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Java Source Code / Java Documentation » Web Framework » aranea mvc 1.1.1 » org.araneaframework.core.util 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:          File: Mutex.java
003:
004:          Originally written by Doug Lea and released into the public domain.
005:          This may be used for any purposes whatsoever without acknowledgment.
006:          Thanks for the assistance and support of Sun Microsystems Labs,
007:          and everyone contributing, testing, and using this code.
008:
009:          History:
010:          Date       Who                What
011:          11Jun1998  dl               Create public version
012:         */
013:
014:        package org.araneaframework.core.util;
015:
016:        /**
017:         * A simple non-reentrant mutual exclusion lock.
018:         * The lock is free upon construction. Each acquire gets the
019:         * lock, and each release frees it. Releasing a lock that
020:         * is already free has no effect. 
021:         * <p>
022:         * This implementation makes no attempt to provide any fairness
023:         * or ordering guarantees. If you need them, consider using one of
024:         * the Semaphore implementations as a locking mechanism.
025:         * <p>
026:         * <b>Sample usage</b><br>
027:         * <p>
028:         * Mutex can be useful in constructions that cannot be
029:         * expressed using java synchronized blocks because the
030:         * acquire/release pairs do not occur in the same method or
031:         * code block. For example, you can use them for hand-over-hand
032:         * locking across the nodes of a linked list. This allows
033:         * extremely fine-grained locking,  and so increases 
034:         * potential concurrency, at the cost of additional complexity and
035:         * overhead that would normally make this worthwhile only in cases of
036:         * extreme contention.
037:         * <pre>
038:         * class Node { 
039:         *   Object item; 
040:         *   Node next; 
041:         *   Mutex lock = new Mutex(); // each node keeps its own lock
042:         *
043:         *   Node(Object x, Node n) { item = x; next = n; }
044:         * }
045:         *
046:         * class List {
047:         *    protected Node head; // pointer to first node of list
048:         *
049:         *    // Use plain java synchronization to protect head field.
050:         *    //  (We could instead use a Mutex here too but there is no
051:         *    //  reason to do so.)
052:         *    protected synchronized Node getHead() { return head; }
053:         *
054:         *    boolean search(Object x) throws InterruptedException {
055:         *      Node p = getHead();
056:         *      if (p == null) return false;
057:         *
058:         *      //  (This could be made more compact, but for clarity of illustration,
059:         *      //  all of the cases that can arise are handled separately.)
060:         *
061:         *      p.lock.acquire();              // Prime loop by acquiring first lock.
062:         *                                     //    (If the acquire fails due to
063:         *                                     //    interrupt, the method will throw
064:         *                                     //    InterruptedException now,
065:         *                                     //    so there is no need for any
066:         *                                     //    further cleanup.)
067:         *      for (;;) {
068:         *        if (x.equals(p.item)) {
069:         *          p.lock.release();          // release current before return
070:         *          return true;
071:         *        }
072:         *        else {
073:         *          Node nextp = p.next;
074:         *          if (nextp == null) {
075:         *            p.lock.release();       // release final lock that was held
076:         *            return false;
077:         *          }
078:         *          else {
079:         *            try {
080:         *              nextp.lock.acquire(); // get next lock before releasing current
081:         *            }
082:         *            catch (InterruptedException ex) {
083:         *              p.lock.release();    // also release current if acquire fails
084:         *              throw ex;
085:         *            }
086:         *            p.lock.release();      // release old lock now that new one held
087:         *            p = nextp;
088:         *          }
089:         *        }
090:         *      }
091:         *    }
092:         *
093:         *    synchronized void add(Object x) { // simple prepend
094:         *      // The use of `synchronized'  here protects only head field.
095:         *      // The method does not need to wait out other traversers 
096:         *      // who have already made it past head.
097:         *
098:         *      head = new Node(x, head);
099:         *    }
100:         *
101:         *    // ...  other similar traversal and update methods ...
102:         * }
103:         * </pre>
104:         * <p>
105:         * @see Semaphore
106:         * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
107:         **/
108:
109:        public class Mutex implements  Sync {
110:
111:            /** The lock status **/
112:            protected boolean inuse_ = false;
113:
114:            public void acquire() throws InterruptedException {
115:                if (Thread.interrupted())
116:                    throw new InterruptedException();
117:                synchronized (this ) {
118:                    try {
119:                        while (inuse_)
120:                            wait();
121:                        inuse_ = true;
122:                    } catch (InterruptedException ex) {
123:                        notify();
124:                        throw ex;
125:                    }
126:                }
127:            }
128:
129:            public synchronized void release() {
130:                inuse_ = false;
131:                notify();
132:            }
133:
134:            public boolean attempt(long msecs) throws InterruptedException {
135:                if (Thread.interrupted())
136:                    throw new InterruptedException();
137:                synchronized (this ) {
138:                    if (!inuse_) {
139:                        inuse_ = true;
140:                        return true;
141:                    } else if (msecs <= 0)
142:                        return false;
143:                    else {
144:                        long waitTime = msecs;
145:                        long start = System.currentTimeMillis();
146:                        try {
147:                            for (;;) {
148:                                wait(waitTime);
149:                                if (!inuse_) {
150:                                    inuse_ = true;
151:                                    return true;
152:                                } else {
153:                                    waitTime = msecs
154:                                            - (System.currentTimeMillis() - start);
155:                                    if (waitTime <= 0)
156:                                        return false;
157:                                }
158:                            }
159:                        } catch (InterruptedException ex) {
160:                            notify();
161:                            throw ex;
162:                        }
163:                    }
164:                }
165:            }
166:
167:        }
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