abstractpreferences.java

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            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 i=kidCache.values().iterator(); i.hasNext(); )
                ((AbstractPreferences)i.next()).removeNode2();
            kidCache.clear();
            
            // 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() {
        Boolean result = (Boolean)
          AccessController.doPrivileged( new PrivilegedAction() {            
            public Object run() {
                return new Boolean(root == Preferences.userRoot());
            }
        });
        return result.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
            NodeChangeListener[] newNl =
                new NodeChangeListener[nodeListeners.length - 1];
            int i = 0;
            while (i < nodeListeners.length && nodeListeners[i] != ncl)
                newNl[i] = nodeListeners[i++];

            if (i == nodeListeners.length)
                throw new IllegalArgumentException("Listener not registered.");
            while (i < newNl.length)
                newNl[i] = nodeListeners[++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.
     */
    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.
     *
     * @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.
     */
    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