1 /**
2 * Copyright Terracotta, Inc.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16 package info.magnolia.cache.concurrent;
17
18 /**
19 * Various bits of black magic garnered from experts on the concurrency-interest@cs.oswego.edu mailing list.
20 *
21 * @author Greg Luck
22 * @version $Id: ConcurrencyUtil.java 7833 2013-07-25 01:00:20Z cschanck $
23 */
24 public final class ConcurrencyUtil {
25
26 private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_1 = 20;
27 private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_2 = 12;
28 private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_3 = 7;
29 private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_4 = 4;
30
31
32 /**
33 * Utility class therefore precent construction.
34 */
35 private ConcurrencyUtil() {
36 //noop;
37 }
38
39 /**
40 * Returns a hash code for non-null Object x.
41 * <p/>
42 * This function ensures that hashCodes that differ only by
43 * constant multiples at each bit position have a bounded
44 * number of collisions. (Doug Lea)
45 *
46 * @param object the object serving as a key
47 * @return the hash code
48 */
49 public static int hash(Object object) {
50 int h = object.hashCode();
51 h ^= (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_1) ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_2);
52 return h ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_3) ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_4);
53 }
54
55 /**
56 * Selects a lock for a key. The same lock is always used for a given key.
57 *
58 * @param key
59 * @return the selected lock index
60 */
61 public static int selectLock(final Object key, int numberOfLocks) {
62 int number = numberOfLocks & (numberOfLocks - 1);
63 if (number != 0) {
64 throw new IllegalStateException("Lock number must be a power of two: " + numberOfLocks);
65 }
66 if (key == null) {
67 return 0;
68 } else {
69 int hash = hash(key) & (numberOfLocks - 1);
70 return hash;
71 }
72
73 }
74 }