J.U.C并发框架源码阅读（十五）CopyOnWriteArrayList
基于版本jdk1.7.0_80
java.util.concurrent.CopyOnWriteArrayList
代码如下
/* * 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); } } }
0. CopyOnWriteArrayList简介
使用写时复制（copy on write）思路设计的一种并发容器，适合于读多写少的场景。
1. 接口分析
CopyOnWriteArrayList继承于List<E>, RandomAccess, Cloneable, java.io.Serializable接口
2. CopyOnWriteArrayList原理简介
CopyOnWriteArrayList维护了一个volatile类型的Object数组array，与一个全局锁lock
读操作无需加锁，直接访问array即可
写操作会加上全局锁（这样写操作就是线程安全的），然后新建一个临时数组，将原array中的元素复制到临时数组的过程中完成写操作，然后将array指向临时数组，最后释放锁。
由于array是volatile的，所以写操作一旦完成，所有的读操作都能得到最新的array引用。
3. CopyOnWriteArrayList.add方法分析
/** * 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();//加上全局锁 try { Object[] elements = getArray();//获取底层数组引用 int len = elements.length; Object[] newElements = Arrays.copyOf(elements, len + 1);//下面两行是将array中的所有原有元素复制到newElements中，然后将新加入的元素复制到newElements数组的尾部 newElements[len] = e; setArray(newElements);//更新array的引用 return true; } finally { lock.unlock();//解锁 } } /** * 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; }
可以清楚的看出写时复制的操作
其他的remove，set这样涉及到修改的方法亦是如此的思路
4. CopyOnWriteArrayList.get方法分析
/** * {@inheritDoc} * * @throws IndexOutOfBoundsException {@inheritDoc} */ public E get(int index) { return get(getArray(), index); } @SuppressWarnings("unchecked") private E get(Object[] a, int index) { return (E) a[index]; }
很简单的代码，不加锁，仅靠volatile提供的happen-before语义实现对最新修改的可见性。
5. CopyOnWriteArrayList的迭代器
先看看CopyOnWriteArrayList.iterator方法的调用链
/** * 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); } 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(); } }
从源码可以看出，在调用iterator方法的时候，会创建一个COWIterator迭代器，这个迭代器维护了一个指向CopyOnWriteArrayList当前使用的array的指针
由于CopyOnWriteArrayList的特性，底层数组只会被更新引用，底层数组本身是不会被修改的，也就是说这个COWIterator维护的是一个指向迭代器创建瞬间CopyOnWriteArrayList的底层数组的快照。
在这个快照上，可以进行任意的遍历操作而不用担心并发修改的问题，但是不能做任何的修改操作：set/remove/add之类，这些操作一律会抛出UnsupportedOperationException异常
