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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/*
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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
1 import java.util.*;
2 import java.util.concurrent.*;
3
4 /*
5 * ConcurrentSkipListSet是“线程安全”的集合，而TreeSet是非线程安全的。
6 *
7 * 下面是“多个线程同时操作并且遍历集合set”的示例
8 * (01) 当set是ConcurrentSkipListSet对象时，程序能正常运行。
9 * (02) 当set是TreeSet对象时，程序会产生ConcurrentModificationException异常。
10 *
11 * @author skywang
12 */
13 public class ConcurrentSkipListSetDemo1 {
14
15 // TODO: set是TreeSet对象时，程序会出错。
16 //private static Set<String> set = new TreeSet<String>();
17 private static Set<String> set = new ConcurrentSkipListSet<String>();
18 public static void main(String[] args) {
19
20 // 同时启动两个线程对set进行操作！
21 new MyThread("a").start();
22 new MyThread("b").start();
23 }
24
25 private static void printAll() {
26 String value = null;
27 Iterator iter = set.iterator();
28 while(iter.hasNext()) {
29 value = (String)iter.next();
30 System.out.print(value+", ");
31 }
32 System.out.println();
33 }
34
35 private static class MyThread extends Thread {
36 MyThread(String name) {
37 super(name);
38 }
39 @Override
40 public void run() {
41 int i = 0;
42 while (i++ < 10) {
43 // “线程名” + "序号"
44 String val = Thread.currentThread().getName() + (i%6);
45 set.add(val);
46 // 通过“Iterator”遍历set。
47 printAll();
48 }
49 }
50 }
51 }
(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。
