1 /*
2 * Copyright (c) 2001, 2002, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 /*
27 File: Mutex.java
28
29 Originally written by Doug Lea and released into the public domain.
30 This may be used for any purposes whatsoever without acknowledgment.
31 Thanks for the assistance and support of Sun Microsystems Labs,
32 and everyone contributing, testing, and using this code.
33
34 History:
35 Date Who What
36 11Jun1998 dl Create public version
37 */
38
39 package com.sun.corba.se.impl.orbutil.concurrent;
40
41 /**
42 * A simple non-reentrant mutual exclusion lock.
43 * The lock is free upon construction. Each acquire gets the
44 * lock, and each release frees it. Releasing a lock that
45 * is already free has no effect.
46 * <p>
47 * This implementation makes no attempt to provide any fairness
48 * or ordering guarantees. If you need them, consider using one of
49 * the Semaphore implementations as a locking mechanism.
50 * <p>
51 * <b>Sample usage</b><br>
52 * <p>
53 * Mutex can be useful in constructions that cannot be
54 * expressed using java synchronized blocks because the
55 * acquire/release pairs do not occur in the same method or
56 * code block. For example, you can use them for hand-over-hand
57 * locking across the nodes of a linked list. This allows
58 * extremely fine-grained locking, and so increases
59 * potential concurrency, at the cost of additional complexity and
60 * overhead that would normally make this worthwhile only in cases of
61 * extreme contention.
62 * <pre>
63 * class Node {
64 * Object item;
65 * Node next;
66 * Mutex lock = new Mutex(); // each node keeps its own lock
67 *
68 * Node(Object x, Node n) { item = x; next = n; }
69 * }
70 *
71 * class List {
72 * protected Node head; // pointer to first node of list
73 *
74 * // Use plain java synchronization to protect head field.
75 * // (We could instead use a Mutex here too but there is no
76 * // reason to do so.)
77 * protected synchronized Node getHead() { return head; }
78 *
79 * boolean search(Object x) throws InterruptedException {
80 * Node p = getHead();
81 * if (p == null) return false;
82 *
83 * // (This could be made more compact, but for clarity of illustration,
84 * // all of the cases that can arise are handled separately.)
85 *
86 * p.lock.acquire(); // Prime loop by acquiring first lock.
87 * // (If the acquire fails due to
88 * // interrupt, the method will throw
89 * // InterruptedException now,
90 * // so there is no need for any
91 * // further cleanup.)
92 * for (;;) {
93 * if (x.equals(p.item)) {
94 * p.lock.release(); // release current before return
95 * return true;
96 * }
97 * else {
98 * Node nextp = p.next;
99 * if (nextp == null) {
100 * p.lock.release(); // release final lock that was held
101 * return false;
102 * }
103 * else {
104 * try {
105 * nextp.lock.acquire(); // get next lock before releasing current
106 * }
107 * catch (InterruptedException ex) {
108 * p.lock.release(); // also release current if acquire fails
109 * throw ex;
110 * }
111 * p.lock.release(); // release old lock now that new one held
112 * p = nextp;
113 * }
114 * }
115 * }
116 * }
117 *
118 * synchronized void add(Object x) { // simple prepend
119 * // The use of `synchronized' here protects only head field.
120 * // The method does not need to wait out other traversers
121 * // who have already made it past head.
122 *
123 * head = new Node(x, head);
124 * }
125 *
126 * // ... other similar traversal and update methods ...
127 * }
128 * </pre>
129 * <p>
130 * <p>This version adds some debugging capability: it will detect an attempt by a thread
131 * that holds the lock to acquire it for a second time, and also an attempt by a thread that
132 * does not hold the mutex to release it.
133 * @see Semaphore
134 * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
135 **/
136
137 import org.omg.CORBA.INTERNAL ;
138
139 public class DebugMutex implements Sync {
140
141 /** The lock status **/
142 protected boolean inuse_ = false;
143 protected Thread holder_ = null;
144
145 public void acquire() throws InterruptedException {
146 if (Thread.interrupted()) throw new InterruptedException();
147 synchronized(this) {
148 Thread thr = Thread.currentThread();
149 if (holder_ == thr)
150 throw new INTERNAL(
151 "Attempt to acquire Mutex by thread holding the Mutex" ) ;
152
153 try {
154 while (inuse_) wait();
155 inuse_ = true;
156 holder_ = Thread.currentThread();
157 }
158 catch (InterruptedException ex) {
159 notify();
160 throw ex;
161 }
162 }
163 }
164
165 public synchronized void release() {
166 Thread thr = Thread.currentThread();
167 if (thr != holder_)
168 throw new INTERNAL(
169 "Attempt to release Mutex by thread not holding the Mutex" ) ;
170 holder_ = null;
171 inuse_ = false;
172 notify();
173 }
174
175
176 public boolean attempt(long msecs) throws InterruptedException {
177 if (Thread.interrupted()) throw new InterruptedException();
178 synchronized(this) {
179 Thread thr = Thread.currentThread() ;
180
181 if (!inuse_) {
182 inuse_ = true;
183 holder_ = thr;
184 return true;
185 } else if (msecs <= 0)
186 return false;
187 else {
188 long waitTime = msecs;
189 long start = System.currentTimeMillis();
190 try {
191 for (;;) {
192 wait(waitTime);
193 if (!inuse_) {
194 inuse_ = true;
195 holder_ = thr;
196 return true;
197 }
198 else {
199 waitTime = msecs - (System.currentTimeMillis() - start);
200 if (waitTime <= 0)
201 return false;
202 }
203 }
204 }
205 catch (InterruptedException ex) {
206 notify();
207 throw ex;
208 }
209 }
210 }
211 }
212 }