1 /****************************************************************
2 * Licensed to the Apache Software Foundation (ASF) under one *
3 * or more contributor license agreements. See the NOTICE file *
4 * distributed with this work for additional information *
5 * regarding copyright ownership. The ASF licenses this file *
6 * to you under the Apache License, Version 2.0 (the *
7 * "License"); you may not use this file except in compliance *
8 * with the License. You may obtain a copy of the License at *
9 * *
10 * http://www.apache.org/licenses/LICENSE-2.0 *
11 * *
12 * Unless required by applicable law or agreed to in writing, *
13 * software distributed under the License is distributed on an *
14 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY *
15 * KIND, either express or implied. See the License for the *
16 * specific language governing permissions and limitations *
17 * under the License. *
18 ****************************************************************/
19
20
21
22 package org.apache.james.util;
23
24 import java.util.Hashtable;
25
26 /**
27 * Provides Lock functionality
28 *
29 */
30 public class Lock {
31 /**
32 * An internal hash table of keys to locks
33 */
34 private Hashtable locks = new Hashtable();
35
36 /**
37 * Check to see if the object is locked
38 *
39 * @param key the Object on which to check the lock
40 * @return true if the object is locked, false otherwise
41 */
42 public boolean isLocked(final Object key) {
43 return (locks.get(key) != null);
44 }
45
46 /**
47 * Check to see if we can lock on a given object.
48 *
49 * @param key the Object on which to lock
50 * @return true if the calling thread can lock, false otherwise
51 */
52 public boolean canI(final Object key) {
53 Object o = locks.get( key );
54
55 if (null == o || o == this.getCallerId()) {
56 return true;
57 }
58
59 return false;
60 }
61
62 /**
63 * Lock on a given object.
64 *
65 * @param key the Object on which to lock
66 * @return true if the locking was successful, false otherwise
67 */
68 public boolean lock(final Object key) {
69 Object theLock;
70
71 synchronized(this) {
72 theLock = locks.get(key);
73
74 if (null == theLock) {
75 locks.put(key, getCallerId());
76 return true;
77 } else if (getCallerId() == theLock) {
78 return true;
79 } else {
80 return false;
81 }
82 }
83 }
84
85 /**
86 * Release the lock on a given object.
87 *
88 * @param key the Object on which the lock is held
89 * @return true if the unlocking was successful, false otherwise
90 */
91 public boolean unlock(final Object key) {
92 Object theLock;
93 synchronized (this) {
94 theLock = locks.get(key);
95
96 if (null == theLock) {
97 return true;
98 } else if (getCallerId() == theLock) {
99 locks.remove(key);
100 return true;
101 } else {
102 return false;
103 }
104 }
105 }
106
107 /**
108 * Private helper method to abstract away caller ID.
109 *
110 * @return the id of the caller (i.e. the Thread reference)
111 */
112 private Object getCallerId() {
113 return Thread.currentThread();
114 }
115 }