J.U.C并发框架源码阅读（十五）CopyOnWriteArrayList
2024-10-19 13:26:07
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.
*
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*/ /*
* 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
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