Java多线程系列--“JUC集合”06之 ConcurrentSkipListSet

  1 /*
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 25 /*
 26  *
 27  *
 28  *
 29  *
 30  *
 31  * Written by Doug Lea with assistance from members of JCP JSR-166
 32  * Expert Group and released to the public domain, as explained at
 33  * http://creativecommons.org/publicdomain/zero/1.0/
 34  */
 35 
 36 package java.util.concurrent;
 37 import java.util.*;
 38 import sun.misc.Unsafe;
 39 
 40 /**
 41  * A scalable concurrent {@link NavigableSet} implementation based on
 42  * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
 43  * sorted according to their {@linkplain Comparable natural ordering},
 44  * or by a {@link Comparator} provided at set creation time, depending
 45  * on which constructor is used.
 46  *
 47  * <p>This implementation provides expected average <i>log(n)</i> time
 48  * cost for the <tt>contains</tt>, <tt>add</tt>, and <tt>remove</tt>
 49  * operations and their variants.  Insertion, removal, and access
 50  * operations safely execute concurrently by multiple threads.
 51  * Iterators are <i>weakly consistent</i>, returning elements
 52  * reflecting the state of the set at some point at or since the
 53  * creation of the iterator.  They do <em>not</em> throw {@link
 54  * ConcurrentModificationException}, and may proceed concurrently with
 55  * other operations.  Ascending ordered views and their iterators are
 56  * faster than descending ones.
 57  *
 58  * <p>Beware that, unlike in most collections, the <tt>size</tt>
 59  * method is <em>not</em> a constant-time operation. Because of the
 60  * asynchronous nature of these sets, determining the current number
 61  * of elements requires a traversal of the elements, and so may report
 62  * inaccurate results if this collection is modified during traversal.
 63  * Additionally, the bulk operations <tt>addAll</tt>,
 64  * <tt>removeAll</tt>, <tt>retainAll</tt>, <tt>containsAll</tt>,
 65  * <tt>equals</tt>, and <tt>toArray</tt> are <em>not</em> guaranteed
 66  * to be performed atomically. For example, an iterator operating
 67  * concurrently with an <tt>addAll</tt> operation might view only some
 68  * of the added elements.
 69  *
 70  * <p>This class and its iterators implement all of the
 71  * <em>optional</em> methods of the {@link Set} and {@link Iterator}
 72  * interfaces. Like most other concurrent collection implementations,
 73  * this class does not permit the use of <tt>null</tt> elements,
 74  * because <tt>null</tt> arguments and return values cannot be reliably
 75  * distinguished from the absence of elements.
 76  *
 77  * <p>This class is a member of the
 78  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 79  * Java Collections Framework</a>.
 80  *
 81  * @author Doug Lea
 82  * @param <E> the type of elements maintained by this set
 83  * @since 1.6
 84  */
 85 public class ConcurrentSkipListSet<E>
 86     extends AbstractSet<E>
 87     implements NavigableSet<E>, Cloneable, java.io.Serializable {
 88 
 89     private static final long serialVersionUID = -2479143111061671589L;
 90 
 91     /**
 92      * The underlying map. Uses Boolean.TRUE as value for each
 93      * element.  This field is declared final for the sake of thread
 94      * safety, which entails some ugliness in clone()
 95      */
 96     private final ConcurrentNavigableMap<E,Object> m;
 97 
 98     /**
 99      * Constructs a new, empty set that orders its elements according to
100      * their {@linkplain Comparable natural ordering}.
101      */
102     public ConcurrentSkipListSet() {
103         m = new ConcurrentSkipListMap<E,Object>();
104     }
105 
106     /**
107      * Constructs a new, empty set that orders its elements according to
108      * the specified comparator.
109      *
110      * @param comparator the comparator that will be used to order this set.
111      *        If <tt>null</tt>, the {@linkplain Comparable natural
112      *        ordering} of the elements will be used.
113      */
114     public ConcurrentSkipListSet(Comparator<? super E> comparator) {
115         m = new ConcurrentSkipListMap<E,Object>(comparator);
116     }
117 
118     /**
119      * Constructs a new set containing the elements in the specified
120      * collection, that orders its elements according to their
121      * {@linkplain Comparable natural ordering}.
122      *
123      * @param c The elements that will comprise the new set
124      * @throws ClassCastException if the elements in <tt>c</tt> are
125      *         not {@link Comparable}, or are not mutually comparable
126      * @throws NullPointerException if the specified collection or any
127      *         of its elements are null
128      */
129     public ConcurrentSkipListSet(Collection<? extends E> c) {
130         m = new ConcurrentSkipListMap<E,Object>();
131         addAll(c);
132     }
133 
134     /**
135      * Constructs a new set containing the same elements and using the
136      * same ordering as the specified sorted set.
137      *
138      * @param s sorted set whose elements will comprise the new set
139      * @throws NullPointerException if the specified sorted set or any
140      *         of its elements are null
141      */
142     public ConcurrentSkipListSet(SortedSet<E> s) {
143         m = new ConcurrentSkipListMap<E,Object>(s.comparator());
144         addAll(s);
145     }
146 
147     /**
148      * For use by submaps
149      */
150     ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
151         this.m = m;
152     }
153 
154     /**
155      * Returns a shallow copy of this <tt>ConcurrentSkipListSet</tt>
156      * instance. (The elements themselves are not cloned.)
157      *
158      * @return a shallow copy of this set
159      */
160     public ConcurrentSkipListSet<E> clone() {
161         ConcurrentSkipListSet<E> clone = null;
162         try {
163             clone = (ConcurrentSkipListSet<E>) super.clone();
164             clone.setMap(new ConcurrentSkipListMap(m));
165         } catch (CloneNotSupportedException e) {
166             throw new InternalError();
167         }
168 
169         return clone;
170     }
171 
172     /* ---------------- Set operations -------------- */
173 
174     /**
175      * Returns the number of elements in this set.  If this set
176      * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
177      * returns <tt>Integer.MAX_VALUE</tt>.
178      *
179      * <p>Beware that, unlike in most collections, this method is
180      * <em>NOT</em> a constant-time operation. Because of the
181      * asynchronous nature of these sets, determining the current
182      * number of elements requires traversing them all to count them.
183      * Additionally, it is possible for the size to change during
184      * execution of this method, in which case the returned result
185      * will be inaccurate. Thus, this method is typically not very
186      * useful in concurrent applications.
187      *
188      * @return the number of elements in this set
189      */
190     public int size() {
191         return m.size();
192     }
193 
194     /**
195      * Returns <tt>true</tt> if this set contains no elements.
196      * @return <tt>true</tt> if this set contains no elements
197      */
198     public boolean isEmpty() {
199         return m.isEmpty();
200     }
201 
202     /**
203      * Returns <tt>true</tt> if this set contains the specified element.
204      * More formally, returns <tt>true</tt> if and only if this set
205      * contains an element <tt>e</tt> such that <tt>o.equals(e)</tt>.
206      *
207      * @param o object to be checked for containment in this set
208      * @return <tt>true</tt> if this set contains the specified element
209      * @throws ClassCastException if the specified element cannot be
210      *         compared with the elements currently in this set
211      * @throws NullPointerException if the specified element is null
212      */
213     public boolean contains(Object o) {
214         return m.containsKey(o);
215     }
216 
217     /**
218      * Adds the specified element to this set if it is not already present.
219      * More formally, adds the specified element <tt>e</tt> to this set if
220      * the set contains no element <tt>e2</tt> such that <tt>e.equals(e2)</tt>.
221      * If this set already contains the element, the call leaves the set
222      * unchanged and returns <tt>false</tt>.
223      *
224      * @param e element to be added to this set
225      * @return <tt>true</tt> if this set did not already contain the
226      *         specified element
227      * @throws ClassCastException if <tt>e</tt> cannot be compared
228      *         with the elements currently in this set
229      * @throws NullPointerException if the specified element is null
230      */
231     public boolean add(E e) {
232         return m.putIfAbsent(e, Boolean.TRUE) == null;
233     }
234 
235     /**
236      * Removes the specified element from this set if it is present.
237      * More formally, removes an element <tt>e</tt> such that
238      * <tt>o.equals(e)</tt>, if this set contains such an element.
239      * Returns <tt>true</tt> if this set contained the element (or
240      * equivalently, if this set changed as a result of the call).
241      * (This set will not contain the element once the call returns.)
242      *
243      * @param o object to be removed from this set, if present
244      * @return <tt>true</tt> if this set contained the specified element
245      * @throws ClassCastException if <tt>o</tt> cannot be compared
246      *         with the elements currently in this set
247      * @throws NullPointerException if the specified element is null
248      */
249     public boolean remove(Object o) {
250         return m.remove(o, Boolean.TRUE);
251     }
252 
253     /**
254      * Removes all of the elements from this set.
255      */
256     public void clear() {
257         m.clear();
258     }
259 
260     /**
261      * Returns an iterator over the elements in this set in ascending order.
262      *
263      * @return an iterator over the elements in this set in ascending order
264      */
265     public Iterator<E> iterator() {
266         return m.navigableKeySet().iterator();
267     }
268 
269     /**
270      * Returns an iterator over the elements in this set in descending order.
271      *
272      * @return an iterator over the elements in this set in descending order
273      */
274     public Iterator<E> descendingIterator() {
275         return m.descendingKeySet().iterator();
276     }
277 
278 
279     /* ---------------- AbstractSet Overrides -------------- */
280 
281     /**
282      * Compares the specified object with this set for equality.  Returns
283      * <tt>true</tt> if the specified object is also a set, the two sets
284      * have the same size, and every member of the specified set is
285      * contained in this set (or equivalently, every member of this set is
286      * contained in the specified set).  This definition ensures that the
287      * equals method works properly across different implementations of the
288      * set interface.
289      *
290      * @param o the object to be compared for equality with this set
291      * @return <tt>true</tt> if the specified object is equal to this set
292      */
293     public boolean equals(Object o) {
294         // Override AbstractSet version to avoid calling size()
295         if (o == this)
296             return true;
297         if (!(o instanceof Set))
298             return false;
299         Collection<?> c = (Collection<?>) o;
300         try {
301             return containsAll(c) && c.containsAll(this);
302         } catch (ClassCastException unused)   {
303             return false;
304         } catch (NullPointerException unused) {
305             return false;
306         }
307     }
308 
309     /**
310      * Removes from this set all of its elements that are contained in
311      * the specified collection.  If the specified collection is also
312      * a set, this operation effectively modifies this set so that its
313      * value is the <i>asymmetric set difference</i> of the two sets.
314      *
315      * @param  c collection containing elements to be removed from this set
316      * @return <tt>true</tt> if this set changed as a result of the call
317      * @throws ClassCastException if the types of one or more elements in this
318      *         set are incompatible with the specified collection
319      * @throws NullPointerException if the specified collection or any
320      *         of its elements are null
321      */
322     public boolean removeAll(Collection<?> c) {
323         // Override AbstractSet version to avoid unnecessary call to size()
324         boolean modified = false;
325         for (Iterator<?> i = c.iterator(); i.hasNext(); )
326             if (remove(i.next()))
327                 modified = true;
328         return modified;
329     }
330 
331     /* ---------------- Relational operations -------------- */
332 
333     /**
334      * @throws ClassCastException {@inheritDoc}
335      * @throws NullPointerException if the specified element is null
336      */
337     public E lower(E e) {
338         return m.lowerKey(e);
339     }
340 
341     /**
342      * @throws ClassCastException {@inheritDoc}
343      * @throws NullPointerException if the specified element is null
344      */
345     public E floor(E e) {
346         return m.floorKey(e);
347     }
348 
349     /**
350      * @throws ClassCastException {@inheritDoc}
351      * @throws NullPointerException if the specified element is null
352      */
353     public E ceiling(E e) {
354         return m.ceilingKey(e);
355     }
356 
357     /**
358      * @throws ClassCastException {@inheritDoc}
359      * @throws NullPointerException if the specified element is null
360      */
361     public E higher(E e) {
362         return m.higherKey(e);
363     }
364 
365     public E pollFirst() {
366         Map.Entry<E,Object> e = m.pollFirstEntry();
367         return (e == null) ? null : e.getKey();
368     }
369 
370     public E pollLast() {
371         Map.Entry<E,Object> e = m.pollLastEntry();
372         return (e == null) ? null : e.getKey();
373     }
374 
375 
376     /* ---------------- SortedSet operations -------------- */
377 
378 
379     public Comparator<? super E> comparator() {
380         return m.comparator();
381     }
382 
383     /**
384      * @throws NoSuchElementException {@inheritDoc}
385      */
386     public E first() {
387         return m.firstKey();
388     }
389 
390     /**
391      * @throws NoSuchElementException {@inheritDoc}
392      */
393     public E last() {
394         return m.lastKey();
395     }
396 
397     /**
398      * @throws ClassCastException {@inheritDoc}
399      * @throws NullPointerException if {@code fromElement} or
400      *         {@code toElement} is null
401      * @throws IllegalArgumentException {@inheritDoc}
402      */
403     public NavigableSet<E> subSet(E fromElement,
404                                   boolean fromInclusive,
405                                   E toElement,
406                                   boolean toInclusive) {
407         return new ConcurrentSkipListSet<E>
408             (m.subMap(fromElement, fromInclusive,
409                       toElement,   toInclusive));
410     }
411 
412     /**
413      * @throws ClassCastException {@inheritDoc}
414      * @throws NullPointerException if {@code toElement} is null
415      * @throws IllegalArgumentException {@inheritDoc}
416      */
417     public NavigableSet<E> headSet(E toElement, boolean inclusive) {
418         return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
419     }
420 
421     /**
422      * @throws ClassCastException {@inheritDoc}
423      * @throws NullPointerException if {@code fromElement} is null
424      * @throws IllegalArgumentException {@inheritDoc}
425      */
426     public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
427         return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
428     }
429 
430     /**
431      * @throws ClassCastException {@inheritDoc}
432      * @throws NullPointerException if {@code fromElement} or
433      *         {@code toElement} is null
434      * @throws IllegalArgumentException {@inheritDoc}
435      */
436     public NavigableSet<E> subSet(E fromElement, E toElement) {
437         return subSet(fromElement, true, toElement, false);
438     }
439 
440     /**
441      * @throws ClassCastException {@inheritDoc}
442      * @throws NullPointerException if {@code toElement} is null
443      * @throws IllegalArgumentException {@inheritDoc}
444      */
445     public NavigableSet<E> headSet(E toElement) {
446         return headSet(toElement, false);
447     }
448 
449     /**
450      * @throws ClassCastException {@inheritDoc}
451      * @throws NullPointerException if {@code fromElement} is null
452      * @throws IllegalArgumentException {@inheritDoc}
453      */
454     public NavigableSet<E> tailSet(E fromElement) {
455         return tailSet(fromElement, true);
456     }
457 
458     /**
459      * Returns a reverse order view of the elements contained in this set.
460      * The descending set is backed by this set, so changes to the set are
461      * reflected in the descending set, and vice-versa.
462      *
463      * <p>The returned set has an ordering equivalent to
464      * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
465      * The expression {@code s.descendingSet().descendingSet()} returns a
466      * view of {@code s} essentially equivalent to {@code s}.
467      *
468      * @return a reverse order view of this set
469      */
470     public NavigableSet<E> descendingSet() {
471         return new ConcurrentSkipListSet(m.descendingMap());
472     }
473 
474     // Support for resetting map in clone
475     private void setMap(ConcurrentNavigableMap<E,Object> map) {
476         UNSAFE.putObjectVolatile(this, mapOffset, map);
477     }
478 
479     private static final sun.misc.Unsafe UNSAFE;
480     private static final long mapOffset;
481     static {
482         try {
483             UNSAFE = sun.misc.Unsafe.getUnsafe();
484             Class k = ConcurrentSkipListSet.class;
485             mapOffset = UNSAFE.objectFieldOffset
486                 (k.getDeclaredField("m"));
487         } catch (Exception e) {
488             throw new Error(e);
489         }
490     }
491 }