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dl |
1.2 |
/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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dl |
1.24 |
* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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dl |
1.2 |
*/ |
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tim |
1.1 |
package java.util.concurrent; |
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dl |
1.16 |
import java.util.*; |
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1.5 |
import java.util.concurrent.locks.*; |
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dl |
1.16 |
import java.util.concurrent.atomic.*; |
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tim |
1.1 |
|
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/** |
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* A counting semaphore. Conceptually, a semaphore maintains a set of |
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* permits. Each {@link #acquire} blocks if necessary until a permit is |
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* available, and then takes it. Each {@link #release} adds a permit, |
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* potentially releasing a blocking acquirer. |
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* However, no actual permit objects are used; the <tt>Semaphore</tt> just |
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* keeps a count of the number available and acts accordingly. |
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* |
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dl |
1.16 |
* <p>Semaphores are often used to restrict the number of threads than can |
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tim |
1.1 |
* access some (physical or logical) resource. For example, here is |
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* a class that uses a semaphore to control access to a pool of items: |
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* <pre> |
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* class Pool { |
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dl |
1.44 |
* private static final int MAX_AVAILABLE = 100; |
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1.16 |
* private final Semaphore available = new Semaphore(MAX_AVAILABLE, true); |
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tim |
1.1 |
* |
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* public Object getItem() throws InterruptedException { |
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* available.acquire(); |
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* return getNextAvailableItem(); |
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* } |
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* |
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* public void putItem(Object x) { |
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* if (markAsUnused(x)) |
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* available.release(); |
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* } |
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* |
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* // Not a particularly efficient data structure; just for demo |
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* |
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* protected Object[] items = ... whatever kinds of items being managed |
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* protected boolean[] used = new boolean[MAX_AVAILABLE]; |
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* |
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* protected synchronized Object getNextAvailableItem() { |
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* for (int i = 0; i < MAX_AVAILABLE; ++i) { |
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* if (!used[i]) { |
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* used[i] = true; |
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* return items[i]; |
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* } |
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* } |
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* return null; // not reached |
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* } |
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* |
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* protected synchronized boolean markAsUnused(Object item) { |
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* for (int i = 0; i < MAX_AVAILABLE; ++i) { |
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* if (item == items[i]) { |
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* if (used[i]) { |
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* used[i] = false; |
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* return true; |
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1.8 |
* } else |
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tim |
1.1 |
* return false; |
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* } |
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* } |
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* return false; |
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* } |
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* |
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* } |
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* </pre> |
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* |
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dl |
1.16 |
* <p>Before obtaining an item each thread must acquire a permit from |
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* the semaphore, guaranteeing that an item is available for use. When |
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* the thread has finished with the item it is returned back to the |
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* pool and a permit is returned to the semaphore, allowing another |
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* thread to acquire that item. Note that no synchronization lock is |
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* held when {@link #acquire} is called as that would prevent an item |
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* from being returned to the pool. The semaphore encapsulates the |
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* synchronization needed to restrict access to the pool, separately |
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* from any synchronization needed to maintain the consistency of the |
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* pool itself. |
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* |
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* <p>A semaphore initialized to one, and which is used such that it |
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* only has at most one permit available, can serve as a mutual |
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* exclusion lock. This is more commonly known as a <em>binary |
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* semaphore</em>, because it only has two states: one permit |
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* available, or zero permits available. When used in this way, the |
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* binary semaphore has the property (unlike many {@link Lock} |
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dl |
1.18 |
* implementations), that the "lock" can be released by a |
87 |
dl |
1.16 |
* thread other than the owner (as semaphores have no notion of |
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* ownership). This can be useful in some specialized contexts, such |
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* as deadlock recovery. |
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* |
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dl |
1.36 |
* <p> The constructor for this class optionally accepts a |
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* <em>fairness</em> parameter. When set false, this class makes no |
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* guarantees about the order in which threads acquire permits. In |
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* particular, <em>barging</em> is permitted, that is, a thread |
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* invoking {@link #acquire} can be allocated a permit ahead of a |
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dl |
1.43 |
* thread that has been waiting - logically the new thread places itself at |
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* the head of the queue of waiting threads. When fairness is set true, the |
98 |
dl |
1.36 |
* semaphore guarantees that threads invoking any of the {@link |
99 |
dl |
1.43 |
* #acquire() acquire} methods are selected to obtain permits in the order in |
100 |
dl |
1.36 |
* which their invocation of those methods was processed |
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* (first-in-first-out; FIFO). Note that FIFO ordering necessarily |
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* applies to specific internal points of execution within these |
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* methods. So, it is possible for one thread to invoke |
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* <tt>acquire</tt> before another, but reach the ordering point after |
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* the other, and similarly upon return from the method. |
106 |
jsr166 |
1.45 |
* Also note that the untimed {@link #tryAcquire() tryAcquire} methods do not |
107 |
dl |
1.43 |
* honor the fairness setting, but will take any permits that are |
108 |
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* available. |
109 |
dl |
1.16 |
* |
110 |
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* <p>Generally, semaphores used to control resource access should be |
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* initialized as fair, to ensure that no thread is starved out from |
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* accessing a resource. When using semaphores for other kinds of |
113 |
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* synchronization control, the throughput advantages of non-fair |
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* ordering often outweigh fairness considerations. |
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* |
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* <p>This class also provides convenience methods to {@link |
117 |
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* #acquire(int) acquire} and {@link #release(int) release} multiple |
118 |
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* permits at a time. Beware of the increased risk of indefinite |
119 |
dl |
1.19 |
* postponement when these methods are used without fairness set true. |
120 |
tim |
1.1 |
* |
121 |
brian |
1.47 |
* <p> Memory visibility effects: Actions in a thread prior to calling |
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* a "release" method such as <tt>release()</tt> <a |
123 |
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* href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> |
124 |
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* actions following a successful "acquire" method such as |
125 |
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* <tt>acquire()</tt> in another thread. |
126 |
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* |
127 |
tim |
1.1 |
* @since 1.5 |
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dl |
1.4 |
* @author Doug Lea |
129 |
tim |
1.1 |
* |
130 |
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*/ |
131 |
dl |
1.16 |
|
132 |
dl |
1.23 |
public class Semaphore implements java.io.Serializable { |
133 |
dl |
1.16 |
private static final long serialVersionUID = -3222578661600680210L; |
134 |
dl |
1.25 |
/** All mechanics via AbstractQueuedSynchronizer subclass */ |
135 |
dl |
1.23 |
private final Sync sync; |
136 |
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137 |
dl |
1.25 |
/** |
138 |
dl |
1.34 |
* Synchronization implementation for semaphore. Uses AQS state |
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* to represent permits. Subclassed into fair and nonfair |
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* versions. |
141 |
dl |
1.25 |
*/ |
142 |
dl |
1.34 |
abstract static class Sync extends AbstractQueuedSynchronizer { |
143 |
dl |
1.46 |
private static final long serialVersionUID = 1192457210091910933L; |
144 |
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145 |
dl |
1.34 |
Sync(int permits) { |
146 |
dl |
1.29 |
setState(permits); |
147 |
dl |
1.23 |
} |
148 |
jsr166 |
1.45 |
|
149 |
dl |
1.34 |
final int getPermits() { |
150 |
dl |
1.29 |
return getState(); |
151 |
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} |
152 |
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153 |
dl |
1.34 |
final int nonfairTryAcquireShared(int acquires) { |
154 |
dl |
1.23 |
for (;;) { |
155 |
dl |
1.29 |
int available = getState(); |
156 |
dl |
1.23 |
int remaining = available - acquires; |
157 |
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if (remaining < 0 || |
158 |
dl |
1.29 |
compareAndSetState(available, remaining)) |
159 |
dl |
1.23 |
return remaining; |
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} |
161 |
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} |
162 |
jsr166 |
1.45 |
|
163 |
dl |
1.34 |
protected final boolean tryReleaseShared(int releases) { |
164 |
dl |
1.23 |
for (;;) { |
165 |
dl |
1.29 |
int p = getState(); |
166 |
jsr166 |
1.45 |
if (compareAndSetState(p, p + releases)) |
167 |
dl |
1.23 |
return true; |
168 |
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} |
169 |
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} |
170 |
dl |
1.29 |
|
171 |
dl |
1.34 |
final void reducePermits(int reductions) { |
172 |
dl |
1.29 |
for (;;) { |
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int current = getState(); |
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int next = current - reductions; |
175 |
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if (compareAndSetState(current, next)) |
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return; |
177 |
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} |
178 |
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} |
179 |
dl |
1.31 |
|
180 |
dl |
1.34 |
final int drainPermits() { |
181 |
dl |
1.31 |
for (;;) { |
182 |
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int current = getState(); |
183 |
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if (current == 0 || compareAndSetState(current, 0)) |
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return current; |
185 |
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} |
186 |
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} |
187 |
dl |
1.23 |
} |
188 |
dl |
1.16 |
|
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tim |
1.1 |
/** |
190 |
dl |
1.34 |
* NonFair version |
191 |
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*/ |
192 |
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final static class NonfairSync extends Sync { |
193 |
dl |
1.46 |
private static final long serialVersionUID = -2694183684443567898L; |
194 |
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195 |
dl |
1.34 |
NonfairSync(int permits) { |
196 |
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super(permits); |
197 |
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} |
198 |
jsr166 |
1.45 |
|
199 |
dl |
1.34 |
protected int tryAcquireShared(int acquires) { |
200 |
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return nonfairTryAcquireShared(acquires); |
201 |
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} |
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} |
203 |
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204 |
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/** |
205 |
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* Fair version |
206 |
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*/ |
207 |
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final static class FairSync extends Sync { |
208 |
dl |
1.46 |
private static final long serialVersionUID = 2014338818796000944L; |
209 |
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210 |
dl |
1.34 |
FairSync(int permits) { |
211 |
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super(permits); |
212 |
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} |
213 |
jsr166 |
1.45 |
|
214 |
dl |
1.34 |
protected int tryAcquireShared(int acquires) { |
215 |
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Thread current = Thread.currentThread(); |
216 |
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for (;;) { |
217 |
dl |
1.35 |
Thread first = getFirstQueuedThread(); |
218 |
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if (first != null && first != current) |
219 |
dl |
1.34 |
return -1; |
220 |
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int available = getState(); |
221 |
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int remaining = available - acquires; |
222 |
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if (remaining < 0 || |
223 |
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compareAndSetState(available, remaining)) |
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return remaining; |
225 |
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} |
226 |
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} |
227 |
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} |
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/** |
230 |
dl |
1.41 |
* Creates a <tt>Semaphore</tt> with the given number of |
231 |
dl |
1.36 |
* permits and nonfair fairness setting. |
232 |
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* @param permits the initial number of permits available. This |
233 |
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* value may be negative, in which case releases must |
234 |
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* occur before any acquires will be granted. |
235 |
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*/ |
236 |
jsr166 |
1.45 |
public Semaphore(int permits) { |
237 |
dl |
1.36 |
sync = new NonfairSync(permits); |
238 |
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} |
239 |
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240 |
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/** |
241 |
dl |
1.41 |
* Creates a <tt>Semaphore</tt> with the given number of |
242 |
dl |
1.19 |
* permits and the given fairness setting. |
243 |
dl |
1.16 |
* @param permits the initial number of permits available. This |
244 |
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* value may be negative, in which case releases must |
245 |
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* occur before any acquires will be granted. |
246 |
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* @param fair true if this semaphore will guarantee first-in |
247 |
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* first-out granting of permits under contention, else false. |
248 |
tim |
1.1 |
*/ |
249 |
jsr166 |
1.45 |
public Semaphore(int permits, boolean fair) { |
250 |
dl |
1.34 |
sync = (fair)? new FairSync(permits) : new NonfairSync(permits); |
251 |
tim |
1.1 |
} |
252 |
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253 |
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/** |
254 |
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* Acquires a permit from this semaphore, blocking until one is |
255 |
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* available, or the thread is {@link Thread#interrupt interrupted}. |
256 |
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* |
257 |
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* <p>Acquires a permit, if one is available and returns immediately, |
258 |
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* reducing the number of available permits by one. |
259 |
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* <p>If no permit is available then the current thread becomes |
260 |
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* disabled for thread scheduling purposes and lies dormant until |
261 |
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* one of two things happens: |
262 |
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* <ul> |
263 |
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* <li>Some other thread invokes the {@link #release} method for this |
264 |
dl |
1.16 |
* semaphore and the current thread is next to be assigned a permit; or |
265 |
tim |
1.1 |
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
266 |
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* thread. |
267 |
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* </ul> |
268 |
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* |
269 |
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* <p>If the current thread: |
270 |
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* <ul> |
271 |
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* <li>has its interrupted status set on entry to this method; or |
272 |
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* <li>is {@link Thread#interrupt interrupted} while waiting |
273 |
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* for a permit, |
274 |
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* </ul> |
275 |
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* then {@link InterruptedException} is thrown and the current thread's |
276 |
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* interrupted status is cleared. |
277 |
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* |
278 |
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* @throws InterruptedException if the current thread is interrupted |
279 |
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* |
280 |
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* @see Thread#interrupt |
281 |
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*/ |
282 |
dl |
1.2 |
public void acquire() throws InterruptedException { |
283 |
dl |
1.23 |
sync.acquireSharedInterruptibly(1); |
284 |
dl |
1.2 |
} |
285 |
tim |
1.1 |
|
286 |
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/** |
287 |
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* Acquires a permit from this semaphore, blocking until one is |
288 |
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* available. |
289 |
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* |
290 |
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* <p>Acquires a permit, if one is available and returns immediately, |
291 |
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* reducing the number of available permits by one. |
292 |
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* <p>If no permit is available then the current thread becomes |
293 |
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* disabled for thread scheduling purposes and lies dormant until |
294 |
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* some other thread invokes the {@link #release} method for this |
295 |
dl |
1.16 |
* semaphore and the current thread is next to be assigned a permit. |
296 |
tim |
1.1 |
* |
297 |
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* <p>If the current thread |
298 |
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* is {@link Thread#interrupt interrupted} while waiting |
299 |
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* for a permit then it will continue to wait, but the time at which |
300 |
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* the thread is assigned a permit may change compared to the time it |
301 |
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* would have received the permit had no interruption occurred. When the |
302 |
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* thread does return from this method its interrupt status will be set. |
303 |
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* |
304 |
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*/ |
305 |
dl |
1.2 |
public void acquireUninterruptibly() { |
306 |
dl |
1.39 |
sync.acquireShared(1); |
307 |
dl |
1.2 |
} |
308 |
tim |
1.1 |
|
309 |
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/** |
310 |
jsr166 |
1.45 |
* Acquires a permit from this semaphore, only if one is available at the |
311 |
tim |
1.1 |
* time of invocation. |
312 |
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* <p>Acquires a permit, if one is available and returns immediately, |
313 |
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* with the value <tt>true</tt>, |
314 |
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* reducing the number of available permits by one. |
315 |
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* |
316 |
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* <p>If no permit is available then this method will return |
317 |
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* immediately with the value <tt>false</tt>. |
318 |
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* |
319 |
dl |
1.27 |
* <p>Even when this semaphore has been set to use a |
320 |
|
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* fair ordering policy, a call to <tt>tryAcquire()</tt> <em>will</em> |
321 |
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* immediately acquire a permit if one is available, whether or not |
322 |
jsr166 |
1.45 |
* other threads are currently waiting. |
323 |
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* This "barging" behavior can be useful in certain |
324 |
dl |
1.27 |
* circumstances, even though it breaks fairness. If you want to honor |
325 |
jsr166 |
1.45 |
* the fairness setting, then use |
326 |
dl |
1.27 |
* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) } |
327 |
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* which is almost equivalent (it also detects interruption). |
328 |
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* |
329 |
tim |
1.1 |
* @return <tt>true</tt> if a permit was acquired and <tt>false</tt> |
330 |
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* otherwise. |
331 |
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*/ |
332 |
|
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public boolean tryAcquire() { |
333 |
dl |
1.34 |
return sync.nonfairTryAcquireShared(1) >= 0; |
334 |
tim |
1.1 |
} |
335 |
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|
336 |
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/** |
337 |
jsr166 |
1.45 |
* Acquires a permit from this semaphore, if one becomes available |
338 |
tim |
1.1 |
* within the given waiting time and the |
339 |
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* current thread has not been {@link Thread#interrupt interrupted}. |
340 |
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* <p>Acquires a permit, if one is available and returns immediately, |
341 |
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* with the value <tt>true</tt>, |
342 |
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* reducing the number of available permits by one. |
343 |
|
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* <p>If no permit is available then |
344 |
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* the current thread becomes disabled for thread scheduling |
345 |
|
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* purposes and lies dormant until one of three things happens: |
346 |
|
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* <ul> |
347 |
|
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* <li>Some other thread invokes the {@link #release} method for this |
348 |
dl |
1.16 |
* semaphore and the current thread is next to be assigned a permit; or |
349 |
tim |
1.1 |
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
350 |
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* thread; or |
351 |
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* <li>The specified waiting time elapses. |
352 |
|
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* </ul> |
353 |
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* <p>If a permit is acquired then the value <tt>true</tt> is returned. |
354 |
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* <p>If the current thread: |
355 |
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* <ul> |
356 |
|
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* <li>has its interrupted status set on entry to this method; or |
357 |
|
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* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
358 |
|
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* a permit, |
359 |
|
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* </ul> |
360 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
361 |
|
|
* interrupted status is cleared. |
362 |
|
|
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
363 |
|
|
* is returned. |
364 |
jsr166 |
1.45 |
* If the time is less than or equal to zero, the method will not wait |
365 |
dl |
1.16 |
* at all. |
366 |
tim |
1.1 |
* |
367 |
|
|
* @param timeout the maximum time to wait for a permit |
368 |
dl |
1.13 |
* @param unit the time unit of the <tt>timeout</tt> argument. |
369 |
tim |
1.1 |
* @return <tt>true</tt> if a permit was acquired and <tt>false</tt> |
370 |
|
|
* if the waiting time elapsed before a permit was acquired. |
371 |
|
|
* |
372 |
|
|
* @throws InterruptedException if the current thread is interrupted |
373 |
|
|
* |
374 |
|
|
* @see Thread#interrupt |
375 |
|
|
* |
376 |
|
|
*/ |
377 |
jsr166 |
1.45 |
public boolean tryAcquire(long timeout, TimeUnit unit) |
378 |
tim |
1.1 |
throws InterruptedException { |
379 |
dl |
1.39 |
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); |
380 |
tim |
1.1 |
} |
381 |
|
|
|
382 |
|
|
/** |
383 |
|
|
* Releases a permit, returning it to the semaphore. |
384 |
|
|
* <p>Releases a permit, increasing the number of available permits |
385 |
|
|
* by one. |
386 |
dl |
1.43 |
* If any threads are trying to acquire a permit, then one |
387 |
tim |
1.1 |
* is selected and given the permit that was just released. |
388 |
dl |
1.43 |
* That thread is (re)enabled for thread scheduling purposes. |
389 |
tim |
1.1 |
* <p>There is no requirement that a thread that releases a permit must |
390 |
|
|
* have acquired that permit by calling {@link #acquire}. |
391 |
|
|
* Correct usage of a semaphore is established by programming convention |
392 |
|
|
* in the application. |
393 |
|
|
*/ |
394 |
dl |
1.2 |
public void release() { |
395 |
dl |
1.23 |
sync.releaseShared(1); |
396 |
dl |
1.16 |
} |
397 |
jsr166 |
1.45 |
|
398 |
dl |
1.16 |
/** |
399 |
jsr166 |
1.45 |
* Acquires the given number of permits from this semaphore, |
400 |
|
|
* blocking until all are available, |
401 |
dl |
1.16 |
* or the thread is {@link Thread#interrupt interrupted}. |
402 |
|
|
* |
403 |
|
|
* <p>Acquires the given number of permits, if they are available, |
404 |
|
|
* and returns immediately, |
405 |
|
|
* reducing the number of available permits by the given amount. |
406 |
|
|
* |
407 |
|
|
* <p>If insufficient permits are available then the current thread becomes |
408 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
409 |
|
|
* one of two things happens: |
410 |
|
|
* <ul> |
411 |
jsr166 |
1.45 |
* <li>Some other thread invokes one of the {@link #release() release} |
412 |
dl |
1.16 |
* methods for this semaphore, the current thread is next to be assigned |
413 |
|
|
* permits and the number of available permits satisfies this request; or |
414 |
|
|
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
415 |
|
|
* thread. |
416 |
|
|
* </ul> |
417 |
|
|
* |
418 |
|
|
* <p>If the current thread: |
419 |
|
|
* <ul> |
420 |
|
|
* <li>has its interrupted status set on entry to this method; or |
421 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting |
422 |
|
|
* for a permit, |
423 |
|
|
* </ul> |
424 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
425 |
jsr166 |
1.45 |
* interrupted status is cleared. |
426 |
|
|
* Any permits that were to be assigned to this thread are instead |
427 |
dl |
1.43 |
* assigned to other threads trying to acquire permits, as if |
428 |
|
|
* permits had been made available by a call to {@link #release()}. |
429 |
dl |
1.16 |
* |
430 |
|
|
* @param permits the number of permits to acquire |
431 |
|
|
* |
432 |
|
|
* @throws InterruptedException if the current thread is interrupted |
433 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
434 |
|
|
* |
435 |
|
|
* @see Thread#interrupt |
436 |
|
|
*/ |
437 |
|
|
public void acquire(int permits) throws InterruptedException { |
438 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
439 |
dl |
1.23 |
sync.acquireSharedInterruptibly(permits); |
440 |
dl |
1.16 |
} |
441 |
|
|
|
442 |
|
|
/** |
443 |
jsr166 |
1.45 |
* Acquires the given number of permits from this semaphore, |
444 |
dl |
1.16 |
* blocking until all are available. |
445 |
|
|
* |
446 |
|
|
* <p>Acquires the given number of permits, if they are available, |
447 |
|
|
* and returns immediately, |
448 |
|
|
* reducing the number of available permits by the given amount. |
449 |
|
|
* |
450 |
|
|
* <p>If insufficient permits are available then the current thread becomes |
451 |
|
|
* disabled for thread scheduling purposes and lies dormant until |
452 |
jsr166 |
1.45 |
* some other thread invokes one of the {@link #release() release} |
453 |
dl |
1.16 |
* methods for this semaphore, the current thread is next to be assigned |
454 |
|
|
* permits and the number of available permits satisfies this request. |
455 |
|
|
* |
456 |
|
|
* <p>If the current thread |
457 |
|
|
* is {@link Thread#interrupt interrupted} while waiting |
458 |
|
|
* for permits then it will continue to wait and its position in the |
459 |
|
|
* queue is not affected. When the |
460 |
|
|
* thread does return from this method its interrupt status will be set. |
461 |
|
|
* |
462 |
|
|
* @param permits the number of permits to acquire |
463 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
464 |
|
|
* |
465 |
|
|
*/ |
466 |
|
|
public void acquireUninterruptibly(int permits) { |
467 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
468 |
dl |
1.39 |
sync.acquireShared(permits); |
469 |
dl |
1.16 |
} |
470 |
|
|
|
471 |
|
|
/** |
472 |
dl |
1.40 |
* Acquires the given number of permits from this semaphore, only |
473 |
|
|
* if all are available at the time of invocation. |
474 |
dl |
1.41 |
* |
475 |
jsr166 |
1.45 |
* <p>Acquires the given number of permits, if they are available, and |
476 |
dl |
1.16 |
* returns immediately, with the value <tt>true</tt>, |
477 |
|
|
* reducing the number of available permits by the given amount. |
478 |
|
|
* |
479 |
|
|
* <p>If insufficient permits are available then this method will return |
480 |
|
|
* immediately with the value <tt>false</tt> and the number of available |
481 |
|
|
* permits is unchanged. |
482 |
|
|
* |
483 |
dl |
1.27 |
* <p>Even when this semaphore has been set to use a fair ordering |
484 |
|
|
* policy, a call to <tt>tryAcquire</tt> <em>will</em> |
485 |
|
|
* immediately acquire a permit if one is available, whether or |
486 |
|
|
* not other threads are currently waiting. This |
487 |
|
|
* "barging" behavior can be useful in certain |
488 |
|
|
* circumstances, even though it breaks fairness. If you want to |
489 |
|
|
* honor the fairness setting, then use {@link #tryAcquire(int, |
490 |
|
|
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) } |
491 |
|
|
* which is almost equivalent (it also detects interruption). |
492 |
|
|
* |
493 |
dl |
1.16 |
* @param permits the number of permits to acquire |
494 |
|
|
* |
495 |
|
|
* @return <tt>true</tt> if the permits were acquired and <tt>false</tt> |
496 |
|
|
* otherwise. |
497 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
498 |
|
|
*/ |
499 |
|
|
public boolean tryAcquire(int permits) { |
500 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
501 |
dl |
1.34 |
return sync.nonfairTryAcquireShared(permits) >= 0; |
502 |
dl |
1.16 |
} |
503 |
|
|
|
504 |
|
|
/** |
505 |
jsr166 |
1.45 |
* Acquires the given number of permits from this semaphore, if all |
506 |
dl |
1.16 |
* become available within the given waiting time and the |
507 |
|
|
* current thread has not been {@link Thread#interrupt interrupted}. |
508 |
jsr166 |
1.45 |
* <p>Acquires the given number of permits, if they are available and |
509 |
dl |
1.16 |
* returns immediately, with the value <tt>true</tt>, |
510 |
|
|
* reducing the number of available permits by the given amount. |
511 |
|
|
* <p>If insufficient permits are available then |
512 |
|
|
* the current thread becomes disabled for thread scheduling |
513 |
|
|
* purposes and lies dormant until one of three things happens: |
514 |
|
|
* <ul> |
515 |
jsr166 |
1.45 |
* <li>Some other thread invokes one of the {@link #release() release} |
516 |
dl |
1.16 |
* methods for this semaphore, the current thread is next to be assigned |
517 |
|
|
* permits and the number of available permits satisfies this request; or |
518 |
|
|
* <li>Some other thread {@link Thread#interrupt interrupts} the current |
519 |
|
|
* thread; or |
520 |
|
|
* <li>The specified waiting time elapses. |
521 |
|
|
* </ul> |
522 |
|
|
* <p>If the permits are acquired then the value <tt>true</tt> is returned. |
523 |
|
|
* <p>If the current thread: |
524 |
|
|
* <ul> |
525 |
|
|
* <li>has its interrupted status set on entry to this method; or |
526 |
|
|
* <li>is {@link Thread#interrupt interrupted} while waiting to acquire |
527 |
|
|
* the permits, |
528 |
|
|
* </ul> |
529 |
|
|
* then {@link InterruptedException} is thrown and the current thread's |
530 |
|
|
* interrupted status is cleared. |
531 |
jsr166 |
1.45 |
* Any permits that were to be assigned to this thread, are instead |
532 |
dl |
1.43 |
* assigned to other threads trying to acquire permits, as if |
533 |
|
|
* the permits had been made available by a call to {@link #release()}. |
534 |
dl |
1.16 |
* |
535 |
|
|
* <p>If the specified waiting time elapses then the value <tt>false</tt> |
536 |
|
|
* is returned. |
537 |
|
|
* If the time is |
538 |
|
|
* less than or equal to zero, the method will not wait at all. |
539 |
jsr166 |
1.45 |
* Any permits that were to be assigned to this thread, are instead |
540 |
dl |
1.43 |
* assigned to other threads trying to acquire permits, as if |
541 |
|
|
* the permits had been made available by a call to {@link #release()}. |
542 |
dl |
1.16 |
* |
543 |
|
|
* @param permits the number of permits to acquire |
544 |
|
|
* @param timeout the maximum time to wait for the permits |
545 |
|
|
* @param unit the time unit of the <tt>timeout</tt> argument. |
546 |
|
|
* @return <tt>true</tt> if all permits were acquired and <tt>false</tt> |
547 |
|
|
* if the waiting time elapsed before all permits were acquired. |
548 |
|
|
* |
549 |
|
|
* @throws InterruptedException if the current thread is interrupted |
550 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
551 |
|
|
* |
552 |
|
|
* @see Thread#interrupt |
553 |
|
|
* |
554 |
|
|
*/ |
555 |
|
|
public boolean tryAcquire(int permits, long timeout, TimeUnit unit) |
556 |
|
|
throws InterruptedException { |
557 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
558 |
dl |
1.39 |
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); |
559 |
dl |
1.16 |
} |
560 |
|
|
|
561 |
|
|
/** |
562 |
|
|
* Releases the given number of permits, returning them to the semaphore. |
563 |
jsr166 |
1.45 |
* <p>Releases the given number of permits, increasing the number of |
564 |
dl |
1.16 |
* available permits by that amount. |
565 |
dl |
1.43 |
* If any threads are trying to acquire permits, then one |
566 |
dl |
1.16 |
* is selected and given the permits that were just released. |
567 |
|
|
* If the number of available permits satisfies that thread's request |
568 |
dl |
1.43 |
* then that thread is (re)enabled for thread scheduling purposes; |
569 |
|
|
* otherwise the thread will wait until sufficient permits are available. |
570 |
|
|
* If there are still permits available |
571 |
|
|
* after this thread's request has been satisfied, then those permits |
572 |
|
|
* are assigned in turn to other threads trying to acquire permits. |
573 |
dl |
1.16 |
* |
574 |
|
|
* <p>There is no requirement that a thread that releases a permit must |
575 |
|
|
* have acquired that permit by calling {@link Semaphore#acquire acquire}. |
576 |
|
|
* Correct usage of a semaphore is established by programming convention |
577 |
|
|
* in the application. |
578 |
|
|
* |
579 |
|
|
* @param permits the number of permits to release |
580 |
|
|
* @throws IllegalArgumentException if permits less than zero. |
581 |
|
|
*/ |
582 |
|
|
public void release(int permits) { |
583 |
|
|
if (permits < 0) throw new IllegalArgumentException(); |
584 |
dl |
1.23 |
sync.releaseShared(permits); |
585 |
dl |
1.2 |
} |
586 |
tim |
1.1 |
|
587 |
|
|
/** |
588 |
dl |
1.41 |
* Returns the current number of permits available in this semaphore. |
589 |
tim |
1.1 |
* <p>This method is typically used for debugging and testing purposes. |
590 |
|
|
* @return the number of permits available in this semaphore. |
591 |
|
|
*/ |
592 |
dl |
1.16 |
public int availablePermits() { |
593 |
dl |
1.29 |
return sync.getPermits(); |
594 |
tim |
1.1 |
} |
595 |
dl |
1.15 |
|
596 |
|
|
/** |
597 |
jsr166 |
1.45 |
* Acquires and returns all permits that are immediately available. |
598 |
|
|
* @return the number of permits |
599 |
dl |
1.31 |
*/ |
600 |
|
|
public int drainPermits() { |
601 |
|
|
return sync.drainPermits(); |
602 |
|
|
} |
603 |
|
|
|
604 |
|
|
/** |
605 |
dl |
1.41 |
* Shrinks the number of available permits by the indicated |
606 |
dl |
1.40 |
* reduction. This method can be useful in subclasses that use |
607 |
|
|
* semaphores to track resources that become unavailable. This |
608 |
|
|
* method differs from <tt>acquire</tt> in that it does not block |
609 |
|
|
* waiting for permits to become available. |
610 |
dl |
1.15 |
* @param reduction the number of permits to remove |
611 |
|
|
* @throws IllegalArgumentException if reduction is negative |
612 |
|
|
*/ |
613 |
dl |
1.16 |
protected void reducePermits(int reduction) { |
614 |
dl |
1.15 |
if (reduction < 0) throw new IllegalArgumentException(); |
615 |
dl |
1.29 |
sync.reducePermits(reduction); |
616 |
dl |
1.16 |
} |
617 |
|
|
|
618 |
|
|
/** |
619 |
dl |
1.42 |
* Returns true if this semaphore has fairness set true. |
620 |
dl |
1.16 |
* @return true if this semaphore has fairness set true. |
621 |
|
|
*/ |
622 |
|
|
public boolean isFair() { |
623 |
dl |
1.34 |
return sync instanceof FairSync; |
624 |
dl |
1.22 |
} |
625 |
|
|
|
626 |
|
|
/** |
627 |
dl |
1.23 |
* Queries whether any threads are waiting to acquire. Note that |
628 |
|
|
* because cancellations may occur at any time, a <tt>true</tt> |
629 |
|
|
* return does not guarantee that any other thread will ever |
630 |
|
|
* acquire. This method is designed primarily for use in |
631 |
|
|
* monitoring of the system state. |
632 |
|
|
* |
633 |
|
|
* @return true if there may be other threads waiting to acquire |
634 |
|
|
* the lock. |
635 |
dl |
1.22 |
*/ |
636 |
jsr166 |
1.45 |
public final boolean hasQueuedThreads() { |
637 |
dl |
1.25 |
return sync.hasQueuedThreads(); |
638 |
dl |
1.22 |
} |
639 |
|
|
|
640 |
|
|
/** |
641 |
dl |
1.23 |
* Returns an estimate of the number of threads waiting to |
642 |
|
|
* acquire. The value is only an estimate because the number of |
643 |
|
|
* threads may change dynamically while this method traverses |
644 |
|
|
* internal data structures. This method is designed for use in |
645 |
|
|
* monitoring of the system state, not for synchronization |
646 |
|
|
* control. |
647 |
|
|
* @return the estimated number of threads waiting for this lock |
648 |
dl |
1.22 |
*/ |
649 |
dl |
1.23 |
public final int getQueueLength() { |
650 |
|
|
return sync.getQueueLength(); |
651 |
dl |
1.22 |
} |
652 |
|
|
|
653 |
|
|
/** |
654 |
dl |
1.23 |
* Returns a collection containing threads that may be waiting to |
655 |
|
|
* acquire. Because the actual set of threads may change |
656 |
|
|
* dynamically while constructing this result, the returned |
657 |
|
|
* collection is only a best-effort estimate. The elements of the |
658 |
|
|
* returned collection are in no particular order. This method is |
659 |
|
|
* designed to facilitate construction of subclasses that provide |
660 |
|
|
* more extensive monitoring facilities. |
661 |
|
|
* @return the collection of threads |
662 |
dl |
1.22 |
*/ |
663 |
dl |
1.23 |
protected Collection<Thread> getQueuedThreads() { |
664 |
|
|
return sync.getQueuedThreads(); |
665 |
dl |
1.16 |
} |
666 |
dl |
1.37 |
|
667 |
|
|
/** |
668 |
dl |
1.40 |
* Returns a string identifying this semaphore, as well as its state. |
669 |
jsr166 |
1.45 |
* The state, in brackets, includes the String |
670 |
dl |
1.38 |
* "Permits =" followed by the number of permits. |
671 |
|
|
* @return a string identifying this semaphore, as well as its |
672 |
|
|
* state |
673 |
dl |
1.37 |
*/ |
674 |
|
|
public String toString() { |
675 |
|
|
return super.toString() + "[Permits = " + sync.getPermits() + "]"; |
676 |
|
|
} |
677 |
|
|
|
678 |
tim |
1.1 |
} |