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概要
本章是"JUC系列"的CopyOnWriteArrayList篇。接下来，会先对CopyOnWriteArrayList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CopyOnWriteArrayList。内容包括：
CopyOnWriteArrayList介绍
CopyOnWriteArrayList原理和数据结构
CopyOnWriteArrayList函数列表
CopyOnWriteArrayList源码分析(JDK1.7.0_40版本)
CopyOnWriteArrayList示例
转载请注明出处：http://www.cnblogs.com/skywang12345/p/3498483.html
CopyOnWriteArrayList介绍
它相当于线程安全的ArrayList。和ArrayList一样，它是个可变数组；但是和ArrayList不同的时，它具有以下特性：
1. 它最适合于具有以下特征的应用程序：List 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03之 ArrayList详细介绍(源码解析)和使用示例"对ArrayList进行了解！
CopyOnWriteArrayList原理和数据结构
CopyOnWriteArrayList的数据结构，如下图所示：
说明：
1. CopyOnWriteArrayList实现了List接口，因此它是一个队列。
2. CopyOnWriteArrayList包含了成员lock。每一个CopyOnWriteArrayList都和一个互斥锁lock绑定，通过lock，实现了对CopyOnWriteArrayList的互斥访问。
3. CopyOnWriteArrayList包含了成员array数组，这说明CopyOnWriteArrayList本质上通过数组实现的。
下面从“动态数组”和“线程安全”两个方面进一步对CopyOnWriteArrayList的原理进行说明。
1. CopyOnWriteArrayList的“动态数组”机制 -- 它内部有个“volatile数组”(array)来保持数据。在“添加/修改/删除”数据时，都会新建一个数组，并将更新后的数据拷贝到新建的数组中，最后再将该数组赋值给“volatile数组”。这就是它叫做CopyOnWriteArrayList的原因！CopyOnWriteArrayList就是通过这种方式实现的动态数组；不过正由于它在“添加/修改/删除”数据时，都会新建数组，所以涉及到修改数据的操作，CopyOnWriteArrayList效率很
低；但是单单只是进行遍历查找的话，效率比较高。
2. CopyOnWriteArrayList的“线程安全”机制 -- 是通过volatile和互斥锁来实现的。(01) CopyOnWriteArrayList是通过“volatile数组”来保存数据的。一个线程读取volatile数组时，总能看到其它线程对该volatile变量最后的写入；就这样，通过volatile提供了“读取到的数据总是最新的”这个机制的
保证。(02) CopyOnWriteArrayList通过互斥锁来保护数据。在“添加/修改/删除”数据时，会先“获取互斥锁”，再修改完毕之后，先将数据更新到“volatile数组”中，然后再“释放互斥锁”；这样，就达到了保护数据的目的。
CopyOnWriteArrayList函数列表
// 创建一个空列表。 CopyOnWriteArrayList() // 创建一个按 collection 的迭代器返回元素的顺序包含指定 collection 元素的列表。 CopyOnWriteArrayList(Collection<? extends E> c) // CopyOnWriteArrayList(E[] toCopyIn) 创建一个保存给定数组的副本的列表。// 将指定元素添加到此列表的尾部。 boolean add(E e) // 在此列表的指定位置上插入指定元素。 void add(int index, E element) // 按照指定 collection 的迭代器返回元素的顺序，将指定 collection 中的所有元素添加此列表的尾部。 boolean addAll(Collection<? extends E> c) // 从指定位置开始，将指定 collection 的所有元素插入此列表。 boolean addAll(int index, Collection<? extends E> c) // 按照指定 collection 的迭代器返回元素的顺序，将指定 collection 中尚未包含在此列表中的所有元素添加列表的尾部。 int addAllAbsent(Collection<? extends E> c) // 添加元素（如果不存在）。 boolean addIfAbsent(E e) // 从此列表移除所有元素。 void clear() // 返回此列表的浅表副本。 Object clone() // 如果此列表包含指定的元素，则返回 true。 boolean contains(Object o) // 如果此列表包含指定 collection 的所有元素，则返回 true。 boolean containsAll(Collection<?> c) // 比较指定对象与此列表的相等性。 boolean equals(Object o) // 返回列表中指定位置的元素。 E get(int index) // 返回此列表的哈希码值。 int hashCode() // 返回第一次出现的指定元素在此列表中的索引，从 index 开始向前搜索，如果没有找到该元素，则返回 -1。 int indexOf(E e, int index) // 返回此列表中第一次出现的指定元素的索引；如果此列表不包含该元素，则返回 -1。 int indexOf(Object o) // 如果此列表不包含任何元素，则返回 true。 boolean isEmpty() // 返回以恰当顺序在此列表元素上进行迭代的迭代器。 Iterator<E> iterator() // 返回最后一次出现的指定元素在此列表中的索引，从 index 开始向后搜索，如果没有找到该元素，则返回 -1。 int lastIndexOf(E e, int index) // 返回此列表中最后出现的指定元素的索引；如果列表不包含此元素，则返回 -1。 int lastIndexOf(Object o) // 返回此列表元素的列表迭代器（按适当顺序）。 ListIterator<E> listIterator() // 返回列表中元素的列表迭代器（按适当顺序），从列表的指定位置开始。 ListIterator<E> listIterator(int index) // 移除此列表指定位置上的元素。 E remove(int index) // 从此列表移除第一次出现的指定元素（如果存在）。 boolean remove(Object o) // 从此列表移除所有包含在指定 collection 中的元素。 boolean removeAll(Collection<?> c) // 只保留此列表中包含在指定 collection 中的元素。 boolean retainAll(Collection<?> c) // 用指定的元素替代此列表指定位置上的元素。 E set(int index, E element) // 返回此列表中的元素数。 int size() // 返回此列表中 fromIndex（包括）和 toIndex（不包括）之间部分的视图。 List<E> subList(int fromIndex, int toIndex) // 返回一个按恰当顺序（从第一个元素到最后一个元素）包含此列表中所有元素的数组。 Object[] toArray() // 返回以恰当顺序（从第一个元素到最后一个元素）包含列表所有元素的数组；返回数组的运行时类型是指定数组的运行时类型。 <T> T[] toArray(T[] a) // 返回此列表的字符串表示形式。 String toString()
CopyOnWriteArrayList源码分析(JDK1.7.0_40版本)
JDK1.7.0_40版本中CopyOnWriteArrayList.java的完整源码如下：
/** Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.*********************//** Written by Doug Lea with assistance from members of JCP JSR-166* Expert Group. Adapted and released, under explicit permission,* from JDK ArrayList.java which carries the following copyright:** Copyright 1997 by Sun Microsystems, Inc.,* 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.* All rights reserved.*/package java.util.concurrent;
import java.util.*;
import java.util.concurrent.locks.*;
import sun.misc.Unsafe;/*** A thread-safe variant of {@link java.util.ArrayList} in which all mutative* operations (<tt>add</tt>, <tt>set</tt>, and so on) are implemented by* making a fresh copy of the underlying array.** <p> This is ordinarily too costly, but may be <em>more</em> efficient* than alternatives when traversal operations vastly outnumber* mutations, and is useful when you cannot or don't want to* synchronize traversals, yet need to preclude interference among* concurrent threads. The "snapshot" style iterator method uses a* reference to the state of the array at the point that the iterator* was created. This array never changes during the lifetime of the* iterator, so interference is impossible and the iterator is* guaranteed not to throw <tt>ConcurrentModificationException</tt>.* The iterator will not reflect additions, removals, or changes to* the list since the iterator was created. Element-changing* operations on iterators themselves (<tt>remove</tt>, <tt>set</tt>, and* <tt>add</tt>) are not supported. These methods throw* <tt>UnsupportedOperationException</tt>.** <p>All elements are permitted, including <tt>null</tt>.** <p>Memory consistency effects: As with other concurrent* collections, actions in a thread prior to placing an object into a* {@code CopyOnWriteArrayList}* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>* actions subsequent to the access or removal of that element from* the {@code CopyOnWriteArrayList} in another thread.** <p>This class is a member of the* <a href="{@docRoot}/../technotes/guides/collections/index.html">* Java Collections Framework</a>.** @since 1.5* @author Doug Lea* @param <E> the type of elements held in this collection*/
public class CopyOnWriteArrayList<E>implements List<E>, RandomAccess, Cloneable, java.io.Serializable {private static final long serialVersionUID = 8673264195747942595L;/** The lock protecting all mutators */transient final ReentrantLock lock = new ReentrantLock();/** The array, accessed only via getArray/setArray. */private volatile transient Object[] array;/*** Gets the array. Non-private so as to also be accessible* from CopyOnWriteArraySet class.*/final Object[] getArray() {return array;}/*** Sets the array.*/final void setArray(Object[] a) {array = a;}/*** Creates an empty list.*/public CopyOnWriteArrayList() {setArray(new Object[0]);}/*** Creates a list containing the elements of the specified* collection, in the order they are returned by the collection's* iterator.** @param c the collection of initially held elements* @throws NullPointerException if the specified collection is null*/public CopyOnWriteArrayList(Collection<? extends E> c) {Object[] elements = c.toArray();// c.toArray might (incorrectly) not return Object[] (see 6260652)if (elements.getClass() != Object[].class)elements = Arrays.copyOf(elements, elements.length, Object[].class);setArray(elements);}/*** Creates a list holding a copy of the given array.** @param toCopyIn the array (a copy of this array is used as the* internal array)* @throws NullPointerException if the specified array is null*/public CopyOnWriteArrayList(E[] toCopyIn) {setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));}/*** Returns the number of elements in this list.** @return the number of elements in this list*/public int size() {return getArray().length;}/*** Returns <tt>true</tt> if this list contains no elements.** @return <tt>true</tt> if this list contains no elements*/public boolean isEmpty() {return size() == 0;}/*** Test for equality, coping with nulls.*/private static boolean eq(Object o1, Object o2) {return (o1 == null ? o2 == null : o1.equals(o2));}/*** static version of indexOf, to allow repeated calls without* needing to re-acquire array each time.* @param o element to search for* @param elements the array* @param index first index to search* @param fence one past last index to search* @return index of element, or -1 if absent*/private static int indexOf(Object o, Object[] elements,int index, int fence) {if (o == null) {for (int i = index; i < fence; i++)if (elements[i] == null)return i;} else {for (int i = index; i < fence; i++)if (o.equals(elements[i]))return i;}return -1;}/*** static version of lastIndexOf.* @param o element to search for* @param elements the array* @param index first index to search* @return index of element, or -1 if absent*/private static int lastIndexOf(Object o, Object[] elements, int index) {if (o == null) {for (int i = index; i >= 0; i--)if (elements[i] == null)return i;} else {for (int i = index; i >= 0; i--)if (o.equals(elements[i]))return i;}return -1;}/*** Returns <tt>true</tt> if this list contains the specified element.* More formally, returns <tt>true</tt> if and only if this list contains* at least one element <tt>e</tt> such that* <tt>(o==null ? e==null : o.equals(e))</tt>.** @param o element whose presence in this list is to be tested* @return <tt>true</tt> if this list contains the specified element*/public boolean contains(Object o) {Object[] elements = getArray();return indexOf(o, elements, 0, elements.length) >= 0;}/*** {@inheritDoc}*/public int indexOf(Object o) {Object[] elements = getArray();return indexOf(o, elements, 0, elements.length);}/*** Returns the index of the first occurrence of the specified element in* this list, searching forwards from <tt>index</tt>, or returns -1 if* the element is not found.* More formally, returns the lowest index <tt>i</tt> such that* <tt>(i >= index && (e==null ? get(i)==null : e.equals(get(i))))</tt>,* or -1 if there is no such index.** @param e element to search for* @param index index to start searching from* @return the index of the first occurrence of the element in* this list at position <tt>index</tt> or later in the list;* <tt>-1</tt> if the element is not found.* @throws IndexOutOfBoundsException if the specified index is negative*/public int indexOf(E e, int index) {Object[] elements = getArray();return indexOf(e, elements, index, elements.length);}/*** {@inheritDoc}*/public int lastIndexOf(Object o) {Object[] elements = getArray();return lastIndexOf(o, elements, elements.length - 1);}/*** Returns the index of the last occurrence of the specified element in* this list, searching backwards from <tt>index</tt>, or returns -1 if* the element is not found.* More formally, returns the highest index <tt>i</tt> such that* <tt>(i <= index && (e==null ? get(i)==null : e.equals(get(i))))</tt>,* or -1 if there is no such index.** @param e element to search for* @param index index to start searching backwards from* @return the index of the last occurrence of the element at position* less than or equal to <tt>index</tt> in this list;* -1 if the element is not found.* @throws IndexOutOfBoundsException if the specified index is greater* than or equal to the current size of this list*/public int lastIndexOf(E e, int index) {Object[] elements = getArray();return lastIndexOf(e, elements, index);}/*** Returns a shallow copy of this list. (The elements themselves* are not copied.)** @return a clone of this list*/public Object clone() {try {CopyOnWriteArrayList c = (CopyOnWriteArrayList)(super.clone());c.resetLock();return c;} catch (CloneNotSupportedException e) {// this shouldn't happen, since we are Cloneablethrow new InternalError();}}/*** Returns an array containing all of the elements in this list* in proper sequence (from first to last element).** <p>The returned array will be "safe" in that no references to it are* maintained by this list. (In other words, this method must allocate* a new 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 list*/public Object[] toArray() {Object[] elements = getArray();return Arrays.copyOf(elements, elements.length);}/*** Returns an array containing all of the elements in this list in* proper sequence (from first to last element); the runtime type of* the returned array is that of the specified array. If the list 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 list.** <p>If this list fits in the specified array with room to spare* (i.e., the array has more elements than this list), the element in* the array immediately following the end of the list is set to* <tt>null</tt>. (This is useful in determining the length of this* list <i>only</i> if the caller knows that this list does not contain* any null elements.)** <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 list known to contain only strings.* The following code can be used to dump the list 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 the list 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 list* @throws ArrayStoreException if the runtime type of the specified array* is not a supertype of the runtime type of every element in* this list* @throws NullPointerException if the specified array is null*/@SuppressWarnings("unchecked")public <T> T[] toArray(T a[]) {Object[] elements = getArray();int len = elements.length;if (a.length < len)return (T[]) Arrays.copyOf(elements, len, a.getClass());else {System.arraycopy(elements, 0, a, 0, len);if (a.length > len)a[len] = null;return a;}}// Positional Access Operations@SuppressWarnings("unchecked")private E get(Object[] a, int index) {return (E) a[index];}/*** {@inheritDoc}** @throws IndexOutOfBoundsException {@inheritDoc}*/public E get(int index) {return get(getArray(), index);}/*** Replaces the element at the specified position in this list with the* specified element.** @throws IndexOutOfBoundsException {@inheritDoc}*/public E set(int index, E element) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();E oldValue = get(elements, index);if (oldValue != element) {int len = elements.length;Object[] newElements = Arrays.copyOf(elements, len);newElements[index] = element;setArray(newElements);} else {// Not quite a no-op; ensures volatile write semanticssetArray(elements);}return oldValue;} finally {lock.unlock();}}/*** Appends the specified element to the end of this list.** @param e element to be appended to this list* @return <tt>true</tt> (as specified by {@link Collection#add})*/public boolean add(E e) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;Object[] newElements = Arrays.copyOf(elements, len + 1);newElements[len] = e;setArray(newElements);return true;} finally {lock.unlock();}}/*** Inserts the specified element at the specified position in this* list. Shifts the element currently at that position (if any) and* any subsequent elements to the right (adds one to their indices).** @throws IndexOutOfBoundsException {@inheritDoc}*/public void add(int index, E element) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (index > len || index < 0)throw new IndexOutOfBoundsException("Index: "+index+", Size: "+len);Object[] newElements;int numMoved = len - index;if (numMoved == 0)newElements = Arrays.copyOf(elements, len + 1);else {newElements = new Object[len + 1];System.arraycopy(elements, 0, newElements, 0, index);System.arraycopy(elements, index, newElements, index + 1,numMoved);}newElements[index] = element;setArray(newElements);} finally {lock.unlock();}}/*** Removes the element at the specified position in this list.* Shifts any subsequent elements to the left (subtracts one from their* indices). Returns the element that was removed from the list.** @throws IndexOutOfBoundsException {@inheritDoc}*/public E remove(int index) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;E oldValue = get(elements, index);int numMoved = len - index - 1;if (numMoved == 0)setArray(Arrays.copyOf(elements, len - 1));else {Object[] newElements = new Object[len - 1];System.arraycopy(elements, 0, newElements, 0, index);System.arraycopy(elements, index + 1, newElements, index,numMoved);setArray(newElements);}return oldValue;} finally {lock.unlock();}}/*** Removes the first occurrence of the specified element from this list,* if it is present. If this list does not contain the element, it is* unchanged. More formally, removes the element with the lowest index* <tt>i</tt> such that* <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt>* (if such an element exists). Returns <tt>true</tt> if this list* contained the specified element (or equivalently, if this list* changed as a result of the call).** @param o element to be removed from this list, if present* @return <tt>true</tt> if this list contained the specified element*/public boolean remove(Object o) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (len != 0) {// Copy while searching for element to remove// This wins in the normal case of element being presentint newlen = len - 1;Object[] newElements = new Object[newlen];for (int i = 0; i < newlen; ++i) {if (eq(o, elements[i])) {// found one; copy remaining and exitfor (int k = i + 1; k < len; ++k)newElements[k-1] = elements[k];setArray(newElements);return true;} elsenewElements[i] = elements[i];}// special handling for last cellif (eq(o, elements[newlen])) {setArray(newElements);return true;}}return false;} finally {lock.unlock();}}/*** Removes from this list all of the elements whose index is between* <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.* Shifts any succeeding elements to the left (reduces their index).* This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements.* (If <tt>toIndex==fromIndex</tt>, this operation has no effect.)** @param fromIndex index of first element to be removed* @param toIndex index after last element to be removed* @throws IndexOutOfBoundsException if fromIndex or toIndex out of range* ({@code{fromIndex < 0 || toIndex > size() || toIndex < fromIndex})*/private void removeRange(int fromIndex, int toIndex) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (fromIndex < 0 || toIndex > len || toIndex < fromIndex)throw new IndexOutOfBoundsException();int newlen = len - (toIndex - fromIndex);int numMoved = len - toIndex;if (numMoved == 0)setArray(Arrays.copyOf(elements, newlen));else {Object[] newElements = new Object[newlen];System.arraycopy(elements, 0, newElements, 0, fromIndex);System.arraycopy(elements, toIndex, newElements,fromIndex, numMoved);setArray(newElements);}} finally {lock.unlock();}}/*** Append the element if not present.** @param e element to be added to this list, if absent* @return <tt>true</tt> if the element was added*/public boolean addIfAbsent(E e) {final ReentrantLock lock = this.lock;lock.lock();try {// Copy while checking if already present.// This wins in the most common case where it is not presentObject[] elements = getArray();int len = elements.length;Object[] newElements = new Object[len + 1];for (int i = 0; i < len; ++i) {if (eq(e, elements[i]))return false; // exit, throwing away copyelsenewElements[i] = elements[i];}newElements[len] = e;setArray(newElements);return true;} finally {lock.unlock();}}/*** Returns <tt>true</tt> if this list contains all of the elements of the* specified collection.** @param c collection to be checked for containment in this list* @return <tt>true</tt> if this list 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) {Object[] elements = getArray();int len = elements.length;for (Object e : c) {if (indexOf(e, elements, 0, len) < 0)return false;}return true;}/*** Removes from this list all of its elements that are contained in* the specified collection. This is a particularly expensive operation* in this class because of the need for an internal temporary array.** @param c collection containing elements to be removed from this list* @return <tt>true</tt> if this list changed as a result of the call* @throws ClassCastException if the class of an element of this list* is incompatible with the specified collection* (<a href="../Collection.html#optional-restrictions">optional</a>)* @throws NullPointerException if this list contains a null element and the* specified collection does not permit null elements* (<a href="../Collection.html#optional-restrictions">optional</a>),* or if the specified collection is null* @see #remove(Object)*/public boolean removeAll(Collection<?> c) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (len != 0) {// temp array holds those elements we know we want to keepint newlen = 0;Object[] temp = new Object[len];for (int i = 0; i < len; ++i) {Object element = elements[i];if (!c.contains(element))temp[newlen++] = element;}if (newlen != len) {setArray(Arrays.copyOf(temp, newlen));return true;}}return false;} finally {lock.unlock();}}/*** Retains only the elements in this list that are contained in the* specified collection. In other words, removes from this list all of* its elements that are not contained in the specified collection.** @param c collection containing elements to be retained in this list* @return <tt>true</tt> if this list changed as a result of the call* @throws ClassCastException if the class of an element of this list* is incompatible with the specified collection* (<a href="../Collection.html#optional-restrictions">optional</a>)* @throws NullPointerException if this list contains a null element and the* specified collection does not permit null elements* (<a href="../Collection.html#optional-restrictions">optional</a>),* or if the specified collection is null* @see #remove(Object)*/public boolean retainAll(Collection<?> c) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (len != 0) {// temp array holds those elements we know we want to keepint newlen = 0;Object[] temp = new Object[len];for (int i = 0; i < len; ++i) {Object element = elements[i];if (c.contains(element))temp[newlen++] = element;}if (newlen != len) {setArray(Arrays.copyOf(temp, newlen));return true;}}return false;} finally {lock.unlock();}}/*** Appends all of the elements in the specified collection that* are not already contained in this list, to the end of* this list, in the order that they are returned by the* specified collection's iterator.** @param c collection containing elements to be added to this list* @return the number of elements added* @throws NullPointerException if the specified collection is null* @see #addIfAbsent(Object)*/public int addAllAbsent(Collection<? extends E> c) {Object[] cs = c.toArray();if (cs.length == 0)return 0;Object[] uniq = new Object[cs.length];final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;int added = 0;for (int i = 0; i < cs.length; ++i) { // scan for duplicatesObject e = cs[i];if (indexOf(e, elements, 0, len) < 0 &&indexOf(e, uniq, 0, added) < 0)uniq[added++] = e;}if (added > 0) {Object[] newElements = Arrays.copyOf(elements, len + added);System.arraycopy(uniq, 0, newElements, len, added);setArray(newElements);}return added;} finally {lock.unlock();}}/*** Removes all of the elements from this list.* The list will be empty after this call returns.*/public void clear() {final ReentrantLock lock = this.lock;lock.lock();try {setArray(new Object[0]);} finally {lock.unlock();}}/*** Appends all of the elements in the specified collection to the end* of this list, in the order that they are returned by the specified* collection's iterator.** @param c collection containing elements to be added to this list* @return <tt>true</tt> if this list 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) {Object[] cs = c.toArray();if (cs.length == 0)return false;final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;Object[] newElements = Arrays.copyOf(elements, len + cs.length);System.arraycopy(cs, 0, newElements, len, cs.length);setArray(newElements);return true;} finally {lock.unlock();}}/*** Inserts all of the elements in the specified collection into this* list, starting at the specified position. Shifts the element* currently at that position (if any) and any subsequent elements to* the right (increases their indices). The new elements will appear* in this list in the order that they are returned by the* specified collection's iterator.** @param index index at which to insert the first element* from the specified collection* @param c collection containing elements to be added to this list* @return <tt>true</tt> if this list changed as a result of the call* @throws IndexOutOfBoundsException {@inheritDoc}* @throws NullPointerException if the specified collection is null* @see #add(int,Object)*/public boolean addAll(int index, Collection<? extends E> c) {Object[] cs = c.toArray();final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (index > len || index < 0)throw new IndexOutOfBoundsException("Index: "+index+", Size: "+len);if (cs.length == 0)return false;int numMoved = len - index;Object[] newElements;if (numMoved == 0)newElements = Arrays.copyOf(elements, len + cs.length);else {newElements = new Object[len + cs.length];System.arraycopy(elements, 0, newElements, 0, index);System.arraycopy(elements, index,newElements, index + cs.length,numMoved);}System.arraycopy(cs, 0, newElements, index, cs.length);setArray(newElements);return true;} finally {lock.unlock();}}/*** Saves the state of the list to a stream (that is, serializes it).** @serialData The length of the array backing the list is emitted* (int), followed by all of its elements (each an Object)* in the proper order.* @param s the stream*/private void writeObject(java.io.ObjectOutputStream s)throws java.io.IOException{s.defaultWriteObject();Object[] elements = getArray();// Write out array lengths.writeInt(elements.length);// Write out all elements in the proper order.for (Object element : elements)s.writeObject(element);}/*** Reconstitutes the list from a stream (that is, deserializes it).** @param s the stream*/private void readObject(java.io.ObjectInputStream s)throws java.io.IOException, ClassNotFoundException {s.defaultReadObject();// bind to new lockresetLock();// Read in array length and allocate arrayint len = s.readInt();Object[] elements = new Object[len];// Read in all elements in the proper order.for (int i = 0; i < len; i++)elements[i] = s.readObject();setArray(elements);}/*** Returns a string representation of this list. The string* representation consists of the string representations of the list's* elements in the order they are returned by its iterator, enclosed in* square brackets (<tt>"[]"</tt>). Adjacent elements are separated by* the characters <tt>", "</tt> (comma and space). Elements are* converted to strings as by {@link String#valueOf(Object)}.** @return a string representation of this list*/public String toString() {return Arrays.toString(getArray());}/*** Compares the specified object with this list for equality.* Returns {@code true} if the specified object is the same object* as this object, or if it is also a {@link List} and the sequence* of elements returned by an {@linkplain List#iterator() iterator}* over the specified list is the same as the sequence returned by* an iterator over this list. The two sequences are considered to* be the same if they have the same length and corresponding* elements at the same position in the sequence are <em>equal</em>.* Two elements {@code e1} and {@code e2} are considered* <em>equal</em> if {@code (e1==null ? e2==null : e1.equals(e2))}.** @param o the object to be compared for equality with this list* @return {@code true} if the specified object is equal to this list*/public boolean equals(Object o) {if (o == this)return true;if (!(o instanceof List))return false;List<?> list = (List<?>)(o);Iterator<?> it = list.iterator();Object[] elements = getArray();int len = elements.length;for (int i = 0; i < len; ++i)if (!it.hasNext() || !eq(elements[i], it.next()))return false;if (it.hasNext())return false;return true;}/*** Returns the hash code value for this list.** <p>This implementation uses the definition in {@link List#hashCode}.** @return the hash code value for this list*/public int hashCode() {int hashCode = 1;Object[] elements = getArray();int len = elements.length;for (int i = 0; i < len; ++i) {Object obj = elements[i];hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());}return hashCode;}/*** Returns an iterator over the elements in this list in proper sequence.** <p>The returned iterator provides a snapshot of the state of the list* 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 list in proper sequence*/public Iterator<E> iterator() {return new COWIterator<E>(getArray(), 0);}/*** {@inheritDoc}** <p>The returned iterator provides a snapshot of the state of the list* when the iterator was constructed. No synchronization is needed while* traversing the iterator. The iterator does <em>NOT</em> support the* <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.*/public ListIterator<E> listIterator() {return new COWIterator<E>(getArray(), 0);}/*** {@inheritDoc}** <p>The returned iterator provides a snapshot of the state of the list* when the iterator was constructed. No synchronization is needed while* traversing the iterator. The iterator does <em>NOT</em> support the* <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods.** @throws IndexOutOfBoundsException {@inheritDoc}*/public ListIterator<E> listIterator(final int index) {Object[] elements = getArray();int len = elements.length;if (index<0 || index>len)throw new IndexOutOfBoundsException("Index: "+index);return new COWIterator<E>(elements, index);}private static class COWIterator<E> implements ListIterator<E> {/** Snapshot of the array */private final Object[] snapshot;/** Index of element to be returned by subsequent call to next. */private int cursor;private COWIterator(Object[] elements, int initialCursor) {cursor = initialCursor;snapshot = elements;}public boolean hasNext() {return cursor < snapshot.length;}public boolean hasPrevious() {return cursor > 0;}@SuppressWarnings("unchecked")public E next() {if (! hasNext())throw new NoSuchElementException();return (E) snapshot[cursor++];}@SuppressWarnings("unchecked")public E previous() {if (! hasPrevious())throw new NoSuchElementException();return (E) snapshot[--cursor];}public int nextIndex() {return cursor;}public int previousIndex() {return cursor-1;}/*** Not supported. Always throws UnsupportedOperationException.* @throws UnsupportedOperationException always; <tt>remove</tt>* is not supported by this iterator.*/public void remove() {throw new UnsupportedOperationException();}/*** Not supported. Always throws UnsupportedOperationException.* @throws UnsupportedOperationException always; <tt>set</tt>* is not supported by this iterator.*/public void set(E e) {throw new UnsupportedOperationException();}/*** Not supported. Always throws UnsupportedOperationException.* @throws UnsupportedOperationException always; <tt>add</tt>* is not supported by this iterator.*/public void add(E e) {throw new UnsupportedOperationException();}}/*** Returns a view of the portion of this list between* <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive.* The returned list is backed by this list, so changes in the* returned list are reflected in this list.** <p>The semantics of the list returned by this method become* undefined if the backing list (i.e., this list) is modified in* any way other than via the returned list.** @param fromIndex low endpoint (inclusive) of the subList* @param toIndex high endpoint (exclusive) of the subList* @return a view of the specified range within this list* @throws IndexOutOfBoundsException {@inheritDoc}*/public List<E> subList(int fromIndex, int toIndex) {final ReentrantLock lock = this.lock;lock.lock();try {Object[] elements = getArray();int len = elements.length;if (fromIndex < 0 || toIndex > len || fromIndex > toIndex)throw new IndexOutOfBoundsException();return new COWSubList<E>(this, fromIndex, toIndex);} finally {lock.unlock();}}/*** Sublist for CopyOnWriteArrayList.* This class extends AbstractList merely for convenience, to* avoid having to define addAll, etc. This doesn't hurt, but* is wasteful. This class does not need or use modCount* mechanics in AbstractList, but does need to check for* concurrent modification using similar mechanics. On each* operation, the array that we expect the backing list to use* is checked and updated. Since we do this for all of the* base operations invoked by those defined in AbstractList,* all is well. While inefficient, this is not worth* improving. The kinds of list operations inherited from* AbstractList are already so slow on COW sublists that* adding a bit more space/time doesn't seem even noticeable.*/private static class COWSubList<E>extends AbstractList<E>implements RandomAccess{private final CopyOnWriteArrayList<E> l;private final int offset;private int size;private Object[] expectedArray;// only call this holding l's lockCOWSubList(CopyOnWriteArrayList<E> list,int fromIndex, int toIndex) {l = list;expectedArray = l.getArray();offset = fromIndex;size = toIndex - fromIndex;}// only call this holding l's lockprivate void checkForComodification() {if (l.getArray() != expectedArray)throw new ConcurrentModificationException();}// only call this holding l's lockprivate void rangeCheck(int index) {if (index<0 || index>=size)throw new IndexOutOfBoundsException("Index: "+index+",Size: "+size);}public E set(int index, E element) {final ReentrantLock lock = l.lock;lock.lock();try {rangeCheck(index);checkForComodification();E x = l.set(index+offset, element);expectedArray = l.getArray();return x;} finally {lock.unlock();}}public E get(int index) {final ReentrantLock lock = l.lock;lock.lock();try {rangeCheck(index);checkForComodification();return l.get(index+offset);} finally {lock.unlock();}}public int size() {final ReentrantLock lock = l.lock;lock.lock();try {checkForComodification();return size;} finally {lock.unlock();}}public void add(int index, E element) {final ReentrantLock lock = l.lock;lock.lock();try {checkForComodification();if (index<0 || index>size)throw new IndexOutOfBoundsException();l.add(index+offset, element);expectedArray = l.getArray();size++;} finally {lock.unlock();}}public void clear() {final ReentrantLock lock = l.lock;lock.lock();try {checkForComodification();l.removeRange(offset, offset+size);expectedArray = l.getArray();size = 0;} finally {lock.unlock();}}public E remove(int index) {final ReentrantLock lock = l.lock;lock.lock();try {rangeCheck(index);checkForComodification();E result = l.remove(index+offset);expectedArray = l.getArray();size--;return result;} finally {lock.unlock();}}public boolean remove(Object o) {int index = indexOf(o);if (index == -1)return false;remove(index);return true;}public Iterator<E> iterator() {final ReentrantLock lock = l.lock;lock.lock();try {checkForComodification();return new COWSubListIterator<E>(l, 0, offset, size);} finally {lock.unlock();}}public ListIterator<E> listIterator(final int index) {final ReentrantLock lock = l.lock;lock.lock();try {checkForComodification();if (index<0 || index>size)throw new IndexOutOfBoundsException("Index: "+index+", Size: "+size);return new COWSubListIterator<E>(l, index, offset, size);} finally {lock.unlock();}}public List<E> subList(int fromIndex, int toIndex) {final ReentrantLock lock = l.lock;lock.lock();try {checkForComodification();if (fromIndex<0 || toIndex>size)throw new IndexOutOfBoundsException();return new COWSubList<E>(l, fromIndex + offset,toIndex + offset);} finally {lock.unlock();}}}private static class COWSubListIterator<E> implements ListIterator<E> {private final ListIterator<E> i;private final int index;private final int offset;private final int size;COWSubListIterator(List<E> l, int index, int offset,int size) {this.index = index;this.offset = offset;this.size = size;i = l.listIterator(index+offset);}public boolean hasNext() {return nextIndex() < size;}public E next() {if (hasNext())return i.next();elsethrow new NoSuchElementException();}public boolean hasPrevious() {return previousIndex() >= 0;}public E previous() {if (hasPrevious())return i.previous();elsethrow new NoSuchElementException();}public int nextIndex() {return i.nextIndex() - offset;}public int previousIndex() {return i.previousIndex() - offset;}public void remove() {throw new UnsupportedOperationException();}public void set(E e) {throw new UnsupportedOperationException();}public void add(E e) {throw new UnsupportedOperationException();}}// Support for resetting lock while deserializingprivate void resetLock() {UNSAFE.putObjectVolatile(this, lockOffset, new ReentrantLock());}private static final sun.misc.Unsafe UNSAFE;private static final long lockOffset;static {try {UNSAFE = sun.misc.Unsafe.getUnsafe();Class k = CopyOnWriteArrayList.class;lockOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("lock"));} catch (Exception e) {throw new Error(e);}}
}
下面我们从“创建，添加，删除，获取，遍历”这5个方面去分析CopyOnWriteArrayList的原理。
1. 创建
CopyOnWriteArrayList共3个构造函数。它们的源码如下：
public CopyOnWriteArrayList() {setArray(new Object[0]); }public CopyOnWriteArrayList(Collection<? extends E> c) {Object[] elements = c.toArray();if (elements.getClass() != Object[].class)elements = Arrays.copyOf(elements, elements.length, Object[].class);setArray(elements); }public CopyOnWriteArrayList(E[] toCopyIn) {setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class)); }
说明：这3个构造函数都调用了setArray()，setArray()的源码如下：
private volatile transient Object[] array;final Object[] getArray() {return array; }final void setArray(Object[] a) {array = a; }
说明：setArray()的作用是给array赋值；其中，array是volatile transient Object[]类型，即array是“volatile数组”。
关于volatile关键字，我们知道“volatile能让变量变得可见”，即对一个volatile变量的读，总是能看到（任意线程）对这个volatile变量最后的写入。正在由于这种特性，每次更新了“volatile数组”之后，其它线程都能看到对它所做的更新。
关于transient关键字，它是在序列化中才起作用，transient变量不会被自动序列化。transient不是本文关注的重点，了解即可。
关于transient的更多内容，请参考：http://www.cnblogs.com/skywang12345/p/io_06.html
2. 添加
以add(E e)为例，来对“CopyOnWriteArrayList的添加操作”进行说明。下面是add(E e)的代码：
public boolean add(E e) {final ReentrantLock lock = this.lock;// 获取“锁” lock.lock();try {// 获取原始”volatile数组“中的数据和数据长度。Object[] elements = getArray();int len = elements.length;// 新建一个数组newElements，并将原始数据拷贝到newElements中；// newElements数组的长度=“原始数组的长度”+1Object[] newElements = Arrays.copyOf(elements, len + 1);// 将“新增加的元素”保存到newElements中。newElements[len] = e;// 将newElements赋值给”volatile数组“。 setArray(newElements);return true;} finally {// 释放“锁” lock.unlock();} }
说明：add(E e)的作用就是将数据e添加到”volatile数组“中。它的实现方式是，新建一个数组，接着将原始的”volatile数组“的数据拷贝到新数组中，然后将新增数据也添加到新数组中；最后，将新数组赋值给”volatile数组“。
在add(E e)中有两点需要关注。
第一，在”添加操作“开始前，获取独占锁(lock)，若此时有需要线程要获取锁，则必须等待；在操作完毕后，释放独占锁(lock)，此时其它线程才能获取锁。通过独占锁，来防止多线程同时修改数据！lock的定义如下：
transient final ReentrantLock lock = new ReentrantLock();
关于ReentrantLock的更多内容，可以参考：Java多线程系列--“JUC锁”02之 互斥锁ReentrantLock
第二，操作完毕时，会通过setArray()来更新”volatile数组“。而且，前面我们提过”即对一个volatile变量的读，总是能看到（任意线程）对这个volatile变量最后的写入“；这样，每次添加元素之后，其它线程都能看到新添加的元素。
3. 获取
以get(int index)为例，来对“CopyOnWriteArrayList的删除操作”进行说明。下面是get(int index)的代码：
public E get(int index) {return get(getArray(), index); }private E get(Object[] a, int index) {return (E) a[index]; }
说明：get(int index)的实现很简单，就是返回”volatile数组“中的第index个元素。
4. 删除
以remove(int index)为例，来对“CopyOnWriteArrayList的删除操作”进行说明。下面是remove(int index)的代码：
public E remove(int index) {final ReentrantLock lock = this.lock;// 获取“锁” lock.lock();try {// 获取原始”volatile数组“中的数据和数据长度。Object[] elements = getArray();int len = elements.length;// 获取elements数组中的第index个数据。E oldValue = get(elements, index);int numMoved = len - index - 1;// 如果被删除的是最后一个元素，则直接通过Arrays.copyOf()进行处理，而不需要新建数组。// 否则，新建数组，然后将”volatile数组中被删除元素之外的其它元素“拷贝到新数组中；最后，将新数组赋值给”volatile数组“。if (numMoved == 0)setArray(Arrays.copyOf(elements, len - 1));else {Object[] newElements = new Object[len - 1];System.arraycopy(elements, 0, newElements, 0, index);System.arraycopy(elements, index + 1, newElements, index,numMoved);setArray(newElements);}return oldValue;} finally {// 释放“锁” lock.unlock();} }
说明：remove(int index)的作用就是将”volatile数组“中第index个元素删除。它的实现方式是，如果被删除的是最后一个元素，则直接通过Arrays.copyOf()进行处理，而不需要新建数组。否则，新建数组，然后将”volatile数组中被删除元素之外的其它元素“拷贝到新数组中；最后，将新数组赋值给”volatile数组“。
和add(E e)一样，remove(int index)也是”在操作之前，获取独占锁；操作完成之后，释放独占是“；并且”在操作完成时，会通过将数据更新到volatile数组中“。
5. 遍历
以iterator()为例，来对“CopyOnWriteArrayList的遍历操作”进行说明。下面是iterator()的代码：
public Iterator<E> iterator() {return new COWIterator<E>(getArray(), 0); }
说明：iterator()会返回COWIterator对象。
COWIterator实现额ListIterator接口，它的源码如下：
private static class COWIterator<E> implements ListIterator<E> {private final Object[] snapshot;private int cursor;private COWIterator(Object[] elements, int initialCursor) {cursor = initialCursor;snapshot = elements;}public boolean hasNext() {return cursor < snapshot.length;}public boolean hasPrevious() {return cursor > 0;}// 获取下一个元素@SuppressWarnings("unchecked")public E next() {if (! hasNext())throw new NoSuchElementException();return (E) snapshot[cursor++];}// 获取上一个元素@SuppressWarnings("unchecked")public E previous() {if (! hasPrevious())throw new NoSuchElementException();return (E) snapshot[--cursor];}public int nextIndex() {return cursor;}public int previousIndex() {return cursor-1;}public void remove() {throw new UnsupportedOperationException();}public void set(E e) {throw new UnsupportedOperationException();}public void add(E e) {throw new UnsupportedOperationException();} }
说明：COWIterator不支持修改元素的操作。例如，对于remove(),set(),add()等操作，COWIterator都会抛出异常！
另外，需要提到的一点是，CopyOnWriteArrayList返回迭代器不会抛出ConcurrentModificationException异常，即它不是fail-fast机制的！
关于fail-fast机制，可以参考“Java 集合系列04之 fail-fast总结(通过ArrayList来说明fail-fast的原理、解决办法)”。
CopyOnWriteArrayList示例
下面，我们通过一个例子去对比ArrayList和CopyOnWriteArrayList。
1 import java.util.*; 2 import java.util.concurrent.*; 3 4 /* 5 * CopyOnWriteArrayList是“线程安全”的动态数组，而ArrayList是非线程安全的。 6 * 7 * 下面是“多个线程同时操作并且遍历list”的示例 8 * (01) 当list是CopyOnWriteArrayList对象时，程序能正常运行。 9 * (02) 当list是ArrayList对象时，程序会产生ConcurrentModificationException异常。 10 * 11 * @author skywang 12 */ 13 public class CopyOnWriteArrayListTest1 { 14 15 // TODO: list是ArrayList对象时，程序会出错。 16 //private static List<String> list = new ArrayList<String>(); 17 private static List<String> list = new CopyOnWriteArrayList<String>(); 18 public static void main(String[] args) { 19 20 // 同时启动两个线程对list进行操作！ 21 new MyThread("ta").start(); 22 new MyThread("tb").start(); 23 } 24 25 private static void printAll() { 26 String value = null; 27 Iterator iter = list.iterator(); 28 while(iter.hasNext()) { 29 value = (String)iter.next(); 30 System.out.print(value+", "); 31 } 32 System.out.println(); 33 } 34 35 private static class MyThread extends Thread { 36 MyThread(String name) { 37 super(name); 38 } 39 @Override 40 public void run() { 41 int i = 0; 42 while (i++ < 6) { 43 // “线程名” + "-" + "序号" 44 String val = Thread.currentThread().getName()+"-"+i; 45 list.add(val); 46 // 通过“Iterator”遍历List。 47 printAll(); 48 } 49 } 50 } 51 }
(某一次)运行结果：
ta-1, tb-1, ta-1, tb-1, ta-1, ta-1, tb-1, tb-1, tb-2, tb-2, ta-1, ta-2, tb-1, ta-1, tb-2, tb-1, ta-2, tb-2, tb-3, ta-2, ta-1, tb-3, tb-1, ta-3, tb-2, ta-1, ta-2, tb-1, tb-3, tb-2, ta-3, ta-2, tb-4, tb-3, ta-1, ta-3, tb-1, tb-4, tb-2, ta-4, ta-2, ta-1, tb-3, tb-1, ta-3, tb-2, tb-4, ta-2, ta-4, tb-3, tb-5, ta-3, ta-1, tb-4, tb-1, ta-4, tb-2, tb-5, ta-2, ta-5, tb-3, ta-1, ta-3, tb-1, tb-4, tb-2, ta-4, ta-2, tb-5, tb-3, ta-5, ta-3, tb-6, tb-4, ta-4, tb-5, ta-5, tb-6, ta-6,
结果说明：如果将源码中的list改成ArrayList对象时，程序会产生ConcurrentModificationException异常。
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