對于CopyOnWriteArrayList來說,讀取之間不加鎖,而且寫入也不阻塞讀取操作,隻有寫入和寫入之間需要同步等待;
讀取:
/** 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];
}
内部數組array不會發生修改,隻會被另一個array替換,是以在讀取代碼中沒有同步控制和鎖操作也可以保證資料的安全;
寫操作:
/**
* 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();
}
}
對于寫-寫操作加鎖,使用Arrays.copyOf()對數組複制,并加入新增元素,并使用setArray()替換老的數組array,并且因為array變量是volatile類型的,是以可以被讀取線程所見;