| 23 |
|
* the lock is held. When it is even (i.e., ({@code lock.getSequence() |
| 24 |
|
* & 1L) == 0L}), the lock is released. Method {@link |
| 25 |
|
* #awaitAvailability} can be used to await availability of the lock, |
| 26 |
< |
* returning its current sequence number. Sequence numbers are of type |
| 27 |
< |
* {@code long} to ensure that they will not wrap around until |
| 28 |
< |
* hundreds of years of use under current processor rates. A |
| 29 |
< |
* SequenceLock can be created with a specified number of |
| 30 |
< |
* spins. Attempts to lock or await release will retry at least the |
| 31 |
< |
* given number of times before blocking. If not specified, a default, |
| 32 |
< |
* possibly platform-specific, value is used. |
| 26 |
> |
* returning its current sequence number. Sequence numbers (as well as |
| 27 |
> |
* reentrant hold counts) are of type {@code long} to ensure that they |
| 28 |
> |
* will not wrap around until hundreds of years of use under current |
| 29 |
> |
* processor rates. A SequenceLock can be created with a specified |
| 30 |
> |
* number of spins. Attempts to acquire the lock in method {@link |
| 31 |
> |
* #lock} will retry at least the given number of times before |
| 32 |
> |
* blocking. If not specified, a default, possibly platform-specific, |
| 33 |
> |
* value is used. |
| 34 |
|
* |
| 35 |
|
* <p>Except for the lack of support for specified fairness policies, |
| 36 |
|
* or {@link Condition} objects, a SequenceLock can be used in the |
| 37 |
|
* same way as {@link ReentrantLock}. It provides similar status and |
| 38 |
< |
* monitoring methods such as {@link #isHeldByCurrentThread}. |
| 38 |
> |
* monitoring methods, such as {@link #isHeldByCurrentThread}. |
| 39 |
|
* SequenceLocks may be preferable in contexts in which multiple |
| 40 |
|
* threads invoke short read-only methods much more frequently than |
| 41 |
|
* fully locked methods. |
| 43 |
|
* <p> Methods {@code awaitAvailability} and {@code getSequence} can |
| 44 |
|
* be used together to define (partially) optimistic read-only methods |
| 45 |
|
* that are usually more efficient than ReadWriteLocks when they |
| 46 |
< |
* apply. These read-only methods typically read multiple field |
| 47 |
< |
* values into local variables when the lock is not held, retrying if |
| 48 |
< |
* the sequence number changed while doing so. Alternatively, because |
| 49 |
< |
* {@code awaitAvailability} accommodates reentrancy, a method can |
| 50 |
< |
* retry a bounded number of times before switching to locking mode. |
| 51 |
< |
* While conceptually straightforward, expressing these ideas can be |
| 52 |
< |
* verbose. For example: |
| 46 |
> |
* apply. These methods should in general be structured as loops that |
| 47 |
> |
* await lock availability, then read {@code volatile} fields into |
| 48 |
> |
* local variables (and may further read other values derived from |
| 49 |
> |
* these, for example the {@code length} of a {@code volatile} array), |
| 50 |
> |
* and retry if the sequence number changed while doing so. |
| 51 |
> |
* Alternatively, because {@code awaitAvailability} accommodates |
| 52 |
> |
* reentrancy, a method can retry a bounded number of times before |
| 53 |
> |
* switching to locking mode. While conceptually straightforward, |
| 54 |
> |
* expressing these ideas can be verbose. For example: |
| 55 |
|
* |
| 56 |
< |
* <pre> {@code |
| 56 |
> |
* <pre> {@code |
| 57 |
|
* class Point { |
| 58 |
< |
* private float x, y; |
| 59 |
< |
* private final SequenceLock sl = new SequenceLock(); |
| 58 |
> |
* private volatile double x, y; |
| 59 |
> |
* private final SequenceLock sl = new SequenceLock(); |
| 60 |
|
* |
| 61 |
< |
* void move(float deltaX, float deltaY) { // an exclusively locked method |
| 62 |
< |
* sl.lock(); |
| 63 |
< |
* try { |
| 64 |
< |
* x += deltaX; |
| 65 |
< |
* y += deltaY; |
| 66 |
< |
* } finally { |
| 67 |
< |
* sl.unlock(); |
| 68 |
< |
* } |
| 69 |
< |
* } |
| 61 |
> |
* // an exclusively locked method |
| 62 |
> |
* void move(double deltaX, double deltaY) { |
| 63 |
> |
* sl.lock(); |
| 64 |
> |
* try { |
| 65 |
> |
* x += deltaX; |
| 66 |
> |
* y += deltaY; |
| 67 |
> |
* } finally { |
| 68 |
> |
* sl.unlock(); |
| 69 |
> |
* } |
| 70 |
> |
* } |
| 71 |
|
* |
| 72 |
< |
* float distanceFromOriginV1() { // A read-only method |
| 73 |
< |
* float currentX, currentY; |
| 74 |
< |
* long seq; |
| 75 |
< |
* do { |
| 76 |
< |
* seq = sl.awaitAvailability(); |
| 77 |
< |
* currentX = x; |
| 78 |
< |
* currentY = y; |
| 79 |
< |
* } while (sl.getSequence() != seq); // retry if sequence changed |
| 80 |
< |
* return (float)Math.sqrt(currentX * currentX + currentY * currentY); |
| 81 |
< |
* } |
| 72 |
> |
* // A read-only method |
| 73 |
> |
* double distanceFromOriginV1() { |
| 74 |
> |
* double currentX, currentY; |
| 75 |
> |
* long seq; |
| 76 |
> |
* do { |
| 77 |
> |
* seq = sl.awaitAvailability(); |
| 78 |
> |
* currentX = x; |
| 79 |
> |
* currentY = y; |
| 80 |
> |
* } while (sl.getSequence() != seq); // retry if sequence changed |
| 81 |
> |
* return Math.sqrt(currentX * currentX + currentY * currentY); |
| 82 |
> |
* } |
| 83 |
|
* |
| 84 |
< |
* float distanceFromOriginV2() { // Uses bounded retries before locking |
| 85 |
< |
* float currentX, currentY; |
| 86 |
< |
* long seq; |
| 87 |
< |
* int retries = RETRIES_BEFORE_LOCKING; // for example 8 |
| 88 |
< |
* try { |
| 89 |
< |
* do { |
| 90 |
< |
* if (--retries < 0) |
| 91 |
< |
* sl.lock(); |
| 92 |
< |
* seq = sl.awaitAvailability(); |
| 93 |
< |
* currentX = x; |
| 94 |
< |
* currentY = y; |
| 95 |
< |
* } while (sl.getSequence() != seq); |
| 96 |
< |
* } finally { |
| 97 |
< |
* if (retries < 0) |
| 98 |
< |
* sl.unlock(); |
| 99 |
< |
* } |
| 100 |
< |
* return (float)Math.sqrt(currentX * currentX + currentY * currentY); |
| 101 |
< |
* } |
| 102 |
< |
*}}</pre> |
| 84 |
> |
* // Uses bounded retries before locking |
| 85 |
> |
* double distanceFromOriginV2() { |
| 86 |
> |
* double currentX, currentY; |
| 87 |
> |
* long seq; |
| 88 |
> |
* int retries = RETRIES_BEFORE_LOCKING; // for example 8 |
| 89 |
> |
* try { |
| 90 |
> |
* do { |
| 91 |
> |
* if (--retries < 0) |
| 92 |
> |
* sl.lock(); |
| 93 |
> |
* seq = sl.awaitAvailability(); |
| 94 |
> |
* currentX = x; |
| 95 |
> |
* currentY = y; |
| 96 |
> |
* } while (sl.getSequence() != seq); |
| 97 |
> |
* } finally { |
| 98 |
> |
* if (retries < 0) |
| 99 |
> |
* sl.unlock(); |
| 100 |
> |
* } |
| 101 |
> |
* return Math.sqrt(currentX * currentX + currentY * currentY); |
| 102 |
> |
* } |
| 103 |
> |
* }}</pre> |
| 104 |
|
* |
| 105 |
|
* @since 1.8 |
| 106 |
|
* @author Doug Lea |
| 117 |
|
/** |
| 118 |
|
* The number of reentrant holds on this lock. Uses a long for |
| 119 |
|
* compatibility with other AbstractQueuedLongSynchronizer |
| 120 |
< |
* operations. |
| 120 |
> |
* operations. Accessed only by lock holder. |
| 121 |
|
*/ |
| 122 |
|
long holds; |
| 123 |
|
|
| 191 |
|
|
| 192 |
|
final long awaitAvailability() { |
| 193 |
|
long s; |
| 188 |
– |
int k = spins; |
| 194 |
|
while (((s = getState()) & 1L) != 0L && |
| 195 |
|
getExclusiveOwnerThread() != Thread.currentThread()) { |
| 196 |
< |
if (k > 0) |
| 197 |
< |
--k; |
| 193 |
< |
else { |
| 194 |
< |
acquireShared(1L); |
| 195 |
< |
releaseShared(1L); |
| 196 |
< |
} |
| 196 |
> |
acquireShared(1L); |
| 197 |
> |
releaseShared(1L); |
| 198 |
|
} |
| 199 |
|
return s; |
| 200 |
|
} |
| 249 |
|
|
| 250 |
|
/** |
| 251 |
|
* Creates an instance of {@code SequenceLock} with the default |
| 252 |
< |
* number of retry attempts to lock or await release before |
| 252 |
< |
* blocking. |
| 252 |
> |
* number of retry attempts to acquire the lock before blocking. |
| 253 |
|
*/ |
| 254 |
|
public SequenceLock() { sync = new Sync(DEFAULT_SPINS); } |
| 255 |
|
|
| 256 |
|
/** |
| 257 |
< |
* Creates an instance of {@code SequenceLock} that |
| 258 |
< |
* will retry attempts to lock or await release |
| 259 |
< |
* at least the given number times before blocking. |
| 257 |
> |
* Creates an instance of {@code SequenceLock} that will retry |
| 258 |
> |
* attempts to acquire the lock at least the given number times |
| 259 |
> |
* before blocking. |
| 260 |
|
*/ |
| 261 |
|
public SequenceLock(int spins) { sync = new Sync(spins); } |
| 262 |
|
|
| 306 |
|
* @param timeout the time to wait for availability |
| 307 |
|
* @param unit the time unit of the timeout argument |
| 308 |
|
* @return the current sequence number if the lock is available |
| 309 |
< |
* upon return from this method. |
| 309 |
> |
* upon return from this method |
| 310 |
|
* @throws InterruptedException if the current thread is interrupted |
| 311 |
|
* @throws TimeoutException if the lock was not available within |
| 312 |
< |
* the specified waiting time. |
| 312 |
> |
* the specified waiting time |
| 313 |
|
* @throws NullPointerException if the time unit is null |
| 314 |
|
*/ |
| 315 |
|
public long tryAwaitAvailability(long timeout, TimeUnit unit) |
