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

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：

/*
 * 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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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&nbsp;?&nbsp;e==null&nbsp;:&nbsp;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&nbsp;?&nbsp;e2==null&nbsp;:&nbsp;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”。

 

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。

 

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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)