01: /**
02: * Copyright 2003-2007 Luck Consulting Pty Ltd
03: *
04: * Licensed under the Apache License, Version 2.0 (the "License");
05: * you may not use this file except in compliance with the License.
06: * You may obtain a copy of the License at
07: *
08: * http://www.apache.org/licenses/LICENSE-2.0
09: *
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: */package net.sf.ehcache.constructs.concurrent;
16:
17: import net.sf.ehcache.CacheException;
18:
19: /**
20: * Various bits of black magic garnered from experts on the concurrency-interest@cs.oswego.edu mailing list.
21: *
22: * @author Greg Luck
23: * @version $Id: ConcurrencyUtil.java 519 2007-07-27 07:11:45Z gregluck $
24: */
25: public final class ConcurrencyUtil {
26:
27: private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_1 = 20;
28: private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_2 = 12;
29: private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_3 = 7;
30: private static final int DOUG_LEA_BLACK_MAGIC_OPERAND_4 = 4;
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)
52: ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_2);
53: return h ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_3)
54: ^ (h >>> DOUG_LEA_BLACK_MAGIC_OPERAND_4);
55: }
56:
57: /**
58: * Selects a lock for a key. The same lock is always used for a given key.
59: *
60: * @param key
61: * @return the selected lock index
62: */
63: public static int selectLock(final Object key, int numberOfLocks)
64: throws CacheException {
65: int number = numberOfLocks & (numberOfLocks - 1);
66: if (number != 0) {
67: throw new CacheException(
68: "Lock number must be a power of two: "
69: + numberOfLocks);
70: }
71: if (key == null) {
72: return 0;
73: } else {
74: int hash = hash(key) & (numberOfLocks - 1);
75: return hash;
76: }
77:
78: }
79:
80: }
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