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

概要

本章对Java.util.concurrent包中的ConcurrentSkipListSet类进行详细的介绍。内容包括：
ConcurrentSkipListSet介绍ConcurrentSkipListSet原理和数据结构
ConcurrentSkipListSet函数列表
ConcurrentSkipListSet源码(JDK1.7.0_40版本)
ConcurrentSkipListSet示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3498634.html

ConcurrentSkipListSet介绍

ConcurrentSkipListSet是线程安全的有序的集合，适用于高并发的场景。
ConcurrentSkipListSet和TreeSet，它们虽然都是有序的集合。但是，第一，它们的线程安全机制不同，TreeSet是非线程安全的，而ConcurrentSkipListSet是线程安全的。第二，ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的，而TreeSet是通过TreeMap实现的。

ConcurrentSkipListSet原理和数据结构

ConcurrentSkipListSet的数据结构，如下图所示：

说明：
(01) ConcurrentSkipListSet继承于AbstractSet。因此，它本质上是一个集合。
(02) ConcurrentSkipListSet实现了NavigableSet接口。因此，ConcurrentSkipListSet是一个有序的集合。
(03) ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的。它包含一个ConcurrentNavigableMap对象m，而m对象实际上是ConcurrentNavigableMap的实现类ConcurrentSkipListMap的实例。ConcurrentSkipListMap中的元素是key-value键值对；而ConcurrentSkipListSet是集合，它只用到了ConcurrentSkipListMap中的key！

ConcurrentSkipListSet函数列表

// 构造一个新的空 set，该 set 按照元素的自然顺序对其进行排序。
ConcurrentSkipListSet()
// 构造一个包含指定 collection 中元素的新 set，这个新 set 按照元素的自然顺序对其进行排序。
ConcurrentSkipListSet(Collection<? extends E> c)
// 构造一个新的空 set，该 set 按照指定的比较器对其元素进行排序。
ConcurrentSkipListSet(Comparator<? super E> comparator)
// 构造一个新 set，该 set 所包含的元素与指定的有序 set 包含的元素相同，使用的顺序也相同。
ConcurrentSkipListSet(SortedSet<E> s)

// 如果此 set 中不包含指定元素，则添加指定元素。
boolean add(E e)
// 返回此 set 中大于等于给定元素的最小元素；如果不存在这样的元素，则返回 null。
E ceiling(E e)
// 从此 set 中移除所有元素。
void clear()
// 返回此 ConcurrentSkipListSet 实例的浅表副本。
ConcurrentSkipListSet<E> clone()
// 返回对此 set 中的元素进行排序的比较器；如果此 set 使用其元素的自然顺序，则返回 null。
Comparator<? super E> comparator()
// 如果此 set 包含指定的元素，则返回 true。
boolean contains(Object o)
// 返回在此 set 的元素上以降序进行迭代的迭代器。
Iterator<E> descendingIterator()
// 返回此 set 中所包含元素的逆序视图。
NavigableSet<E> descendingSet()
// 比较指定对象与此 set 的相等性。
boolean equals(Object o)
// 返回此 set 中当前第一个（最低）元素。
E first()
// 返回此 set 中小于等于给定元素的最大元素；如果不存在这样的元素，则返回 null。
E floor(E e)
// 返回此 set 的部分视图，其元素严格小于 toElement。
NavigableSet<E> headSet(E toElement)
// 返回此 set 的部分视图，其元素小于（或等于，如果 inclusive 为 true）toElement。
NavigableSet<E> headSet(E toElement, boolean inclusive)
// 返回此 set 中严格大于给定元素的最小元素；如果不存在这样的元素，则返回 null。
E higher(E e)
// 如果此 set 不包含任何元素，则返回 true。
boolean isEmpty()
// 返回在此 set 的元素上以升序进行迭代的迭代器。
Iterator<E> iterator()
// 返回此 set 中当前最后一个（最高）元素。
E last()
// 返回此 set 中严格小于给定元素的最大元素；如果不存在这样的元素，则返回 null。
E lower(E e)
// 获取并移除第一个（最低）元素；如果此 set 为空，则返回 null。
E pollFirst()
// 获取并移除最后一个（最高）元素；如果此 set 为空，则返回 null。
E pollLast()
// 如果此 set 中存在指定的元素，则将其移除。
boolean remove(Object o)
// 从此 set 中移除包含在指定 collection 中的所有元素。
boolean removeAll(Collection<?> c)
// 返回此 set 中的元素数目。
int size()
// 返回此 set 的部分视图，其元素范围从 fromElement 到 toElement。
NavigableSet<E> subSet(E fromElement, boolean fromInclusive, E toElement, boolean toInclusive)
// 返回此 set 的部分视图，其元素从 fromElement（包括）到 toElement（不包括）。
NavigableSet<E> subSet(E fromElement, E toElement)
// 返回此 set 的部分视图，其元素大于等于 fromElement。
NavigableSet<E> tailSet(E fromElement)
// 返回此 set 的部分视图，其元素大于（或等于，如果 inclusive 为 true）fromElement。
NavigableSet<E> tailSet(E fromElement, boolean inclusive)

ConcurrentSkipListSet源码(JDK1.7.0_40版本)

ConcurrentSkipListSet.java的完整源码如下：

 /*
  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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 /*
  *
  *
  *
  *
  *
  * Written by Doug Lea with assistance from members of JCP JSR-166
  * Expert Group and released to the public domain, as explained at
  * http://creativecommons.org/publicdomain/zero/1.0/
  */

 package java.util.concurrent;
 import java.util.*;
 import sun.misc.Unsafe;

 /**
  * A scalable concurrent {@link NavigableSet} implementation based on
  * a {@link ConcurrentSkipListMap}.  The elements of the set are kept
  * sorted according to their {@linkplain Comparable natural ordering},
  * or by a {@link Comparator} provided at set creation time, depending
  * on which constructor is used.
  *
  * <p>This implementation provides expected average <i>log(n)</i> time
  * cost for the <tt>contains</tt>, <tt>add</tt>, and <tt>remove</tt>
  * operations and their variants.  Insertion, removal, and access
  * operations safely execute concurrently by multiple threads.
  * Iterators are <i>weakly consistent</i>, returning elements
  * reflecting the state of the set at some point at or since the
  * creation of the iterator.  They do <em>not</em> throw {@link
  * ConcurrentModificationException}, and may proceed concurrently with
  * other operations.  Ascending ordered views and their iterators are
  * faster than descending ones.
  *
  * <p>Beware that, unlike in most collections, the <tt>size</tt>
  * method is <em>not</em> a constant-time operation. Because of the
  * asynchronous nature of these sets, determining the current number
  * of elements requires a traversal of the elements, and so may report
  * inaccurate results if this collection is modified during traversal.
  * Additionally, the bulk operations <tt>addAll</tt>,
  * <tt>removeAll</tt>, <tt>retainAll</tt>, <tt>containsAll</tt>,
  * <tt>equals</tt>, and <tt>toArray</tt> are <em>not</em> guaranteed
  * to be performed atomically. For example, an iterator operating
  * concurrently with an <tt>addAll</tt> operation might view only some
  * of the added elements.
  *
  * <p>This class and its iterators implement all of the
  * <em>optional</em> methods of the {@link Set} and {@link Iterator}
  * interfaces. Like most other concurrent collection implementations,
  * this class does not permit the use of <tt>null</tt> elements,
  * because <tt>null</tt> arguments and return values cannot be reliably
  * distinguished from the absence of elements.
  *
  * <p>This class is a member of the
  * <a href="{@docRoot}/../technotes/guides/collections/index.html">
  * Java Collections Framework</a>.
  *
  * @author Doug Lea
  * @param <E> the type of elements maintained by this set
  * @since 1.6
  */
 public class ConcurrentSkipListSet<E>
     extends AbstractSet<E>
     implements NavigableSet<E>, Cloneable, java.io.Serializable {

     private static final long serialVersionUID = -2479143111061671589L;

     /**
      * The underlying map. Uses Boolean.TRUE as value for each
      * element.  This field is declared final for the sake of thread
      * safety, which entails some ugliness in clone()
      */
     private final ConcurrentNavigableMap<E,Object> m;

     /**
      * Constructs a new, empty set that orders its elements according to
      * their {@linkplain Comparable natural ordering}.
      */
     public ConcurrentSkipListSet() {
         m = new ConcurrentSkipListMap<E,Object>();
     }

     /**
      * Constructs a new, empty set that orders its elements according to
      * the specified comparator.
      *
      * @param comparator the comparator that will be used to order this set.
      *        If <tt>null</tt>, the {@linkplain Comparable natural
      *        ordering} of the elements will be used.
      */
     public ConcurrentSkipListSet(Comparator<? super E> comparator) {
         m = new ConcurrentSkipListMap<E,Object>(comparator);
     }

     /**
      * Constructs a new set containing the elements in the specified
      * collection, that orders its elements according to their
      * {@linkplain Comparable natural ordering}.
      *
      * @param c The elements that will comprise the new set
      * @throws ClassCastException if the elements in <tt>c</tt> are
      *         not {@link Comparable}, or are not mutually comparable
      * @throws NullPointerException if the specified collection or any
      *         of its elements are null
      */
     public ConcurrentSkipListSet(Collection<? extends E> c) {
         m = new ConcurrentSkipListMap<E,Object>();
         addAll(c);
     }

     /**
      * Constructs a new set containing the same elements and using the
      * same ordering as the specified sorted set.
      *
      * @param s sorted set whose elements will comprise the new set
      * @throws NullPointerException if the specified sorted set or any
      *         of its elements are null
      */
     public ConcurrentSkipListSet(SortedSet<E> s) {
         m = new ConcurrentSkipListMap<E,Object>(s.comparator());
         addAll(s);
     }

     /**
      * For use by submaps
      */
     ConcurrentSkipListSet(ConcurrentNavigableMap<E,Object> m) {
         this.m = m;
     }

     /**
      * Returns a shallow copy of this <tt>ConcurrentSkipListSet</tt>
      * instance. (The elements themselves are not cloned.)
      *
      * @return a shallow copy of this set
      */
     public ConcurrentSkipListSet<E> clone() {
         ConcurrentSkipListSet<E> clone = null;
         try {
             clone = (ConcurrentSkipListSet<E>) super.clone();
             clone.setMap(new ConcurrentSkipListMap(m));
         } catch (CloneNotSupportedException e) {
             throw new InternalError();
         }

         return clone;
     }

     /* ---------------- Set operations -------------- */

     /**
      * Returns the number of elements in this set.  If this set
      * contains more than <tt>Integer.MAX_VALUE</tt> elements, it
      * returns <tt>Integer.MAX_VALUE</tt>.
      *
      * <p>Beware that, unlike in most collections, this method is
      * <em>NOT</em> a constant-time operation. Because of the
      * asynchronous nature of these sets, determining the current
      * number of elements requires traversing them all to count them.
      * Additionally, it is possible for the size to change during
      * execution of this method, in which case the returned result
      * will be inaccurate. Thus, this method is typically not very
      * useful in concurrent applications.
      *
      * @return the number of elements in this set
      */
     public int size() {
         return m.size();
     }

     /**
      * Returns <tt>true</tt> if this set contains no elements.
      * @return <tt>true</tt> if this set contains no elements
      */
     public boolean isEmpty() {
         return m.isEmpty();
     }

     /**
      * Returns <tt>true</tt> if this set contains the specified element.
      * More formally, returns <tt>true</tt> if and only if this set
      * contains an element <tt>e</tt> such that <tt>o.equals(e)</tt>.
      *
      * @param o object to be checked for containment in this set
      * @return <tt>true</tt> if this set contains the specified element
      * @throws ClassCastException if the specified element cannot be
      *         compared with the elements currently in this set
      * @throws NullPointerException if the specified element is null
      */
     public boolean contains(Object o) {
         return m.containsKey(o);
     }

     /**
      * Adds the specified element to this set if it is not already present.
      * More formally, adds the specified element <tt>e</tt> to this set if
      * the set contains no element <tt>e2</tt> such that <tt>e.equals(e2)</tt>.
      * If this set already contains the element, the call leaves the set
      * unchanged and returns <tt>false</tt>.
      *
      * @param e element to be added to this set
      * @return <tt>true</tt> if this set did not already contain the
      *         specified element
      * @throws ClassCastException if <tt>e</tt> cannot be compared
      *         with the elements currently in this set
      * @throws NullPointerException if the specified element is null
      */
     public boolean add(E e) {
         return m.putIfAbsent(e, Boolean.TRUE) == null;
     }

     /**
      * Removes the specified element from this set if it is present.
      * More formally, removes an element <tt>e</tt> such that
      * <tt>o.equals(e)</tt>, if this set contains such an element.
      * Returns <tt>true</tt> if this set contained the element (or
      * equivalently, if this set changed as a result of the call).
      * (This set will not contain the element once the call returns.)
      *
      * @param o object to be removed from this set, if present
      * @return <tt>true</tt> if this set contained the specified element
      * @throws ClassCastException if <tt>o</tt> cannot be compared
      *         with the elements currently in this set
      * @throws NullPointerException if the specified element is null
      */
     public boolean remove(Object o) {
         return m.remove(o, Boolean.TRUE);
     }

     /**
      * Removes all of the elements from this set.
      */
     public void clear() {
         m.clear();
     }

     /**
      * Returns an iterator over the elements in this set in ascending order.
      *
      * @return an iterator over the elements in this set in ascending order
      */
     public Iterator<E> iterator() {
         return m.navigableKeySet().iterator();
     }

     /**
      * Returns an iterator over the elements in this set in descending order.
      *
      * @return an iterator over the elements in this set in descending order
      */
     public Iterator<E> descendingIterator() {
         return m.descendingKeySet().iterator();
     }

     /* ---------------- AbstractSet Overrides -------------- */

     /**
      * Compares the specified object with this set for equality.  Returns
      * <tt>true</tt> if the specified object is also a set, the two sets
      * have the same size, and every member of the specified set is
      * contained in this set (or equivalently, every member of this set is
      * contained in the specified set).  This definition ensures that the
      * equals method works properly across different implementations of the
      * set interface.
      *
      * @param o the object to be compared for equality with this set
      * @return <tt>true</tt> if the specified object is equal to this set
      */
     public boolean equals(Object o) {
         // Override AbstractSet version to avoid calling size()
         if (o == this)
             return true;
         if (!(o instanceof Set))
             return false;
         Collection<?> c = (Collection<?>) o;
         try {
             return containsAll(c) && c.containsAll(this);
         } catch (ClassCastException unused)   {
             return false;
         } catch (NullPointerException unused) {
             return false;
         }
     }

     /**
      * Removes from this set all of its elements that are contained in
      * the specified collection.  If the specified collection is also
      * a set, this operation effectively modifies this set so that its
      * value is the <i>asymmetric set difference</i> of the two sets.
      *
      * @param  c collection containing elements to be removed from this set
      * @return <tt>true</tt> if this set changed as a result of the call
      * @throws ClassCastException if the types of one or more elements in this
      *         set are incompatible with the specified collection
      * @throws NullPointerException if the specified collection or any
      *         of its elements are null
      */
     public boolean removeAll(Collection<?> c) {
         // Override AbstractSet version to avoid unnecessary call to size()
         boolean modified = false;
         for (Iterator<?> i = c.iterator(); i.hasNext(); )
             if (remove(i.next()))
                 modified = true;
         return modified;
     }

     /* ---------------- Relational operations -------------- */

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if the specified element is null
      */
     public E lower(E e) {
         return m.lowerKey(e);
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if the specified element is null
      */
     public E floor(E e) {
         return m.floorKey(e);
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if the specified element is null
      */
     public E ceiling(E e) {
         return m.ceilingKey(e);
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if the specified element is null
      */
     public E higher(E e) {
         return m.higherKey(e);
     }

     public E pollFirst() {
         Map.Entry<E,Object> e = m.pollFirstEntry();
         return (e == null) ? null : e.getKey();
     }

     public E pollLast() {
         Map.Entry<E,Object> e = m.pollLastEntry();
         return (e == null) ? null : e.getKey();
     }

     /* ---------------- SortedSet operations -------------- */

     public Comparator<? super E> comparator() {
         return m.comparator();
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E first() {
         return m.firstKey();
     }

     /**
      * @throws NoSuchElementException {@inheritDoc}
      */
     public E last() {
         return m.lastKey();
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if {@code fromElement} or
      *         {@code toElement} is null
      * @throws IllegalArgumentException {@inheritDoc}
      */
     public NavigableSet<E> subSet(E fromElement,
                                   boolean fromInclusive,
                                   E toElement,
                                   boolean toInclusive) {
         return new ConcurrentSkipListSet<E>
             (m.subMap(fromElement, fromInclusive,
                       toElement,   toInclusive));
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if {@code toElement} is null
      * @throws IllegalArgumentException {@inheritDoc}
      */
     public NavigableSet<E> headSet(E toElement, boolean inclusive) {
         return new ConcurrentSkipListSet<E>(m.headMap(toElement, inclusive));
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if {@code fromElement} is null
      * @throws IllegalArgumentException {@inheritDoc}
      */
     public NavigableSet<E> tailSet(E fromElement, boolean inclusive) {
         return new ConcurrentSkipListSet<E>(m.tailMap(fromElement, inclusive));
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if {@code fromElement} or
      *         {@code toElement} is null
      * @throws IllegalArgumentException {@inheritDoc}
      */
     public NavigableSet<E> subSet(E fromElement, E toElement) {
         return subSet(fromElement, true, toElement, false);
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if {@code toElement} is null
      * @throws IllegalArgumentException {@inheritDoc}
      */
     public NavigableSet<E> headSet(E toElement) {
         return headSet(toElement, false);
     }

     /**
      * @throws ClassCastException {@inheritDoc}
      * @throws NullPointerException if {@code fromElement} is null
      * @throws IllegalArgumentException {@inheritDoc}
      */
     public NavigableSet<E> tailSet(E fromElement) {
         return tailSet(fromElement, true);
     }

     /**
      * Returns a reverse order view of the elements contained in this set.
      * The descending set is backed by this set, so changes to the set are
      * reflected in the descending set, and vice-versa.
      *
      * <p>The returned set has an ordering equivalent to
      * <tt>{@link Collections#reverseOrder(Comparator) Collections.reverseOrder}(comparator())</tt>.
      * The expression {@code s.descendingSet().descendingSet()} returns a
      * view of {@code s} essentially equivalent to {@code s}.
      *
      * @return a reverse order view of this set
      */
     public NavigableSet<E> descendingSet() {
         return new ConcurrentSkipListSet(m.descendingMap());
     }

     // Support for resetting map in clone
     private void setMap(ConcurrentNavigableMap<E,Object> map) {
         UNSAFE.putObjectVolatile(this, mapOffset, map);
     }

     private static final sun.misc.Unsafe UNSAFE;
     private static final long mapOffset;
     static {
         try {
             UNSAFE = sun.misc.Unsafe.getUnsafe();
             Class k = ConcurrentSkipListSet.class;
             mapOffset = UNSAFE.objectFieldOffset
                 (k.getDeclaredField("m"));
         } catch (Exception e) {
             throw new Error(e);
         }
     }
 }

ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的，它的接口基本上都是通过调用ConcurrentSkipListMap接口来实现的。这里就不再对它的源码进行分析了。

ConcurrentSkipListSet示例

 import java.util.*;
 import java.util.concurrent.*;

 /*
  *   ConcurrentSkipListSet是“线程安全”的集合，而TreeSet是非线程安全的。
  *
  *   下面是“多个线程同时操作并且遍历集合set”的示例
  *   (01) 当set是ConcurrentSkipListSet对象时，程序能正常运行。
  *   (02) 当set是TreeSet对象时，程序会产生ConcurrentModificationException异常。
  *
  * @author skywang
  */
 public class ConcurrentSkipListSetDemo1 {

     // TODO: set是TreeSet对象时，程序会出错。
     //private static Set<String> set = new TreeSet<String>();
     private static Set<String> set = new ConcurrentSkipListSet<String>();
     public static void main(String[] args) {

         // 同时启动两个线程对set进行操作！
         new MyThread("a").start();
         new MyThread("b").start();
     }

     private static void printAll() {
         String value = null;
         Iterator iter = set.iterator();
         while(iter.hasNext()) {
             value = (String)iter.next();
             System.out.print(value+", ");
         }
         System.out.println();
     }

     private static class MyThread extends Thread {
         MyThread(String name) {
             super(name);
         }
         @Override
         public void run() {
                 int i = 0;
             while (i++ < 10) {
                 // “线程名” + "序号"
                 String val = Thread.currentThread().getName() + (i%6);
                 set.add(val);
                 // 通过“Iterator”遍历set。
                 printAll();
             }
         }
     }
 }

(某一次)运行结果：

a1, b1,
a1, a1, a2, b1,
b1, a1, a2, a3, b1,

a1, a2, a3, a1, a4, b1, b2,
a2, a1, a2, a3, a4, a5, b1, b2,
a3, a0, a4, a5, a1, b1, a2, b2,
a3, a0, a4, a1, a5, a2, b1, a3, b2, a4, b3,
a5, a0, b1, a1, b2, a2, b3,
a3, a0, a4, a1, a5, a2, b1, a3, b2, a4, b3, a5, b4,
b1, a0, b2, a1, b3, a2, b4,
a3, a0, a4, a1, a5, a2, b1, a3, b2, a4, b3, a5, b4, b1, b5,
b2, a0, a1, a2, a3, a4, a5, b3, b1, b4, b2, b5,
b3, a0, b4, a1, b5,
a2, a0, a3, a1, a4, a2, a5, a3, b0, a4, b1, a5, b2, b0, b3, b1, b4, b2, b5, b3,
b4, a0, b5,
a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5,
a0, a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5,
a0, a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5,
a0, a1, a2, a3, a4, a5, b0, b1, b2, b3, b4, b5, 

结果说明：
示例程序中，启动两个线程(线程a和线程b)分别对ConcurrentSkipListSet进行操作。以线程a而言，它会先获取“线程名”+“序号”，然后将该字符串添加到ConcurrentSkipListSet集合中；接着，遍历并输出集合中的全部元素。 线程b的操作和线程a一样，只不过线程b的名字和线程a的名字不同。
当set是ConcurrentSkipListSet对象时，程序能正常运行。如果将set改为TreeSet时，程序会产生ConcurrentModificationException异常。

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