001: /*
002: * Written by Doug Lea with assistance from members of JCP JSR-166
003: * Expert Group and released to the public domain, as explained at
004: * http://creativecommons.org/licenses/publicdomain
005: */
006:
007: package org.drools.util.concurrent.locks;
008:
009: import java.util.Collection;
010:
011: /**
012: * A {@link edu.emory.mathcs.backport.java.util.Queue} that additionally supports operations
013: * that wait for the queue to become non-empty when retrieving an
014: * element, and wait for space to become available in the queue when
015: * storing an element.
016: *
017: * <p><tt>BlockingQueue</tt> methods come in four forms, with different ways
018: * of handling operations that cannot be satisfied immediately, but may be
019: * satisfied at some point in the future:
020: * one throws an exception, the second returns a special value (either
021: * <tt>null</tt> or <tt>false</tt>, depending on the operation), the third
022: * blocks the current thread indefinitely until the operation can succeed,
023: * and the fourth blocks for only a given maximum time limit before giving
024: * up. These methods are summarized in the following table:
025: *
026: * <p>
027: * <table BORDER CELLPADDING=3 CELLSPACING=1>
028: * <tr>
029: * <td></td>
030: * <td ALIGN=CENTER><em>Throws exception</em></td>
031: * <td ALIGN=CENTER><em>Special value</em></td>
032: * <td ALIGN=CENTER><em>Blocks</em></td>
033: * <td ALIGN=CENTER><em>Times out</em></td>
034: * </tr>
035: * <tr>
036: * <td><b>Insert</b></td>
037: * <td>{@link #add add(e)}</td>
038: * <td>{@link #offer offer(e)}</td>
039: * <td>{@link #put put(e)}</td>
040: * <td>{@link #offer(Object, long, TimeUnit) offer(e, time, unit)}</td>
041: * </tr>
042: * <tr>
043: * <td><b>Remove</b></td>
044: * <td>{@link #remove remove()}</td>
045: * <td>{@link #poll poll()}</td>
046: * <td>{@link #take take()}</td>
047: * <td>{@link #poll(long, TimeUnit) poll(time, unit)}</td>
048: * </tr>
049: * <tr>
050: * <td><b>Examine</b></td>
051: * <td>{@link #element element()}</td>
052: * <td>{@link #peek peek()}</td>
053: * <td><em>not applicable</em></td>
054: * <td><em>not applicable</em></td>
055: * </tr>
056: * </table>
057: *
058: * <p>A <tt>BlockingQueue</tt> does not accept <tt>null</tt> elements.
059: * Implementations throw <tt>NullPointerException</tt> on attempts
060: * to <tt>add</tt>, <tt>put</tt> or <tt>offer</tt> a <tt>null</tt>. A
061: * <tt>null</tt> is used as a sentinel value to indicate failure of
062: * <tt>poll</tt> operations.
063: *
064: * <p>A <tt>BlockingQueue</tt> may be capacity bounded. At any given
065: * time it may have a <tt>remainingCapacity</tt> beyond which no
066: * additional elements can be <tt>put</tt> without blocking.
067: * A <tt>BlockingQueue</tt> without any intrinsic capacity constraints always
068: * reports a remaining capacity of <tt>Integer.MAX_VALUE</tt>.
069: *
070: * <p> <tt>BlockingQueue</tt> implementations are designed to be used
071: * primarily for producer-consumer queues, but additionally support
072: * the {@link java.util.Collection} interface. So, for example, it is
073: * possible to remove an arbitrary element from a queue using
074: * <tt>remove(x)</tt>. However, such operations are in general
075: * <em>not</em> performed very efficiently, and are intended for only
076: * occasional use, such as when a queued message is cancelled.
077: *
078: * <p> <tt>BlockingQueue</tt> implementations are thread-safe. All
079: * queuing methods achieve their effects atomically using internal
080: * locks or other forms of concurrency control. However, the
081: * <em>bulk</em> Collection operations <tt>addAll</tt>,
082: * <tt>containsAll</tt>, <tt>retainAll</tt> and <tt>removeAll</tt> are
083: * <em>not</em> necessarily performed atomically unless specified
084: * otherwise in an implementation. So it is possible, for example, for
085: * <tt>addAll(c)</tt> to fail (throwing an exception) after adding
086: * only some of the elements in <tt>c</tt>.
087: *
088: * <p>A <tt>BlockingQueue</tt> does <em>not</em> intrinsically support
089: * any kind of "close" or "shutdown" operation to
090: * indicate that no more items will be added. The needs and usage of
091: * such features tend to be implementation-dependent. For example, a
092: * common tactic is for producers to insert special
093: * <em>end-of-stream</em> or <em>poison</em> objects, that are
094: * interpreted accordingly when taken by consumers.
095: *
096: * <p>
097: * Usage example, based on a typical producer-consumer scenario.
098: * Note that a <tt>BlockingQueue</tt> can safely be used with multiple
099: * producers and multiple consumers.
100: * <pre>
101: * class Producer implements Runnable {
102: * private final BlockingQueue queue;
103: * Producer(BlockingQueue q) { queue = q; }
104: * public void run() {
105: * try {
106: * while (true) { queue.put(produce()); }
107: * } catch (InterruptedException ex) { ... handle ...}
108: * }
109: * Object produce() { ... }
110: * }
111: *
112: * class Consumer implements Runnable {
113: * private final BlockingQueue queue;
114: * Consumer(BlockingQueue q) { queue = q; }
115: * public void run() {
116: * try {
117: * while (true) { consume(queue.take()); }
118: * } catch (InterruptedException ex) { ... handle ...}
119: * }
120: * void consume(Object x) { ... }
121: * }
122: *
123: * class Setup {
124: * void main() {
125: * BlockingQueue q = new SomeQueueImplementation();
126: * Producer p = new Producer(q);
127: * Consumer c1 = new Consumer(q);
128: * Consumer c2 = new Consumer(q);
129: * new Thread(p).start();
130: * new Thread(c1).start();
131: * new Thread(c2).start();
132: * }
133: * }
134: * </pre>
135: *
136: * <p>Memory consistency effects: As with other concurrent
137: * collections, actions in a thread prior to placing an object into a
138: * {@code BlockingQueue}
139: * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
140: * actions subsequent to the access or removal of that element from
141: * the {@code BlockingQueue} in another thread.
142: *
143: * <p>This interface is a member of the
144: * <a href="{@docRoot}/../technotes/guides/collections/index.html">
145: * Java Collections Framework</a>.
146: *
147: * @since 1.5
148: * @author Doug Lea
149: */
150: public interface BlockingQueue extends Queue {
151: /**
152: * Inserts the specified element into this queue if it is possible to do
153: * so immediately without violating capacity restrictions, returning
154: * <tt>true</tt> upon success and throwing an
155: * <tt>IllegalStateException</tt> if no space is currently available.
156: * When using a capacity-restricted queue, it is generally preferable to
157: * use {@link #offer(Object) offer}.
158: *
159: * @param e the element to add
160: * @return <tt>true</tt> (as specified by {@link java.util.Collection#add})
161: * @throws IllegalStateException if the element cannot be added at this
162: * time due to capacity restrictions
163: * @throws ClassCastException if the class of the specified element
164: * prevents it from being added to this queue
165: * @throws NullPointerException if the specified element is null
166: * @throws IllegalArgumentException if some property of the specified
167: * element prevents it from being added to this queue
168: */
169: boolean add(Object e);
170:
171: /**
172: * Inserts the specified element into this queue if it is possible to do
173: * so immediately without violating capacity restrictions, returning
174: * <tt>true</tt> upon success and <tt>false</tt> if no space is currently
175: * available. When using a capacity-restricted queue, this method is
176: * generally preferable to {@link #add}, which can fail to insert an
177: * element only by throwing an exception.
178: *
179: * @param e the element to add
180: * @return <tt>true</tt> if the element was added to this queue, else
181: * <tt>false</tt>
182: * @throws ClassCastException if the class of the specified element
183: * prevents it from being added to this queue
184: * @throws NullPointerException if the specified element is null
185: * @throws IllegalArgumentException if some property of the specified
186: * element prevents it from being added to this queue
187: */
188: boolean offer(Object e);
189:
190: /**
191: * Inserts the specified element into this queue, waiting if necessary
192: * for space to become available.
193: *
194: * @param e the element to add
195: * @throws InterruptedException if interrupted while waiting
196: * @throws ClassCastException if the class of the specified element
197: * prevents it from being added to this queue
198: * @throws NullPointerException if the specified element is null
199: * @throws IllegalArgumentException if some property of the specified
200: * element prevents it from being added to this queue
201: */
202: void put(Object e) throws InterruptedException;
203:
204: /**
205: * Inserts the specified element into this queue, waiting up to the
206: * specified wait time if necessary for space to become available.
207: *
208: * @param e the element to add
209: * @param timeout how long to wait before giving up, in units of
210: * <tt>unit</tt>
211: * @param unit a <tt>TimeUnit</tt> determining how to interpret the
212: * <tt>timeout</tt> parameter
213: * @return <tt>true</tt> if successful, or <tt>false</tt> if
214: * the specified waiting time elapses before space is available
215: * @throws InterruptedException if interrupted while waiting
216: * @throws ClassCastException if the class of the specified element
217: * prevents it from being added to this queue
218: * @throws NullPointerException if the specified element is null
219: * @throws IllegalArgumentException if some property of the specified
220: * element prevents it from being added to this queue
221: */
222: boolean offer(Object e, long timeout, TimeUnit unit)
223: throws InterruptedException;
224:
225: /**
226: * Retrieves and removes the head of this queue, waiting if necessary
227: * until an element becomes available.
228: *
229: * @return the head of this queue
230: * @throws InterruptedException if interrupted while waiting
231: */
232: Object take() throws InterruptedException;
233:
234: /**
235: * Retrieves and removes the head of this queue, waiting up to the
236: * specified wait time if necessary for an element to become available.
237: *
238: * @param timeout how long to wait before giving up, in units of
239: * <tt>unit</tt>
240: * @param unit a <tt>TimeUnit</tt> determining how to interpret the
241: * <tt>timeout</tt> parameter
242: * @return the head of this queue, or <tt>null</tt> if the
243: * specified waiting time elapses before an element is available
244: * @throws InterruptedException if interrupted while waiting
245: */
246: Object poll(long timeout, TimeUnit unit)
247: throws InterruptedException;
248:
249: /**
250: * Returns the number of additional elements that this queue can ideally
251: * (in the absence of memory or resource constraints) accept without
252: * blocking, or <tt>Integer.MAX_VALUE</tt> if there is no intrinsic
253: * limit.
254: *
255: * <p>Note that you <em>cannot</em> always tell if an attempt to insert
256: * an element will succeed by inspecting <tt>remainingCapacity</tt>
257: * because it may be the case that another thread is about to
258: * insert or remove an element.
259: *
260: * @return the remaining capacity
261: */
262: int remainingCapacity();
263:
264: /**
265: * Removes a single instance of the specified element from this queue,
266: * if it is present. More formally, removes an element <tt>e</tt> such
267: * that <tt>o.equals(e)</tt>, if this queue contains one or more such
268: * elements.
269: * Returns <tt>true</tt> if this queue contained the specified element
270: * (or equivalently, if this queue changed as a result of the call).
271: *
272: * @param o element to be removed from this queue, if present
273: * @return <tt>true</tt> if this queue changed as a result of the call
274: * @throws ClassCastException if the class of the specified element
275: * is incompatible with this queue (optional)
276: * @throws NullPointerException if the specified element is null (optional)
277: */
278: boolean remove(Object o);
279:
280: /**
281: * Returns <tt>true</tt> if this queue contains the specified element.
282: * More formally, returns <tt>true</tt> if and only if this queue contains
283: * at least one element <tt>e</tt> such that <tt>o.equals(e)</tt>.
284: *
285: * @param o object to be checked for containment in this queue
286: * @return <tt>true</tt> if this queue contains the specified element
287: * @throws ClassCastException if the class of the specified element
288: * is incompatible with this queue (optional)
289: * @throws NullPointerException if the specified element is null (optional)
290: */
291: public boolean contains(Object o);
292:
293: /**
294: * Removes all available elements from this queue and adds them
295: * to the given collection. This operation may be more
296: * efficient than repeatedly polling this queue. A failure
297: * encountered while attempting to add elements to
298: * collection <tt>c</tt> may result in elements being in neither,
299: * either or both collections when the associated exception is
300: * thrown. Attempts to drain a queue to itself result in
301: * <tt>IllegalArgumentException</tt>. Further, the behavior of
302: * this operation is undefined if the specified collection is
303: * modified while the operation is in progress.
304: *
305: * @param c the collection to transfer elements into
306: * @return the number of elements transferred
307: * @throws UnsupportedOperationException if addition of elements
308: * is not supported by the specified collection
309: * @throws ClassCastException if the class of an element of this queue
310: * prevents it from being added to the specified collection
311: * @throws NullPointerException if the specified collection is null
312: * @throws IllegalArgumentException if the specified collection is this
313: * queue, or some property of an element of this queue prevents
314: * it from being added to the specified collection
315: */
316: int drainTo(Collection c);
317:
318: /**
319: * Removes at most the given number of available elements from
320: * this queue and adds them to the given collection. A failure
321: * encountered while attempting to add elements to
322: * collection <tt>c</tt> may result in elements being in neither,
323: * either or both collections when the associated exception is
324: * thrown. Attempts to drain a queue to itself result in
325: * <tt>IllegalArgumentException</tt>. Further, the behavior of
326: * this operation is undefined if the specified collection is
327: * modified while the operation is in progress.
328: *
329: * @param c the collection to transfer elements into
330: * @param maxElements the maximum number of elements to transfer
331: * @return the number of elements transferred
332: * @throws UnsupportedOperationException if addition of elements
333: * is not supported by the specified collection
334: * @throws ClassCastException if the class of an element of this queue
335: * prevents it from being added to the specified collection
336: * @throws NullPointerException if the specified collection is null
337: * @throws IllegalArgumentException if the specified collection is this
338: * queue, or some property of an element of this queue prevents
339: * it from being added to the specified collection
340: */
341: int drainTo(Collection c, int maxElements);
342: }
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