Java多线程
Java并发
JUC集合

Java多线程 35 - ConcurrentSkipListSet详解

简介:ConcurrentSkipListSet是线程安全的有序的集合,适用于高并发的场景。

1. 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,但有以下的不同点:

  1. 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。
  2. TreeSet是通过TreeMap实现的,而ConcurrentSkipListSet是通过ConcurrentSkipListMap实现的。

ConcurrentSkipListSet的类图结构如下:

1.ConcurrentSkipListSet类图结构.png

  1. ConcurrentSkipListSet继承于AbstractSet,因此它本质上是一个集合。
  2. ConcurrentSkipListSet实现了NavigableSet接口,因此它是一个有序的集合。
  3. 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中的元素是键值对;而ConcurrentSkipListSet是集合,它只用到了ConcurrentSkipListMap中的键。

ConcurrentSkipListSet函数列表如下:

  • // 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)

2. ConcurrentSkipListSet源码解析

下面是ConcurrentSkipListSet的源码解析,基于JDK 1.7.0_07:

  • package java.util.concurrent;
  • import java.util.*;
  • import sun.misc.Unsafe;
  • 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()
  • * 内部实际使用的ConcurrentNavigableMap类型的对象进行数据的保存
  • * 键是实际存储在集合中的值,而值使用Boolean.TRUE表示
  • * 使用final修饰保证现场安全性
  • */
  • private final ConcurrentNavigableMap<E,Object> m;
  • /**
  • * Constructs a new, empty set that orders its elements according to
  • * their {@linkplain Comparable natural ordering}.
  • * 构造方法,将m实例化为一个ConcurrentSkipListMap对象
  • */
  • public ConcurrentSkipListSet() {
  • m = new ConcurrentSkipListMap<E,Object>();
  • }
  • /**
  • * Constructs a new, empty set that orders its elements according to
  • * the specified comparator.
  • *
  • * 指定特定比较器的ConcurrentSkipListMap对象
  • *
  • * @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}.
  • *
  • * 通过一个Collection集合创建ConcurrentSkipListSet
  • *
  • * @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>();
  • // 通过addAll()将Collection中的元素添加到ConcurrentSkipListSet
  • addAll(c);
  • }
  • /**
  • * Constructs a new set containing the same elements and using the
  • * same ordering as the specified sorted set.
  • *
  • * 通过一个SortedSet有序集合创建ConcurrentSkipListSet
  • *
  • * @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) {
  • // 直接使用SortedSet的比较器
  • m = new ConcurrentSkipListMap<E,Object>(s.comparator());
  • // 通过addAll()将SortedSet中的元素添加到ConcurrentSkipListSet
  • addAll(s);
  • }
  • /**
  • * For use by submaps
  • * 通过传入ConcurrentNavigableMap实例构造ConcurrentSkipListSet
  • * 主要用于SubMap中
  • */
  • 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 {
  • // 通过super.clone()克隆一个ConcurrentSkipListSet
  • clone = (ConcurrentSkipListSet<E>) super.clone();
  • // 根据m构造一个新的ConcurrentSkipListMap,添加到clone的集合中
  • 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() {
  • // 返回m的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() {
  • // 返回m的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) {
  • // 返回m的containsKey(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) {
  • // 使用putIfAbsent()添加元素,注意值为Boolean.TRUE
  • 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) {
  • // 使用remove()移除元素,注意值为Boolean.TRUE
  • return m.remove(o, Boolean.TRUE);
  • }
  • /**
  • * Removes all of the elements from this set.
  • */
  • public void clear() {
  • // 使用m的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() {
  • // 返回m的navigableKeySet迭代器
  • 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() {
  • // 返回m的逆序KeySet迭代器
  • 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()
  • // 判断对象是否是同一个对象
  • if (o == this)
  • return true;
  • // 判断o是否是Set
  • if (!(o instanceof Set))
  • return false;
  • // 将o转为Collection
  • Collection<?> c = (Collection<?>) o;
  • try {
  • // 使用containsAll()进行比较
  • 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.
  • *
  • * 从ConcurrentSkipListSet中移除c中的元素
  • *
  • * @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()
  • // 记录是否发生移除操作
  • boolean modified = false;
  • // 遍历c中的元素,依次进行移除
  • for (Iterator<?> i = c.iterator(); i.hasNext(); )
  • if (remove(i.next()))
  • // 如果发生移除操作就将modified置为true
  • modified = true;
  • return modified;
  • }
  • /** 下面的关系判断操作的方法、子Set视图方法都是使用的ConcurrentSkipListMap相关的API,这里不再赘述 */
  • /* ---------------- 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;
  • // 记录m在ConcurrentSkipListSet中的偏移量
  • private static final long mapOffset;
  • static {
  • try {
  • UNSAFE = sun.misc.Unsafe.getUnsafe();
  • Class k = ConcurrentSkipListSet.class;
  • // 使用Unsafe获取m的偏移量
  • mapOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("m"));
  • } catch (Exception e) {
  • throw new Error(e);
  • }
  • }
  • }

从源码中可以得知,ConcurrentSkipListSet其实是基于ConcurrentSkipListMap实现的,它只是用了ConcurrentSkipListMap的键存储集合数据,而值则统一置为Boolean.TRUE。由于前面已经讲过ConcurrentSkipListMap的原理,理解ConcurrentSkipListSet就非常简单了,这里就不多赘述。

3. ConcurrentSkipListSet示例

下面提供一个简单的ConcurrentSkipListSet的示例:

  • package com.coderap.collection;
  • import java.util.Iterator;
  • import java.util.Set;
  • import java.util.TreeSet;
  • import java.util.concurrent.ConcurrentSkipListSet;
  • public class ConcurrentSkipListSetTest {
  • // 循环次数
  • private final static int loopCount = 5;
  • // 操作的TreeSet和ConcurrentSkipListSet
  • private final static Set<String> treeSet = new TreeSet<>();
  • private final static ConcurrentSkipListSet<String> concurrentSkipListSet = new ConcurrentSkipListSet<>();
  • public static void main(String[] args) {
  • // 开启两个线程同时操作
  • new Thread(new ConcurrentSkipListSetTest.OperateThread(concurrentSkipListSet, loopCount), "T-1").start();
  • new Thread(new ConcurrentSkipListSetTest.OperateThread(concurrentSkipListSet, loopCount), "T-2").start();
  • // new Thread(new ConcurrentSkipListSetTest.OperateThread(treeSet, loopCount), "T-3").start();
  • // new Thread(new ConcurrentSkipListSetTest.OperateThread(treeSet, loopCount), "T-4").start();
  • }
  • // 遍历方法
  • private static void iterate(Set<String> set) {
  • StringBuilder stringBuilder = new StringBuilder();
  • Iterator<String> iterator = set.iterator();
  • while (iterator.hasNext()) {
  • stringBuilder.append(iterator.next());
  • stringBuilder.append(", ");
  • }
  • // 删除最后多余的字符
  • stringBuilder.delete(stringBuilder.length() - 2, stringBuilder.length() - 1);
  • // 打印
  • System.out.println(Thread.currentThread().getName() + " iterate: " + stringBuilder.toString());
  • }
  • private static class OperateThread implements Runnable {
  • private Set set;
  • private int loopCount;
  • public OperateThread(Set set, int loopCount) {
  • this.set = set;
  • this.loopCount = loopCount;
  • }
  • @Override
  • public void run() {
  • // 循环添加并遍历打印
  • while (loopCount >= 0) {
  • set.add(Thread.currentThread().getName() + " - " + loopCount);
  • iterate(set);
  • loopCount--;
  • }
  • }
  • }
  • }

某一次运行结果如下:

  • T-2 iterate: T-1 - 5, T-2 - 5
  • T-1 iterate: T-1 - 5, T-2 - 5
  • T-2 iterate: T-1 - 5, T-2 - 4, T-2 - 5
  • T-1 iterate: T-1 - 4, T-1 - 5, T-2 - 4, T-2 - 5
  • T-1 iterate: T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 3, T-2 - 4, T-2 - 5
  • T-2 iterate: T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 3, T-2 - 4, T-2 - 5
  • T-1 iterate: T-1 - 2, T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 3, T-2 - 4, T-2 - 5
  • T-2 iterate: T-1 - 2, T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 2, T-2 - 3, T-2 - 4, T-2 - 5
  • T-1 iterate: T-1 - 1, T-1 - 2, T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 2, T-2 - 3, T-2 - 4, T-2 - 5
  • T-2 iterate: T-1 - 1, T-1 - 2, T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 1, T-2 - 2, T-2 - 3, T-2 - 4, T-2 - 5
  • T-1 iterate: T-1 - 0, T-1 - 1, T-1 - 2, T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 1, T-2 - 2, T-2 - 3, T-2 - 4, T-2 - 5
  • T-2 iterate: T-1 - 0, T-1 - 1, T-1 - 2, T-1 - 3, T-1 - 4, T-1 - 5, T-2 - 0, T-2 - 1, T-2 - 2, T-2 - 3, T-2 - 4, T-2 - 5

如果将源码中的ConcurrentSkipListSet类型集合改成TreeSet类型集合时,程序可能会产生ConcurrentModificationException异常。