Java多线程----CopyOnWriteArraySet

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概要

本章是JUC系列中的CopyOnWriteArraySet篇。接下来，会先对CopyOnWriteArraySet进行基本介绍，然后再说明它的原理，接着通过代码去分析，最后通过示例更进一步的了解CopyOnWriteArraySet。内容包括：
CopyOnWriteArraySet介绍
CopyOnWriteArraySet原理和数据结构
CopyOnWriteArraySet函数列表
CopyOnWriteArraySet源码(JDK1.7.0_40版本)
CopyOnWriteArraySet示例

转载请注明出处：http://www.cnblogs.com/skywang12345/p/3498497.html

 

CopyOnWriteArraySet介绍

它是线程安全的无序的集合，可以将它理解成线程安全的HashSet。有意思的是，CopyOnWriteArraySet和HashSet虽然都继承于共同的父类AbstractSet；但是，HashSet是通过“散列表(HashMap)”实现的，而CopyOnWriteArraySet则是通过“动态数组(CopyOnWriteArrayList)”实现的，并不是散列表。
和CopyOnWriteArrayList类似，CopyOnWriteArraySet具有以下特性：
1. 它最适合于具有以下特征的应用程序：Set 大小通常保持很小，只读操作远多于可变操作，需要在遍历期间防止线程间的冲突。
2. 它是线程安全的。
3. 因为通常需要复制整个基础数组，所以可变操作（add()、set() 和 remove() 等等）的开销很大。
4. 迭代器支持hasNext(), next()等不可变操作，但不支持可变 remove()等 操作。
5. 使用迭代器进行遍历的速度很快，并且不会与其他线程发生冲突。在构造迭代器时，迭代器依赖于不变的数组快照。

建议：在学习CopyOnWriteArraySet之前，先通过"Java 集合系列16之 HashSet详细介绍(源码解析)和使用示例"对HashSet进行了解。

 

CopyOnWriteArraySet原理和数据结构

CopyOnWriteArraySet的数据结构，如下图所示：

说明：
  1. CopyOnWriteArraySet继承于AbstractSet，这就意味着它是一个集合。
  2. CopyOnWriteArraySet包含CopyOnWriteArrayList对象，它是通过CopyOnWriteArrayList实现的。而CopyOnWriteArrayList本质是个动态数组队列，
所以CopyOnWriteArraySet相当于通过通过动态数组实现的“集合”！ CopyOnWriteArrayList中允许有重复的元素；但是，CopyOnWriteArraySet是一个集合，所以它不能有重复集合。因此，CopyOnWriteArrayList额外提供了addIfAbsent()和addAllAbsent()这两个添加元素的API，通过这些API来添加元素时，只有当元素不存在时才执行添加操作！
   至于CopyOnWriteArraySet的“线程安全”机制，和CopyOnWriteArrayList一样，是通过volatile和互斥锁来实现的。这个在前一章节介绍CopyOnWriteArrayList时数据结构时，已经进行了说明，这里就不再重复叙述了。

 

CopyOnWriteArraySet函数列表

 

CopyOnWriteArraySet源码(JDK1.7.0_40版本)

CopyOnWriteArraySet.java的完整源码如下：

/** ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.*********************//******* 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.*;/*** A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList}* for all of its operations.  Thus, it shares the same basic properties:* <ul>*  <li>It is best suited for applications in which set sizes generally*       stay small, read-only operations*       vastly outnumber mutative operations, and you need*       to prevent interference among threads during traversal.*  <li>It is thread-safe.*  <li>Mutative operations (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.)*      are expensive since they usually entail copying the entire underlying*      array.*  <li>Iterators do not support the mutative <tt>remove</tt> operation.*  <li>Traversal via iterators is fast and cannot encounter*      interference from other threads. Iterators rely on*      unchanging snapshots of the array at the time the iterators were*      constructed.* </ul>** <p> <b>Sample Usage.</b> The following code sketch uses a* copy-on-write set to maintain a set of Handler objects that* perform some action upon state updates.**  <pre> {@code* class Handler { void handle(); ... }** class X {*   private final CopyOnWriteArraySet<Handler> handlers*     = new CopyOnWriteArraySet<Handler>();*   public void addHandler(Handler h) { handlers.add(h); }**   private long internalState;*   private synchronized void changeState() { internalState = ...; }**   public void update() {*     changeState();*     for (Handler handler : handlers)*        handler.handle();*   }* }}</pre>** <p>This class is a member of the* <a href="{@docRoot}/../technotes/guides/collections/index.html">* Java Collections Framework</a>.** @see CopyOnWriteArrayList* @since 1.5* @author Doug Lea* @param <E> the type of elements held in this collection*/
public class CopyOnWriteArraySet<E> extends AbstractSet<E>implements java.io.Serializable {private static final long serialVersionUID = 5457747651344034263L;private final CopyOnWriteArrayList<E> al;/*** Creates an empty set.*/public CopyOnWriteArraySet() {al = new CopyOnWriteArrayList<E>();}/*** Creates a set containing all of the elements of the specified* collection.** @param c the collection of elements to initially contain* @throws NullPointerException if the specified collection is null*/public CopyOnWriteArraySet(Collection<? extends E> c) {al = new CopyOnWriteArrayList<E>();al.addAllAbsent(c);}/*** Returns the number of elements in this set.** @return the number of elements in this set*/public int size() {return al.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 al.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==null ? e==null : o.equals(e))</tt>.** @param o element whose presence in this set is to be tested* @return <tt>true</tt> if this set contains the specified element*/public boolean contains(Object o) {return al.contains(o);}/*** Returns an array containing all of the elements in this set.* If this set makes any guarantees as to what order its elements* are returned by its iterator, this method must return the* elements in the same order.** <p>The returned array will be "safe" in that no references to it* are maintained by this set.  (In other words, this method must* allocate a new array even if this set is backed by an array).* The caller is thus free to modify the returned array.** <p>This method acts as bridge between array-based and collection-based* APIs.** @return an array containing all the elements in this set*/public Object[] toArray() {return al.toArray();}/*** Returns an array containing all of the elements in this set; the* runtime type of the returned array is that of the specified array.* If the set fits in the specified array, it is returned therein.* Otherwise, a new array is allocated with the runtime type of the* specified array and the size of this set.** <p>If this set fits in the specified array with room to spare* (i.e., the array has more elements than this set), the element in* the array immediately following the end of the set is set to* <tt>null</tt>.  (This is useful in determining the length of this* set <i>only</i> if the caller knows that this set does not contain* any null elements.)** <p>If this set makes any guarantees as to what order its elements* are returned by its iterator, this method must return the elements* in the same order.** <p>Like the {@link #toArray()} method, this method acts as bridge between* array-based and collection-based APIs.  Further, this method allows* precise control over the runtime type of the output array, and may,* under certain circumstances, be used to save allocation costs.** <p>Suppose <tt>x</tt> is a set known to contain only strings.* The following code can be used to dump the set into a newly allocated* array of <tt>String</tt>:** <pre>*     String[] y = x.toArray(new String[0]);</pre>** Note that <tt>toArray(new Object[0])</tt> is identical in function to* <tt>toArray()</tt>.** @param a the array into which the elements of this set are to be*        stored, if it is big enough; otherwise, a new array of the same*        runtime type is allocated for this purpose.* @return an array containing all the elements in this set* @throws ArrayStoreException if the runtime type of the specified array*         is not a supertype of the runtime type of every element in this*         set* @throws NullPointerException if the specified array is null*/public <T> T[] toArray(T[] a) {return al.toArray(a);}/*** Removes all of the elements from this set.* The set will be empty after this call returns.*/public void clear() {al.clear();}/*** Removes the specified element from this set if it is present.* More formally, removes an element <tt>e</tt> such that* <tt>(o==null ? e==null : 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*/public boolean remove(Object o) {return al.remove(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==null ? e2==null : 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*/public boolean add(E e) {return al.addIfAbsent(e);}/*** Returns <tt>true</tt> if this set contains all of the elements of the* specified collection.  If the specified collection is also a set, this* method returns <tt>true</tt> if it is a <i>subset</i> of this set.** @param  c collection to be checked for containment in this set* @return <tt>true</tt> if this set contains all of the elements of the*         specified collection* @throws NullPointerException if the specified collection is null* @see #contains(Object)*/public boolean containsAll(Collection<?> c) {return al.containsAll(c);}/*** Adds all of the elements in the specified collection to this set if* they're not already present.  If the specified collection is also a* set, the <tt>addAll</tt> operation effectively modifies this set so* that its value is the <i>union</i> of the two sets.  The behavior of* this operation is undefined if the specified collection is modified* while the operation is in progress.** @param  c collection containing elements to be added to this set* @return <tt>true</tt> if this set changed as a result of the call* @throws NullPointerException if the specified collection is null* @see #add(Object)*/public boolean addAll(Collection<? extends E> c) {return al.addAllAbsent(c) > 0;}/*** 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 class of an element of this set*         is incompatible with the specified collection (optional)* @throws NullPointerException if this set contains a null element and the*         specified collection does not permit null elements (optional),*         or if the specified collection is null* @see #remove(Object)*/public boolean removeAll(Collection<?> c) {return al.removeAll(c);}/*** Retains only the elements in this set that are contained in the* specified collection.  In other words, removes from this set all of* its elements that are not 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>intersection</i> of the* two sets.** @param  c collection containing elements to be retained in this set* @return <tt>true</tt> if this set changed as a result of the call* @throws ClassCastException if the class of an element of this set*         is incompatible with the specified collection (optional)* @throws NullPointerException if this set contains a null element and the*         specified collection does not permit null elements (optional),*         or if the specified collection is null* @see #remove(Object)*/public boolean retainAll(Collection<?> c) {return al.retainAll(c);}/*** Returns an iterator over the elements contained in this set* in the order in which these elements were added.** <p>The returned iterator provides a snapshot of the state of the set* when the iterator was constructed. No synchronization is needed while* traversing the iterator. The iterator does <em>NOT</em> support the* <tt>remove</tt> method.** @return an iterator over the elements in this set*/public Iterator<E> iterator() {return al.iterator();}/*** Compares the specified object with this set for equality.* Returns {@code true} if the specified object is the same object* as this object, or if it is also a {@link Set} and the elements* returned by an {@linkplain List#iterator() iterator} over the* specified set are the same as the elements returned by an* iterator over this set.  More formally, the two iterators are* considered to return the same elements if they return the same* number of elements and for every element {@code e1} returned by* the iterator over the specified set, there is an element* {@code e2} returned by the iterator over this set such that* {@code (e1==null ? e2==null : e1.equals(e2))}.** @param o object to be compared for equality with this set* @return {@code true} if the specified object is equal to this set*/public boolean equals(Object o) {if (o == this)return true;if (!(o instanceof Set))return false;Set<?> set = (Set<?>)(o);Iterator<?> it = set.iterator();// Uses O(n^2) algorithm that is only appropriate// for small sets, which CopyOnWriteArraySets should be.//  Use a single snapshot of underlying arrayObject[] elements = al.getArray();int len = elements.length;// Mark matched elements to avoid re-checkingboolean[] matched = new boolean[len];int k = 0;outer: while (it.hasNext()) {if (++k > len)return false;Object x = it.next();for (int i = 0; i < len; ++i) {if (!matched[i] && eq(x, elements[i])) {matched[i] = true;continue outer;}}return false;}return k == len;}/*** Test for equality, coping with nulls.*/private static boolean eq(Object o1, Object o2) {return (o1 == null ? o2 == null : o1.equals(o2));}
}

CopyOnWriteArraySet是通过CopyOnWriteArrayList实现的，它的API基本上都是通过调用CopyOnWriteArrayList的API来实现的。相信对CopyOnWriteArrayList了解的话，对CopyOnWriteArraySet的了解是水到渠成的事；所以，这里就不再对CopyOnWriteArraySet的代码进行详细的解析了。若对CopyOnWriteArrayList不了解，请参考“ Java多线程系列--“JUC集合”02之 CopyOnWriteArrayList ”。

 

CopyOnWriteArraySet示例

下面，我们通过一个例子去对比HashSet和CopyOnWriteArraySet。

(某一次)运行结果：

结果说明：
由于set是集合对象，因此它不会包含重复的元素。
如果将源码中的set改成HashSet对象时，程序会产生ConcurrentModificationException异常。

CopyOnWriteArraySet基于CopyOnWriteArrayList实现，其唯一的不同是在add时调用的是CopyOnWriteArrayList的addIfAbsent方法.adIfAbsent方法同样采用锁保护，并创建一个新的大小+1的Object数组。遍历当前Object数组，如Object数组中已有了当前元素，则直接返回，如果没有则放入Object数组的尾部，并返回。

从以上分析可见，CopyOnWriteArraySet在add时每次都要进行数组的遍历，因此其性能会低于CopyOnWriteArrayList.

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