Java多线程系列--“JUC集合”03之 CopyOnWriteArraySet

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

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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：

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* 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.*;

/**
* 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));
}
}

View Code
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)