本节一起学习CopyOnWriteArrayList类的源码
1.首先看一下类的定义
public class CopyOnWriteArrayList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable
2.类的变量
/** 重入锁用来保护所有的存取器 */
final transient ReentrantLock lock = new ReentrantLock();
/** 存储数据的数组，多线程可见 */
private transient volatile Object[] array;
3.构造方法
/**
* 创建一个空的数组.
*/
public CopyOnWriteArrayList() {
setArray(new Object[0]);
}
/**
* 创建一个包含集合c的数组
*/
public CopyOnWriteArrayList(Collection<? extends E> c) {
Object[] elements;
if (c.getClass() == CopyOnWriteArrayList.class)
elements = ((CopyOnWriteArrayList<?>)c).getArray();
else {
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);
}
/**
* 创建一个包含数组toCopyIn的数组
*/
public CopyOnWriteArrayList(E[] toCopyIn) {
setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
}
/**
* Sets the array.
*/
final void setArray(Object[] a) {
array = a;
}
4.添加元素
/**
* 添加元素e到list的结尾
*/
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);//拷贝长度+1的新数组
newElements[len] = e;//设置结尾下标的值
setArray(newElements);//赋值给array数组
return true;
} finally {
lock.unlock();
}
}
/**
* 插入element在下标index位置
*/
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)//下标不在有效范围内
throw new IndexOutOfBoundsException("Index: "+index+
", Size: "+len);
Object[] newElements;
int numMoved = len - index;//需要移动的元素个数
if (numMoved == 0)//如果不需要移动元素个数，也既是插入到结尾
newElements = Arrays.copyOf(elements, len + 1);
else {//否则，分段复制，留出index位置
newElements = new Object[len + 1];
System.arraycopy(elements, 0, newElements, 0, index);
System.arraycopy(elements, index, newElements, index + 1,
numMoved);
}
newElements[index] = element;//赋值
setArray(newElements);//设置数组
} finally {
lock.unlock();
}
}
5. 获取元素
// 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);
}
6.移除元素
/**
* 移除下标位置index的元素
*/
public E remove(int index) {
final ReentrantLock lock = this.lock;
lock.lock();//加锁
try {
Object[] elements = getArray();
int len = elements.length;
E oldValue = get(elements, index);//获取下标位置index的元素
int numMoved = len - index - 1;//需要移动的元素个数
if (numMoved == 0)//不需要移动，既是删除最后一个元素
setArray(Arrays.copyOf(elements, len - 1));
else {
Object[] newElements = new Object[len - 1];//创建新的数组
System.arraycopy(elements, 0, newElements, 0, index);//复制旧的数组数据到新的数组，不包含index位置的元素
System.arraycopy(elements, index + 1, newElements, index,
numMoved);
setArray(newElements);
}
return oldValue;
} finally {
lock.unlock();
}
}
/**
* 移除元素o第一次出现的下标位置的元素
*/
public boolean remove(Object o) {
Object[] snapshot = getArray();
int index = indexOf(o, snapshot, 0, snapshot.length);
return (index < 0) ? false : 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) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] current = getArray();//获取最新的数组
int len = current.length;
if (snapshot != current) findIndex: {//在查找元素下标后，数组发生了改变，需要再次查找下标
int prefix = Math.min(index, len);
for (int i = 0; i < prefix; i++) {//在前半部分重新查找下标位置
if (current[i] != snapshot[i] && eq(o, current[i])) {
index = i;
break findIndex;
}
}
if (index >= len)//如果下标位置超过len的长度，返回false
return false;
if (current[index] == o)//仍然在index位置
break findIndex;
index = indexOf(o, current, index, len);//在后半部分查找元素
if (index < 0)
return false;
}
Object[] newElements = new Object[len - 1];//将旧的数组元素移动到新的数组中
System.arraycopy(current, 0, newElements, 0, index);
System.arraycopy(current, index + 1,
newElements, index,
len - index - 1);
setArray(newElements);//设置数组
return true;
} finally {
lock.unlock();
}
}
7.修改元素
/**
* 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);//复制，修改，赋值
} else {
// Not quite a no-op; ensures volatile write semantics
setArray(elements);
}
return oldValue;
} finally {
lock.unlock();
}
}8.遍历操作
public Iterator<E> iterator() {
return new COWIterator<E>(getArray(), 0);//使用全局变量数组赋值给副本，进行操作
}
static final class COWIterator<E> implements ListIterator<E> {
/** 副本 */
private final Object[] snapshot;
/** 指针位置，初始0 */
private int cursor;
private COWIterator(Object[] elements, int initialCursor) {
cursor = initialCursor;
snapshot = elements;
}
public boolean hasNext() {//判断是否还存在下一个元素
return cursor < snapshot.length;
}
public boolean hasPrevious() {//判断是否有前一个元素
return cursor > 0;
}
@SuppressWarnings("unchecked")
public E next() {//获取下一个元素
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; {@code remove}
* is not supported by this iterator.
*/
public void remove() {
throw new UnsupportedOperationException();
}
/**
* Not supported. Always throws UnsupportedOperationException.
* @throws UnsupportedOperationException always; {@code set}
* is not supported by this iterator.
*/
public void set(E e) {
throw new UnsupportedOperationException();
}
/**
* Not supported. Always throws UnsupportedOperationException.
* @throws UnsupportedOperationException always; {@code add}
* is not supported by this iterator.
*/
public void add(E e) {
throw new UnsupportedOperationException();
}
@Override
public void forEachRemaining(Consumer<? super E> action) {
Objects.requireNonNull(action);
Object[] elements = snapshot;
final int size = elements.length;
for (int i = cursor; i < size; i++) {
@SuppressWarnings("unchecked") E e = (E) elements[i];
action.accept(e);
}
cursor = size;
}
}
总结：
1.CopyOnWriteArrayList读操作无锁，线程安全
2.底层使用一个线程可见的数组维护元素，初始容量为0，每增加一个元素，则数组长度+1，通过复制将旧的数组移动到新数组。
3.增删改会使用重入锁进行加锁操作，读取不加锁
4.遍历是针对副本的一个遍历，不会出现并发异常
5.针对读多写少的情况，推荐使用CopyOnWriteArrayList