How can CopyOnWriteArrayList be thread-safe?
- by Shooshpanchick
I've taken a look into OpenJDK's sources of CopyOnWriteArrayList and it seems that all write operations are protected by the same lock and read operations are not protected at all. As I understand, under JMM all accesses to a variable (both read and write) should be protected by lock or reordering effects may occur.
For example, set(int, E) method contains these lines (under lock):
/* 1 */ int len = elements.length;
/* 2 */ Object[] newElements = Arrays.copyOf(elements, len);
/* 3 */ newElements[index] = element;
/* 4 */ setArray(newElements);
The get(int) method, on the other hand, only does return get(getArray(), index);.
In my understanding of JMM, this means that get may observe the array in an inconsistent state if statements 1-4 are reordered like 1-2(new)-4-2(copyOf)-3.
Do I understand JMM incorrectly or is there any other explanations on why CopyOnWriteArrayList is thread-safe?
