对于CopyOnWriteArrayList来说,读取之间不加锁,而且写入也不阻塞读取操作,只有写入和写入之间需要同步等待;
读取:
/** The array, accessed only via getArray/setArray. */
private transient volatile Object[] array;
public E get(int index) {
return get(getArray(), index);
}
/**
* Gets the array. Non-private so as to also be accessible
* from CopyOnWriteArraySet class.
*/
final Object[] getArray() {
return array;
}
@SuppressWarnings("unchecked")
private E get(Object[] a, int index) {
return (E) a[index];
}
内部数组array不会发生修改,只会被另一个array替换,所以在读取代码中没有同步控制和锁操作也可以保证数据的安全;
写操作:
/**
* Appends the specified element to the end of this list.
*
* @param e element to be appended to this list
* @return {@code true} (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();
}
}
对于写-写操作加锁,使用Arrays.copyOf()对数组复制,并加入新增元素,并使用setArray()替换老的数组array,并且因为array变量是volatile类型的,所以可以被读取线程所见;