View Javadoc
1   /*
2    * Copyright (c) 2000, 2013, Oracle and/or its affiliates. All rights reserved.
3    * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4    *
5    * This code is free software; you can redistribute it and/or modify it
6    * under the terms of the GNU General Public License version 2 only, as
7    * published by the Free Software Foundation.  Oracle designates this
8    * particular file as subject to the "Classpath" exception as provided
9    * by Oracle in the LICENSE file that accompanied this code.
10   *
11   * This code is distributed in the hope that it will be useful, but WITHOUT
12   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
14   * version 2 for more details (a copy is included in the LICENSE file that
15   * accompanied this code).
16   *
17   * You should have received a copy of the GNU General Public License version
18   * 2 along with this work; if not, write to the Free Software Foundation,
19   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20   *
21   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22   * or visit www.oracle.com if you need additional information or have any
23   * questions.
24   */
25  
26  package java.util.prefs;
27  
28  import java.util.*;
29  import java.io.*;
30  import java.security.AccessController;
31  import java.security.PrivilegedAction;
32  // These imports needed only as a workaround for a JavaDoc bug
33  import java.lang.Integer;
34  import java.lang.Long;
35  import java.lang.Float;
36  import java.lang.Double;
37  
38  /**
39   * This class provides a skeletal implementation of the {@link Preferences}
40   * class, greatly easing the task of implementing it.
41   *
42   * <p><strong>This class is for <tt>Preferences</tt> implementers only.
43   * Normal users of the <tt>Preferences</tt> facility should have no need to
44   * consult this documentation.  The {@link Preferences} documentation
45   * should suffice.</strong>
46   *
47   * <p>Implementors must override the nine abstract service-provider interface
48   * (SPI) methods: {@link #getSpi(String)}, {@link #putSpi(String,String)},
49   * {@link #removeSpi(String)}, {@link #childSpi(String)}, {@link
50   * #removeNodeSpi()}, {@link #keysSpi()}, {@link #childrenNamesSpi()}, {@link
51   * #syncSpi()} and {@link #flushSpi()}.  All of the concrete methods specify
52   * precisely how they are implemented atop these SPI methods.  The implementor
53   * may, at his discretion, override one or more of the concrete methods if the
54   * default implementation is unsatisfactory for any reason, such as
55   * performance.
56   *
57   * <p>The SPI methods fall into three groups concerning exception
58   * behavior. The <tt>getSpi</tt> method should never throw exceptions, but it
59   * doesn't really matter, as any exception thrown by this method will be
60   * intercepted by {@link #get(String,String)}, which will return the specified
61   * default value to the caller.  The <tt>removeNodeSpi, keysSpi,
62   * childrenNamesSpi, syncSpi</tt> and <tt>flushSpi</tt> methods are specified
63   * to throw {@link BackingStoreException}, and the implementation is required
64   * to throw this checked exception if it is unable to perform the operation.
65   * The exception propagates outward, causing the corresponding API method
66   * to fail.
67   *
68   * <p>The remaining SPI methods {@link #putSpi(String,String)}, {@link
69   * #removeSpi(String)} and {@link #childSpi(String)} have more complicated
70   * exception behavior.  They are not specified to throw
71   * <tt>BackingStoreException</tt>, as they can generally obey their contracts
72   * even if the backing store is unavailable.  This is true because they return
73   * no information and their effects are not required to become permanent until
74   * a subsequent call to {@link Preferences#flush()} or
75   * {@link Preferences#sync()}. Generally speaking, these SPI methods should not
76   * throw exceptions.  In some implementations, there may be circumstances
77   * under which these calls cannot even enqueue the requested operation for
78   * later processing.  Even under these circumstances it is generally better to
79   * simply ignore the invocation and return, rather than throwing an
80   * exception.  Under these circumstances, however, all subsequent invocations
81   * of <tt>flush()</tt> and <tt>sync</tt> should return <tt>false</tt>, as
82   * returning <tt>true</tt> would imply that all previous operations had
83   * successfully been made permanent.
84   *
85   * <p>There is one circumstance under which <tt>putSpi, removeSpi and
86   * childSpi</tt> <i>should</i> throw an exception: if the caller lacks
87   * sufficient privileges on the underlying operating system to perform the
88   * requested operation.  This will, for instance, occur on most systems
89   * if a non-privileged user attempts to modify system preferences.
90   * (The required privileges will vary from implementation to
91   * implementation.  On some implementations, they are the right to modify the
92   * contents of some directory in the file system; on others they are the right
93   * to modify contents of some key in a registry.)  Under any of these
94   * circumstances, it would generally be undesirable to let the program
95   * continue executing as if these operations would become permanent at a later
96   * time.  While implementations are not required to throw an exception under
97   * these circumstances, they are encouraged to do so.  A {@link
98   * SecurityException} would be appropriate.
99   *
100  * <p>Most of the SPI methods require the implementation to read or write
101  * information at a preferences node.  The implementor should beware of the
102  * fact that another VM may have concurrently deleted this node from the
103  * backing store.  It is the implementation's responsibility to recreate the
104  * node if it has been deleted.
105  *
106  * <p>Implementation note: In Sun's default <tt>Preferences</tt>
107  * implementations, the user's identity is inherited from the underlying
108  * operating system and does not change for the lifetime of the virtual
109  * machine.  It is recognized that server-side <tt>Preferences</tt>
110  * implementations may have the user identity change from request to request,
111  * implicitly passed to <tt>Preferences</tt> methods via the use of a
112  * static {@link ThreadLocal} instance.  Authors of such implementations are
113  * <i>strongly</i> encouraged to determine the user at the time preferences
114  * are accessed (for example by the {@link #get(String,String)} or {@link
115  * #put(String,String)} method) rather than permanently associating a user
116  * with each <tt>Preferences</tt> instance.  The latter behavior conflicts
117  * with normal <tt>Preferences</tt> usage and would lead to great confusion.
118  *
119  * @author  Josh Bloch
120  * @see     Preferences
121  * @since   1.4
122  */
123 public abstract class AbstractPreferences extends Preferences {
124     /**
125      * Our name relative to parent.
126      */
127     private final String name;
128 
129     /**
130      * Our absolute path name.
131      */
132     private final String absolutePath;
133 
134     /**
135      * Our parent node.
136      */
137     final AbstractPreferences parent;
138 
139     /**
140      * Our root node.
141      */
142     private final AbstractPreferences root; // Relative to this node
143 
144     /**
145      * This field should be <tt>true</tt> if this node did not exist in the
146      * backing store prior to the creation of this object.  The field
147      * is initialized to false, but may be set to true by a subclass
148      * constructor (and should not be modified thereafter).  This field
149      * indicates whether a node change event should be fired when
150      * creation is complete.
151      */
152     protected boolean newNode = false;
153 
154     /**
155      * All known unremoved children of this node.  (This "cache" is consulted
156      * prior to calling childSpi() or getChild().
157      */
158     private Map<String, AbstractPreferences> kidCache = new HashMap<>();
159 
160     /**
161      * This field is used to keep track of whether or not this node has
162      * been removed.  Once it's set to true, it will never be reset to false.
163      */
164     private boolean removed = false;
165 
166     /**
167      * Registered preference change listeners.
168      */
169     private PreferenceChangeListener[] prefListeners =
170         new PreferenceChangeListener[0];
171 
172     /**
173      * Registered node change listeners.
174      */
175     private NodeChangeListener[] nodeListeners = new NodeChangeListener[0];
176 
177     /**
178      * An object whose monitor is used to lock this node.  This object
179      * is used in preference to the node itself to reduce the likelihood of
180      * intentional or unintentional denial of service due to a locked node.
181      * To avoid deadlock, a node is <i>never</i> locked by a thread that
182      * holds a lock on a descendant of that node.
183      */
184     protected final Object lock = new Object();
185 
186     /**
187      * Creates a preference node with the specified parent and the specified
188      * name relative to its parent.
189      *
190      * @param parent the parent of this preference node, or null if this
191      *               is the root.
192      * @param name the name of this preference node, relative to its parent,
193      *             or <tt>""</tt> if this is the root.
194      * @throws IllegalArgumentException if <tt>name</tt> contains a slash
195      *          (<tt>'/'</tt>),  or <tt>parent</tt> is <tt>null</tt> and
196      *          name isn't <tt>""</tt>.
197      */
198     protected AbstractPreferences(AbstractPreferences parent, String name) {
199         if (parent==null) {
200             if (!name.equals(""))
201                 throw new IllegalArgumentException("Root name '"+name+
202                                                    "' must be \"\"");
203             this.absolutePath = "/";
204             root = this;
205         } else {
206             if (name.indexOf('/') != -1)
207                 throw new IllegalArgumentException("Name '" + name +
208                                                  "' contains '/'");
209             if (name.equals(""))
210               throw new IllegalArgumentException("Illegal name: empty string");
211 
212             root = parent.root;
213             absolutePath = (parent==root ? "/" + name
214                                          : parent.absolutePath() + "/" + name);
215         }
216         this.name = name;
217         this.parent = parent;
218     }
219 
220     /**
221      * Implements the <tt>put</tt> method as per the specification in
222      * {@link Preferences#put(String,String)}.
223      *
224      * <p>This implementation checks that the key and value are legal,
225      * obtains this preference node's lock, checks that the node
226      * has not been removed, invokes {@link #putSpi(String,String)}, and if
227      * there are any preference change listeners, enqueues a notification
228      * event for processing by the event dispatch thread.
229      *
230      * @param key key with which the specified value is to be associated.
231      * @param value value to be associated with the specified key.
232      * @throws NullPointerException if key or value is <tt>null</tt>.
233      * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
234      *       <tt>MAX_KEY_LENGTH</tt> or if <tt>value.length</tt> exceeds
235      *       <tt>MAX_VALUE_LENGTH</tt>.
236      * @throws IllegalStateException if this node (or an ancestor) has been
237      *         removed with the {@link #removeNode()} method.
238      */
239     public void put(String key, String value) {
240         if (key==null || value==null)
241             throw new NullPointerException();
242         if (key.length() > MAX_KEY_LENGTH)
243             throw new IllegalArgumentException("Key too long: "+key);
244         if (value.length() > MAX_VALUE_LENGTH)
245             throw new IllegalArgumentException("Value too long: "+value);
246 
247         synchronized(lock) {
248             if (removed)
249                 throw new IllegalStateException("Node has been removed.");
250 
251             putSpi(key, value);
252             enqueuePreferenceChangeEvent(key, value);
253         }
254     }
255 
256     /**
257      * Implements the <tt>get</tt> method as per the specification in
258      * {@link Preferences#get(String,String)}.
259      *
260      * <p>This implementation first checks to see if <tt>key</tt> is
261      * <tt>null</tt> throwing a <tt>NullPointerException</tt> if this is
262      * the case.  Then it obtains this preference node's lock,
263      * checks that the node has not been removed, invokes {@link
264      * #getSpi(String)}, and returns the result, unless the <tt>getSpi</tt>
265      * invocation returns <tt>null</tt> or throws an exception, in which case
266      * this invocation returns <tt>def</tt>.
267      *
268      * @param key key whose associated value is to be returned.
269      * @param def the value to be returned in the event that this
270      *        preference node has no value associated with <tt>key</tt>.
271      * @return the value associated with <tt>key</tt>, or <tt>def</tt>
272      *         if no value is associated with <tt>key</tt>.
273      * @throws IllegalStateException if this node (or an ancestor) has been
274      *         removed with the {@link #removeNode()} method.
275      * @throws NullPointerException if key is <tt>null</tt>.  (A
276      *         <tt>null</tt> default <i>is</i> permitted.)
277      */
278     public String get(String key, String def) {
279         if (key==null)
280             throw new NullPointerException("Null key");
281         synchronized(lock) {
282             if (removed)
283                 throw new IllegalStateException("Node has been removed.");
284 
285             String result = null;
286             try {
287                 result = getSpi(key);
288             } catch (Exception e) {
289                 // Ignoring exception causes default to be returned
290             }
291             return (result==null ? def : result);
292         }
293     }
294 
295     /**
296      * Implements the <tt>remove(String)</tt> method as per the specification
297      * in {@link Preferences#remove(String)}.
298      *
299      * <p>This implementation obtains this preference node's lock,
300      * checks that the node has not been removed, invokes
301      * {@link #removeSpi(String)} and if there are any preference
302      * change listeners, enqueues a notification event for processing by the
303      * event dispatch thread.
304      *
305      * @param key key whose mapping is to be removed from the preference node.
306      * @throws IllegalStateException if this node (or an ancestor) has been
307      *         removed with the {@link #removeNode()} method.
308      * @throws NullPointerException {@inheritDoc}.
309      */
310     public void remove(String key) {
311         Objects.requireNonNull(key, "Specified key cannot be null");
312         synchronized(lock) {
313             if (removed)
314                 throw new IllegalStateException("Node has been removed.");
315 
316             removeSpi(key);
317             enqueuePreferenceChangeEvent(key, null);
318         }
319     }
320 
321     /**
322      * Implements the <tt>clear</tt> method as per the specification in
323      * {@link Preferences#clear()}.
324      *
325      * <p>This implementation obtains this preference node's lock,
326      * invokes {@link #keys()} to obtain an array of keys, and
327      * iterates over the array invoking {@link #remove(String)} on each key.
328      *
329      * @throws BackingStoreException if this operation cannot be completed
330      *         due to a failure in the backing store, or inability to
331      *         communicate with it.
332      * @throws IllegalStateException if this node (or an ancestor) has been
333      *         removed with the {@link #removeNode()} method.
334      */
335     public void clear() throws BackingStoreException {
336         synchronized(lock) {
337             String[] keys = keys();
338             for (int i=0; i<keys.length; i++)
339                 remove(keys[i]);
340         }
341     }
342 
343     /**
344      * Implements the <tt>putInt</tt> method as per the specification in
345      * {@link Preferences#putInt(String,int)}.
346      *
347      * <p>This implementation translates <tt>value</tt> to a string with
348      * {@link Integer#toString(int)} and invokes {@link #put(String,String)}
349      * on the result.
350      *
351      * @param key key with which the string form of value is to be associated.
352      * @param value value whose string form is to be associated with key.
353      * @throws NullPointerException if key is <tt>null</tt>.
354      * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
355      *         <tt>MAX_KEY_LENGTH</tt>.
356      * @throws IllegalStateException if this node (or an ancestor) has been
357      *         removed with the {@link #removeNode()} method.
358      */
359     public void putInt(String key, int value) {
360         put(key, Integer.toString(value));
361     }
362 
363     /**
364      * Implements the <tt>getInt</tt> method as per the specification in
365      * {@link Preferences#getInt(String,int)}.
366      *
367      * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
368      * null)</tt>}.  If the return value is non-null, the implementation
369      * attempts to translate it to an <tt>int</tt> with
370      * {@link Integer#parseInt(String)}.  If the attempt succeeds, the return
371      * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
372      *
373      * @param key key whose associated value is to be returned as an int.
374      * @param def the value to be returned in the event that this
375      *        preference node has no value associated with <tt>key</tt>
376      *        or the associated value cannot be interpreted as an int.
377      * @return the int value represented by the string associated with
378      *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
379      *         associated value does not exist or cannot be interpreted as
380      *         an int.
381      * @throws IllegalStateException if this node (or an ancestor) has been
382      *         removed with the {@link #removeNode()} method.
383      * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
384      */
385     public int getInt(String key, int def) {
386         int result = def;
387         try {
388             String value = get(key, null);
389             if (value != null)
390                 result = Integer.parseInt(value);
391         } catch (NumberFormatException e) {
392             // Ignoring exception causes specified default to be returned
393         }
394 
395         return result;
396     }
397 
398     /**
399      * Implements the <tt>putLong</tt> method as per the specification in
400      * {@link Preferences#putLong(String,long)}.
401      *
402      * <p>This implementation translates <tt>value</tt> to a string with
403      * {@link Long#toString(long)} and invokes {@link #put(String,String)}
404      * on the result.
405      *
406      * @param key key with which the string form of value is to be associated.
407      * @param value value whose string form is to be associated with key.
408      * @throws NullPointerException if key is <tt>null</tt>.
409      * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
410      *         <tt>MAX_KEY_LENGTH</tt>.
411      * @throws IllegalStateException if this node (or an ancestor) has been
412      *         removed with the {@link #removeNode()} method.
413      */
414     public void putLong(String key, long value) {
415         put(key, Long.toString(value));
416     }
417 
418     /**
419      * Implements the <tt>getLong</tt> method as per the specification in
420      * {@link Preferences#getLong(String,long)}.
421      *
422      * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
423      * null)</tt>}.  If the return value is non-null, the implementation
424      * attempts to translate it to a <tt>long</tt> with
425      * {@link Long#parseLong(String)}.  If the attempt succeeds, the return
426      * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
427      *
428      * @param key key whose associated value is to be returned as a long.
429      * @param def the value to be returned in the event that this
430      *        preference node has no value associated with <tt>key</tt>
431      *        or the associated value cannot be interpreted as a long.
432      * @return the long value represented by the string associated with
433      *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
434      *         associated value does not exist or cannot be interpreted as
435      *         a long.
436      * @throws IllegalStateException if this node (or an ancestor) has been
437      *         removed with the {@link #removeNode()} method.
438      * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
439      */
440     public long getLong(String key, long def) {
441         long result = def;
442         try {
443             String value = get(key, null);
444             if (value != null)
445                 result = Long.parseLong(value);
446         } catch (NumberFormatException e) {
447             // Ignoring exception causes specified default to be returned
448         }
449 
450         return result;
451     }
452 
453     /**
454      * Implements the <tt>putBoolean</tt> method as per the specification in
455      * {@link Preferences#putBoolean(String,boolean)}.
456      *
457      * <p>This implementation translates <tt>value</tt> to a string with
458      * {@link String#valueOf(boolean)} and invokes {@link #put(String,String)}
459      * on the result.
460      *
461      * @param key key with which the string form of value is to be associated.
462      * @param value value whose string form is to be associated with key.
463      * @throws NullPointerException if key is <tt>null</tt>.
464      * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
465      *         <tt>MAX_KEY_LENGTH</tt>.
466      * @throws IllegalStateException if this node (or an ancestor) has been
467      *         removed with the {@link #removeNode()} method.
468      */
469     public void putBoolean(String key, boolean value) {
470         put(key, String.valueOf(value));
471     }
472 
473     /**
474      * Implements the <tt>getBoolean</tt> method as per the specification in
475      * {@link Preferences#getBoolean(String,boolean)}.
476      *
477      * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
478      * null)</tt>}.  If the return value is non-null, it is compared with
479      * <tt>"true"</tt> using {@link String#equalsIgnoreCase(String)}.  If the
480      * comparison returns <tt>true</tt>, this invocation returns
481      * <tt>true</tt>.  Otherwise, the original return value is compared with
482      * <tt>"false"</tt>, again using {@link String#equalsIgnoreCase(String)}.
483      * If the comparison returns <tt>true</tt>, this invocation returns
484      * <tt>false</tt>.  Otherwise, this invocation returns <tt>def</tt>.
485      *
486      * @param key key whose associated value is to be returned as a boolean.
487      * @param def the value to be returned in the event that this
488      *        preference node has no value associated with <tt>key</tt>
489      *        or the associated value cannot be interpreted as a boolean.
490      * @return the boolean value represented by the string associated with
491      *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
492      *         associated value does not exist or cannot be interpreted as
493      *         a boolean.
494      * @throws IllegalStateException if this node (or an ancestor) has been
495      *         removed with the {@link #removeNode()} method.
496      * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
497      */
498     public boolean getBoolean(String key, boolean def) {
499         boolean result = def;
500         String value = get(key, null);
501         if (value != null) {
502             if (value.equalsIgnoreCase("true"))
503                 result = true;
504             else if (value.equalsIgnoreCase("false"))
505                 result = false;
506         }
507 
508         return result;
509     }
510 
511     /**
512      * Implements the <tt>putFloat</tt> method as per the specification in
513      * {@link Preferences#putFloat(String,float)}.
514      *
515      * <p>This implementation translates <tt>value</tt> to a string with
516      * {@link Float#toString(float)} and invokes {@link #put(String,String)}
517      * on the result.
518      *
519      * @param key key with which the string form of value is to be associated.
520      * @param value value whose string form is to be associated with key.
521      * @throws NullPointerException if key is <tt>null</tt>.
522      * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
523      *         <tt>MAX_KEY_LENGTH</tt>.
524      * @throws IllegalStateException if this node (or an ancestor) has been
525      *         removed with the {@link #removeNode()} method.
526      */
527     public void putFloat(String key, float value) {
528         put(key, Float.toString(value));
529     }
530 
531     /**
532      * Implements the <tt>getFloat</tt> method as per the specification in
533      * {@link Preferences#getFloat(String,float)}.
534      *
535      * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
536      * null)</tt>}.  If the return value is non-null, the implementation
537      * attempts to translate it to an <tt>float</tt> with
538      * {@link Float#parseFloat(String)}.  If the attempt succeeds, the return
539      * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
540      *
541      * @param key key whose associated value is to be returned as a float.
542      * @param def the value to be returned in the event that this
543      *        preference node has no value associated with <tt>key</tt>
544      *        or the associated value cannot be interpreted as a float.
545      * @return the float value represented by the string associated with
546      *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
547      *         associated value does not exist or cannot be interpreted as
548      *         a float.
549      * @throws IllegalStateException if this node (or an ancestor) has been
550      *         removed with the {@link #removeNode()} method.
551      * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
552      */
553     public float getFloat(String key, float def) {
554         float result = def;
555         try {
556             String value = get(key, null);
557             if (value != null)
558                 result = Float.parseFloat(value);
559         } catch (NumberFormatException e) {
560             // Ignoring exception causes specified default to be returned
561         }
562 
563         return result;
564     }
565 
566     /**
567      * Implements the <tt>putDouble</tt> method as per the specification in
568      * {@link Preferences#putDouble(String,double)}.
569      *
570      * <p>This implementation translates <tt>value</tt> to a string with
571      * {@link Double#toString(double)} and invokes {@link #put(String,String)}
572      * on the result.
573      *
574      * @param key key with which the string form of value is to be associated.
575      * @param value value whose string form is to be associated with key.
576      * @throws NullPointerException if key is <tt>null</tt>.
577      * @throws IllegalArgumentException if <tt>key.length()</tt> exceeds
578      *         <tt>MAX_KEY_LENGTH</tt>.
579      * @throws IllegalStateException if this node (or an ancestor) has been
580      *         removed with the {@link #removeNode()} method.
581      */
582     public void putDouble(String key, double value) {
583         put(key, Double.toString(value));
584     }
585 
586     /**
587      * Implements the <tt>getDouble</tt> method as per the specification in
588      * {@link Preferences#getDouble(String,double)}.
589      *
590      * <p>This implementation invokes {@link #get(String,String) <tt>get(key,
591      * null)</tt>}.  If the return value is non-null, the implementation
592      * attempts to translate it to an <tt>double</tt> with
593      * {@link Double#parseDouble(String)}.  If the attempt succeeds, the return
594      * value is returned by this method.  Otherwise, <tt>def</tt> is returned.
595      *
596      * @param key key whose associated value is to be returned as a double.
597      * @param def the value to be returned in the event that this
598      *        preference node has no value associated with <tt>key</tt>
599      *        or the associated value cannot be interpreted as a double.
600      * @return the double value represented by the string associated with
601      *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
602      *         associated value does not exist or cannot be interpreted as
603      *         a double.
604      * @throws IllegalStateException if this node (or an ancestor) has been
605      *         removed with the {@link #removeNode()} method.
606      * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.
607      */
608     public double getDouble(String key, double def) {
609         double result = def;
610         try {
611             String value = get(key, null);
612             if (value != null)
613                 result = Double.parseDouble(value);
614         } catch (NumberFormatException e) {
615             // Ignoring exception causes specified default to be returned
616         }
617 
618         return result;
619     }
620 
621     /**
622      * Implements the <tt>putByteArray</tt> method as per the specification in
623      * {@link Preferences#putByteArray(String,byte[])}.
624      *
625      * @param key key with which the string form of value is to be associated.
626      * @param value value whose string form is to be associated with key.
627      * @throws NullPointerException if key or value is <tt>null</tt>.
628      * @throws IllegalArgumentException if key.length() exceeds MAX_KEY_LENGTH
629      *         or if value.length exceeds MAX_VALUE_LENGTH*3/4.
630      * @throws IllegalStateException if this node (or an ancestor) has been
631      *         removed with the {@link #removeNode()} method.
632      */
633     public void putByteArray(String key, byte[] value) {
634         put(key, Base64.byteArrayToBase64(value));
635     }
636 
637     /**
638      * Implements the <tt>getByteArray</tt> method as per the specification in
639      * {@link Preferences#getByteArray(String,byte[])}.
640      *
641      * @param key key whose associated value is to be returned as a byte array.
642      * @param def the value to be returned in the event that this
643      *        preference node has no value associated with <tt>key</tt>
644      *        or the associated value cannot be interpreted as a byte array.
645      * @return the byte array value represented by the string associated with
646      *         <tt>key</tt> in this preference node, or <tt>def</tt> if the
647      *         associated value does not exist or cannot be interpreted as
648      *         a byte array.
649      * @throws IllegalStateException if this node (or an ancestor) has been
650      *         removed with the {@link #removeNode()} method.
651      * @throws NullPointerException if <tt>key</tt> is <tt>null</tt>.  (A
652      *         <tt>null</tt> value for <tt>def</tt> <i>is</i> permitted.)
653      */
654     public byte[] getByteArray(String key, byte[] def) {
655         byte[] result = def;
656         String value = get(key, null);
657         try {
658             if (value != null)
659                 result = Base64.base64ToByteArray(value);
660         }
661         catch (RuntimeException e) {
662             // Ignoring exception causes specified default to be returned
663         }
664 
665         return result;
666     }
667 
668     /**
669      * Implements the <tt>keys</tt> method as per the specification in
670      * {@link Preferences#keys()}.
671      *
672      * <p>This implementation obtains this preference node's lock, checks that
673      * the node has not been removed and invokes {@link #keysSpi()}.
674      *
675      * @return an array of the keys that have an associated value in this
676      *         preference node.
677      * @throws BackingStoreException if this operation cannot be completed
678      *         due to a failure in the backing store, or inability to
679      *         communicate with it.
680      * @throws IllegalStateException if this node (or an ancestor) has been
681      *         removed with the {@link #removeNode()} method.
682      */
683     public String[] keys() throws BackingStoreException {
684         synchronized(lock) {
685             if (removed)
686                 throw new IllegalStateException("Node has been removed.");
687 
688             return keysSpi();
689         }
690     }
691 
692     /**
693      * Implements the <tt>children</tt> method as per the specification in
694      * {@link Preferences#childrenNames()}.
695      *
696      * <p>This implementation obtains this preference node's lock, checks that
697      * the node has not been removed, constructs a <tt>TreeSet</tt> initialized
698      * to the names of children already cached (the children in this node's
699      * "child-cache"), invokes {@link #childrenNamesSpi()}, and adds all of the
700      * returned child-names into the set.  The elements of the tree set are
701      * dumped into a <tt>String</tt> array using the <tt>toArray</tt> method,
702      * and this array is returned.
703      *
704      * @return the names of the children of this preference node.
705      * @throws BackingStoreException if this operation cannot be completed
706      *         due to a failure in the backing store, or inability to
707      *         communicate with it.
708      * @throws IllegalStateException if this node (or an ancestor) has been
709      *         removed with the {@link #removeNode()} method.
710      * @see #cachedChildren()
711      */
712     public String[] childrenNames() throws BackingStoreException {
713         synchronized(lock) {
714             if (removed)
715                 throw new IllegalStateException("Node has been removed.");
716 
717             Set<String> s = new TreeSet<>(kidCache.keySet());
718             for (String kid : childrenNamesSpi())
719                 s.add(kid);
720             return s.toArray(EMPTY_STRING_ARRAY);
721         }
722     }
723 
724     private static final String[] EMPTY_STRING_ARRAY = new String[0];
725 
726     /**
727      * Returns all known unremoved children of this node.
728      *
729      * @return all known unremoved children of this node.
730      */
731     protected final AbstractPreferences[] cachedChildren() {
732         return kidCache.values().toArray(EMPTY_ABSTRACT_PREFS_ARRAY);
733     }
734 
735     private static final AbstractPreferences[] EMPTY_ABSTRACT_PREFS_ARRAY
736         = new AbstractPreferences[0];
737 
738     /**
739      * Implements the <tt>parent</tt> method as per the specification in
740      * {@link Preferences#parent()}.
741      *
742      * <p>This implementation obtains this preference node's lock, checks that
743      * the node has not been removed and returns the parent value that was
744      * passed to this node's constructor.
745      *
746      * @return the parent of this preference node.
747      * @throws IllegalStateException if this node (or an ancestor) has been
748      *         removed with the {@link #removeNode()} method.
749      */
750     public Preferences parent() {
751         synchronized(lock) {
752             if (removed)
753                 throw new IllegalStateException("Node has been removed.");
754 
755             return parent;
756         }
757     }
758 
759     /**
760      * Implements the <tt>node</tt> method as per the specification in
761      * {@link Preferences#node(String)}.
762      *
763      * <p>This implementation obtains this preference node's lock and checks
764      * that the node has not been removed.  If <tt>path</tt> is <tt>""</tt>,
765      * this node is returned; if <tt>path</tt> is <tt>"/"</tt>, this node's
766      * root is returned.  If the first character in <tt>path</tt> is
767      * not <tt>'/'</tt>, the implementation breaks <tt>path</tt> into
768      * tokens and recursively traverses the path from this node to the
769      * named node, "consuming" a name and a slash from <tt>path</tt> at
770      * each step of the traversal.  At each step, the current node is locked
771      * and the node's child-cache is checked for the named node.  If it is
772      * not found, the name is checked to make sure its length does not
773      * exceed <tt>MAX_NAME_LENGTH</tt>.  Then the {@link #childSpi(String)}
774      * method is invoked, and the result stored in this node's child-cache.
775      * If the newly created <tt>Preferences</tt> object's {@link #newNode}
776      * field is <tt>true</tt> and there are any node change listeners,
777      * a notification event is enqueued for processing by the event dispatch
778      * thread.
779      *
780      * <p>When there are no more tokens, the last value found in the
781      * child-cache or returned by <tt>childSpi</tt> is returned by this
782      * method.  If during the traversal, two <tt>"/"</tt> tokens occur
783      * consecutively, or the final token is <tt>"/"</tt> (rather than a name),
784      * an appropriate <tt>IllegalArgumentException</tt> is thrown.
785      *
786      * <p> If the first character of <tt>path</tt> is <tt>'/'</tt>
787      * (indicating an absolute path name) this preference node's
788      * lock is dropped prior to breaking <tt>path</tt> into tokens, and
789      * this method recursively traverses the path starting from the root
790      * (rather than starting from this node).  The traversal is otherwise
791      * identical to the one described for relative path names.  Dropping
792      * the lock on this node prior to commencing the traversal at the root
793      * node is essential to avoid the possibility of deadlock, as per the
794      * {@link #lock locking invariant}.
795      *
796      * @param path the path name of the preference node to return.
797      * @return the specified preference node.
798      * @throws IllegalArgumentException if the path name is invalid (i.e.,
799      *         it contains multiple consecutive slash characters, or ends
800      *         with a slash character and is more than one character long).
801      * @throws IllegalStateException if this node (or an ancestor) has been
802      *         removed with the {@link #removeNode()} method.
803      */
804     public Preferences node(String path) {
805         synchronized(lock) {
806             if (removed)
807                 throw new IllegalStateException("Node has been removed.");
808             if (path.equals(""))
809                 return this;
810             if (path.equals("/"))
811                 return root;
812             if (path.charAt(0) != '/')
813                 return node(new StringTokenizer(path, "/", true));
814         }
815 
816         // Absolute path.  Note that we've dropped our lock to avoid deadlock
817         return root.node(new StringTokenizer(path.substring(1), "/", true));
818     }
819 
820     /**
821      * tokenizer contains <name> {'/' <name>}*
822      */
823     private Preferences node(StringTokenizer path) {
824         String token = path.nextToken();
825         if (token.equals("/"))  // Check for consecutive slashes
826             throw new IllegalArgumentException("Consecutive slashes in path");
827         synchronized(lock) {
828             AbstractPreferences child = kidCache.get(token);
829             if (child == null) {
830                 if (token.length() > MAX_NAME_LENGTH)
831                     throw new IllegalArgumentException(
832                         "Node name " + token + " too long");
833                 child = childSpi(token);
834                 if (child.newNode)
835                     enqueueNodeAddedEvent(child);
836                 kidCache.put(token, child);
837             }
838             if (!path.hasMoreTokens())
839                 return child;
840             path.nextToken();  // Consume slash
841             if (!path.hasMoreTokens())
842                 throw new IllegalArgumentException("Path ends with slash");
843             return child.node(path);
844         }
845     }
846 
847     /**
848      * Implements the <tt>nodeExists</tt> method as per the specification in
849      * {@link Preferences#nodeExists(String)}.
850      *
851      * <p>This implementation is very similar to {@link #node(String)},
852      * except that {@link #getChild(String)} is used instead of {@link
853      * #childSpi(String)}.
854      *
855      * @param path the path name of the node whose existence is to be checked.
856      * @return true if the specified node exists.
857      * @throws BackingStoreException if this operation cannot be completed
858      *         due to a failure in the backing store, or inability to
859      *         communicate with it.
860      * @throws IllegalArgumentException if the path name is invalid (i.e.,
861      *         it contains multiple consecutive slash characters, or ends
862      *         with a slash character and is more than one character long).
863      * @throws IllegalStateException if this node (or an ancestor) has been
864      *         removed with the {@link #removeNode()} method and
865      *         <tt>pathname</tt> is not the empty string (<tt>""</tt>).
866      */
867     public boolean nodeExists(String path)
868         throws BackingStoreException
869     {
870         synchronized(lock) {
871             if (path.equals(""))
872                 return !removed;
873             if (removed)
874                 throw new IllegalStateException("Node has been removed.");
875             if (path.equals("/"))
876                 return true;
877             if (path.charAt(0) != '/')
878                 return nodeExists(new StringTokenizer(path, "/", true));
879         }
880 
881         // Absolute path.  Note that we've dropped our lock to avoid deadlock
882         return root.nodeExists(new StringTokenizer(path.substring(1), "/",
883                                                    true));
884     }
885 
886     /**
887      * tokenizer contains <name> {'/' <name>}*
888      */
889     private boolean nodeExists(StringTokenizer path)
890         throws BackingStoreException
891     {
892         String token = path.nextToken();
893         if (token.equals("/"))  // Check for consecutive slashes
894             throw new IllegalArgumentException("Consecutive slashes in path");
895         synchronized(lock) {
896             AbstractPreferences child = kidCache.get(token);
897             if (child == null)
898                 child = getChild(token);
899             if (child==null)
900                 return false;
901             if (!path.hasMoreTokens())
902                 return true;
903             path.nextToken();  // Consume slash
904             if (!path.hasMoreTokens())
905                 throw new IllegalArgumentException("Path ends with slash");
906             return child.nodeExists(path);
907         }
908     }
909 
910     /**
911 
912      * Implements the <tt>removeNode()</tt> method as per the specification in
913      * {@link Preferences#removeNode()}.
914      *
915      * <p>This implementation checks to see that this node is the root; if so,
916      * it throws an appropriate exception.  Then, it locks this node's parent,
917      * and calls a recursive helper method that traverses the subtree rooted at
918      * this node.  The recursive method locks the node on which it was called,
919      * checks that it has not already been removed, and then ensures that all
920      * of its children are cached: The {@link #childrenNamesSpi()} method is
921      * invoked and each returned child name is checked for containment in the
922      * child-cache.  If a child is not already cached, the {@link
923      * #childSpi(String)} method is invoked to create a <tt>Preferences</tt>
924      * instance for it, and this instance is put into the child-cache.  Then
925      * the helper method calls itself recursively on each node contained in its
926      * child-cache.  Next, it invokes {@link #removeNodeSpi()}, marks itself
927      * as removed, and removes itself from its parent's child-cache.  Finally,
928      * if there are any node change listeners, it enqueues a notification
929      * event for processing by the event dispatch thread.
930      *
931      * <p>Note that the helper method is always invoked with all ancestors up
932      * to the "closest non-removed ancestor" locked.
933      *
934      * @throws IllegalStateException if this node (or an ancestor) has already
935      *         been removed with the {@link #removeNode()} method.
936      * @throws UnsupportedOperationException if this method is invoked on
937      *         the root node.
938      * @throws BackingStoreException if this operation cannot be completed
939      *         due to a failure in the backing store, or inability to
940      *         communicate with it.
941      */
942     public void removeNode() throws BackingStoreException {
943         if (this==root)
944             throw new UnsupportedOperationException("Can't remove the root!");
945         synchronized(parent.lock) {
946             removeNode2();
947             parent.kidCache.remove(name);
948         }
949     }
950 
951     /*
952      * Called with locks on all nodes on path from parent of "removal root"
953      * to this (including the former but excluding the latter).
954      */
955     private void removeNode2() throws BackingStoreException {
956         synchronized(lock) {
957             if (removed)
958                 throw new IllegalStateException("Node already removed.");
959 
960             // Ensure that all children are cached
961             String[] kidNames = childrenNamesSpi();
962             for (int i=0; i<kidNames.length; i++)
963                 if (!kidCache.containsKey(kidNames[i]))
964                     kidCache.put(kidNames[i], childSpi(kidNames[i]));
965 
966             // Recursively remove all cached children
967             for (Iterator<AbstractPreferences> i = kidCache.values().iterator();
968                  i.hasNext();) {
969                 try {
970                     i.next().removeNode2();
971                     i.remove();
972                 } catch (BackingStoreException x) { }
973             }
974 
975             // Now we have no descendants - it's time to die!
976             removeNodeSpi();
977             removed = true;
978             parent.enqueueNodeRemovedEvent(this);
979         }
980     }
981 
982     /**
983      * Implements the <tt>name</tt> method as per the specification in
984      * {@link Preferences#name()}.
985      *
986      * <p>This implementation merely returns the name that was
987      * passed to this node's constructor.
988      *
989      * @return this preference node's name, relative to its parent.
990      */
991     public String name() {
992         return name;
993     }
994 
995     /**
996      * Implements the <tt>absolutePath</tt> method as per the specification in
997      * {@link Preferences#absolutePath()}.
998      *
999      * <p>This implementation merely returns the absolute path name that
1000      * was computed at the time that this node was constructed (based on
1001      * the name that was passed to this node's constructor, and the names
1002      * that were passed to this node's ancestors' constructors).
1003      *
1004      * @return this preference node's absolute path name.
1005      */
1006     public String absolutePath() {
1007         return absolutePath;
1008     }
1009 
1010     /**
1011      * Implements the <tt>isUserNode</tt> method as per the specification in
1012      * {@link Preferences#isUserNode()}.
1013      *
1014      * <p>This implementation compares this node's root node (which is stored
1015      * in a private field) with the value returned by
1016      * {@link Preferences#userRoot()}.  If the two object references are
1017      * identical, this method returns true.
1018      *
1019      * @return <tt>true</tt> if this preference node is in the user
1020      *         preference tree, <tt>false</tt> if it's in the system
1021      *         preference tree.
1022      */
1023     public boolean isUserNode() {
1024         return AccessController.doPrivileged(
1025             new PrivilegedAction<Boolean>() {
1026                 public Boolean run() {
1027                     return root == Preferences.userRoot();
1028             }
1029             }).booleanValue();
1030     }
1031 
1032     public void addPreferenceChangeListener(PreferenceChangeListener pcl) {
1033         if (pcl==null)
1034             throw new NullPointerException("Change listener is null.");
1035         synchronized(lock) {
1036             if (removed)
1037                 throw new IllegalStateException("Node has been removed.");
1038 
1039             // Copy-on-write
1040             PreferenceChangeListener[] old = prefListeners;
1041             prefListeners = new PreferenceChangeListener[old.length + 1];
1042             System.arraycopy(old, 0, prefListeners, 0, old.length);
1043             prefListeners[old.length] = pcl;
1044         }
1045         startEventDispatchThreadIfNecessary();
1046     }
1047 
1048     public void removePreferenceChangeListener(PreferenceChangeListener pcl) {
1049         synchronized(lock) {
1050             if (removed)
1051                 throw new IllegalStateException("Node has been removed.");
1052             if ((prefListeners == null) || (prefListeners.length == 0))
1053                 throw new IllegalArgumentException("Listener not registered.");
1054 
1055             // Copy-on-write
1056             PreferenceChangeListener[] newPl =
1057                 new PreferenceChangeListener[prefListeners.length - 1];
1058             int i = 0;
1059             while (i < newPl.length && prefListeners[i] != pcl)
1060                 newPl[i] = prefListeners[i++];
1061 
1062             if (i == newPl.length &&  prefListeners[i] != pcl)
1063                 throw new IllegalArgumentException("Listener not registered.");
1064             while (i < newPl.length)
1065                 newPl[i] = prefListeners[++i];
1066             prefListeners = newPl;
1067         }
1068     }
1069 
1070     public void addNodeChangeListener(NodeChangeListener ncl) {
1071         if (ncl==null)
1072             throw new NullPointerException("Change listener is null.");
1073         synchronized(lock) {
1074             if (removed)
1075                 throw new IllegalStateException("Node has been removed.");
1076 
1077             // Copy-on-write
1078             if (nodeListeners == null) {
1079                 nodeListeners = new NodeChangeListener[1];
1080                 nodeListeners[0] = ncl;
1081             } else {
1082                 NodeChangeListener[] old = nodeListeners;
1083                 nodeListeners = new NodeChangeListener[old.length + 1];
1084                 System.arraycopy(old, 0, nodeListeners, 0, old.length);
1085                 nodeListeners[old.length] = ncl;
1086             }
1087         }
1088         startEventDispatchThreadIfNecessary();
1089     }
1090 
1091     public void removeNodeChangeListener(NodeChangeListener ncl) {
1092         synchronized(lock) {
1093             if (removed)
1094                 throw new IllegalStateException("Node has been removed.");
1095             if ((nodeListeners == null) || (nodeListeners.length == 0))
1096                 throw new IllegalArgumentException("Listener not registered.");
1097 
1098             // Copy-on-write
1099             int i = 0;
1100             while (i < nodeListeners.length && nodeListeners[i] != ncl)
1101                 i++;
1102             if (i == nodeListeners.length)
1103                 throw new IllegalArgumentException("Listener not registered.");
1104             NodeChangeListener[] newNl =
1105                 new NodeChangeListener[nodeListeners.length - 1];
1106             if (i != 0)
1107                 System.arraycopy(nodeListeners, 0, newNl, 0, i);
1108             if (i != newNl.length)
1109                 System.arraycopy(nodeListeners, i + 1,
1110                                  newNl, i, newNl.length - i);
1111             nodeListeners = newNl;
1112         }
1113     }
1114 
1115     // "SPI" METHODS
1116 
1117     /**
1118      * Put the given key-value association into this preference node.  It is
1119      * guaranteed that <tt>key</tt> and <tt>value</tt> are non-null and of
1120      * legal length.  Also, it is guaranteed that this node has not been
1121      * removed.  (The implementor needn't check for any of these things.)
1122      *
1123      * <p>This method is invoked with the lock on this node held.
1124      * @param key the key
1125      * @param value the value
1126      */
1127     protected abstract void putSpi(String key, String value);
1128 
1129     /**
1130      * Return the value associated with the specified key at this preference
1131      * node, or <tt>null</tt> if there is no association for this key, or the
1132      * association cannot be determined at this time.  It is guaranteed that
1133      * <tt>key</tt> is non-null.  Also, it is guaranteed that this node has
1134      * not been removed.  (The implementor needn't check for either of these
1135      * things.)
1136      *
1137      * <p> Generally speaking, this method should not throw an exception
1138      * under any circumstances.  If, however, if it does throw an exception,
1139      * the exception will be intercepted and treated as a <tt>null</tt>
1140      * return value.
1141      *
1142      * <p>This method is invoked with the lock on this node held.
1143      *
1144      * @param key the key
1145      * @return the value associated with the specified key at this preference
1146      *          node, or <tt>null</tt> if there is no association for this
1147      *          key, or the association cannot be determined at this time.
1148      */
1149     protected abstract String getSpi(String key);
1150 
1151     /**
1152      * Remove the association (if any) for the specified key at this
1153      * preference node.  It is guaranteed that <tt>key</tt> is non-null.
1154      * Also, it is guaranteed that this node has not been removed.
1155      * (The implementor needn't check for either of these things.)
1156      *
1157      * <p>This method is invoked with the lock on this node held.
1158      * @param key the key
1159      */
1160     protected abstract void removeSpi(String key);
1161 
1162     /**
1163      * Removes this preference node, invalidating it and any preferences that
1164      * it contains.  The named child will have no descendants at the time this
1165      * invocation is made (i.e., the {@link Preferences#removeNode()} method
1166      * invokes this method repeatedly in a bottom-up fashion, removing each of
1167      * a node's descendants before removing the node itself).
1168      *
1169      * <p>This method is invoked with the lock held on this node and its
1170      * parent (and all ancestors that are being removed as a
1171      * result of a single invocation to {@link Preferences#removeNode()}).
1172      *
1173      * <p>The removal of a node needn't become persistent until the
1174      * <tt>flush</tt> method is invoked on this node (or an ancestor).
1175      *
1176      * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
1177      * will propagate out beyond the enclosing {@link #removeNode()}
1178      * invocation.
1179      *
1180      * @throws BackingStoreException if this operation cannot be completed
1181      *         due to a failure in the backing store, or inability to
1182      *         communicate with it.
1183      */
1184     protected abstract void removeNodeSpi() throws BackingStoreException;
1185 
1186     /**
1187      * Returns all of the keys that have an associated value in this
1188      * preference node.  (The returned array will be of size zero if
1189      * this node has no preferences.)  It is guaranteed that this node has not
1190      * been removed.
1191      *
1192      * <p>This method is invoked with the lock on this node held.
1193      *
1194      * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
1195      * will propagate out beyond the enclosing {@link #keys()} invocation.
1196      *
1197      * @return an array of the keys that have an associated value in this
1198      *         preference node.
1199      * @throws BackingStoreException if this operation cannot be completed
1200      *         due to a failure in the backing store, or inability to
1201      *         communicate with it.
1202      */
1203     protected abstract String[] keysSpi() throws BackingStoreException;
1204 
1205     /**
1206      * Returns the names of the children of this preference node.  (The
1207      * returned array will be of size zero if this node has no children.)
1208      * This method need not return the names of any nodes already cached,
1209      * but may do so without harm.
1210      *
1211      * <p>This method is invoked with the lock on this node held.
1212      *
1213      * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
1214      * will propagate out beyond the enclosing {@link #childrenNames()}
1215      * invocation.
1216      *
1217      * @return an array containing the names of the children of this
1218      *         preference node.
1219      * @throws BackingStoreException if this operation cannot be completed
1220      *         due to a failure in the backing store, or inability to
1221      *         communicate with it.
1222      */
1223     protected abstract String[] childrenNamesSpi()
1224         throws BackingStoreException;
1225 
1226     /**
1227      * Returns the named child if it exists, or <tt>null</tt> if it does not.
1228      * It is guaranteed that <tt>nodeName</tt> is non-null, non-empty,
1229      * does not contain the slash character ('/'), and is no longer than
1230      * {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed
1231      * that this node has not been removed.  (The implementor needn't check
1232      * for any of these things if he chooses to override this method.)
1233      *
1234      * <p>Finally, it is guaranteed that the named node has not been returned
1235      * by a previous invocation of this method or {@link #childSpi} after the
1236      * last time that it was removed.  In other words, a cached value will
1237      * always be used in preference to invoking this method.  (The implementor
1238      * needn't maintain his own cache of previously returned children if he
1239      * chooses to override this method.)
1240      *
1241      * <p>This implementation obtains this preference node's lock, invokes
1242      * {@link #childrenNames()} to get an array of the names of this node's
1243      * children, and iterates over the array comparing the name of each child
1244      * with the specified node name.  If a child node has the correct name,
1245      * the {@link #childSpi(String)} method is invoked and the resulting
1246      * node is returned.  If the iteration completes without finding the
1247      * specified name, <tt>null</tt> is returned.
1248      *
1249      * @param nodeName name of the child to be searched for.
1250      * @return the named child if it exists, or null if it does not.
1251      * @throws BackingStoreException if this operation cannot be completed
1252      *         due to a failure in the backing store, or inability to
1253      *         communicate with it.
1254      */
1255     protected AbstractPreferences getChild(String nodeName)
1256             throws BackingStoreException {
1257         synchronized(lock) {
1258             // assert kidCache.get(nodeName)==null;
1259             String[] kidNames = childrenNames();
1260             for (int i=0; i<kidNames.length; i++)
1261                 if (kidNames[i].equals(nodeName))
1262                     return childSpi(kidNames[i]);
1263         }
1264         return null;
1265     }
1266 
1267     /**
1268      * Returns the named child of this preference node, creating it if it does
1269      * not already exist.  It is guaranteed that <tt>name</tt> is non-null,
1270      * non-empty, does not contain the slash character ('/'), and is no longer
1271      * than {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed that
1272      * this node has not been removed.  (The implementor needn't check for any
1273      * of these things.)
1274      *
1275      * <p>Finally, it is guaranteed that the named node has not been returned
1276      * by a previous invocation of this method or {@link #getChild(String)}
1277      * after the last time that it was removed.  In other words, a cached
1278      * value will always be used in preference to invoking this method.
1279      * Subclasses need not maintain their own cache of previously returned
1280      * children.
1281      *
1282      * <p>The implementer must ensure that the returned node has not been
1283      * removed.  If a like-named child of this node was previously removed, the
1284      * implementer must return a newly constructed <tt>AbstractPreferences</tt>
1285      * node; once removed, an <tt>AbstractPreferences</tt> node
1286      * cannot be "resuscitated."
1287      *
1288      * <p>If this method causes a node to be created, this node is not
1289      * guaranteed to be persistent until the <tt>flush</tt> method is
1290      * invoked on this node or one of its ancestors (or descendants).
1291      *
1292      * <p>This method is invoked with the lock on this node held.
1293      *
1294      * @param name The name of the child node to return, relative to
1295      *        this preference node.
1296      * @return The named child node.
1297      */
1298     protected abstract AbstractPreferences childSpi(String name);
1299 
1300     /**
1301      * Returns the absolute path name of this preferences node.
1302      */
1303     public String toString() {
1304         return (this.isUserNode() ? "User" : "System") +
1305                " Preference Node: " + this.absolutePath();
1306     }
1307 
1308     /**
1309      * Implements the <tt>sync</tt> method as per the specification in
1310      * {@link Preferences#sync()}.
1311      *
1312      * <p>This implementation calls a recursive helper method that locks this
1313      * node, invokes syncSpi() on it, unlocks this node, and recursively
1314      * invokes this method on each "cached child."  A cached child is a child
1315      * of this node that has been created in this VM and not subsequently
1316      * removed.  In effect, this method does a depth first traversal of the
1317      * "cached subtree" rooted at this node, calling syncSpi() on each node in
1318      * the subTree while only that node is locked. Note that syncSpi() is
1319      * invoked top-down.
1320      *
1321      * @throws BackingStoreException if this operation cannot be completed
1322      *         due to a failure in the backing store, or inability to
1323      *         communicate with it.
1324      * @throws IllegalStateException if this node (or an ancestor) has been
1325      *         removed with the {@link #removeNode()} method.
1326      * @see #flush()
1327      */
1328     public void sync() throws BackingStoreException {
1329         sync2();
1330     }
1331 
1332     private void sync2() throws BackingStoreException {
1333         AbstractPreferences[] cachedKids;
1334 
1335         synchronized(lock) {
1336             if (removed)
1337                 throw new IllegalStateException("Node has been removed");
1338             syncSpi();
1339             cachedKids = cachedChildren();
1340         }
1341 
1342         for (int i=0; i<cachedKids.length; i++)
1343             cachedKids[i].sync2();
1344     }
1345 
1346     /**
1347      * This method is invoked with this node locked.  The contract of this
1348      * method is to synchronize any cached preferences stored at this node
1349      * with any stored in the backing store.  (It is perfectly possible that
1350      * this node does not exist on the backing store, either because it has
1351      * been deleted by another VM, or because it has not yet been created.)
1352      * Note that this method should <i>not</i> synchronize the preferences in
1353      * any subnodes of this node.  If the backing store naturally syncs an
1354      * entire subtree at once, the implementer is encouraged to override
1355      * sync(), rather than merely overriding this method.
1356      *
1357      * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
1358      * will propagate out beyond the enclosing {@link #sync()} invocation.
1359      *
1360      * @throws BackingStoreException if this operation cannot be completed
1361      *         due to a failure in the backing store, or inability to
1362      *         communicate with it.
1363      */
1364     protected abstract void syncSpi() throws BackingStoreException;
1365 
1366     /**
1367      * Implements the <tt>flush</tt> method as per the specification in
1368      * {@link Preferences#flush()}.
1369      *
1370      * <p>This implementation calls a recursive helper method that locks this
1371      * node, invokes flushSpi() on it, unlocks this node, and recursively
1372      * invokes this method on each "cached child."  A cached child is a child
1373      * of this node that has been created in this VM and not subsequently
1374      * removed.  In effect, this method does a depth first traversal of the
1375      * "cached subtree" rooted at this node, calling flushSpi() on each node in
1376      * the subTree while only that node is locked. Note that flushSpi() is
1377      * invoked top-down.
1378      *
1379      * <p> If this method is invoked on a node that has been removed with
1380      * the {@link #removeNode()} method, flushSpi() is invoked on this node,
1381      * but not on others.
1382      *
1383      * @throws BackingStoreException if this operation cannot be completed
1384      *         due to a failure in the backing store, or inability to
1385      *         communicate with it.
1386      * @see #flush()
1387      */
1388     public void flush() throws BackingStoreException {
1389         flush2();
1390     }
1391 
1392     private void flush2() throws BackingStoreException {
1393         AbstractPreferences[] cachedKids;
1394 
1395         synchronized(lock) {
1396             flushSpi();
1397             if(removed)
1398                 return;
1399             cachedKids = cachedChildren();
1400         }
1401 
1402         for (int i = 0; i < cachedKids.length; i++)
1403             cachedKids[i].flush2();
1404     }
1405 
1406     /**
1407      * This method is invoked with this node locked.  The contract of this
1408      * method is to force any cached changes in the contents of this
1409      * preference node to the backing store, guaranteeing their persistence.
1410      * (It is perfectly possible that this node does not exist on the backing
1411      * store, either because it has been deleted by another VM, or because it
1412      * has not yet been created.)  Note that this method should <i>not</i>
1413      * flush the preferences in any subnodes of this node.  If the backing
1414      * store naturally flushes an entire subtree at once, the implementer is
1415      * encouraged to override flush(), rather than merely overriding this
1416      * method.
1417      *
1418      * <p>If this node throws a <tt>BackingStoreException</tt>, the exception
1419      * will propagate out beyond the enclosing {@link #flush()} invocation.
1420      *
1421      * @throws BackingStoreException if this operation cannot be completed
1422      *         due to a failure in the backing store, or inability to
1423      *         communicate with it.
1424      */
1425     protected abstract void flushSpi() throws BackingStoreException;
1426 
1427     /**
1428      * Returns <tt>true</tt> iff this node (or an ancestor) has been
1429      * removed with the {@link #removeNode()} method.  This method
1430      * locks this node prior to returning the contents of the private
1431      * field used to track this state.
1432      *
1433      * @return <tt>true</tt> iff this node (or an ancestor) has been
1434      *       removed with the {@link #removeNode()} method.
1435      */
1436     protected boolean isRemoved() {
1437         synchronized(lock) {
1438             return removed;
1439         }
1440     }
1441 
1442     /**
1443      * Queue of pending notification events.  When a preference or node
1444      * change event for which there are one or more listeners occurs,
1445      * it is placed on this queue and the queue is notified.  A background
1446      * thread waits on this queue and delivers the events.  This decouples
1447      * event delivery from preference activity, greatly simplifying
1448      * locking and reducing opportunity for deadlock.
1449      */
1450     private static final List<EventObject> eventQueue = new LinkedList<>();
1451 
1452     /**
1453      * These two classes are used to distinguish NodeChangeEvents on
1454      * eventQueue so the event dispatch thread knows whether to call
1455      * childAdded or childRemoved.
1456      */
1457     private class NodeAddedEvent extends NodeChangeEvent {
1458         private static final long serialVersionUID = -6743557530157328528L;
1459         NodeAddedEvent(Preferences parent, Preferences child) {
1460             super(parent, child);
1461         }
1462     }
1463     private class NodeRemovedEvent extends NodeChangeEvent {
1464         private static final long serialVersionUID = 8735497392918824837L;
1465         NodeRemovedEvent(Preferences parent, Preferences child) {
1466             super(parent, child);
1467         }
1468     }
1469 
1470     /**
1471      * A single background thread ("the event notification thread") monitors
1472      * the event queue and delivers events that are placed on the queue.
1473      */
1474     private static class EventDispatchThread extends Thread {
1475         public void run() {
1476             while(true) {
1477                 // Wait on eventQueue till an event is present
1478                 EventObject event = null;
1479                 synchronized(eventQueue) {
1480                     try {
1481                         while (eventQueue.isEmpty())
1482                             eventQueue.wait();
1483                         event = eventQueue.remove(0);
1484                     } catch (InterruptedException e) {
1485                         // XXX Log "Event dispatch thread interrupted. Exiting"
1486                         return;
1487                     }
1488                 }
1489 
1490                 // Now we have event & hold no locks; deliver evt to listeners
1491                 AbstractPreferences src=(AbstractPreferences)event.getSource();
1492                 if (event instanceof PreferenceChangeEvent) {
1493                     PreferenceChangeEvent pce = (PreferenceChangeEvent)event;
1494                     PreferenceChangeListener[] listeners = src.prefListeners();
1495                     for (int i=0; i<listeners.length; i++)
1496                         listeners[i].preferenceChange(pce);
1497                 } else {
1498                     NodeChangeEvent nce = (NodeChangeEvent)event;
1499                     NodeChangeListener[] listeners = src.nodeListeners();
1500                     if (nce instanceof NodeAddedEvent) {
1501                         for (int i=0; i<listeners.length; i++)
1502                             listeners[i].childAdded(nce);
1503                     } else {
1504                         // assert nce instanceof NodeRemovedEvent;
1505                         for (int i=0; i<listeners.length; i++)
1506                             listeners[i].childRemoved(nce);
1507                     }
1508                 }
1509             }
1510         }
1511     }
1512 
1513     private static Thread eventDispatchThread = null;
1514 
1515     /**
1516      * This method starts the event dispatch thread the first time it
1517      * is called.  The event dispatch thread will be started only
1518      * if someone registers a listener.
1519      */
1520     private static synchronized void startEventDispatchThreadIfNecessary() {
1521         if (eventDispatchThread == null) {
1522             // XXX Log "Starting event dispatch thread"
1523             eventDispatchThread = new EventDispatchThread();
1524             eventDispatchThread.setDaemon(true);
1525             eventDispatchThread.start();
1526         }
1527     }
1528 
1529     /**
1530      * Return this node's preference/node change listeners.  Even though
1531      * we're using a copy-on-write lists, we use synchronized accessors to
1532      * ensure information transmission from the writing thread to the
1533      * reading thread.
1534      */
1535     PreferenceChangeListener[] prefListeners() {
1536         synchronized(lock) {
1537             return prefListeners;
1538         }
1539     }
1540     NodeChangeListener[] nodeListeners() {
1541         synchronized(lock) {
1542             return nodeListeners;
1543         }
1544     }
1545 
1546     /**
1547      * Enqueue a preference change event for delivery to registered
1548      * preference change listeners unless there are no registered
1549      * listeners.  Invoked with this.lock held.
1550      */
1551     private void enqueuePreferenceChangeEvent(String key, String newValue) {
1552         if (prefListeners.length != 0) {
1553             synchronized(eventQueue) {
1554                 eventQueue.add(new PreferenceChangeEvent(this, key, newValue));
1555                 eventQueue.notify();
1556             }
1557         }
1558     }
1559 
1560     /**
1561      * Enqueue a "node added" event for delivery to registered node change
1562      * listeners unless there are no registered listeners.  Invoked with
1563      * this.lock held.
1564      */
1565     private void enqueueNodeAddedEvent(Preferences child) {
1566         if (nodeListeners.length != 0) {
1567             synchronized(eventQueue) {
1568                 eventQueue.add(new NodeAddedEvent(this, child));
1569                 eventQueue.notify();
1570             }
1571         }
1572     }
1573 
1574     /**
1575      * Enqueue a "node removed" event for delivery to registered node change
1576      * listeners unless there are no registered listeners.  Invoked with
1577      * this.lock held.
1578      */
1579     private void enqueueNodeRemovedEvent(Preferences child) {
1580         if (nodeListeners.length != 0) {
1581             synchronized(eventQueue) {
1582                 eventQueue.add(new NodeRemovedEvent(this, child));
1583                 eventQueue.notify();
1584             }
1585         }
1586     }
1587 
1588     /**
1589      * Implements the <tt>exportNode</tt> method as per the specification in
1590      * {@link Preferences#exportNode(OutputStream)}.
1591      *
1592      * @param os the output stream on which to emit the XML document.
1593      * @throws IOException if writing to the specified output stream
1594      *         results in an <tt>IOException</tt>.
1595      * @throws BackingStoreException if preference data cannot be read from
1596      *         backing store.
1597      */
1598     public void exportNode(OutputStream os)
1599         throws IOException, BackingStoreException
1600     {
1601         XmlSupport.export(os, this, false);
1602     }
1603 
1604     /**
1605      * Implements the <tt>exportSubtree</tt> method as per the specification in
1606      * {@link Preferences#exportSubtree(OutputStream)}.
1607      *
1608      * @param os the output stream on which to emit the XML document.
1609      * @throws IOException if writing to the specified output stream
1610      *         results in an <tt>IOException</tt>.
1611      * @throws BackingStoreException if preference data cannot be read from
1612      *         backing store.
1613      */
1614     public void exportSubtree(OutputStream os)
1615         throws IOException, BackingStoreException
1616     {
1617         XmlSupport.export(os, this, true);
1618     }
1619 }