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。
目录
1. CopyOnWriteArraySet介绍
2. CopyOnWriteArraySet原理和数据结构
3. CopyOnWriteArraySet函数列表
4. CopyOnWriteArraySet源码(JDK1.7.0_40版本)
5. CopyOnWriteArraySet示例
1. 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：
(01). 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。
(02). 它是线程安全的。
(03). 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。
(04). 迭代器支持hasNext(), next()等不可变操作，但不支持可变 remove()等 操作。
**(05). 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。
2. CopyOnWriteArraySet原理和数据结构
CopyOnWriteArraySet的数据结构，如下图所示：
说明：
(01). CopyOnWriteArraySet继承于AbstractSet，这就意味着它是一个集合。
(02). 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。
3. 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)
4. CopyOnWriteArraySet源码(JDK1.7.0_40版本)
CopyOnWriteArraySet.java的完整源码如下：
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 array
Object[] elements = al.getArray();
int len = elements.length;
// Mark matched elements to avoid re-checking
boolean[] 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”。
5. 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。