ImportNewXXT0101

Java多线程系列--【JUC集合02】- CopyOnWriteArrayList

参考：http://www.cnblogs.com/skywang12345/p/3498483.html

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 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 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 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 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 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 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 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 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[] 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：

   1 /*
   2  * Copyright (c) 2003, 2011, Oracle and/or its affiliates. All rights reserved.
   3  * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
   4  *
   5  *
   6  *
   7  *
   8  *
   9  *
  10  *
  11  *
  12  *
  13  *
  14  *
  15  *
  16  *
  17  *
  18  *
  19  *
  20  *
  21  *
  22  *
  23  *
  24  */
  25 
  26 /*
  27  * Written by Doug Lea with assistance from members of JCP JSR-166
  28  * Expert Group.  Adapted and released, under explicit permission,
  29  * from JDK ArrayList.java which carries the following copyright:
  30  *
  31  * Copyright 1997 by Sun Microsystems, Inc.,
  32  * 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
  33  * All rights reserved.
  34  */
  35 
  36 package java.util.concurrent;
  37 import java.util.*;
  38 import java.util.concurrent.locks.*;
  39 import sun.misc.Unsafe;
  40 
  41 /**
  42  * A thread-safe variant of {@link java.util.ArrayList} in which all mutative
  43  * operations (add, set, and so on) are implemented by
  44  * making a fresh copy of the underlying array.
  45  *
  46  *  This is ordinarily too costly, but may be more efficient
  47  * than alternatives when traversal operations vastly outnumber
  48  * mutations, and is useful when you cannot or don't want to
  49  * synchronize traversals, yet need to preclude interference among
  50  * concurrent threads.  The "snapshot" style iterator method uses a
  51  * reference to the state of the array at the point that the iterator
  52  * was created. This array never changes during the lifetime of the
  53  * iterator, so interference is impossible and the iterator is
  54  * guaranteed not to throw ConcurrentModificationException.
  55  * The iterator will not reflect additions, removals, or changes to
  56  * the list since the iterator was created.  Element-changing
  57  * operations on iterators themselves (remove, set, and
  58  * add) are not supported. These methods throw
  59  * UnsupportedOperationException.
  60  *
  61  * 
All elements are permitted, including null.
  62  *
  63  * 
Memory consistency effects: As with other concurrent
  64  * collections, actions in a thread prior to placing an object into a
  65  * {@code CopyOnWriteArrayList}
  66  * happen-before
  67  * actions subsequent to the access or removal of that element from
  68  * the {@code CopyOnWriteArrayList} in another thread.
  69  *
  70  * 
This class is a member of the
  71  * docRoot}/../technotes/guides/collections/index.html">
  72  * Java Collections Framework.
  73  *
  74  * @since 1.5
  75  * @author Doug Lea
  76  * @param  the type of elements held in this collection
  77  */
  78 public class CopyOnWriteArrayList
  79     implements List, RandomAccess, Cloneable, java.io.Serializable {
  80     private static final long serialVersionUID = 8673264195747942595L;
  81 
  82     /** The lock protecting all mutators */
  83     transient final ReentrantLock lock = new ReentrantLock();
  84 
  85     /** The array, accessed only via getArray/setArray. */
  86     private volatile transient Object[] array;
  87 
  88     /**
  89      * Gets the array.  Non-private so as to also be accessible
  90      * from CopyOnWriteArraySet class.
  91      */
  92     final Object[] getArray() {
  93         return array;
  94     }
  95 
  96     /**
  97      * Sets the array.
  98      */
  99     final void setArray(Object[] a) {
 100         array = a;
 101     }
 102 
 103     /**
 104      * Creates an empty list.
 105      */
 106     public CopyOnWriteArrayList() {
 107         setArray(new Object[0]);
 108     }
 109 
 110     /**
 111      * Creates a list containing the elements of the specified
 112      * collection, in the order they are returned by the collection's
 113      * iterator.
 114      *
 115      * @param c the collection of initially held elements
 116      * @throws NullPointerException if the specified collection is null
 117      */
 118     public CopyOnWriteArrayList(Collectionextends E> c) {
 119         Object[] elements = c.toArray();
 120         // c.toArray might (incorrectly) not return Object[] (see 6260652)
 121         if (elements.getClass() != Object[].class)
 122             elements = Arrays.copyOf(elements, elements.length, Object[].class);
 123         setArray(elements);
 124     }
 125 
 126     /**
 127      * Creates a list holding a copy of the given array.
 128      *
 129      * @param toCopyIn the array (a copy of this array is used as the
 130      *        internal array)
 131      * @throws NullPointerException if the specified array is null
 132      */
 133     public CopyOnWriteArrayList(E[] toCopyIn) {
 134         setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
 135     }
 136 
 137     /**
 138      * Returns the number of elements in this list.
 139      *
 140      * @return the number of elements in this list
 141      */
 142     public int size() {
 143         return getArray().length;
 144     }
 145 
 146     /**
 147      * Returns true if this list contains no elements.
 148      *
 149      * @return true if this list contains no elements
 150      */
 151     public boolean isEmpty() {
 152         return size() == 0;
 153     }
 154 
 155     /**
 156      * Test for equality, coping with nulls.
 157      */
 158     private static boolean eq(Object o1, Object o2) {
 159         return (o1 == null ? o2 == null : o1.equals(o2));
 160     }
 161 
 162     /**
 163      * static version of indexOf, to allow repeated calls without
 164      * needing to re-acquire array each time.
 165      * @param o element to search for
 166      * @param elements the array
 167      * @param index first index to search
 168      * @param fence one past last index to search
 169      * @return index of element, or -1 if absent
 170      */
 171     private static int indexOf(Object o, Object[] elements,
 172                                int index, int fence) {
 173         if (o == null) {
 174             for (int i = index; i < fence; i++)
 175                 if (elements[i] == null)
 176                     return i;
 177         } else {
 178             for (int i = index; i < fence; i++)
 179                 if (o.equals(elements[i]))
 180                     return i;
 181         }
 182         return -1;
 183     }
 184 
 185     /**
 186      * static version of lastIndexOf.
 187      * @param o element to search for
 188      * @param elements the array
 189      * @param index first index to search
 190      * @return index of element, or -1 if absent
 191      */
 192     private static int lastIndexOf(Object o, Object[] elements, int index) {
 193         if (o == null) {
 194             for (int i = index; i >= 0; i--)
 195                 if (elements[i] == null)
 196                     return i;
 197         } else {
 198             for (int i = index; i >= 0; i--)
 199                 if (o.equals(elements[i]))
 200                     return i;
 201         }
 202         return -1;
 203     }
 204 
 205     /**
 206      * Returns true if this list contains the specified element.
 207      * More formally, returns true if and only if this list contains
 208      * at least one element e such that
 209      * (o==null ? e==null : o.equals(e)).
 210      *
 211      * @param o element whose presence in this list is to be tested
 212      * @return true if this list contains the specified element
 213      */
 214     public boolean contains(Object o) {
 215         Object[] elements = getArray();
 216         return indexOf(o, elements, 0, elements.length) >= 0;
 217     }
 218 
 219     /**
 220      * {@inheritDoc}
 221      */
 222     public int indexOf(Object o) {
 223         Object[] elements = getArray();
 224         return indexOf(o, elements, 0, elements.length);
 225     }
 226 
 227     /**
 228      * Returns the index of the first occurrence of the specified element in
 229      * this list, searching forwards from index, or returns -1 if
 230      * the element is not found.
 231      * More formally, returns the lowest index i such that
 232      * (i >= index && (e==null ? get(i)==null : e.equals(get(i)))),
 233      * or -1 if there is no such index.
 234      *
 235      * @param e element to search for
 236      * @param index index to start searching from
 237      * @return the index of the first occurrence of the element in
 238      *         this list at position index or later in the list;
 239      *         -1 if the element is not found.
 240      * @throws IndexOutOfBoundsException if the specified index is negative
 241      */
 242     public int indexOf(E e, int index) {
 243         Object[] elements = getArray();
 244         return indexOf(e, elements, index, elements.length);
 245     }
 246 
 247     /**
 248      * {@inheritDoc}
 249      */
 250     public int lastIndexOf(Object o) {
 251         Object[] elements = getArray();
 252         return lastIndexOf(o, elements, elements.length - 1);
 253     }
 254 
 255     /**
 256      * Returns the index of the last occurrence of the specified element in
 257      * this list, searching backwards from index, or returns -1 if
 258      * the element is not found.
 259      * More formally, returns the highest index i such that
 260      * (i <= index && (e==null ? get(i)==null : e.equals(get(i)))),
 261      * or -1 if there is no such index.
 262      *
 263      * @param e element to search for
 264      * @param index index to start searching backwards from
 265      * @return the index of the last occurrence of the element at position
 266      *         less than or equal to index in this list;
 267      *         -1 if the element is not found.
 268      * @throws IndexOutOfBoundsException if the specified index is greater
 269      *         than or equal to the current size of this list
 270      */
 271     public int lastIndexOf(E e, int index) {
 272         Object[] elements = getArray();
 273         return lastIndexOf(e, elements, index);
 274     }
 275 
 276     /**
 277      * Returns a shallow copy of this list.  (The elements themselves
 278      * are not copied.)
 279      *
 280      * @return a clone of this list
 281      */
 282     public Object clone() {
 283         try {
 284             CopyOnWriteArrayList c = (CopyOnWriteArrayList)(super.clone());
 285             c.resetLock();
 286             return c;
 287         } catch (CloneNotSupportedException e) {
 288             // this shouldn't happen, since we are Cloneable
 289             throw new InternalError();
 290         }
 291     }
 292 
 293     /**
 294      * Returns an array containing all of the elements in this list
 295      * in proper sequence (from first to last element).
 296      *
 297      * 
The returned array will be "safe" in that no references to it are
 298      * maintained by this list.  (In other words, this method must allocate
 299      * a new array).  The caller is thus free to modify the returned array.
 300      *
 301      * 
This method acts as bridge between array-based and collection-based
 302      * APIs.
 303      *
 304      * @return an array containing all the elements in this list
 305      */
 306     public Object[] toArray() {
 307         Object[] elements = getArray();
 308         return Arrays.copyOf(elements, elements.length);
 309     }
 310 
 311     /**
 312      * Returns an array containing all of the elements in this list in
 313      * proper sequence (from first to last element); the runtime type of
 314      * the returned array is that of the specified array.  If the list fits
 315      * in the specified array, it is returned therein.  Otherwise, a new
 316      * array is allocated with the runtime type of the specified array and
 317      * the size of this list.
 318      *
 319      * 
If this list fits in the specified array with room to spare
 320      * (i.e., the array has more elements than this list), the element in
 321      * the array immediately following the end of the list is set to
 322      * null.  (This is useful in determining the length of this
 323      * list only if the caller knows that this list does not contain
 324      * any null elements.)
 325      *
 326      * 
Like the {@link #toArray()} method, this method acts as bridge between
 327      * array-based and collection-based APIs.  Further, this method allows
 328      * precise control over the runtime type of the output array, and may,
 329      * under certain circumstances, be used to save allocation costs.
 330      *
 331      * 
Suppose x is a list known to contain only strings.
 332      * The following code can be used to dump the list into a newly
 333      * allocated array of String:
 334      *
 335      * 
 336      *     String[] y = x.toArray(new String[0]);
 337      *
 338      * Note that toArray(new Object[0]) is identical in function to
 339      * toArray().
 340      *
 341      * @param a the array into which the elements of the list are to
 342      *          be stored, if it is big enough; otherwise, a new array of the
 343      *          same runtime type is allocated for this purpose.
 344      * @return an array containing all the elements in this list
 345      * @throws ArrayStoreException if the runtime type of the specified array
 346      *         is not a supertype of the runtime type of every element in
 347      *         this list
 348      * @throws NullPointerException if the specified array is null
 349      */
 350     @SuppressWarnings("unchecked")
 351     public  T[] toArray(T a[]) {
 352         Object[] elements = getArray();
 353         int len = elements.length;
 354         if (a.length < len)
 355             return (T[]) Arrays.copyOf(elements, len, a.getClass());
 356         else {
 357             System.arraycopy(elements, 0, a, 0, len);
 358             if (a.length > len)
 359                 a[len] = null;
 360             return a;
 361         }
 362     }
 363 
 364     // Positional Access Operations
 365 
 366     @SuppressWarnings("unchecked")
 367     private E get(Object[] a, int index) {
 368         return (E) a[index];
 369     }
 370 
 371     /**
 372      * {@inheritDoc}
 373      *
 374      * @throws IndexOutOfBoundsException {@inheritDoc}
 375      */
 376     public E get(int index) {
 377         return get(getArray(), index);
 378     }
 379 
 380     /**
 381      * Replaces the element at the specified position in this list with the
 382      * specified element.
 383      *
 384      * @throws IndexOutOfBoundsException {@inheritDoc}
 385      */
 386     public E set(int index, E element) {
 387         final ReentrantLock lock = this.lock;
 388         lock.lock();
 389         try {
 390             Object[] elements = getArray();
 391             E oldValue = get(elements, index);
 392 
 393             if (oldValue != element) {
 394                 int len = elements.length;
 395                 Object[] newElements = Arrays.copyOf(elements, len);
 396                 newElements[index] = element;
 397                 setArray(newElements);
 398             } else {
 399                 // Not quite a no-op; ensures volatile write semantics
 400                 setArray(elements);
 401             }
 402             return oldValue;
 403         } finally {
 404             lock.unlock();
 405         }
 406     }
 407 
 408     /**
 409      * Appends the specified element to the end of this list.
 410      *
 411      * @param e element to be appended to this list
 412      * @return true (as specified by {@link Collection#add})
 413      */
 414     public boolean add(E e) {
 415         final ReentrantLock lock = this.lock;
 416         lock.lock();
 417         try {
 418             Object[] elements = getArray();
 419             int len = elements.length;
 420             Object[] newElements = Arrays.copyOf(elements, len + 1);
 421             newElements[len] = e;
 422             setArray(newElements);
 423             return true;
 424         } finally {
 425             lock.unlock();
 426         }
 427     }
 428 
 429     /**
 430      * Inserts the specified element at the specified position in this
 431      * list. Shifts the element currently at that position (if any) and
 432      * any subsequent elements to the right (adds one to their indices).
 433      *
 434      * @throws IndexOutOfBoundsException {@inheritDoc}
 435      */
 436     public void add(int index, E element) {
 437         final ReentrantLock lock = this.lock;
 438         lock.lock();
 439         try {
 440             Object[] elements = getArray();
 441             int len = elements.length;
 442             if (index > len || index < 0)
 443                 throw new IndexOutOfBoundsException("Index: "+index+
 444                                                     ", Size: "+len);
 445             Object[] newElements;
 446             int numMoved = len - index;
 447             if (numMoved == 0)
 448                 newElements = Arrays.copyOf(elements, len + 1);
 449             else {
 450                 newElements = new Object[len + 1];
 451                 System.arraycopy(elements, 0, newElements, 0, index);
 452                 System.arraycopy(elements, index, newElements, index + 1,
 453                                  numMoved);
 454             }
 455             newElements[index] = element;
 456             setArray(newElements);
 457         } finally {
 458             lock.unlock();
 459         }
 460     }
 461 
 462     /**
 463      * Removes the element at the specified position in this list.
 464      * Shifts any subsequent elements to the left (subtracts one from their
 465      * indices).  Returns the element that was removed from the list.
 466      *
 467      * @throws IndexOutOfBoundsException {@inheritDoc}
 468      */
 469     public E remove(int index) {
 470         final ReentrantLock lock = this.lock;
 471         lock.lock();
 472         try {
 473             Object[] elements = getArray();
 474             int len = elements.length;
 475             E oldValue = get(elements, index);
 476             int numMoved = len - index - 1;
 477             if (numMoved == 0)
 478                 setArray(Arrays.copyOf(elements, len - 1));
 479             else {
 480                 Object[] newElements = new Object[len - 1];
 481                 System.arraycopy(elements, 0, newElements, 0, index);
 482                 System.arraycopy(elements, index + 1, newElements, index,
 483                                  numMoved);
 484                 setArray(newElements);
 485             }
 486             return oldValue;
 487         } finally {
 488             lock.unlock();
 489         }
 490     }
 491 
 492     /**
 493      * Removes the first occurrence of the specified element from this list,
 494      * if it is present.  If this list does not contain the element, it is
 495      * unchanged.  More formally, removes the element with the lowest index
 496      * i such that
 497      * (o==null ? get(i)==null : o.equals(get(i)))
 498      * (if such an element exists).  Returns true if this list
 499      * contained the specified element (or equivalently, if this list
 500      * changed as a result of the call).
 501      *
 502      * @param o element to be removed from this list, if present
 503      * @return true if this list contained the specified element
 504      */
 505     public boolean remove(Object o) {
 506         final ReentrantLock lock = this.lock;
 507         lock.lock();
 508         try {
 509             Object[] elements = getArray();
 510             int len = elements.length;
 511             if (len != 0) {
 512                 // Copy while searching for element to remove
 513                 // This wins in the normal case of element being present
 514                 int newlen = len - 1;
 515                 Object[] newElements = new Object[newlen];
 516 
 517                 for (int i = 0; i < newlen; ++i) {
 518                     if (eq(o, elements[i])) {
 519                         // found one;  copy remaining and exit
 520                         for (int k = i + 1; k < len; ++k)
 521                             newElements[k-1] = elements[k];
 522                         setArray(newElements);
 523                         return true;
 524                     } else
 525                         newElements[i] = elements[i];
 526                 }
 527 
 528                 // special handling for last cell
 529                 if (eq(o, elements[newlen])) {
 530                     setArray(newElements);
 531                     return true;
 532                 }
 533             }
 534             return false;
 535         } finally {
 536             lock.unlock();
 537         }
 538     }
 539 
 540     /**
 541      * Removes from this list all of the elements whose index is between
 542      * fromIndex, inclusive, and toIndex, exclusive.
 543      * Shifts any succeeding elements to the left (reduces their index).
 544      * This call shortens the list by (toIndex - fromIndex) elements.
 545      * (If toIndex==fromIndex, this operation has no effect.)
 546      *
 547      * @param fromIndex index of first element to be removed
 548      * @param toIndex index after last element to be removed
 549      * @throws IndexOutOfBoundsException if fromIndex or toIndex out of range
 550      *         ({@code{fromIndex < 0 || toIndex > size() || toIndex < fromIndex})
 551      */
 552     private void removeRange(int fromIndex, int toIndex) {
 553         final ReentrantLock lock = this.lock;
 554         lock.lock();
 555         try {
 556             Object[] elements = getArray();
 557             int len = elements.length;
 558 
 559             if (fromIndex < 0 || toIndex > len || toIndex < fromIndex)
 560                 throw new IndexOutOfBoundsException();
 561             int newlen = len - (toIndex - fromIndex);
 562             int numMoved = len - toIndex;
 563             if (numMoved == 0)
 564                 setArray(Arrays.copyOf(elements, newlen));
 565             else {
 566                 Object[] newElements = new Object[newlen];
 567                 System.arraycopy(elements, 0, newElements, 0, fromIndex);
 568                 System.arraycopy(elements, toIndex, newElements,
 569                                  fromIndex, numMoved);
 570                 setArray(newElements);
 571             }
 572         } finally {
 573             lock.unlock();
 574         }
 575     }
 576 
 577     /**
 578      * Append the element if not present.
 579      *
 580      * @param e element to be added to this list, if absent
 581      * @return true if the element was added
 582      */
 583     public boolean addIfAbsent(E e) {
 584         final ReentrantLock lock = this.lock;
 585         lock.lock();
 586         try {
 587             // Copy while checking if already present.
 588             // This wins in the most common case where it is not present
 589             Object[] elements = getArray();
 590             int len = elements.length;
 591             Object[] newElements = new Object[len + 1];
 592             for (int i = 0; i < len; ++i) {
 593                 if (eq(e, elements[i]))
 594                     return false; // exit, throwing away copy
 595                 else
 596                     newElements[i] = elements[i];
 597             }
 598             newElements[len] = e;
 599             setArray(newElements);
 600             return true;
 601         } finally {
 602             lock.unlock();
 603         }
 604     }
 605 
 606     /**
 607      * Returns true if this list contains all of the elements of the
 608      * specified collection.
 609      *
 610      * @param c collection to be checked for containment in this list
 611      * @return true if this list contains all of the elements of the
 612      *         specified collection
 613      * @throws NullPointerException if the specified collection is null
 614      * @see #contains(Object)
 615      */
 616     public boolean containsAll(Collection c) {
 617         Object[] elements = getArray();
 618         int len = elements.length;
 619         for (Object e : c) {
 620             if (indexOf(e, elements, 0, len) < 0)
 621                 return false;
 622         }
 623         return true;
 624     }
 625 
 626     /**
 627      * Removes from this list all of its elements that are contained in
 628      * the specified collection. This is a particularly expensive operation
 629      * in this class because of the need for an internal temporary array.
 630      *
 631      * @param c collection containing elements to be removed from this list
 632      * @return true if this list changed as a result of the call
 633      * @throws ClassCastException if the class of an element of this list
 634      *         is incompatible with the specified collection
 635      *         (optional)
 636      * @throws NullPointerException if this list contains a null element and the
 637      *         specified collection does not permit null elements
 638      *         (optional),
 639      *         or if the specified collection is null
 640      * @see #remove(Object)
 641      */
 642     public boolean removeAll(Collection c) {
 643         final ReentrantLock lock = this.lock;
 644         lock.lock();
 645         try {
 646             Object[] elements = getArray();
 647             int len = elements.length;
 648             if (len != 0) {
 649                 // temp array holds those elements we know we want to keep
 650                 int newlen = 0;
 651                 Object[] temp = new Object[len];
 652                 for (int i = 0; i < len; ++i) {
 653                     Object element = elements[i];
 654                     if (!c.contains(element))
 655                         temp[newlen++] = element;
 656                 }
 657                 if (newlen != len) {
 658                     setArray(Arrays.copyOf(temp, newlen));
 659                     return true;
 660                 }
 661             }
 662             return false;
 663         } finally {
 664             lock.unlock();
 665         }
 666     }
 667 
 668     /**
 669      * Retains only the elements in this list that are contained in the
 670      * specified collection.  In other words, removes from this list all of
 671      * its elements that are not contained in the specified collection.
 672      *
 673      * @param c collection containing elements to be retained in this list
 674      * @return true if this list changed as a result of the call
 675      * @throws ClassCastException if the class of an element of this list
 676      *         is incompatible with the specified collection
 677      *         (optional)
 678      * @throws NullPointerException if this list contains a null element and the
 679      *         specified collection does not permit null elements
 680      *         (optional),
 681      *         or if the specified collection is null
 682      * @see #remove(Object)
 683      */
 684     public boolean retainAll(Collection c) {
 685         final ReentrantLock lock = this.lock;
 686         lock.lock();
 687         try {
 688             Object[] elements = getArray();
 689             int len = elements.length;
 690             if (len != 0) {
 691                 // temp array holds those elements we know we want to keep
 692                 int newlen = 0;
 693                 Object[] temp = new Object[len];
 694                 for (int i = 0; i < len; ++i) {
 695                     Object element = elements[i];
 696                     if (c.contains(element))
 697                         temp[newlen++] = element;
 698                 }
 699                 if (newlen != len) {
 700                     setArray(Arrays.copyOf(temp, newlen));
 701                     return true;
 702                 }
 703             }
 704             return false;
 705         } finally {
 706             lock.unlock();
 707         }
 708     }
 709 
 710     /**
 711      * Appends all of the elements in the specified collection that
 712      * are not already contained in this list, to the end of
 713      * this list, in the order that they are returned by the
 714      * specified collection's iterator.
 715      *
 716      * @param c collection containing elements to be added to this list
 717      * @return the number of elements added
 718      * @throws NullPointerException if the specified collection is null
 719      * @see #addIfAbsent(Object)
 720      */
 721     public int addAllAbsent(Collectionextends E> c) {
 722         Object[] cs = c.toArray();
 723         if (cs.length == 0)
 724             return 0;
 725         Object[] uniq = new Object[cs.length];
 726         final ReentrantLock lock = this.lock;
 727         lock.lock();
 728         try {
 729             Object[] elements = getArray();
 730             int len = elements.length;
 731             int added = 0;
 732             for (int i = 0; i < cs.length; ++i) { // scan for duplicates
 733                 Object e = cs[i];
 734                 if (indexOf(e, elements, 0, len) < 0 &&
 735                     indexOf(e, uniq, 0, added) < 0)
 736                     uniq[added++] = e;
 737             }
 738             if (added > 0) {
 739                 Object[] newElements = Arrays.copyOf(elements, len + added);
 740                 System.arraycopy(uniq, 0, newElements, len, added);
 741                 setArray(newElements);
 742             }
 743             return added;
 744         } finally {
 745             lock.unlock();
 746         }
 747     }
 748 
 749     /**
 750      * Removes all of the elements from this list.
 751      * The list will be empty after this call returns.
 752      */
 753     public void clear() {
 754         final ReentrantLock lock = this.lock;
 755         lock.lock();
 756         try {
 757             setArray(new Object[0]);
 758         } finally {
 759             lock.unlock();
 760         }
 761     }
 762 
 763     /**
 764      * Appends all of the elements in the specified collection to the end
 765      * of this list, in the order that they are returned by the specified
 766      * collection's iterator.
 767      *
 768      * @param c collection containing elements to be added to this list
 769      * @return true if this list changed as a result of the call
 770      * @throws NullPointerException if the specified collection is null
 771      * @see #add(Object)
 772      */
 773     public boolean addAll(Collectionextends E> c) {
 774         Object[] cs = c.toArray();
 775         if (cs.length == 0)
 776             return false;
 777         final ReentrantLock lock = this.lock;
 778         lock.lock();
 779         try {
 780             Object[] elements = getArray();
 781             int len = elements.length;
 782             Object[] newElements = Arrays.copyOf(elements, len + cs.length);
 783             System.arraycopy(cs, 0, newElements, len, cs.length);
 784             setArray(newElements);
 785             return true;
 786         } finally {
 787             lock.unlock();
 788         }
 789     }
 790 
 791     /**
 792      * Inserts all of the elements in the specified collection into this
 793      * list, starting at the specified position.  Shifts the element
 794      * currently at that position (if any) and any subsequent elements to
 795      * the right (increases their indices).  The new elements will appear
 796      * in this list in the order that they are returned by the
 797      * specified collection's iterator.
 798      *
 799      * @param index index at which to insert the first element
 800      *        from the specified collection
 801      * @param c collection containing elements to be added to this list
 802      * @return true if this list changed as a result of the call
 803      * @throws IndexOutOfBoundsException {@inheritDoc}
 804      * @throws NullPointerException if the specified collection is null
 805      * @see #add(int,Object)
 806      */
 807     public boolean addAll(int index, Collectionextends E> c) {
 808         Object[] cs = c.toArray();
 809         final ReentrantLock lock = this.lock;
 810         lock.lock();
 811         try {
 812             Object[] elements = getArray();
 813             int len = elements.length;
 814             if (index > len || index < 0)
 815                 throw new IndexOutOfBoundsException("Index: "+index+
 816                                                     ", Size: "+len);
 817             if (cs.length == 0)
 818                 return false;
 819             int numMoved = len - index;
 820             Object[] newElements;
 821             if (numMoved == 0)
 822                 newElements = Arrays.copyOf(elements, len + cs.length);
 823             else {
 824                 newElements = new Object[len + cs.length];
 825                 System.arraycopy(elements, 0, newElements, 0, index);
 826                 System.arraycopy(elements, index,
 827                                  newElements, index + cs.length,
 828                                  numMoved);
 829             }
 830             System.arraycopy(cs, 0, newElements, index, cs.length);
 831             setArray(newElements);
 832             return true;
 833         } finally {
 834             lock.unlock();
 835         }
 836     }
 837 
 838     /**
 839      * Saves the state of the list to a stream (that is, serializes it).
 840      *
 841      * @serialData The length of the array backing the list is emitted
 842      *               (int), followed by all of its elements (each an Object)
 843      *               in the proper order.
 844      * @param s the stream
 845      */
 846     private void writeObject(java.io.ObjectOutputStream s)
 847         throws java.io.IOException{
 848 
 849         s.defaultWriteObject();
 850 
 851         Object[] elements = getArray();
 852         // Write out array length
 853         s.writeInt(elements.length);
 854 
 855         // Write out all elements in the proper order.
 856         for (Object element : elements)
 857             s.writeObject(element);
 858     }
 859 
 860     /**
 861      * Reconstitutes the list from a stream (that is, deserializes it).
 862      *
 863      * @param s the stream
 864      */
 865     private void readObject(java.io.ObjectInputStream s)
 866         throws java.io.IOException, ClassNotFoundException {
 867 
 868         s.defaultReadObject();
 869 
 870         // bind to new lock
 871         resetLock();
 872 
 873         // Read in array length and allocate array
 874         int len = s.readInt();
 875         Object[] elements = new Object[len];
 876 
 877         // Read in all elements in the proper order.
 878         for (int i = 0; i < len; i++)
 879             elements[i] = s.readObject();
 880         setArray(elements);
 881     }
 882 
 883     /**
 884      * Returns a string representation of this list.  The string
 885      * representation consists of the string representations of the list's
 886      * elements in the order they are returned by its iterator, enclosed in
 887      * square brackets ("[]").  Adjacent elements are separated by
 888      * the characters ", " (comma and space).  Elements are
 889      * converted to strings as by {@link String#valueOf(Object)}.
 890      *
 891      * @return a string representation of this list
 892      */
 893     public String toString() {
 894         return Arrays.toString(getArray());
 895     }
 896 
 897     /**
 898      * Compares the specified object with this list for equality.
 899      * Returns {@code true} if the specified object is the same object
 900      * as this object, or if it is also a {@link List} and the sequence
 901      * of elements returned by an {@linkplain List#iterator() iterator}
 902      * over the specified list is the same as the sequence returned by
 903      * an iterator over this list.  The two sequences are considered to
 904      * be the same if they have the same length and corresponding
 905      * elements at the same position in the sequence are equal.
 906      * Two elements {@code e1} and {@code e2} are considered
 907      * equal if {@code (e1==null ? e2==null : e1.equals(e2))}.
 908      *
 909      * @param o the object to be compared for equality with this list
 910      * @return {@code true} if the specified object is equal to this list
 911      */
 912     public boolean equals(Object o) {
 913         if (o == this)
 914             return true;
 915         if (!(o instanceof List))
 916             return false;
 917 
 918         List list = (List)(o);
 919         Iterator it = list.iterator();
 920         Object[] elements = getArray();
 921         int len = elements.length;
 922         for (int i = 0; i < len; ++i)
 923             if (!it.hasNext() || !eq(elements[i], it.next()))
 924                 return false;
 925         if (it.hasNext())
 926             return false;
 927         return true;
 928     }
 929 
 930     /**
 931      * Returns the hash code value for this list.
 932      *
 933      * This implementation uses the definition in {@link List#hashCode}.
 934      *
 935      * @return the hash code value for this list
 936      */
 937     public int hashCode() {
 938         int hashCode = 1;
 939         Object[] elements = getArray();
 940         int len = elements.length;
 941         for (int i = 0; i < len; ++i) {
 942             Object obj = elements[i];
 943             hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode());
 944         }
 945         return hashCode;
 946     }
 947 
 948     /**
 949      * Returns an iterator over the elements in this list in proper sequence.
 950      *
 951      * 
The returned iterator provides a snapshot of the state of the list
 952      * when the iterator was constructed. No synchronization is needed while
 953      * traversing the iterator. The iterator does NOT support the
 954      * remove method.
 955      *
 956      * @return an iterator over the elements in this list in proper sequence
 957      */
 958     public Iterator iterator() {
 959         return new COWIterator(getArray(), 0);
 960     }
 961 
 962     /**
 963      * {@inheritDoc}
 964      *
 965      * 
The returned iterator provides a snapshot of the state of the list
 966      * when the iterator was constructed. No synchronization is needed while
 967      * traversing the iterator. The iterator does NOT support the
 968      * remove, set or add methods.
 969      */
 970     public ListIterator listIterator() {
 971         return new COWIterator(getArray(), 0);
 972     }
 973 
 974     /**
 975      * {@inheritDoc}
 976      *
 977      * 
The returned iterator provides a snapshot of the state of the list
 978      * when the iterator was constructed. No synchronization is needed while
 979      * traversing the iterator. The iterator does NOT support the
 980      * remove, set or add methods.
 981      *
 982      * @throws IndexOutOfBoundsException {@inheritDoc}
 983      */
 984     public ListIterator listIterator(final int index) {
 985         Object[] elements = getArray();
 986         int len = elements.length;
 987         if (index<0 || index>len)
 988             throw new IndexOutOfBoundsException("Index: "+index);
 989 
 990         return new COWIterator(elements, index);
 991     }
 992 
 993     private static class COWIterator implements ListIterator {
 994         /** Snapshot of the array */
 995         private final Object[] snapshot;
 996         /** Index of element to be returned by subsequent call to next.  */
 997         private int cursor;
 998 
 999         private COWIterator(Object[] elements, int initialCursor) {
1000             cursor = initialCursor;
1001             snapshot = elements;
1002         }
1003 
1004         public boolean hasNext() {
1005             return cursor < snapshot.length;
1006         }
1007 
1008         public boolean hasPrevious() {
1009             return cursor > 0;
1010         }
1011 
1012         @SuppressWarnings("unchecked")
1013         public E next() {
1014             if (! hasNext())
1015                 throw new NoSuchElementException();
1016             return (E) snapshot[cursor++];
1017         }
1018 
1019         @SuppressWarnings("unchecked")
1020         public E previous() {
1021             if (! hasPrevious())
1022                 throw new NoSuchElementException();
1023             return (E) snapshot[--cursor];
1024         }
1025 
1026         public int nextIndex() {
1027             return cursor;
1028         }
1029 
1030         public int previousIndex() {
1031             return cursor-1;
1032         }
1033 
1034         /**
1035          * Not supported. Always throws UnsupportedOperationException.
1036          * @throws UnsupportedOperationException always; remove
1037          *         is not supported by this iterator.
1038          */
1039         public void remove() {
1040             throw new UnsupportedOperationException();
1041         }
1042 
1043         /**
1044          * Not supported. Always throws UnsupportedOperationException.
1045          * @throws UnsupportedOperationException always; set
1046          *         is not supported by this iterator.
1047          */
1048         public void set(E e) {
1049             throw new UnsupportedOperationException();
1050         }
1051 
1052         /**
1053          * Not supported. Always throws UnsupportedOperationException.
1054          * @throws UnsupportedOperationException always; add
1055          *         is not supported by this iterator.
1056          */
1057         public void add(E e) {
1058             throw new UnsupportedOperationException();
1059         }
1060     }
1061 
1062     /**
1063      * Returns a view of the portion of this list between
1064      * fromIndex, inclusive, and toIndex, exclusive.
1065      * The returned list is backed by this list, so changes in the
1066      * returned list are reflected in this list.
1067      *
1068      * 
The semantics of the list returned by this method become
1069      * undefined if the backing list (i.e., this list) is modified in
1070      * any way other than via the returned list.
1071      *
1072      * @param fromIndex low endpoint (inclusive) of the subList
1073      * @param toIndex high endpoint (exclusive) of the subList
1074      * @return a view of the specified range within this list
1075      * @throws IndexOutOfBoundsException {@inheritDoc}
1076      */
1077     public List subList(int fromIndex, int toIndex) {
1078         final ReentrantLock lock = this.lock;
1079         lock.lock();
1080         try {
1081             Object[] elements = getArray();
1082             int len = elements.length;
1083             if (fromIndex < 0 || toIndex > len || fromIndex > toIndex)
1084                 throw new IndexOutOfBoundsException();
1085             return new COWSubList(this, fromIndex, toIndex);
1086         } finally {
1087             lock.unlock();
1088         }
1089     }
1090 
1091     /**
1092      * Sublist for CopyOnWriteArrayList.
1093      * This class extends AbstractList merely for convenience, to
1094      * avoid having to define addAll, etc. This doesn't hurt, but
1095      * is wasteful.  This class does not need or use modCount
1096      * mechanics in AbstractList, but does need to check for
1097      * concurrent modification using similar mechanics.  On each
1098      * operation, the array that we expect the backing list to use
1099      * is checked and updated.  Since we do this for all of the
1100      * base operations invoked by those defined in AbstractList,
1101      * all is well.  While inefficient, this is not worth
1102      * improving.  The kinds of list operations inherited from
1103      * AbstractList are already so slow on COW sublists that
1104      * adding a bit more space/time doesn't seem even noticeable.
1105      */
1106     private static class COWSubList
1107         extends AbstractList
1108         implements RandomAccess
1109     {
1110         private final CopyOnWriteArrayList l;
1111         private final int offset;
1112         private int size;
1113         private Object[] expectedArray;
1114 
1115         // only call this holding l's lock
1116         COWSubList(CopyOnWriteArrayList list,
1117                    int fromIndex, int toIndex) {
1118             l = list;
1119             expectedArray = l.getArray();
1120             offset = fromIndex;
1121             size = toIndex - fromIndex;
1122         }
1123 
1124         // only call this holding l's lock
1125         private void checkForComodification() {
1126             if (l.getArray() != expectedArray)
1127                 throw new ConcurrentModificationException();
1128         }
1129 
1130         // only call this holding l's lock
1131         private void rangeCheck(int index) {
1132             if (index<0 || index>=size)
1133                 throw new IndexOutOfBoundsException("Index: "+index+
1134                                                     ",Size: "+size);
1135         }
1136 
1137         public E set(int index, E element) {
1138             final ReentrantLock lock = l.lock;
1139             lock.lock();
1140             try {
1141                 rangeCheck(index);
1142                 checkForComodification();
1143                 E x = l.set(index+offset, element);
1144                 expectedArray = l.getArray();
1145                 return x;
1146             } finally {
1147                 lock.unlock();
1148             }
1149         }
1150 
1151         public E get(int index) {
1152             final ReentrantLock lock = l.lock;
1153             lock.lock();
1154             try {
1155                 rangeCheck(index);
1156                 checkForComodification();
1157                 return l.get(index+offset);
1158             } finally {
1159                 lock.unlock();
1160             }
1161         }
1162 
1163         public int size() {
1164             final ReentrantLock lock = l.lock;
1165             lock.lock();
1166             try {
1167                 checkForComodification();
1168                 return size;
1169             } finally {
1170                 lock.unlock();
1171             }
1172         }
1173 
1174         public void add(int index, E element) {
1175             final ReentrantLock lock = l.lock;
1176             lock.lock();
1177             try {
1178                 checkForComodification();
1179                 if (index<0 || index>size)
1180                     throw new IndexOutOfBoundsException();
1181                 l.add(index+offset, element);
1182                 expectedArray = l.getArray();
1183                 size++;
1184             } finally {
1185                 lock.unlock();
1186             }
1187         }
1188 
1189         public void clear() {
1190             final ReentrantLock lock = l.lock;
1191             lock.lock();
1192             try {
1193                 checkForComodification();
1194                 l.removeRange(offset, offset+size);
1195                 expectedArray = l.getArray();
1196                 size = 0;
1197             } finally {
1198                 lock.unlock();
1199             }
1200         }
1201 
1202         public E remove(int index) {
1203             final ReentrantLock lock = l.lock;
1204             lock.lock();
1205             try {
1206                 rangeCheck(index);
1207                 checkForComodification();
1208                 E result = l.remove(index+offset);
1209                 expectedArray = l.getArray();
1210                 size--;
1211                 return result;
1212             } finally {
1213                 lock.unlock();
1214             }
1215         }
1216 
1217         public boolean remove(Object o) {
1218             int index = indexOf(o);
1219             if (index == -1)
1220                 return false;
1221             remove(index);
1222             return true;
1223         }
1224 
1225         public Iterator iterator() {
1226             final ReentrantLock lock = l.lock;
1227             lock.lock();
1228             try {
1229                 checkForComodification();
1230                 return new COWSubListIterator(l, 0, offset, size);
1231             } finally {
1232                 lock.unlock();
1233             }
1234         }
1235 
1236         public ListIterator listIterator(final int index) {
1237             final ReentrantLock lock = l.lock;
1238             lock.lock();
1239             try {
1240                 checkForComodification();
1241                 if (index<0 || index>size)
1242                     throw new IndexOutOfBoundsException("Index: "+index+
1243                                                         ", Size: "+size);
1244                 return new COWSubListIterator(l, index, offset, size);
1245             } finally {
1246                 lock.unlock();
1247             }
1248         }
1249 
1250         public List subList(int fromIndex, int toIndex) {
1251             final ReentrantLock lock = l.lock;
1252             lock.lock();
1253             try {
1254                 checkForComodification();
1255                 if (fromIndex<0 || toIndex>size)
1256                     throw new IndexOutOfBoundsException();
1257                 return new COWSubList(l, fromIndex + offset,
1258                                          toIndex + offset);
1259             } finally {
1260                 lock.unlock();
1261             }
1262         }
1263 
1264     }
1265 
1266 
1267     private static class COWSubListIterator implements ListIterator {
1268         private final ListIterator i;
1269         private final int index;
1270         private final int offset;
1271         private final int size;
1272 
1273         COWSubListIterator(List l, int index, int offset,
1274                            int size) {
1275             this.index = index;
1276             this.offset = offset;
1277             this.size = size;
1278             i = l.listIterator(index+offset);
1279         }
1280 
1281         public boolean hasNext() {
1282             return nextIndex() < size;
1283         }
1284 
1285         public E next() {
1286             if (hasNext())
1287                 return i.next();
1288             else
1289                 throw new NoSuchElementException();
1290         }
1291 
1292         public boolean hasPrevious() {
1293             return previousIndex() >= 0;
1294         }
1295 
1296         public E previous() {
1297             if (hasPrevious())
1298                 return i.previous();
1299             else
1300                 throw new NoSuchElementException();
1301         }
1302 
1303         public int nextIndex() {
1304             return i.nextIndex() - offset;
1305         }
1306 
1307         public int previousIndex() {
1308             return i.previousIndex() - offset;
1309         }
1310 
1311         public void remove() {
1312             throw new UnsupportedOperationException();
1313         }
1314 
1315         public void set(E e) {
1316             throw new UnsupportedOperationException();
1317         }
1318 
1319         public void add(E e) {
1320             throw new UnsupportedOperationException();
1321         }
1322     }
1323 
1324     // Support for resetting lock while deserializing
1325     private void resetLock() {
1326         UNSAFE.putObjectVolatile(this, lockOffset, new ReentrantLock());
1327     }
1328     private static final sun.misc.Unsafe UNSAFE;
1329     private static final long lockOffset;
1330     static {
1331         try {
1332             UNSAFE = sun.misc.Unsafe.getUnsafe();
1333             Class k = CopyOnWriteArrayList.class;
1334             lockOffset = UNSAFE.objectFieldOffset
1335                 (k.getDeclaredField("lock"));
1336         } catch (Exception e) {
1337             throw new Error(e);
1338         }
1339     }
1340 }

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(Collectionextends 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”+1
        Object[] 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 iterator() {
    return new COWIterator(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 implements ListIterator {
    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 list = new ArrayList();
17     private static List list = new CopyOnWriteArrayList();
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。

概要