Source Code Cross Referenced for WriterPreferenceReadWriteLock.java in  » Database-JDBC-Connection-Pool » proxool » org » logicalcobwebs » concurrent » Java Source Code / Java DocumentationJava Source Code and Java Documentation

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Java Source Code / Java Documentation » Database JDBC Connection Pool » proxool » org.logicalcobwebs.concurrent 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:          File: WriterPreferenceReadWriteLock.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:           5Aug1998  dl               replaced int counters with longs
013:          25aug1998  dl               record writer thread
014:           3May1999  dl               add notifications on interrupt/timeout
015:
016:         */
017:
018:        package org.logicalcobwebs.concurrent;
019:
020:        /**
021:         * A ReadWriteLock that prefers waiting writers over
022:         * waiting readers when there is contention. This class
023:         * is adapted from the versions described in CPJ, improving
024:         * on the ones there a bit by segregating reader and writer
025:         * wait queues, which is typically more efficient.
026:         * <p>
027:         * The locks are <em>NOT</em> reentrant. In particular,
028:         * even though it may appear to usually work OK,
029:         * a thread holding a read lock should not attempt to
030:         * re-acquire it. Doing so risks lockouts when there are
031:         * also waiting writers.
032:         * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
033:         **/
034:
035:        public class WriterPreferenceReadWriteLock implements  ReadWriteLock {
036:
037:            protected long activeReaders_ = 0;
038:            protected Thread activeWriter_ = null;
039:            protected long waitingReaders_ = 0;
040:            protected long waitingWriters_ = 0;
041:
042:            protected final ReaderLock readerLock_ = new ReaderLock();
043:            protected final WriterLock writerLock_ = new WriterLock();
044:
045:            public Sync writeLock() {
046:                return writerLock_;
047:            }
048:
049:            public Sync readLock() {
050:                return readerLock_;
051:            }
052:
053:            /*
054:              A bunch of small synchronized methods are needed
055:              to allow communication from the Lock objects
056:              back to this object, that serves as controller
057:             */
058:
059:            protected synchronized void cancelledWaitingReader() {
060:                --waitingReaders_;
061:            }
062:
063:            protected synchronized void cancelledWaitingWriter() {
064:                --waitingWriters_;
065:            }
066:
067:            /** Override this method to change to reader preference **/
068:            protected boolean allowReader() {
069:                return activeWriter_ == null && waitingWriters_ == 0;
070:            }
071:
072:            protected synchronized boolean startRead() {
073:                boolean allowRead = allowReader();
074:                if (allowRead)
075:                    ++activeReaders_;
076:                return allowRead;
077:            }
078:
079:            protected synchronized boolean startWrite() {
080:
081:                // The allowWrite expression cannot be modified without
082:                // also changing startWrite, so is hard-wired
083:
084:                boolean allowWrite = (activeWriter_ == null && activeReaders_ == 0);
085:                if (allowWrite)
086:                    activeWriter_ = Thread.currentThread();
087:                return allowWrite;
088:            }
089:
090:            /*
091:               Each of these variants is needed to maintain atomicity
092:               of wait counts during wait loops. They could be
093:               made faster by manually inlining each other. We hope that
094:               compilers do this for us though.
095:             */
096:
097:            protected synchronized boolean startReadFromNewReader() {
098:                boolean pass = startRead();
099:                if (!pass)
100:                    ++waitingReaders_;
101:                return pass;
102:            }
103:
104:            protected synchronized boolean startWriteFromNewWriter() {
105:                boolean pass = startWrite();
106:                if (!pass)
107:                    ++waitingWriters_;
108:                return pass;
109:            }
110:
111:            protected synchronized boolean startReadFromWaitingReader() {
112:                boolean pass = startRead();
113:                if (pass)
114:                    --waitingReaders_;
115:                return pass;
116:            }
117:
118:            protected synchronized boolean startWriteFromWaitingWriter() {
119:                boolean pass = startWrite();
120:                if (pass)
121:                    --waitingWriters_;
122:                return pass;
123:            }
124:
125:            /**
126:             * Called upon termination of a read.
127:             * Returns the object to signal to wake up a waiter, or null if no such
128:             **/
129:            protected synchronized Signaller endRead() {
130:                if (--activeReaders_ == 0 && waitingWriters_ > 0)
131:                    return writerLock_;
132:                else
133:                    return null;
134:            }
135:
136:            /**
137:             * Called upon termination of a write.
138:             * Returns the object to signal to wake up a waiter, or null if no such
139:             **/
140:            protected synchronized Signaller endWrite() {
141:                activeWriter_ = null;
142:                if (waitingReaders_ > 0 && allowReader())
143:                    return readerLock_;
144:                else if (waitingWriters_ > 0)
145:                    return writerLock_;
146:                else
147:                    return null;
148:            }
149:
150:            /**
151:             * Reader and Writer requests are maintained in two different
152:             * wait sets, by two different objects. These objects do not
153:             * know whether the wait sets need notification since they
154:             * don't know preference rules. So, each supports a
155:             * method that can be selected by main controlling object
156:             * to perform the notifications.  This base class simplifies mechanics.
157:             **/
158:
159:            protected abstract class Signaller { // base for ReaderLock and WriterLock
160:                abstract void signalWaiters();
161:            }
162:
163:            protected class ReaderLock extends Signaller implements  Sync {
164:
165:                public void acquire() throws InterruptedException {
166:                    if (Thread.interrupted())
167:                        throw new InterruptedException();
168:                    InterruptedException ie = null;
169:                    synchronized (this ) {
170:                        if (!startReadFromNewReader()) {
171:                            for (;;) {
172:                                try {
173:                                    ReaderLock.this .wait();
174:                                    if (startReadFromWaitingReader())
175:                                        return;
176:                                } catch (InterruptedException ex) {
177:                                    cancelledWaitingReader();
178:                                    ie = ex;
179:                                    break;
180:                                }
181:                            }
182:                        }
183:                    }
184:                    if (ie != null) {
185:                        // fall through outside synch on interrupt.
186:                        // This notification is not really needed here,
187:                        //   but may be in plausible subclasses
188:                        writerLock_.signalWaiters();
189:                        throw ie;
190:                    }
191:                }
192:
193:                public void release() {
194:                    Signaller s = endRead();
195:                    if (s != null)
196:                        s.signalWaiters();
197:                }
198:
199:                synchronized void signalWaiters() {
200:                    ReaderLock.this .notifyAll();
201:                }
202:
203:                public boolean attempt(long msecs) throws InterruptedException {
204:                    if (Thread.interrupted())
205:                        throw new InterruptedException();
206:                    InterruptedException ie = null;
207:                    synchronized (this ) {
208:                        if (msecs <= 0)
209:                            return startRead();
210:                        else if (startReadFromNewReader())
211:                            return true;
212:                        else {
213:                            long waitTime = msecs;
214:                            long start = System.currentTimeMillis();
215:                            for (;;) {
216:                                try {
217:                                    ReaderLock.this .wait(waitTime);
218:                                } catch (InterruptedException ex) {
219:                                    cancelledWaitingReader();
220:                                    ie = ex;
221:                                    break;
222:                                }
223:                                if (startReadFromWaitingReader())
224:                                    return true;
225:                                else {
226:                                    waitTime = msecs
227:                                            - (System.currentTimeMillis() - start);
228:                                    if (waitTime <= 0) {
229:                                        cancelledWaitingReader();
230:                                        break;
231:                                    }
232:                                }
233:                            }
234:                        }
235:                    }
236:                    // safeguard on interrupt or timeout:
237:                    writerLock_.signalWaiters();
238:                    if (ie != null)
239:                        throw ie;
240:                    else
241:                        return false; // timed out
242:                }
243:
244:            }
245:
246:            protected class WriterLock extends Signaller implements  Sync {
247:
248:                public void acquire() throws InterruptedException {
249:                    if (Thread.interrupted())
250:                        throw new InterruptedException();
251:                    InterruptedException ie = null;
252:                    synchronized (this ) {
253:                        if (!startWriteFromNewWriter()) {
254:                            for (;;) {
255:                                try {
256:                                    WriterLock.this .wait();
257:                                    if (startWriteFromWaitingWriter())
258:                                        return;
259:                                } catch (InterruptedException ex) {
260:                                    cancelledWaitingWriter();
261:                                    WriterLock.this .notify();
262:                                    ie = ex;
263:                                    break;
264:                                }
265:                            }
266:                        }
267:                    }
268:                    if (ie != null) {
269:                        // Fall through outside synch on interrupt.
270:                        //  On exception, we may need to signal readers.
271:                        //  It is not worth checking here whether it is strictly necessary.
272:                        readerLock_.signalWaiters();
273:                        throw ie;
274:                    }
275:                }
276:
277:                public void release() {
278:                    Signaller s = endWrite();
279:                    if (s != null)
280:                        s.signalWaiters();
281:                }
282:
283:                synchronized void signalWaiters() {
284:                    WriterLock.this .notify();
285:                }
286:
287:                public boolean attempt(long msecs) throws InterruptedException {
288:                    if (Thread.interrupted())
289:                        throw new InterruptedException();
290:                    InterruptedException ie = null;
291:                    synchronized (this ) {
292:                        if (msecs <= 0)
293:                            return startWrite();
294:                        else if (startWriteFromNewWriter())
295:                            return true;
296:                        else {
297:                            long waitTime = msecs;
298:                            long start = System.currentTimeMillis();
299:                            for (;;) {
300:                                try {
301:                                    WriterLock.this .wait(waitTime);
302:                                } catch (InterruptedException ex) {
303:                                    cancelledWaitingWriter();
304:                                    WriterLock.this .notify();
305:                                    ie = ex;
306:                                    break;
307:                                }
308:                                if (startWriteFromWaitingWriter())
309:                                    return true;
310:                                else {
311:                                    waitTime = msecs
312:                                            - (System.currentTimeMillis() - start);
313:                                    if (waitTime <= 0) {
314:                                        cancelledWaitingWriter();
315:                                        WriterLock.this .notify();
316:                                        break;
317:                                    }
318:                                }
319:                            }
320:                        }
321:                    }
322:
323:                    readerLock_.signalWaiters();
324:                    if (ie != null)
325:                        throw ie;
326:                    else
327:                        return false; // timed out
328:                }
329:
330:            }
331:
332:        }
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