ActiveMQ example source code file (DNSCache.java)

This example ActiveMQ source code file (DNSCache.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" ^TM.

Java - ActiveMQ tags/keywords

cachenode, cachenode, dnscache, dnscache, dnsentry, dnsentry, hashmap, iterator, iterator, log, logger, logging, string, stringbuffer, util

The ActiveMQ DNSCache.java source code

/**
 * Copyright 2003-2005 Arthur van Hoff, Rick Blair
 *
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.activemq.jmdns;

import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.logging.Logger;

/**
 * A table of DNS entries. This is a hash table which
 * can handle multiple entries with the same name.
 * <p/>
 * Storing multiple entries with the same name is implemented using a
 * linked list of <code>CacheNode's.
 * <p/>
 * The current implementation of the API of DNSCache does expose the
 * cache nodes to clients. Clients must explicitly deal with the nodes
 * when iterating over entries in the cache. Here's how to iterate over
 * all entries in the cache:
 * <pre>
 * for (Iterator i=dnscache.iterator(); i.hasNext(); ) {
 *    for (DNSCache.CacheNode n = (DNSCache.CacheNode) i.next(); n != null; n.next()) {
 *       DNSEntry entry = n.getValue();
 *       ...do something with entry...
 *    }
 * }
 * </pre>
 * <p/>
 * And here's how to iterate over all entries having a given name:
 * <pre>
 * for (DNSCache.CacheNode n = (DNSCache.CacheNode) dnscache.find(name); n != null; n.next()) {
 *     DNSEntry entry = n.getValue();
 *     ...do something with entry...
 * }
 * </pre>
 *
 * @version %I%, %G%
 * @author	Arthur van Hoff, Werner Randelshofer, Rick Blair
 */
class DNSCache
{
    private static Logger logger = Logger.getLogger(DNSCache.class.toString());
    // Implementation note:
    // We might completely hide the existence of CacheNode's in a future version
    // of DNSCache. But this will require to implement two (inner) classes for
    // the  iterators that will be returned by method <code>iterator() and
    // method <code>find(name).
    // Since DNSCache is not a public class, it does not seem worth the effort
    // to clean its API up that much.

    // [PJYF Oct 15 2004] This should implements Collections that would be amuch cleaner implementation
    
    /**
     * The number of DNSEntry's in the cache.
     */
    private int size;

    /**
     * The hashtable used internally to store the entries of the cache.
     * Keys are instances of String. The String contains an unqualified service
     * name.
     * Values are linked lists of CacheNode instances.
     */
    private HashMap hashtable;

    /**
     * Cache nodes are used to implement storage of multiple DNSEntry's of the
     * same name in the cache.
     */
    public static class CacheNode
    {
        private static Logger logger = Logger.getLogger(CacheNode.class.toString());
        private DNSEntry value;
        private CacheNode next;

        public CacheNode(DNSEntry value)
        {
            this.value = value;
        }

        public CacheNode next()
        {
            return next;
        }

        public DNSEntry getValue()
        {
            return value;
        }
    }


    /**
     * Create a table with a given initial size.
     */
    public DNSCache(final int size)
    {
        hashtable = new HashMap(size);
    }

    /**
     * Clears the cache.
     */
    public synchronized void clear()
    {
        hashtable.clear();
        size = 0;
    }

    /**
     * Adds an entry to the table.
     */
    public synchronized void add(final DNSEntry entry)
    {
        //logger.log("DNSCache.add("+entry.getName()+")");
        CacheNode newValue = new CacheNode(entry);
        CacheNode node = (CacheNode) hashtable.get(entry.getName());
        if (node == null)
        {
            hashtable.put(entry.getName(), newValue);
        }
        else
        {
            newValue.next = node.next;
            node.next = newValue;
        }
        size++;
    }

    /**
     * Remove a specific entry from the table. Returns true if the
     * entry was found.
     */
    public synchronized boolean remove(DNSEntry entry)
    {
        CacheNode node = (CacheNode) hashtable.get(entry.getName());
        if (node != null)
        {
            if (node.value == entry)
            {
                if (node.next == null)
                {
                    hashtable.remove(entry.getName());
                }
                else
                {
                    hashtable.put(entry.getName(), node.next);
                }
                size--;
                return true;
            }

            CacheNode previous = node;
            node = node.next;
            while (node != null)
            {
                if (node.value == entry)
                {
                    previous.next = node.next;
                    size--;
                    return true;
                }
                previous = node;
                node = node.next;
            }
            ;
        }
        return false;
    }

    /**
     * Get a matching DNS entry from the table (using equals).
     * Returns the entry that was found.
     */
    public synchronized DNSEntry get(DNSEntry entry)
    {
        for (CacheNode node = find(entry.getName()); node != null; node = node.next)
        {
            if (node.value.equals(entry))
            {
                return node.value;
            }
        }
        return null;
    }

    /**
     * Get a matching DNS entry from the table.
     */
    public synchronized DNSEntry get(String name, int type, int clazz)
    {
        for (CacheNode node = find(name); node != null; node = node.next)
        {
            if (node.value.type == type && node.value.clazz == clazz)
            {
                return node.value;
            }
        }
        return null;
    }

    /**
     * Iterates over all cache nodes.
     * The iterator returns instances of DNSCache.CacheNode.
     * Each instance returned is the first node of a linked list.
     * To retrieve all entries, one must iterate over this linked list. See
     * code snippets in the header of the class.
     */
    public Iterator iterator()
    {
        return Collections.unmodifiableCollection(hashtable.values()).iterator();
    }

    /**
     * Iterate only over items with matching name.
     * Returns an instance of DNSCache.CacheNode or null.
     * If an instance is returned, it is the first node of a linked list.
     * To retrieve all entries, one must iterate over this linked list.
     */
    public synchronized CacheNode find(String name)
    {
        return (CacheNode) hashtable.get(name);
    }

    /**
     * List all entries for debugging.
     */
    public synchronized void print()
    {
        for (Iterator i = iterator(); i.hasNext();)
        {
            for (CacheNode n = (CacheNode) i.next(); n != null; n = n.next)
            {
                System.out.println(n.value);
            }
        }
    }

    public synchronized String toString()
    {
        StringBuffer aLog = new StringBuffer();
        aLog.append("\t---- cache ----");
        for (Iterator i = iterator(); i.hasNext();)
        {
            for (CacheNode n = (CacheNode) i.next(); n != null; n = n.next)
            {
                aLog.append("\n\t\t" + n.value);
            }
        }
        return aLog.toString();
    }

}