Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
来源：互联网 发布：四个系统数据库是什么 编辑：程序博客网 时间：2024/06/04 18:58
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
// 创建一个空 set。CopyOnWriteArraySet()// 创建一个包含指定 collection 所有元素的 set。CopyOnWriteArraySet(Collection<? extends E> c)// 如果指定元素并不存在于此 set 中，则添加它。boolean add(E e)// 如果此 set 中没有指定 collection 中的所有元素，则将它们都添加到此 set 中。boolean addAll(Collection<? extends E> c)// 移除此 set 中的所有元素。void clear()// 如果此 set 包含指定元素，则返回 true。boolean contains(Object o)// 如果此 set 包含指定 collection 的所有元素，则返回 true。boolean containsAll(Collection<?> c)// 比较指定对象与此 set 的相等性。boolean equals(Object o)// 如果此 set 不包含任何元素，则返回 true。boolean isEmpty()// 返回按照元素添加顺序在此 set 中包含的元素上进行迭代的迭代器。Iterator<E> iterator()// 如果指定元素存在于此 set 中，则将其移除。boolean remove(Object o)// 移除此 set 中包含在指定 collection 中的所有元素。boolean removeAll(Collection<?> c)// 仅保留此 set 中那些包含在指定 collection 中的元素。boolean retainAll(Collection<?> c)// 返回此 set 中的元素数目。int size()// 返回一个包含此 set 所有元素的数组。Object[] toArray()// 返回一个包含此 set 所有元素的数组；返回数组的运行时类型是指定数组的类型。<T> T[] toArray(T[] a)
CopyOnWriteArraySet源码(JDK1.7.0_40版本)
CopyOnWriteArraySet.java的完整源码如下：
1 /* 2 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms. 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10 * 11 * 12 * 13 * 14 * 15 * 16 * 17 * 18 * 19 * 20 * 21 * 22 * 23 */ 24 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 39 /** 40 * A {@link java.util.Set} that uses an internal {@link CopyOnWriteArrayList} 41 * for all of its operations. Thus, it shares the same basic properties: 42 * <ul> 43 * <li>It is best suited for applications in which set sizes generally 44 * stay small, read-only operations 45 * vastly outnumber mutative operations, and you need 46 * to prevent interference among threads during traversal. 47 * <li>It is thread-safe. 48 * <li>Mutative operations (<tt>add</tt>, <tt>set</tt>, <tt>remove</tt>, etc.) 49 * are expensive since they usually entail copying the entire underlying 50 * array. 51 * <li>Iterators do not support the mutative <tt>remove</tt> operation. 52 * <li>Traversal via iterators is fast and cannot encounter 53 * interference from other threads. Iterators rely on 54 * unchanging snapshots of the array at the time the iterators were 55 * constructed. 56 * </ul> 57 * 58 * <p> <b>Sample Usage.</b> The following code sketch uses a 59 * copy-on-write set to maintain a set of Handler objects that 60 * perform some action upon state updates. 61 * 62 * <pre> {@code 63 * class Handler { void handle(); ... } 64 * 65 * class X { 66 * private final CopyOnWriteArraySet<Handler> handlers 67 * = new CopyOnWriteArraySet<Handler>(); 68 * public void addHandler(Handler h) { handlers.add(h); } 69 * 70 * private long internalState; 71 * private synchronized void changeState() { internalState = ...; } 72 * 73 * public void update() { 74 * changeState(); 75 * for (Handler handler : handlers) 76 * handler.handle(); 77 * } 78 * }}</pre> 79 * 80 * <p>This class is a member of the 81 * <a href="{@docRoot}/../technotes/guides/collections/index.html"> 82 * Java Collections Framework</a>. 83 * 84 * @see CopyOnWriteArrayList 85 * @since 1.5 86 * @author Doug Lea 87 * @param <E> the type of elements held in this collection 88 */ 89 public class CopyOnWriteArraySet<E> extends AbstractSet<E> 90 implements java.io.Serializable { 91 private static final long serialVersionUID = 5457747651344034263L; 92 93 private final CopyOnWriteArrayList<E> al; 94 95 /** 96 * Creates an empty set. 97 */ 98 public CopyOnWriteArraySet() { 99 al = new CopyOnWriteArrayList<E>();100 }101 102 /**103 * Creates a set containing all of the elements of the specified104 * collection.105 *106 * @param c the collection of elements to initially contain107 * @throws NullPointerException if the specified collection is null108 */109 public CopyOnWriteArraySet(Collection<? extends E> c) {110 al = new CopyOnWriteArrayList<E>();111 al.addAllAbsent(c);112 }113 114 /**115 * Returns the number of elements in this set.116 *117 * @return the number of elements in this set118 */119 public int size() {120 return al.size();121 }122 123 /**124 * Returns <tt>true</tt> if this set contains no elements.125 *126 * @return <tt>true</tt> if this set contains no elements127 */128 public boolean isEmpty() {129 return al.isEmpty();130 }131 132 /**133 * Returns <tt>true</tt> if this set contains the specified element.134 * More formally, returns <tt>true</tt> if and only if this set135 * contains an element <tt>e</tt> such that136 * <tt>(o==null ? e==null : o.equals(e))</tt>.137 *138 * @param o element whose presence in this set is to be tested139 * @return <tt>true</tt> if this set contains the specified element140 */141 public boolean contains(Object o) {142 return al.contains(o);143 }144 145 /**146 * Returns an array containing all of the elements in this set.147 * If this set makes any guarantees as to what order its elements148 * are returned by its iterator, this method must return the149 * elements in the same order.150 *151 * <p>The returned array will be "safe" in that no references to it152 * are maintained by this set. (In other words, this method must153 * allocate a new array even if this set is backed by an array).154 * The caller is thus free to modify the returned array.155 *156 * <p>This method acts as bridge between array-based and collection-based157 * APIs.158 *159 * @return an array containing all the elements in this set160 */161 public Object[] toArray() {162 return al.toArray();163 }164 165 /**166 * Returns an array containing all of the elements in this set; the167 * runtime type of the returned array is that of the specified array.168 * If the set fits in the specified array, it is returned therein.169 * Otherwise, a new array is allocated with the runtime type of the170 * specified array and the size of this set.171 *172 * <p>If this set fits in the specified array with room to spare173 * (i.e., the array has more elements than this set), the element in174 * the array immediately following the end of the set is set to175 * <tt>null</tt>. (This is useful in determining the length of this176 * set <i>only</i> if the caller knows that this set does not contain177 * any null elements.)178 *179 * <p>If this set makes any guarantees as to what order its elements180 * are returned by its iterator, this method must return the elements181 * in the same order.182 *183 * <p>Like the {@link #toArray()} method, this method acts as bridge between184 * array-based and collection-based APIs. Further, this method allows185 * precise control over the runtime type of the output array, and may,186 * under certain circumstances, be used to save allocation costs.187 *188 * <p>Suppose <tt>x</tt> is a set known to contain only strings.189 * The following code can be used to dump the set into a newly allocated190 * array of <tt>String</tt>:191 *192 * <pre>193 * String[] y = x.toArray(new String[0]);</pre>194 *195 * Note that <tt>toArray(new Object[0])</tt> is identical in function to196 * <tt>toArray()</tt>.197 *198 * @param a the array into which the elements of this set are to be199 * stored, if it is big enough; otherwise, a new array of the same200 * runtime type is allocated for this purpose.201 * @return an array containing all the elements in this set202 * @throws ArrayStoreException if the runtime type of the specified array203 * is not a supertype of the runtime type of every element in this204 * set205 * @throws NullPointerException if the specified array is null206 */207 public <T> T[] toArray(T[] a) {208 return al.toArray(a);209 }210 211 /**212 * Removes all of the elements from this set.213 * The set will be empty after this call returns.214 */215 public void clear() {216 al.clear();217 }218 219 /**220 * Removes the specified element from this set if it is present.221 * More formally, removes an element <tt>e</tt> such that222 * <tt>(o==null ? e==null : o.equals(e))</tt>,223 * if this set contains such an element. Returns <tt>true</tt> if224 * this set contained the element (or equivalently, if this set225 * changed as a result of the call). (This set will not contain the226 * element once the call returns.)227 *228 * @param o object to be removed from this set, if present229 * @return <tt>true</tt> if this set contained the specified element230 */231 public boolean remove(Object o) {232 return al.remove(o);233 }234 235 /**236 * Adds the specified element to this set if it is not already present.237 * More formally, adds the specified element <tt>e</tt> to this set if238 * the set contains no element <tt>e2</tt> such that239 * <tt>(e==null ? e2==null : e.equals(e2))</tt>.240 * If this set already contains the element, the call leaves the set241 * unchanged and returns <tt>false</tt>.242 *243 * @param e element to be added to this set244 * @return <tt>true</tt> if this set did not already contain the specified245 * element246 */247 public boolean add(E e) {248 return al.addIfAbsent(e);249 }250 251 /**252 * Returns <tt>true</tt> if this set contains all of the elements of the253 * specified collection. If the specified collection is also a set, this254 * method returns <tt>true</tt> if it is a <i>subset</i> of this set.255 *256 * @param c collection to be checked for containment in this set257 * @return <tt>true</tt> if this set contains all of the elements of the258 * specified collection259 * @throws NullPointerException if the specified collection is null260 * @see #contains(Object)261 */262 public boolean containsAll(Collection<?> c) {263 return al.containsAll(c);264 }265 266 /**267 * Adds all of the elements in the specified collection to this set if268 * they're not already present. If the specified collection is also a269 * set, the <tt>addAll</tt> operation effectively modifies this set so270 * that its value is the <i>union</i> of the two sets. The behavior of271 * this operation is undefined if the specified collection is modified272 * while the operation is in progress.273 *274 * @param c collection containing elements to be added to this set275 * @return <tt>true</tt> if this set changed as a result of the call276 * @throws NullPointerException if the specified collection is null277 * @see #add(Object)278 */279 public boolean addAll(Collection<? extends E> c) {280 return al.addAllAbsent(c) > 0;281 }282 283 /**284 * Removes from this set all of its elements that are contained in the285 * specified collection. If the specified collection is also a set,286 * this operation effectively modifies this set so that its value is the287 * <i>asymmetric set difference</i> of the two sets.288 *289 * @param c collection containing elements to be removed from this set290 * @return <tt>true</tt> if this set changed as a result of the call291 * @throws ClassCastException if the class of an element of this set292 * is incompatible with the specified collection (optional)293 * @throws NullPointerException if this set contains a null element and the294 * specified collection does not permit null elements (optional),295 * or if the specified collection is null296 * @see #remove(Object)297 */298 public boolean removeAll(Collection<?> c) {299 return al.removeAll(c);300 }301 302 /**303 * Retains only the elements in this set that are contained in the304 * specified collection. In other words, removes from this set all of305 * its elements that are not contained in the specified collection. If306 * the specified collection is also a set, this operation effectively307 * modifies this set so that its value is the <i>intersection</i> of the308 * two sets.309 *310 * @param c collection containing elements to be retained in this set311 * @return <tt>true</tt> if this set changed as a result of the call312 * @throws ClassCastException if the class of an element of this set313 * is incompatible with the specified collection (optional)314 * @throws NullPointerException if this set contains a null element and the315 * specified collection does not permit null elements (optional),316 * or if the specified collection is null317 * @see #remove(Object)318 */319 public boolean retainAll(Collection<?> c) {320 return al.retainAll(c);321 }322 323 /**324 * Returns an iterator over the elements contained in this set325 * in the order in which these elements were added.326 *327 * <p>The returned iterator provides a snapshot of the state of the set328 * when the iterator was constructed. No synchronization is needed while329 * traversing the iterator. The iterator does <em>NOT</em> support the330 * <tt>remove</tt> method.331 *332 * @return an iterator over the elements in this set333 */334 public Iterator<E> iterator() {335 return al.iterator();336 }337 338 /**339 * Compares the specified object with this set for equality.340 * Returns {@code true} if the specified object is the same object341 * as this object, or if it is also a {@link Set} and the elements342 * returned by an {@linkplain List#iterator() iterator} over the343 * specified set are the same as the elements returned by an344 * iterator over this set. More formally, the two iterators are345 * considered to return the same elements if they return the same346 * number of elements and for every element {@code e1} returned by347 * the iterator over the specified set, there is an element348 * {@code e2} returned by the iterator over this set such that349 * {@code (e1==null ? e2==null : e1.equals(e2))}.350 *351 * @param o object to be compared for equality with this set352 * @return {@code true} if the specified object is equal to this set353 */354 public boolean equals(Object o) {355 if (o == this)356 return true;357 if (!(o instanceof Set))358 return false;359 Set<?> set = (Set<?>)(o);360 Iterator<?> it = set.iterator();361 362 // Uses O(n^2) algorithm that is only appropriate363 // for small sets, which CopyOnWriteArraySets should be.364 365 // Use a single snapshot of underlying array366 Object[] elements = al.getArray();367 int len = elements.length;368 // Mark matched elements to avoid re-checking369 boolean[] matched = new boolean[len];370 int k = 0;371 outer: while (it.hasNext()) {372 if (++k > len)373 return false;374 Object x = it.next();375 for (int i = 0; i < len; ++i) {376 if (!matched[i] && eq(x, elements[i])) {377 matched[i] = true;378 continue outer;379 }380 }381 return false;382 }383 return k == len;384 }385 386 /**387 * Test for equality, coping with nulls.388 */389 private static boolean eq(Object o1, Object o2) {390 return (o1 == null ? o2 == null : o1.equals(o2));391 }392 }
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。
import java.util.*;import java.util.concurrent.*;/* * CopyOnWriteArraySet是“线程安全”的集合，而HashSet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CopyOnWriteArraySet对象时，程序能正常运行。 * (02) 当set是HashSet对象时，程序会产生ConcurrentModificationException异常。 * * @author skywang */public class CopyOnWriteArraySetTest1 { // TODO: set是HashSet对象时，程序会出错。 //private static Set<String> set = new HashSet<String>(); private static Set<String> set = new CopyOnWriteArraySet<String>(); public static void main(String[] args) { // 同时启动两个线程对set进行操作！ new MyThread("ta").start(); new MyThread("tb").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：
ta-1, tb-1, ta-1, tb-1, ta-1, tb-1, ta-1, ta-2, tb-1, ta-1, ta-2, tb-1, tb-2, ta-2, ta-1, tb-2, tb-1, ta-3, ta-2, ta-1, tb-2, tb-1, ta-3, ta-2, tb-3, tb-2, ta-1, ta-3, tb-1, tb-3, ta-2, ta-4, tb-2, ta-1, ta-3, tb-1, tb-3, ta-2, ta-4, tb-2, tb-4, ta-3, ta-1, tb-3, tb-1, ta-4, ta-2, tb-4, tb-2, ta-5, ta-3, ta-1, tb-3, tb-1, ta-4, ta-2, tb-4, tb-2, ta-5, ta-3, tb-5, tb-3, ta-1, ta-4, tb-1, tb-4, ta-2, ta-5, tb-2, tb-5, ta-3, ta-0, tb-3, ta-1, ta-4, tb-1, tb-4, ta-2, ta-5, tb-2, tb-5, ta-3, ta-0, tb-3, tb-0, ta-4, ta-1, tb-4, tb-1, ta-5, ta-2, tb-5, tb-2, ta-0, ta-3, tb-0, tb-3, ta-1, ta-4, tb-1, tb-4, ta-2, ta-5, tb-5, ta-0, tb-0, ta-1, tb-2, tb-1, ta-3, ta-2, tb-3, tb-2, ta-4, ta-3, tb-4, tb-3, ta-5, ta-4, tb-5, tb-4, ta-0, ta-5, tb-0, tb-5, ta-1, ta-0, tb-1, tb-0, ta-2, ta-1, tb-2, tb-1, ta-3, ta-2, tb-3, tb-2, ta-4, ta-3, tb-4, tb-3, ta-5, tb-5, ta-0, tb-0, ta-4, ta-1, tb-4, tb-1, ta-5, ta-2, tb-5, tb-2, ta-0, ta-3, tb-0, tb-3, ta-1, ta-4, tb-1, tb-4, ta-2, ta-5, tb-2, tb-5, ta-3, ta-0, tb-3, tb-0, ta-4, tb-4, ta-5, tb-5, ta-0, tb-0,
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。
更多内容
1. Java多线程系列--“JUC集合”01之 框架
2. Java多线程系列--“JUC集合”02之 CopyOnWriteArrayLis
3. Java多线程系列目录(共xx篇)
4. Java 集合系列目录(Category)
0 0
- Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
- Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
- Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
- Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
- Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
- JUC集合-03之 CopyOnWriteArraySet
- Java多线程”JUC”集合中的CopyOnWriteArraySet
- Java多线程系列--“JUC集合”
- Java多线程系列--“JUC集合”04之 ConcurrentHashMap
- Java多线程系列--“JUC集合”01之 框架
- Java多线程系列--“JUC集合”01之 框架
- Java多线程系列--“JUC集合”02之 CopyOnWriteArrayList
- Java多线程系列--“JUC集合”04之 ConcurrentHashMap
- Java多线程系列--“JUC集合”01之 框架
- Java多线程系列--“JUC集合”02之 CopyOnWriteArrayList
- Java多线程系列--“JUC集合”04之 ConcurrentHashMap
- Java多线程系列--“JUC集合”05之 ConcurrentSkipListMap
- Java多线程系列--“JUC集合”06之 ConcurrentSkipListSet
- Activity与Fragment搭配使用时，Fragment中使用Toast频繁出现空指针异常的解决办法
- Spring整合Struts2
- bat知识点3_for循环_指定变数
- HDU1754 I Hate It （分块入门）
- ural 2073 - Log Files 模拟
- Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet
- Android-绑定服务
- win7系统mscomctl.ocx不能注册
- java调用opencv的exe程序
- bat知识点5_EnableDelayedExpansion
- errorPage、jsp:include、jsp:forward、get\post乱码解决
- Android中dip、dp、sp、pt和px的区别
- 读书笔记-柒-《腾讯网UED体验设计之旅》
- bat知识点6_if
