Java集合——CopyOnWriteArrayList

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public class CopyOnWriteArrayList<E>
implements List<E>, RandomAccess, Cloneable, java.io.Serializable {
private static final long serialVersionUID = 8673264195747942595L;
//互斥锁
final transient ReentrantLock lock = new ReentrantLock();
//底层存储数据数组，只能通过getArray/setArray访问设置，volatile动态数组
private transient volatile Object[] array;
final Object[] getArray() {
return array;
}
final void setArray(Object[] a) {
array = a;
}
public CopyOnWriteArrayList() {
setArray(new Object[0]);
}
//传入Collection集合对象，将集合中元素存入CopyOnWriteArrayList
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);
}
//传入数组
public CopyOnWriteArrayList(E[] toCopyIn) {
setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class));
}
}

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public boolean add(E e) {
//获得互斥锁
final ReentrantLock lock = this.lock;
lock.lock();
try {
//获取原始数组
Object[] elements = getArray();
int len = elements.length;
Object[] newElements = Arrays.copyOf(elements, len + 1);
newElements[len] = e;
//用新的拷贝数组代替原始数组
setArray(newElements);
return true;
} finally {
lock.unlock();
}
}
public void add(int index, E element) {
//互斥锁
final ReentrantLock lock = this.lock;
lock.lock();
try {
//原始数组
Object[] elements = getArray();
int len = elements.length;
//检查index有效性
if (index > len || index < 0)
throw new IndexOutOfBoundsException("Index: "+index+
", Size: "+len);
//拷贝数组
Object[] newElements;
//从index到数组末尾要向后移动一位数组元素的个数
int numMoved = len - index;
//如果index==length，直接把原数组复制到新数组
if (numMoved == 0)
newElements = Arrays.copyOf(elements, len + 1);
//否则分成两段复制，原始数组index前面的元素位置一一对应赋值到新数组，原数组index开始的元素复制到
//新数组index+1到length+1，相当于依次后移。空出来的index就是新元素插入的位置
else {
newElements = new Object[len + 1];
System.arraycopy(elements, 0, newElements, 0, index);
System.arraycopy(elements, index, newElements, index + 1,
numMoved);
}
//插入新元素
newElements[index] = element;
setArray(newElements);
} finally {
lock.unlock();
}
}
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 {
setArray(elements);
}
return oldValue;
} finally {
lock.unlock();
}
}

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public E remove(int index) {
final ReentrantLock lock = this.lock;
lock.lock();
try {
Object[] elements = getArray();
int len = elements.length;
//数组index处要移除的元素
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();
}
}

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public E get(int index) {
return get(getArray(), index);
}
@SuppressWarnings("unchecked")
private E get(Object[] a, int index) {
//返回数组index处位置
return (E) a[index];
}

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public static <T> List<T> synchronizedList(List<T> list) {
return (list instanceof RandomAccess ?
new SynchronizedRandomAccessList<>(list) :
new SynchronizedList<>(list));
}