定义
package java.util.concurrent;
public class CopyOnWriteArrayList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable { /*/ }
- JUC 中的 CopyOnWriteArrayList，线程安全的 ArrayList
- 通过对底层数组复制保障线程安全
属性
/**
* The lock protecting all mutators. (We have a mild preference
* for builtin monitors over ReentrantLock when either will do.)
*/
final transient Object lock = new Object();
/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;
lock 作为被加锁的对象array 被 volatile 修饰，保证修改后被所有线程可见
方法
public boolean add(E e) {
// 加锁 + 复制
synchronized (lock) {
Object[] es = getArray();
int len = es.length;
es = Arrays.copyOf(es, len + 1);
es[len] = e;
setArray(es);
return true;
}
}
public void add(int index, E element) {
synchronized (lock) {
Object[] es = getArray();
int len = es.length;
if (index > len || index < 0)
throw new IndexOutOfBoundsException(outOfBounds(index, len));
Object[] newElements;
int numMoved = len - index;
if (numMoved == 0)
newElements = Arrays.copyOf(es, len + 1);
else {
newElements = new Object[len + 1];
System.arraycopy(es, 0, newElements, 0, index);
System.arraycopy(es, index, newElements, index + 1, numMoved);
}
newElements[index] = element;
setArray(newElements);
}
}
// getArray/setArray
final Object[] getArray() {
return array;
}
final void setArray(Object[] a) {
array = a;
}
public boolean addAll(int index, Collection<? extends E> c) {
Object[] cs = c.toArray();
synchronized (lock) {
Object[] es = getArray();
int len = es.length;
if (index > len || index < 0)
throw new IndexOutOfBoundsException(outOfBounds(index, len));
if (cs.length == 0)
return false;
int numMoved = len - index;
Object[] newElements;
// 数组复制
if (numMoved == 0)
newElements = Arrays.copyOf(es, len + cs.length);
else {
newElements = new Object[len + cs.length];
System.arraycopy(es, 0, newElements, 0, index);
System.arraycopy(es, index,
newElements, index + cs.length,
numMoved);
}
System.arraycopy(cs, 0, newElements, index, cs.length);
setArray(newElements);
return true;
}
}
public boolean addAll(Collection<? extends E> c) {
Object[] cs = (c.getClass() == CopyOnWriteArrayList.class) ?
((CopyOnWriteArrayList<?>)c).getArray() : c.toArray();
if (cs.length == 0)
return false;
synchronized (lock) {
// 实现类似
}
}
public void clear() {
synchronized (lock) {
setArray(new Object[0]);
}
}
// 读取操作是不需要加锁的
public boolean contains(Object o) {
return indexOf(o) >= 0;
}
public E remove(int index) {
synchronized (lock) {
Object[] es = getArray();
int len = es.length;
E oldValue = elementAt(es, index);
int numMoved = len - index - 1;
Object[] newElements;
if (numMoved == 0)
newElements = Arrays.copyOf(es, len - 1);
else {
newElements = new Object[len - 1];
System.arraycopy(es, 0, newElements, 0, index);
System.arraycopy(es, index + 1, newElements, index, numMoved);
}
setArray(newElements);
return oldValue;
}
}
public boolean remove(Object o) {
Object[] snapshot = getArray(); // 保存快照
int index = indexOfRange(o, snapshot, 0, snapshot.length); // 寻找下标
return index >= 0 && remove(o, snapshot, index);
}
/**
* A version of remove(Object) using the strong hint that given
* recent snapshot contains o at the given index.
*/
private boolean remove(Object o, Object[] snapshot, int index) {
synchronized (lock) { // 加锁
Object[] current = getArray();
int len = current.length;
// 更新下标
if (snapshot != current) findIndex: {
// 新下标在旧下标之前
int prefix = Math.min(index, len);
for (int i = 0; i < prefix; i++) {
if (current[i] != snapshot[i] && Objects.equals(o, current[i])) {
index = i;
break findIndex;
}
}
if (index >= len)
return false;
// 新旧下标一致
if (current[index] == o)
break findIndex;
// 新下标在旧下标之后
index = indexOfRange(o, current, index, len);
if (index < 0)
return false;
}
Object[] newElements = new Object[len - 1];
System.arraycopy(current, 0, newElements, 0, index);
System.arraycopy(current, index + 1,
newElements, index,
len - index - 1);
setArray(newElements);
return true;
}
}
public int indexOf(Object o) {
Object[] es = getArray();
return indexOfRange(o, es, 0, es.length);
}
public int indexOf(E e, int index) {
Object[] es = getArray();
return indexOfRange(e, es, index, es.length);
}
private static int indexOfRange(Object o, Object[] es, int from, int to) {
if (o == null) {
for (int i = from; i < to; i++)
if (es[i] == null)
return i;
} else {
for (int i = from; i < to; i++)
if (o.equals(es[i]))
return i;
}
return -1;
}
public Iterator<E> iterator() {
return new COWIterator<E>(getArray(), 0);
}
// 内部类实现
static final class COWIterator<E> implements ListIterator<E> {
/** Snapshot of the array */
private final Object[] snapshot; // 被 final 修饰 迭代过程中引用不会发生改变
/** Index of element to be returned by subsequent call to next. */
private int cursor;
// 数组改变(add/remove...)时 会创建新数组 使用新的内存地址
// 而该迭代器始终引用的是原数组 即保存了原数组的一个快照
// 因此不会对遍历结果造成影响 也就不需要加锁
COWIterator(Object[] es, int initialCursor) {
cursor = initialCursor; // 直接引用该数组
snapshot = es;
}
@SuppressWarnings("unchecked")
public E next() {
if (! hasNext())
throw new NoSuchElementException();
return (E) snapshot[cursor++];
}
// 无法进行 add set remove 操作
/**
* Not supported. Always throws UnsupportedOperationException.
* @throws UnsupportedOperationException always; {@code remove}
* is not supported by this iterator.
*/
public void remove() {
throw new UnsupportedOperationException();
}
}