DNSCache.java

//Copyright 2003-2005 Arthur van Hoff Rick Blair
//Licensed under Apache License version 2.0
//Original license LGPL

package javax.jmdns.impl;

import java.util.*;
//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</code>'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:
 * 
 * @version %I%, %G%
 * @author Arthur van Hoff, Werner Randelshofer, Rick Blair
 */
public class DNSCache {
	//    private static Logger logger = Logger.getLogger(DNSCache.class.getName());
	// 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()</code> and
	// method <code>find(name)</code>.
	// 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 final 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.getName());
		private final 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()+")");
		final CacheNode newValue = new CacheNode(entry);
		final 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 synchronized Iterator iterator() {
		return new ArrayList(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 (final 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() {
		final StringBuffer aLog = new StringBuffer();
		aLog.append("\t---- cache ----");
		for (final 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();
	}

}