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Java Source Code / Java Documentation » Database DBMS » Quadcap Embeddable Database » javax.concurrent 
Source Cross Referenced  Class Diagram Java Document (Java Doc) 


001:        /*
002:          File: Sync.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               Added some convenient time constants
013:         */
014:
015:        package javax.concurrent;
016:
017:        /**
018:         * Main interface for locks, gates, and conditions.
019:         * <p>
020:         * Sync objects isolate waiting and notification for particular
021:         * logical states, resource availability, events, and the like that are
022:         * shared across multiple threads. Use of Syncs sometimes
023:         * (but by no means always) adds flexibility and efficiency
024:         * compared to the use of plain java monitor methods
025:         * and locking, and are sometimes (but by no means always)
026:         * simpler to program with.
027:         * <p>
028:         *
029:         * Most Syncs are intended to be used primarily (although
030:         * not exclusively) in  before/after constructions such as:
031:         * <pre>
032:         * class X {
033:         *   Sync gate;
034:         *   // ...
035:         *
036:         *   public void m() { 
037:         *     try {
038:         *       gate.acquire();  // block until condition holds
039:         *       try {
040:         *         // ... method body
041:         *       }
042:         *       finally {
043:         *         gate.release()
044:         *       }
045:         *     }
046:         *     catch (InterruptedException ex) {
047:         *       // ... evasive action
048:         *     }
049:         *   }
050:         *
051:         *   public void m2(Sync cond) { // use supplied condition
052:         *     try {
053:         *       if (cond.attempt(10)) {         // try the condition for 10 ms
054:         *         try {
055:         *           // ... method body
056:         *         }
057:         *         finally {
058:         *           cond.release()
059:         *         }
060:         *       }
061:         *     }
062:         *     catch (InterruptedException ex) {
063:         *       // ... evasive action
064:         *     }
065:         *   }
066:         * }
067:         * </pre>
068:         * Syncs may be used in somewhat tedious but more flexible replacements
069:         * for built-in Java synchronized blocks. For example:
070:         * <pre>
071:         * class HandSynched {          
072:         *   private double state_ = 0.0; 
073:         *   private final Sync lock;  // use lock type supplied in constructor
074:         *   public HandSynched(Sync l) { lock = l; } 
075:         *
076:         *   public void changeState(double d) {
077:         *     try {
078:         *       lock.acquire(); 
079:         *       try     { state_ = updateFunction(d); } 
080:         *       finally { lock.release(); }
081:         *     } 
082:         *     catch(InterruptedException ex) { }
083:         *   }
084:         *
085:         *   public double getState() {
086:         *     double d = 0.0;
087:         *     try {
088:         *       lock.acquire(); 
089:         *       try     { d = accessFunction(state_); }
090:         *       finally { lock.release(); }
091:         *     } 
092:         *     catch(InterruptedException ex){}
093:         *     return d;
094:         *   }
095:         *   private double updateFunction(double d) { ... }
096:         *   private double accessFunction(double d) { ... }
097:         * }
098:         * </pre>
099:         * If you have a lot of such methods, and they take a common
100:         * form, you can standardize this using wrappers. Some of these
101:         * wrappers are standardized in LockedExecutor, but you can make others.
102:         * For example:
103:         * <pre>
104:         * class HandSynchedV2 {          
105:         *   private double state_ = 0.0; 
106:         *   private final Sync lock;  // use lock type supplied in constructor
107:         *   public HandSynchedV2(Sync l) { lock = l; } 
108:         *
109:         *   protected void runSafely(Runnable r) {
110:         *     try {
111:         *       lock.acquire();
112:         *       try { r.run(); }
113:         *       finally { lock.release(); }
114:         *     }
115:         *     catch (InterruptedException ex) { // propagate without throwing
116:         *       Thread.currentThread().interrupt();
117:         *     }
118:         *   }
119:         *
120:         *   public void changeState(double d) {
121:         *     runSafely(new Runnable() {
122:         *       public void run() { state_ = updateFunction(d); } 
123:         *     });
124:         *   }
125:         *   // ...
126:         * }
127:         * </pre>
128:         * <p>
129:         * One reason to bother with such constructions is to use deadlock-
130:         * avoiding back-offs when dealing with locks involving multiple objects.
131:         * For example, here is a Cell class that uses attempt to back-off
132:         * and retry if two Cells are trying to swap values with each other 
133:         * at the same time.
134:         * <pre>
135:         * class Cell {
136:         *   long value;
137:         *   Sync lock = ... // some sync implementation class
138:         *   void swapValue(Cell other) {
139:         *     for (;;) { 
140:         *       try {
141:         *         lock.acquire();
142:         *         try {
143:         *           if (other.lock.attempt(100)) {
144:         *             try { 
145:         *               long t = value; 
146:         *               value = other.value;
147:         *               other.value = t;
148:         *               return;
149:         *             }
150:         *             finally { other.lock.release(); }
151:         *           }
152:         *         }
153:         *         finally { lock.release(); }
154:         *       } 
155:         *       catch (InterruptedException ex) { return; }
156:         *     }
157:         *   }
158:         * }
159:         *</pre>
160:         * <p>
161:         * Here is an even fancier version, that uses lock re-ordering
162:         * upon conflict:
163:         * <pre>
164:         * class Cell { 
165:         *   long value;
166:         *   Sync lock = ...;
167:         *   private static boolean trySwap(Cell a, Cell b) {
168:         *     a.lock.acquire();
169:         *     try {
170:         *       if (!b.lock.attempt(0)) 
171:         *         return false;
172:         *       try { 
173:         *         long t = a.value;
174:         *         a.value = b.value;
175:         *         b.value = t;
176:         *         return true;
177:         *       }
178:         *       finally { other.lock.release(); }
179:         *     }
180:         *     finally { lock.release(); }
181:         *     return false;
182:         *   }
183:         *
184:         *  void swapValue(Cell other) {
185:         *    try {
186:         *      while (!trySwap(this, other) &&
187:         *            !tryswap(other, this)) 
188:         *        Thread.sleep(1);
189:         *    }
190:         *    catch (InterruptedException ex) { return; }
191:         *  }
192:         *}
193:         *</pre>
194:         * <p>
195:         * Interruptions are in general handled as early as possible.
196:         * Normally, InterruptionExceptions are thrown
197:         * in acquire and attempt(msec) if interruption
198:         * is detected upon entry to the method, as well as in any
199:         * later context surrounding waits. 
200:         * However, interruption status is ignored in release();
201:         * <p>
202:         * Timed versions of attempt report failure via return value.
203:         * If so desired, you can transform such constructions to use exception
204:         * throws via 
205:         * <pre>
206:         *   if (!c.attempt(timeval)) throw new TimeoutException(timeval);
207:         * </pre>
208:         * <p>
209:         * The TimoutSync wrapper class can be used to automate such usages.
210:         * <p>
211:         * All time values are expressed in milliseconds as longs, which have a maximum
212:         * value of Long.MAX_VALUE, or almost 300,000 centuries. It is not
213:         * known whether JVMs actually deal correctly with such extreme values. 
214:         * For convenience, some useful time values are defined as static constants.
215:         * <p>
216:         * All implementations of the three Sync methods guarantee to
217:         * somehow employ Java <code>synchronized</code> methods or blocks,
218:         * and so entail the memory operations described in JLS
219:         * chapter 17 which ensure that variables are loaded and flushed
220:         * within before/after constructions.
221:         * <p>
222:         * Syncs may also be used in spinlock constructions. Although
223:         * it is normally best to just use acquire(), various forms
224:         * of busy waits can be implemented. For a simple example 
225:         * (but one that would probably never be preferable to using acquire()):
226:         * <pre>
227:         * class X {
228:         *   Sync lock = ...
229:         *   void spinUntilAcquired() throws InterruptedException {
230:         *     // Two phase. 
231:         *     // First spin without pausing.
232:         *     int purespins = 10; 
233:         *     for (int i = 0; i < purespins; ++i) {
234:         *       if (lock.attempt(0))
235:         *         return true;
236:         *     }
237:         *     // Second phase - use timed waits
238:         *     long waitTime = 1; // 1 millisecond
239:         *     for (;;) {
240:         *       if (lock.attempt(waitTime))
241:         *         return true;
242:         *       else 
243:         *         waitTime = waitTime * 3 / 2 + 1; // increase 50% 
244:         *     }
245:         *   }
246:         * }
247:         * </pre>
248:         * <p>
249:         * In addition pure synchronization control, Syncs
250:         * may be useful in any context requiring before/after methods.
251:         * For example, you can use an ObservableSync
252:         * (perhaps as part of a LayeredSync) in order to obtain callbacks
253:         * before and after each method invocation for a given class.
254:         * <p>
255:
256:         * <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
257:         **/
258:
259:        public interface Sync {
260:
261:            /** 
262:             *  Wait (possibly forever) until successful passage.
263:             *  Fail only upon interuption. Interruptions always result in
264:             *  `clean' failures. On failure,  you can be sure that it has not 
265:             *  been acquired, and that no 
266:             *  corresponding release should be performed. Conversely,
267:             *  a normal return guarantees that the acquire was successful.
268:             **/
269:
270:            public void acquire() throws InterruptedException;
271:
272:            /** 
273:             * Wait at most msecs to pass; report whether passed.
274:             * <p>
275:             * The method has best-effort semantics:
276:             * The msecs bound cannot
277:             * be guaranteed to be a precise upper bound on wait time in Java.
278:             * Implementations generally can only attempt to return as soon as possible
279:             * after the specified bound. Also, timers in Java do not stop during garbage
280:             * collection, so timeouts can occur just because a GC intervened.
281:             * So, msecs arguments should be used in
282:             * a coarse-grained manner. Further,
283:             * implementations cannot always guarantee that this method
284:             * will return at all without blocking indefinitely when used in
285:             * unintended ways. For example, deadlocks may be encountered
286:             * when called in an unintended context.
287:             * <p>
288:             * @param msecs the number of milleseconds to wait.
289:             * An argument less than or equal to zero means not to wait at all. 
290:             * However, this may still require
291:             * access to a synchronization lock, which can impose unbounded
292:             * delay if there is a lot of contention among threads.
293:             * @return true if acquired
294:             **/
295:
296:            public boolean attempt(long msecs) throws InterruptedException;
297:
298:            /** 
299:             * Potentially enable others to pass.
300:             * <p>
301:             * Because release does not raise exceptions, 
302:             * it can be used in `finally' clauses without requiring extra
303:             * embedded try/catch blocks. But keep in mind that
304:             * as with any java method, implementations may 
305:             * still throw unchecked exceptions such as Error or NullPointerException
306:             * when faced with uncontinuable errors. However, these should normally
307:             * only be caught by higher-level error handlers.
308:             **/
309:
310:            public void release();
311:
312:            /**  One second, in milliseconds; convenient as a time-out value **/
313:            public static final long ONE_SECOND = 1000;
314:
315:            /**  One minute, in milliseconds; convenient as a time-out value **/
316:            public static final long ONE_MINUTE = 60 * ONE_SECOND;
317:
318:            /**  One hour, in milliseconds; convenient as a time-out value **/
319:            public static final long ONE_HOUR = 60 * ONE_MINUTE;
320:
321:            /**  One day, in milliseconds; convenient as a time-out value **/
322:            public static final long ONE_DAY = 24 * ONE_HOUR;
323:
324:            /**  One week, in milliseconds; convenient as a time-out value **/
325:            public static final long ONE_WEEK = 7 * ONE_DAY;
326:
327:            /**  One year in milliseconds; convenient as a time-out value  **/
328:            // Not that it matters, but there is some variation across
329:            // standard sources about value at msec precision.
330:            // The value used is the same as in java.util.GregorianCalendar
331:            public static final long ONE_YEAR = (long) (365.2425 * ONE_DAY);
332:
333:            /**  One century in milliseconds; convenient as a time-out value **/
334:            public static final long ONE_CENTURY = 100 * ONE_YEAR;
335:
336:        }
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