1 /*
2 * $Source$
3 * $Revision$
4 *
5 * Copyright (C) 2000 William Chesters
6 *
7 * Part of Melati (http://melati.org), a framework for the rapid
8 * development of clean, maintainable web applications.
9 *
10 * Melati is free software; Permission is granted to copy, distribute
11 * and/or modify this software under the terms either:
12 *
13 * a) the GNU General Public License as published by the Free Software
14 * Foundation; either version 2 of the License, or (at your option)
15 * any later version,
16 *
17 * or
18 *
19 * b) any version of the Melati Software License, as published
20 * at http://melati.org
21 *
22 * You should have received a copy of the GNU General Public License and
23 * the Melati Software License along with this program;
24 * if not, write to the Free Software Foundation, Inc.,
25 * 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA to obtain the
26 * GNU General Public License and visit http://melati.org to obtain the
27 * Melati Software License.
28 *
29 * Feel free to contact the Developers of Melati (http://melati.org),
30 * if you would like to work out a different arrangement than the options
31 * outlined here. It is our intention to allow Melati to be used by as
32 * wide an audience as possible.
33 *
34 * This program is distributed in the hope that it will be useful,
35 * but WITHOUT ANY WARRANTY; without even the implied warranty of
36 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
37 * GNU General Public License for more details.
38 *
39 * Contact details for copyright holder:
40 *
41 * William Chesters <williamc At paneris.org>
42 * http://paneris.org/~williamc
43 * Obrechtstraat 114, 2517VX Den Haag, The Netherlands
44 */
45
46 package org.melati.poem.util;
47
48 import java.util.Vector;
49
50 /**
51 * Supply and cache objects identified by a numeric index.
52 * <p>
53 * Subtypes define how the object for a given index is obtained,
54 * and hence the mapping of indexes to objects used in the
55 * caller.
56 * <p>
57 * The name is a touch misleading - the objects returned are
58 * not (necessarily) indexes.
59 * <p>
60 * <code>null</code> object references can be cached and returned.
61 * <p>
62 * Individual elements can be removed from the cache or all elements
63 * may be removed.
64 */
65
66 public abstract class CachedIndexFactory implements IndexFactory {
67
68 private Vector<Object> cache = new Vector<Object>();
69 private static final Object nullFromFactory = new Object();
70
71 /**
72 * @param index of the item, possibly a troid
73 * @return the cached Object
74 */
75 protected abstract Object reallyGet(int index);
76
77 /**
78 * Get either from cache or, failing that, really get it.
79 * {@inheritDoc}
80 * @see org.melati.poem.util.IndexFactory#get(int)
81 */
82 public Object get(int index) {
83 synchronized (cache) {
84 if (cache.size() <= index) {
85 cache.setSize(index + 1);
86 Object it = reallyGet(index);
87 cache.setElementAt(it == null ? nullFromFactory : it, index);
88 return it;
89 }
90 else {
91 Object it = cache.elementAt(index);
92 if (it == null) {
93 it = reallyGet(index);
94 cache.setElementAt(it == null ? nullFromFactory : it, index);
95 return it;
96 }
97 else if (it == nullFromFactory)
98 return null;
99 else
100 return it;
101 }
102 }
103 }
104
105 /**
106 * Invalidate an entry in the cache.
107 * @param index the entry's index to invalidate
108 */
109 public void invalidate(int index) {
110 cache.setElementAt(null, index);
111 }
112
113 /**
114 * Invalidate whole cache.
115 */
116 public void invalidate() {
117 cache.removeAllElements();
118 }
119 }
120
121
122
123