转载：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，虽然是1.7，但是1.8和1.7没有什么区别)
CopyOnWriteArrayList示例
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。
CopyOnWriteArrayList使用了一种叫写时复制的方法，当有新元素添加到CopyOnWriteArrayList时，先从原有的数组中拷贝一份出来，然后在新的数组做写操作，写完之后，再将原来的数组引用指向到新数组。
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”，并在新数组操作，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 Cloneable throw 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 semantics setArray(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 present int 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 exit for (int k = i + 1; k < len; ++k) newElements[k-1] = elements[k]; setArray(newElements); return true; } else newElements[i] = elements[i]; } // special handling for last cell if (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 present Object[] 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 copy else newElements[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 keep int 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 keep int 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 duplicates Object 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 length s.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 lock resetLock(); // Read in array length and allocate array int 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 lock COWSubList(CopyOnWriteArrayList<E> list, int fromIndex, int toIndex) { l = list; expectedArray = l.getArray(); offset = fromIndex; size = toIndex - fromIndex; } // only call this holding l's lock private void checkForComodification() { if (l.getArray() != expectedArray) throw new ConcurrentModificationException(); } // only call this holding l's lock private 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(); else throw new NoSuchElementException(); } public boolean hasPrevious() { return previousIndex() >= 0; } public E previous() { if (hasPrevious()) return i.previous(); else throw 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 deserializing private 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); } } }
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下面我们从“创建，添加，修改，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：这里使用的是数组的拷贝，所以不需要初始化数组为一定长度。
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。
修改
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 semantics setArray(elements); } return oldValue; } finally { lock.unlock(); } }
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(); } }
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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)”。
