</tt> contains a slash
	* (<tt>'/'</tt>), or <tt>parent</tt> is <tt>null</tt> and
	* name isn't <tt>""</tt>.
	*/
	protected AbstractPreferences(AbstractPreferences parent, String name) {
	if (parent==null) {
	if (!name.equals(""))
	throw new IllegalArgumentException("Root name '"+name+
	"' must be \"\"");
	this.absolutePath = "/";
	root = this;
	} else {
	if (name.indexOf('/') != -1)
	throw new IllegalArgumentException("Name '" + name +
	"' contains '/'");
	if (name.equals(""))
	throw new IllegalArgumentException("Illegal name: empty string");

	root = parent.root;
	absolutePath = (parent==root ? "/" + name
	: parent.absolutePath() + "/" + name);
	}
	this.name = name;
	this.parent = parent;
	}

	/**
	* Implements the <tt>put</tt> method as per the specification in
	* {@link Preferences#put(String,String)}.
	*
	* <p>This implementation checks that the key and value are legal,
	* obtains this preference node's lock, checks that the node
	* has not been removed, invokes {@link #putSpi(String,String)}, and if
	* there are any preference change listeners, enqueues a notification
	* event for processing by the event dispatch thread.
	*
	* @param key key with which the specified value is to be associated.
	* @param value value to be associated with the specified key.
	* @throws NullPointerException if key or value is <tt>null</tt>.
	* @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
	* <tt>MAX_KEY_LENGTH</tt> or if <tt>value.length</tt> exceeds
	* <tt>MAX_VALUE_LENGTH</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void put(String key, String value) {
	if (key==null \|\| value==null)
	throw new NullPointerException();
	if (key.length() > MAX_KEY_LENGTH)
	throw new IllegalArgumentException("Key too long: "+key);
	if (value.length() > MAX_VALUE_LENGTH)
	throw new IllegalArgumentException("Value too long: "+value);

	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	putSpi(key, value);
	enqueuePreferenceChangeEvent(key, value);
	}
	}

	/**
	* Implements the <tt>get</tt> method as per the specification in
	* {@link Preferences#get(String,String)}.
	*
	* <p>This implementation first checks to see if <tt>key</tt> is
	* <tt>null</tt> throwing a <tt>NullPointerException</tt> if this is
	* the case. Then it obtains this preference node's lock,
	* checks that the node has not been removed, invokes {@link
	* #getSpi(String)}, and returns the result, unless the <tt>getSpi</tt>
	* invocation returns <tt>null</tt> or throws an exception, in which case
	* this invocation returns <tt>def</tt>.
	*
	* @param key key whose associated value is to be returned.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>.
	* @return the value associated with <tt>key</tt>, or <tt>def</tt>
	* if no value is associated with <tt>key</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if key is <tt>null</tt>. (A
	* <tt>null</tt> default <i>is</i> permitted.)
	*/
	public String get(String key, String def) {
	if (key==null)
	throw new NullPointerException("Null key");
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	String result = null;
	try {
	result = getSpi(key);
	} catch (Exception e) {
	// Ignoring exception causes default to be returned
	}
	return (result==null ? def : result);
	}
	}

	/**
	* Implements the <tt>remove(String)</tt> method as per the specification
	* in {@link Preferences#remove(String)}.
	*
	* <p>This implementation obtains this preference node's lock,
	* checks that the node has not been removed, invokes
	* {@link #removeSpi(String)} and if there are any preference
	* change listeners, enqueues a notification event for processing by the
	* event dispatch thread.
	*
	* @param key key whose mapping is to be removed from the preference node.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException {@inheritDoc}.
	*/
	public void remove(String key) {
	Objects.requireNonNull(key, "Specified key cannot be null");
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	removeSpi(key);
	enqueuePreferenceChangeEvent(key, null);
	}
	}

	/**
	* Implements the <tt>clear</tt> method as per the specification in
	* {@link Preferences#clear()}.
	*
	* <p>This implementation obtains this preference node's lock,
	* invokes {@link #keys()} to obtain an array of keys, and
	* iterates over the array invoking {@link #remove(String)} on each key.
	*
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void clear() throws BackingStoreException {
	synchronized(lock) {
	String[] keys = keys();
	for (int i=0; i<keys.length; i++)
	remove(keys[i]);
	}
	}

	/**
	* Implements the <tt>putInt</tt> method as per the specification in
	* {@link Preferences#putInt(String,int)}.
	*
	* <p>This implementation translates <tt>value</tt> to a string with
	* {@link Integer#toString(int)} and invokes {@link #put(String,String)}
	* on the result.
	*
	* @param key key with which the string form of value is to be associated.
	* @param value value whose string form is to be associated with key.
	* @throws NullPointerException if key is <tt>null</tt>.
	* @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
	* <tt>MAX_KEY_LENGTH</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void putInt(String key, int value) {
	put(key, Integer.toString(value));
	}

	/**
	* Implements the <tt>getInt</tt> method as per the specification in
	* {@link Preferences#getInt(String,int)}.
	*
	* <p>This implementation invokes {@link #get(String,String) <tt>get(key,
	* null)</tt>}. If the return value is non-null, the implementation
	* attempts to translate it to an <tt>int</tt> with
	* {@link Integer#parseInt(String)}. If the attempt succeeds, the return
	* value is returned by this method. Otherwise, <tt>def</tt> is returned.
	*
	* @param key key whose associated value is to be returned as an int.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>
	* or the associated value cannot be interpreted as an int.
	* @return the int value represented by the string associated with
	* <tt>key</tt> in this preference node, or <tt>def</tt> if the
	* associated value does not exist or cannot be interpreted as
	* an int.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
	*/
	public int getInt(String key, int def) {
	int result = def;
	try {
	String value = get(key, null);
	if (value != null)
	result = Integer.parseInt(value);
	} catch (NumberFormatException e) {
	// Ignoring exception causes specified default to be returned
	}

	return result;
	}

	/**
	* Implements the <tt>putLong</tt> method as per the specification in
	* {@link Preferences#putLong(String,long)}.
	*
	* <p>This implementation translates <tt>value</tt> to a string with
	* {@link Long#toString(long)} and invokes {@link #put(String,String)}
	* on the result.
	*
	* @param key key with which the string form of value is to be associated.
	* @param value value whose string form is to be associated with key.
	* @throws NullPointerException if key is <tt>null</tt>.
	* @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
	* <tt>MAX_KEY_LENGTH</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void putLong(String key, long value) {
	put(key, Long.toString(value));
	}

	/**
	* Implements the <tt>getLong</tt> method as per the specification in
	* {@link Preferences#getLong(String,long)}.
	*
	* <p>This implementation invokes {@link #get(String,String) <tt>get(key,
	* null)</tt>}. If the return value is non-null, the implementation
	* attempts to translate it to a <tt>long</tt> with
	* {@link Long#parseLong(String)}. If the attempt succeeds, the return
	* value is returned by this method. Otherwise, <tt>def</tt> is returned.
	*
	* @param key key whose associated value is to be returned as a long.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>
	* or the associated value cannot be interpreted as a long.
	* @return the long value represented by the string associated with
	* <tt>key</tt> in this preference node, or <tt>def</tt> if the
	* associated value does not exist or cannot be interpreted as
	* a long.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
	*/
	public long getLong(String key, long def) {
	long result = def;
	try {
	String value = get(key, null);
	if (value != null)
	result = Long.parseLong(value);
	} catch (NumberFormatException e) {
	// Ignoring exception causes specified default to be returned
	}

	return result;
	}

	/**
	* Implements the <tt>putBoolean</tt> method as per the specification in
	* {@link Preferences#putBoolean(String,boolean)}.
	*
	* <p>This implementation translates <tt>value</tt> to a string with
	* {@link String#valueOf(boolean)} and invokes {@link #put(String,String)}
	* on the result.
	*
	* @param key key with which the string form of value is to be associated.
	* @param value value whose string form is to be associated with key.
	* @throws NullPointerException if key is <tt>null</tt>.
	* @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
	* <tt>MAX_KEY_LENGTH</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void putBoolean(String key, boolean value) {
	put(key, String.valueOf(value));
	}

	/**
	* Implements the <tt>getBoolean</tt> method as per the specification in
	* {@link Preferences#getBoolean(String,boolean)}.
	*
	* <p>This implementation invokes {@link #get(String,String) <tt>get(key,
	* null)</tt>}. If the return value is non-null, it is compared with
	* <tt>"true"</tt> using {@link String#equalsIgnoreCase(String)}. If the
	* comparison returns <tt>true</tt>, this invocation returns
	* <tt>true</tt>. Otherwise, the original return value is compared with
	* <tt>"false"</tt>, again using {@link String#equalsIgnoreCase(String)}.
	* If the comparison returns <tt>true</tt>, this invocation returns
	* <tt>false</tt>. Otherwise, this invocation returns <tt>def</tt>.
	*
	* @param key key whose associated value is to be returned as a boolean.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>
	* or the associated value cannot be interpreted as a boolean.
	* @return the boolean value represented by the string associated with
	* <tt>key</tt> in this preference node, or <tt>def</tt> if the
	* associated value does not exist or cannot be interpreted as
	* a boolean.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
	*/
	public boolean getBoolean(String key, boolean def) {
	boolean result = def;
	String value = get(key, null);
	if (value != null) {
	if (value.equalsIgnoreCase("true"))
	result = true;
	else if (value.equalsIgnoreCase("false"))
	result = false;
	}

	return result;
	}

	/**
	* Implements the <tt>putFloat</tt> method as per the specification in
	* {@link Preferences#putFloat(String,float)}.
	*
	* <p>This implementation translates <tt>value</tt> to a string with
	* {@link Float#toString(float)} and invokes {@link #put(String,String)}
	* on the result.
	*
	* @param key key with which the string form of value is to be associated.
	* @param value value whose string form is to be associated with key.
	* @throws NullPointerException if key is <tt>null</tt>.
	* @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
	* <tt>MAX_KEY_LENGTH</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void putFloat(String key, float value) {
	put(key, Float.toString(value));
	}

	/**
	* Implements the <tt>getFloat</tt> method as per the specification in
	* {@link Preferences#getFloat(String,float)}.
	*
	* <p>This implementation invokes {@link #get(String,String) <tt>get(key,
	* null)</tt>}. If the return value is non-null, the implementation
	* attempts to translate it to an <tt>float</tt> with
	* {@link Float#parseFloat(String)}. If the attempt succeeds, the return
	* value is returned by this method. Otherwise, <tt>def</tt> is returned.
	*
	* @param key key whose associated value is to be returned as a float.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>
	* or the associated value cannot be interpreted as a float.
	* @return the float value represented by the string associated with
	* <tt>key</tt> in this preference node, or <tt>def</tt> if the
	* associated value does not exist or cannot be interpreted as
	* a float.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
	*/
	public float getFloat(String key, float def) {
	float result = def;
	try {
	String value = get(key, null);
	if (value != null)
	result = Float.parseFloat(value);
	} catch (NumberFormatException e) {
	// Ignoring exception causes specified default to be returned
	}

	return result;
	}

	/**
	* Implements the <tt>putDouble</tt> method as per the specification in
	* {@link Preferences#putDouble(String,double)}.
	*
	* <p>This implementation translates <tt>value</tt> to a string with
	* {@link Double#toString(double)} and invokes {@link #put(String,String)}
	* on the result.
	*
	* @param key key with which the string form of value is to be associated.
	* @param value value whose string form is to be associated with key.
	* @throws NullPointerException if key is <tt>null</tt>.
	* @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
	* <tt>MAX_KEY_LENGTH</tt>.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void putDouble(String key, double value) {
	put(key, Double.toString(value));
	}

	/**
	* Implements the <tt>getDouble</tt> method as per the specification in
	* {@link Preferences#getDouble(String,double)}.
	*
	* <p>This implementation invokes {@link #get(String,String) <tt>get(key,
	* null)</tt>}. If the return value is non-null, the implementation
	* attempts to translate it to an <tt>double</tt> with
	* {@link Double#parseDouble(String)}. If the attempt succeeds, the return
	* value is returned by this method. Otherwise, <tt>def</tt> is returned.
	*
	* @param key key whose associated value is to be returned as a double.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>
	* or the associated value cannot be interpreted as a double.
	* @return the double value represented by the string associated with
	* <tt>key</tt> in this preference node, or <tt>def</tt> if the
	* associated value does not exist or cannot be interpreted as
	* a double.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
	*/
	public double getDouble(String key, double def) {
	double result = def;
	try {
	String value = get(key, null);
	if (value != null)
	result = Double.parseDouble(value);
	} catch (NumberFormatException e) {
	// Ignoring exception causes specified default to be returned
	}

	return result;
	}

	/**
	* Implements the <tt>putByteArray</tt> method as per the specification in
	* {@link Preferences#putByteArray(String,byte[])}.
	*
	* @param key key with which the string form of value is to be associated.
	* @param value value whose string form is to be associated with key.
	* @throws NullPointerException if key or value is <tt>null</tt>.
	* @throws IllegalArgumentException if key.length() exceeds MAX_KEY_LENGTH
	* or if value.length exceeds MAX_VALUE_LENGTH*3/4.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public void putByteArray(String key, byte[] value) {
	put(key, Base64.byteArrayToBase64(value));
	}

	/**
	* Implements the <tt>getByteArray</tt> method as per the specification in
	* {@link Preferences#getByteArray(String,byte[])}.
	*
	* @param key key whose associated value is to be returned as a byte array.
	* @param def the value to be returned in the event that this
	* preference node has no value associated with <tt>key</tt>
	* or the associated value cannot be interpreted as a byte array.
	* @return the byte array value represented by the string associated with
	* <tt>key</tt> in this preference node, or <tt>def</tt> if the
	* associated value does not exist or cannot be interpreted as
	* a byte array.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @throws NullPointerException if <tt>key</tt> is <tt>null</tt>. (A
	* <tt>null</tt> value for <tt>def</tt> <i>is</i> permitted.)
	*/
	public byte[] getByteArray(String key, byte[] def) {
	byte[] result = def;
	String value = get(key, null);
	try {
	if (value != null)
	result = Base64.base64ToByteArray(value);
	}
	catch (RuntimeException e) {
	// Ignoring exception causes specified default to be returned
	}

	return result;
	}

	/**
	* Implements the <tt>keys</tt> method as per the specification in
	* {@link Preferences#keys()}.
	*
	* <p>This implementation obtains this preference node's lock, checks that
	* the node has not been removed and invokes {@link #keysSpi()}.
	*
	* @return an array of the keys that have an associated value in this
	* preference node.
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public String[] keys() throws BackingStoreException {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	return keysSpi();
	}
	}

	/**
	* Implements the <tt>children</tt> method as per the specification in
	* {@link Preferences#childrenNames()}.
	*
	* <p>This implementation obtains this preference node's lock, checks that
	* the node has not been removed, constructs a <tt>TreeSet</tt> initialized
	* to the names of children already cached (the children in this node's
	* "child-cache"), invokes {@link #childrenNamesSpi()}, and adds all of the
	* returned child-names into the set. The elements of the tree set are
	* dumped into a <tt>String</tt> array using the <tt>toArray</tt> method,
	* and this array is returned.
	*
	* @return the names of the children of this preference node.
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @see #cachedChildren()
	*/
	public String[] childrenNames() throws BackingStoreException {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	Set<String> s = new TreeSet<>(kidCache.keySet());
	for (String kid : childrenNamesSpi())
	s.add(kid);
	return s.toArray(EMPTY_STRING_ARRAY);
	}
	}

	private static final String[] EMPTY_STRING_ARRAY = new String[0];

	/**
	* Returns all known unremoved children of this node.
	*
	* @return all known unremoved children of this node.
	*/
	protected final AbstractPreferences[] cachedChildren() {
	return kidCache.values().toArray(EMPTY_ABSTRACT_PREFS_ARRAY);
	}

	private static final AbstractPreferences[] EMPTY_ABSTRACT_PREFS_ARRAY
	= new AbstractPreferences[0];

	/**
	* Implements the <tt>parent</tt> method as per the specification in
	* {@link Preferences#parent()}.
	*
	* <p>This implementation obtains this preference node's lock, checks that
	* the node has not been removed and returns the parent value that was
	* passed to this node's constructor.
	*
	* @return the parent of this preference node.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public Preferences parent() {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	return parent;
	}
	}

	/**
	* Implements the <tt>node</tt> method as per the specification in
	* {@link Preferences#node(String)}.
	*
	* <p>This implementation obtains this preference node's lock and checks
	* that the node has not been removed. If <tt>path</tt> is <tt>""</tt>,
	* this node is returned; if <tt>path</tt> is <tt>"/"</tt>, this node's
	* root is returned. If the first character in <tt>path</tt> is
	* not <tt>'/'</tt>, the implementation breaks <tt>path</tt> into
	* tokens and recursively traverses the path from this node to the
	* named node, "consuming" a name and a slash from <tt>path</tt> at
	* each step of the traversal. At each step, the current node is locked
	* and the node's child-cache is checked for the named node. If it is
	* not found, the name is checked to make sure its length does not
	* exceed <tt>MAX_NAME_LENGTH</tt>. Then the {@link #childSpi(String)}
	* method is invoked, and the result stored in this node's child-cache.
	* If the newly created <tt>Preferences</tt> object's {@link #newNode}
	* field is <tt>true</tt> and there are any node change listeners,
	* a notification event is enqueued for processing by the event dispatch
	* thread.
	*
	* <p>When there are no more tokens, the last value found in the
	* child-cache or returned by <tt>childSpi</tt> is returned by this
	* method. If during the traversal, two <tt>"/"</tt> tokens occur
	* consecutively, or the final token is <tt>"/"</tt> (rather than a name),
	* an appropriate <tt>IllegalArgumentException</tt> is thrown.
	*
	* <p> If the first character of <tt>path</tt> is <tt>'/'</tt>
	* (indicating an absolute path name) this preference node's
	* lock is dropped prior to breaking <tt>path</tt> into tokens, and
	* this method recursively traverses the path starting from the root
	* (rather than starting from this node). The traversal is otherwise
	* identical to the one described for relative path names. Dropping
	* the lock on this node prior to commencing the traversal at the root
	* node is essential to avoid the possibility of deadlock, as per the
	* {@link #lock locking invariant}.
	*
	* @param path the path name of the preference node to return.
	* @return the specified preference node.
	* @throws IllegalArgumentException if the path name is invalid (i.e.,
	* it contains multiple consecutive slash characters, or ends
	* with a slash character and is more than one character long).
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	public Preferences node(String path) {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");
	if (path.equals(""))
	return this;
	if (path.equals("/"))
	return root;
	if (path.charAt(0) != '/')
	return node(new StringTokenizer(path, "/", true));
	}

	// Absolute path. Note that we've dropped our lock to avoid deadlock
	return root.node(new StringTokenizer(path.substring(1), "/", true));
	}

	/**
	* tokenizer contains <name> {'/' <name>}*
	*/
	private Preferences node(StringTokenizer path) {
	String token = path.nextToken();
	if (token.equals("/")) // Check for consecutive slashes
	throw new IllegalArgumentException("Consecutive slashes in path");
	synchronized(lock) {
	AbstractPreferences child = kidCache.get(token);
	if (child == null) {
	if (token.length() > MAX_NAME_LENGTH)
	throw new IllegalArgumentException(
	"Node name " + token + " too long");
	child = childSpi(token);
	if (child.newNode)
	enqueueNodeAddedEvent(child);
	kidCache.put(token, child);
	}
	if (!path.hasMoreTokens())
	return child;
	path.nextToken(); // Consume slash
	if (!path.hasMoreTokens())
	throw new IllegalArgumentException("Path ends with slash");
	return child.node(path);
	}
	}

	/**
	* Implements the <tt>nodeExists</tt> method as per the specification in
	* {@link Preferences#nodeExists(String)}.
	*
	* <p>This implementation is very similar to {@link #node(String)},
	* except that {@link #getChild(String)} is used instead of {@link
	* #childSpi(String)}.
	*
	* @param path the path name of the node whose existence is to be checked.
	* @return true if the specified node exists.
	* @throws BackingStoreException if this operation cannot be completed
	* due 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 ends
	* with a slash character and is more than one character long).
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method and
	* <tt>pathname</tt> is not the empty string (<tt>""</tt>).
	*/
	public boolean nodeExists(String path)
	throws BackingStoreException
	{
	synchronized(lock) {
	if (path.equals(""))
	return !removed;
	if (removed)
	throw new IllegalStateException("Node has been removed.");
	if (path.equals("/"))
	return true;
	if (path.charAt(0) != '/')
	return nodeExists(new StringTokenizer(path, "/", true));
	}

	// Absolute path. Note that we've dropped our lock to avoid deadlock
	return root.nodeExists(new StringTokenizer(path.substring(1), "/",
	true));
	}

	/**
	* tokenizer contains <name> {'/' <name>}*
	*/
	private boolean nodeExists(StringTokenizer path)
	throws BackingStoreException
	{
	String token = path.nextToken();
	if (token.equals("/")) // Check for consecutive slashes
	throw new IllegalArgumentException("Consecutive slashes in path");
	synchronized(lock) {
	AbstractPreferences child = kidCache.get(token);
	if (child == null)
	child = getChild(token);
	if (child==null)
	return false;
	if (!path.hasMoreTokens())
	return true;
	path.nextToken(); // Consume slash
	if (!path.hasMoreTokens())
	throw new IllegalArgumentException("Path ends with slash");
	return child.nodeExists(path);
	}
	}

	/**

	* Implements the <tt>removeNode()</tt> method as per the specification in
	* {@link Preferences#removeNode()}.
	*
	* <p>This implementation checks to see that this node is the root; if so,
	* it throws an appropriate exception. Then, it locks this node's parent,
	* and calls a recursive helper method that traverses the subtree rooted at
	* this node. The recursive method locks the node on which it was called,
	* checks that it has not already been removed, and then ensures that all
	* of its children are cached: The {@link #childrenNamesSpi()} method is
	* invoked and each returned child name is checked for containment in the
	* child-cache. If a child is not already cached, the {@link
	* #childSpi(String)} method is invoked to create a <tt>Preferences</tt>
	* instance for it, and this instance is put into the child-cache. Then
	* the helper method calls itself recursively on each node contained in its
	* child-cache. Next, it invokes {@link #removeNodeSpi()}, marks itself
	* as removed, and removes itself from its parent's child-cache. Finally,
	* if there are any node change listeners, it enqueues a notification
	* event for processing by the event dispatch thread.
	*
	* <p>Note that the helper method is always invoked with all ancestors up
	* to the "closest non-removed ancestor" locked.
	*
	* @throws IllegalStateException if this node (or an ancestor) has already
	* been removed with the {@link #removeNode()} method.
	* @throws UnsupportedOperationException if this method is invoked on
	* the root node.
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	public void removeNode() throws BackingStoreException {
	if (this==root)
	throw new UnsupportedOperationException("Can't remove the root!");
	synchronized(parent.lock) {
	removeNode2();
	parent.kidCache.remove(name);
	}
	}

	/*
	* Called with locks on all nodes on path from parent of "removal root"
	* to this (including the former but excluding the latter).
	*/
	private void removeNode2() throws BackingStoreException {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node already removed.");

	// Ensure that all children are cached
	String[] kidNames = childrenNamesSpi();
	for (int i=0; i<kidNames.length; i++)
	if (!kidCache.containsKey(kidNames[i]))
	kidCache.put(kidNames[i], childSpi(kidNames[i]));

	// Recursively remove all cached children
	for (Iterator<AbstractPreferences> i = kidCache.values().iterator();
	i.hasNext();) {
	try {
	i.next().removeNode2();
	i.remove();
	} catch (BackingStoreException x) { }
	}

	// Now we have no descendants - it's time to die!
	removeNodeSpi();
	removed = true;
	parent.enqueueNodeRemovedEvent(this);
	}
	}

	/**
	* Implements the <tt>name</tt> method as per the specification in
	* {@link Preferences#name()}.
	*
	* <p>This implementation merely returns the name that was
	* passed to this node's constructor.
	*
	* @return this preference node's name, relative to its parent.
	*/
	public String name() {
	return name;
	}

	/**
	* Implements the <tt>absolutePath</tt> method as per the specification in
	* {@link Preferences#absolutePath()}.
	*
	* <p>This implementation merely returns the absolute path name that
	* was computed at the time that this node was constructed (based on
	* the name that was passed to this node's constructor, and the names
	* that were passed to this node's ancestors' constructors).
	*
	* @return this preference node's absolute path name.
	*/
	public String absolutePath() {
	return absolutePath;
	}

	/**
	* Implements the <tt>isUserNode</tt> method as per the specification in
	* {@link Preferences#isUserNode()}.
	*
	* <p>This implementation compares this node's root node (which is stored
	* in a private field) with the value returned by
	* {@link Preferences#userRoot()}. If the two object references are
	* identical, this method returns true.
	*
	* @return <tt>true</tt> if this preference node is in the user
	* preference tree, <tt>false</tt> if it's in the system
	* preference tree.
	*/
	public boolean isUserNode() {
	return AccessController.doPrivileged(
	new PrivilegedAction<Boolean>() {
	public Boolean run() {
	return root == Preferences.userRoot();
	}
	}).booleanValue();
	}

	public void addPreferenceChangeListener(PreferenceChangeListener pcl) {
	if (pcl==null)
	throw new NullPointerException("Change listener is null.");
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	// Copy-on-write
	PreferenceChangeListener[] old = prefListeners;
	prefListeners = new PreferenceChangeListener[old.length + 1];
	System.arraycopy(old, 0, prefListeners, 0, old.length);
	prefListeners[old.length] = pcl;
	}
	startEventDispatchThreadIfNecessary();
	}

	public void removePreferenceChangeListener(PreferenceChangeListener pcl) {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");
	if ((prefListeners == null) \|\| (prefListeners.length == 0))
	throw new IllegalArgumentException("Listener not registered.");

	// Copy-on-write
	PreferenceChangeListener[] newPl =
	new PreferenceChangeListener[prefListeners.length - 1];
	int i = 0;
	while (i < newPl.length && prefListeners[i] != pcl)
	newPl[i] = prefListeners[i++];

	if (i == newPl.length && prefListeners[i] != pcl)
	throw new IllegalArgumentException("Listener not registered.");
	while (i < newPl.length)
	newPl[i] = prefListeners[++i];
	prefListeners = newPl;
	}
	}

	public void addNodeChangeListener(NodeChangeListener ncl) {
	if (ncl==null)
	throw new NullPointerException("Change listener is null.");
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");

	// Copy-on-write
	if (nodeListeners == null) {
	nodeListeners = new NodeChangeListener[1];
	nodeListeners[0] = ncl;
	} else {
	NodeChangeListener[] old = nodeListeners;
	nodeListeners = new NodeChangeListener[old.length + 1];
	System.arraycopy(old, 0, nodeListeners, 0, old.length);
	nodeListeners[old.length] = ncl;
	}
	}
	startEventDispatchThreadIfNecessary();
	}

	public void removeNodeChangeListener(NodeChangeListener ncl) {
	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed.");
	if ((nodeListeners == null) \|\| (nodeListeners.length == 0))
	throw new IllegalArgumentException("Listener not registered.");

	// Copy-on-write
	int i = 0;
	while (i < nodeListeners.length && nodeListeners[i] != ncl)
	i++;
	if (i == nodeListeners.length)
	throw new IllegalArgumentException("Listener not registered.");
	NodeChangeListener[] newNl =
	new NodeChangeListener[nodeListeners.length - 1];
	if (i != 0)
	System.arraycopy(nodeListeners, 0, newNl, 0, i);
	if (i != newNl.length)
	System.arraycopy(nodeListeners, i + 1,
	newNl, i, newNl.length - i);
	nodeListeners = newNl;
	}
	}

	// "SPI" METHODS

	/**
	* Put the given key-value association into this preference node. It is
	* guaranteed that <tt>key</tt> and <tt>value</tt> are non-null and of
	* legal length. Also, it is guaranteed that this node has not been
	* removed. (The implementor needn't check for any of these things.)
	*
	* <p>This method is invoked with the lock on this node held.
	* @param key the key
	* @param value the value
	*/
	protected abstract void putSpi(String key, String value);

	/**
	* Return the value associated with the specified key at this preference
	* node, or <tt>null</tt> if there is no association for this key, or the
	* association cannot be determined at this time. It is guaranteed that
	* <tt>key</tt> is non-null. Also, it is guaranteed that this node has
	* not been removed. (The implementor needn't check for either of these
	* things.)
	*
	* <p> Generally speaking, this method should not throw an exception
	* under any circumstances. If, however, if it does throw an exception,
	* the exception will be intercepted and treated as a <tt>null</tt>
	* return value.
	*
	* <p>This method is invoked with the lock on this node held.
	*
	* @param key the key
	* @return the value associated with the specified key at this preference
	* node, or <tt>null</tt> if there is no association for this
	* key, or the association cannot be determined at this time.
	*/
	protected abstract String getSpi(String key);

	/**
	* Remove the association (if any) for the specified key at this
	* preference node. It is guaranteed that <tt>key</tt> is non-null.
	* Also, it is guaranteed that this node has not been removed.
	* (The implementor needn't check for either of these things.)
	*
	* <p>This method is invoked with the lock on this node held.
	* @param key the key
	*/
	protected abstract void removeSpi(String key);

	/**
	* Removes this preference node, invalidating it and any preferences that
	* it contains. The named child will have no descendants at the time this
	* invocation is made (i.e., the {@link Preferences#removeNode()} method
	* invokes this method repeatedly in a bottom-up fashion, removing each of
	* a node's descendants before removing the node itself).
	*
	* <p>This method is invoked with the lock held on this node and its
	* parent (and all ancestors that are being removed as a
	* result of a single invocation to {@link Preferences#removeNode()}).
	*
	* <p>The removal of a node needn't become persistent until the
	* <tt>flush</tt> method is invoked on this node (or an ancestor).
	*
	* <p>If this node throws a <tt>BackingStoreException</tt>, the exception
	* will propagate out beyond the enclosing {@link #removeNode()}
	* invocation.
	*
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	protected abstract void removeNodeSpi() throws BackingStoreException;

	/**
	* Returns all of the keys that have an associated value in this
	* preference node. (The returned array will be of size zero if
	* this node has no preferences.) It is guaranteed that this node has not
	* been removed.
	*
	* <p>This method is invoked with the lock on this node held.
	*
	* <p>If this node throws a <tt>BackingStoreException</tt>, the exception
	* will propagate out beyond the enclosing {@link #keys()} invocation.
	*
	* @return an array of the keys that have an associated value in this
	* preference node.
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	protected abstract String[] keysSpi() throws BackingStoreException;

	/**
	* Returns the names of the children of this preference node. (The
	* returned array will be of size zero if this node has no children.)
	* This method need not return the names of any nodes already cached,
	* but may do so without harm.
	*
	* <p>This method is invoked with the lock on this node held.
	*
	* <p>If this node throws a <tt>BackingStoreException</tt>, the exception
	* will propagate out beyond the enclosing {@link #childrenNames()}
	* invocation.
	*
	* @return an array containing the names of the children of this
	* preference node.
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	protected abstract String[] childrenNamesSpi()
	throws BackingStoreException;

	/**
	* Returns the named child if it exists, or <tt>null</tt> if it does not.
	* It is guaranteed that <tt>nodeName</tt> is non-null, non-empty,
	* does not contain the slash character ('/'), and is no longer than
	* {@link #MAX_NAME_LENGTH} characters. Also, it is guaranteed
	* that this node has not been removed. (The implementor needn't check
	* for any of these things if he chooses to override this method.)
	*
	* <p>Finally, it is guaranteed that the named node has not been returned
	* by a previous invocation of this method or {@link #childSpi} after the
	* last time that it was removed. In other words, a cached value will
	* always be used in preference to invoking this method. (The implementor
	* needn't maintain his own cache of previously returned children if he
	* chooses to override this method.)
	*
	* <p>This implementation obtains this preference node's lock, invokes
	* {@link #childrenNames()} to get an array of the names of this node's
	* children, and iterates over the array comparing the name of each child
	* with the specified node name. If a child node has the correct name,
	* the {@link #childSpi(String)} method is invoked and the resulting
	* node is returned. If the iteration completes without finding the
	* specified name, <tt>null</tt> is returned.
	*
	* @param nodeName name of the child to be searched for.
	* @return the named child if it exists, or null if it does not.
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	protected AbstractPreferences getChild(String nodeName)
	throws BackingStoreException {
	synchronized(lock) {
	// assert kidCache.get(nodeName)==null;
	String[] kidNames = childrenNames();
	for (int i=0; i<kidNames.length; i++)
	if (kidNames[i].equals(nodeName))
	return childSpi(kidNames[i]);
	}
	return null;
	}

	/**
	* Returns the named child of this preference node, creating it if it does
	* not already exist. It is guaranteed that <tt>name</tt> is non-null,
	* non-empty, does not contain the slash character ('/'), and is no longer
	* than {@link #MAX_NAME_LENGTH} characters. Also, it is guaranteed that
	* this node has not been removed. (The implementor needn't check for any
	* of these things.)
	*
	* <p>Finally, it is guaranteed that the named node has not been returned
	* by a previous invocation of this method or {@link #getChild(String)}
	* after the last time that it was removed. In other words, a cached
	* value will always be used in preference to invoking this method.
	* Subclasses need not maintain their own cache of previously returned
	* children.
	*
	* <p>The implementer must ensure that the returned node has not been
	* removed. If a like-named child of this node was previously removed, the
	* implementer must return a newly constructed <tt>AbstractPreferences</tt>
	* node; once removed, an <tt>AbstractPreferences</tt> node
	* cannot be "resuscitated."
	*
	* <p>If this method causes a node to be created, this node is not
	* guaranteed to be persistent until the <tt>flush</tt> method is
	* invoked on this node or one of its ancestors (or descendants).
	*
	* <p>This method is invoked with the lock on this node held.
	*
	* @param name The name of the child node to return, relative to
	* this preference node.
	* @return The named child node.
	*/
	protected abstract AbstractPreferences childSpi(String name);

	/**
	* Returns the absolute path name of this preferences node.
	*/
	public String toString() {
	return (this.isUserNode() ? "User" : "System") +
	" Preference Node: " + this.absolutePath();
	}

	/**
	* Implements the <tt>sync</tt> method as per the specification in
	* {@link Preferences#sync()}.
	*
	* <p>This implementation calls a recursive helper method that locks this
	* node, invokes syncSpi() on it, unlocks this node, and recursively
	* invokes this method on each "cached child." A cached child is a child
	* of this node that has been created in this VM and not subsequently
	* removed. In effect, this method does a depth first traversal of the
	* "cached subtree" rooted at this node, calling syncSpi() on each node in
	* the subTree while only that node is locked. Note that syncSpi() is
	* invoked top-down.
	*
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	* @throws IllegalStateException if this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	* @see #flush()
	*/
	public void sync() throws BackingStoreException {
	sync2();
	}

	private void sync2() throws BackingStoreException {
	AbstractPreferences[] cachedKids;

	synchronized(lock) {
	if (removed)
	throw new IllegalStateException("Node has been removed");
	syncSpi();
	cachedKids = cachedChildren();
	}

	for (int i=0; i<cachedKids.length; i++)
	cachedKids[i].sync2();
	}

	/**
	* This method is invoked with this node locked. The contract of this
	* method is to synchronize any cached preferences stored at this node
	* with any stored in the backing store. (It is perfectly possible that
	* this node does not exist on the backing store, either because it has
	* been deleted by another VM, or because it has not yet been created.)
	* Note that this method should <i>not</i> synchronize the preferences in
	* any subnodes of this node. If the backing store naturally syncs an
	* entire subtree at once, the implementer is encouraged to override
	* sync(), rather than merely overriding this method.
	*
	* <p>If this node throws a <tt>BackingStoreException</tt>, the exception
	* will propagate out beyond the enclosing {@link #sync()} invocation.
	*
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	protected abstract void syncSpi() throws BackingStoreException;

	/**
	* Implements the <tt>flush</tt> method as per the specification in
	* {@link Preferences#flush()}.
	*
	* <p>This implementation calls a recursive helper method that locks this
	* node, invokes flushSpi() on it, unlocks this node, and recursively
	* invokes this method on each "cached child." A cached child is a child
	* of this node that has been created in this VM and not subsequently
	* removed. In effect, this method does a depth first traversal of the
	* "cached subtree" rooted at this node, calling flushSpi() on each node in
	* the subTree while only that node is locked. Note that flushSpi() is
	* invoked top-down.
	*
	* <p> If this method is invoked on a node that has been removed with
	* the {@link #removeNode()} method, flushSpi() is invoked on this node,
	* but not on others.
	*
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	* @see #flush()
	*/
	public void flush() throws BackingStoreException {
	flush2();
	}

	private void flush2() throws BackingStoreException {
	AbstractPreferences[] cachedKids;

	synchronized(lock) {
	flushSpi();
	if(removed)
	return;
	cachedKids = cachedChildren();
	}

	for (int i = 0; i < cachedKids.length; i++)
	cachedKids[i].flush2();
	}

	/**
	* This method is invoked with this node locked. The contract of this
	* method is to force any cached changes in the contents of this
	* preference node to the backing store, guaranteeing their persistence.
	* (It is perfectly possible that this node does not exist on the backing
	* store, either because it has been deleted by another VM, or because it
	* has not yet been created.) Note that this method should <i>not</i>
	* flush the preferences in any subnodes of this node. If the backing
	* store naturally flushes an entire subtree at once, the implementer is
	* encouraged to override flush(), rather than merely overriding this
	* method.
	*
	* <p>If this node throws a <tt>BackingStoreException</tt>, the exception
	* will propagate out beyond the enclosing {@link #flush()} invocation.
	*
	* @throws BackingStoreException if this operation cannot be completed
	* due to a failure in the backing store, or inability to
	* communicate with it.
	*/
	protected abstract void flushSpi() throws BackingStoreException;

	/**
	* Returns <tt>true</tt> iff this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method. This method
	* locks this node prior to returning the contents of the private
	* field used to track this state.
	*
	* @return <tt>true</tt> iff this node (or an ancestor) has been
	* removed with the {@link #removeNode()} method.
	*/
	protected boolean isRemoved() {
	synchronized(lock) {
	return removed;
	}
	}

	/**
	* Queue of pending notification events. When a preference or node
	* change event for which there are one or more listeners occurs,
	* it is placed on this queue and the queue is notified. A background
	* thread waits on this queue and delivers the events. This decouples
	* event delivery from preference activity, greatly simplifying
	* locking and reducing opportunity for deadlock.
	*/
	private static final List<EventObject> eventQueue = new LinkedList<>();

	/**
	* These two classes are used to distinguish NodeChangeEvents on
	* eventQueue so the event dispatch thread knows whether to call
	* childAdded or childRemoved.
	*/
	private class NodeAddedEvent extends NodeChangeEvent {
	private static final long serialVersionUID = -6743557530157328528L;
	NodeAddedEvent(Preferences parent, Preferences child) {
	super(parent, child);
	}
	}
	private class NodeRemovedEvent extends NodeChangeEvent {
	private static final long serialVersionUID = 8735497392918824837L;
	NodeRemovedEvent(Preferences parent, Preferences child) {
	super(parent, child);
	}
	}

	/**
	* A single background thread ("the event notification thread") monitors
	* the event queue and delivers events that are placed on the queue.
	*/
	private static class EventDispatchThread extends Thread {
	public void run() {
	while(true) {
	// Wait on eventQueue till an event is present
	EventObject event = null;
	synchronized(eventQueue) {
	try {
	while (eventQueue.isEmpty())
	eventQueue.wait();
	event = eventQueue.remove(0);
	} catch (InterruptedException e) {
	// XXX Log "Event dispatch thread interrupted. Exiting"
	return;
	}
	}

	// Now we have event & hold no locks; deliver evt to listeners
	AbstractPreferences src=(AbstractPreferences)event.getSource();
	if (event instanceof PreferenceChangeEvent) {
	PreferenceChangeEvent pce = (PreferenceChangeEvent)event;
	PreferenceChangeListener[] listeners = src.prefListeners();
	for (int i=0; i<listeners.length; i++)
	listeners[i].preferenceChange(pce);
	} else {
	NodeChangeEvent nce = (NodeChangeEvent)event;
	NodeChangeListener[] listeners = src.nodeListeners();
	if (nce instanceof NodeAddedEvent) {
	for (int i=0; i<listeners.length; i++)
	listeners[i].childAdded(nce);
	} else {
	// assert nce instanceof NodeRemovedEvent;
	for (int i=0; i<listeners.length; i++)
	listeners[i].childRemoved(nce);
	}
	}
	}
	}
	}

	private static Thread eventDispatchThread = null;

	/**
	* This method starts the event dispatch thread the first time it
	* is called. The event dispatch thread will be started only
	* if someone registers a listener.
	*/
	private static synchronized void startEventDispatchThreadIfNecessary() {
	if (eventDispatchThread == null) {
	// XXX Log "Starting event dispatch thread"
	eventDispatchThread = new EventDispatchThread();
	eventDispatchThread.setDaemon(true);
	eventDispatchThread.start();
	}
	}

	/**
	* Return this node's preference/node change listeners. Even though
	* we're using a copy-on-write lists, we use synchronized accessors to
	* ensure information transmission from the writing thread to the
	* reading thread.
	*/
	PreferenceChangeListener[] prefListeners() {
	synchronized(lock) {
	return prefListeners;
	}
	}
	NodeChangeListener[] nodeListeners() {
	synchronized(lock) {
	return nodeListeners;
	}
	}

	/**
	* Enqueue a preference change event for delivery to registered
	* preference change listeners unless there are no registered
	* listeners. Invoked with this.lock held.
	*/
	private void enqueuePreferenceChangeEvent(String key, String newValue) {
	if (prefListeners.length != 0) {
	synchronized(eventQueue) {
	eventQueue.add(new PreferenceChangeEvent(this, key, newValue));
	eventQueue.notify();
	}
	}
	}

	/**
	* Enqueue a "node added" event for delivery to registered node change
	* listeners unless there are no registered listeners. Invoked with
	* this.lock held.
	*/
	private void enqueueNodeAddedEvent(Preferences child) {
	if (nodeListeners.length != 0) {
	synchronized(eventQueue) {
	eventQueue.add(new NodeAddedEvent(this, child));
	eventQueue.notify();
	}
	}
	}

	/**
	* Enqueue a "node removed" event for delivery to registered node change
	* listeners unless there are no registered listeners. Invoked with
	* this.lock held.
	*/
	private void enqueueNodeRemovedEvent(Preferences child) {
	if (nodeListeners.length != 0) {
	synchronized(eventQueue) {
	eventQueue.add(new NodeRemovedEvent(this, child));
	eventQueue.notify();
	}
	}
	}

	/**
	* Implements the <tt>exportNode</tt> method as per the specification in
	* {@link Preferences#exportNode(OutputStream)}.
	*
	* @param os the output stream on which to emit the XML document.
	* @throws IOException if writing to the specified output stream
	* results in an <tt>IOException</tt>.
	* @throws BackingStoreException if preference data cannot be read from
	* backing store.
	*/
	public void exportNode(OutputStream os)
	throws IOException, BackingStoreException
	{
	XmlSupport.export(os, this, false);
	}

	/**
	* Implements the <tt>exportSubtree</tt> method as per the specification in
	* {@link Preferences#exportSubtree(OutputStream)}.
	*
	* @param os the output stream on which to emit the XML document.
	* @throws IOException if writing to the specified output stream
	* results in an <tt>IOException</tt>.
	* @throws BackingStoreException if preference data cannot be read from
	* backing store.
	*/
	public void exportSubtree(OutputStream os)
	throws IOException, BackingStoreException
	{
	XmlSupport.export(os, this, true);
	}
	}

Back to index...