CopyOnWriteArrayList is too slow
I have following case,
public class Test {
private static final int MAX_NUMBER = 10_00_00;
public static void main(String[] args) {
List<Integer> list = new CopyOnWriteArrayList<>();
long start = System.nanoTime();
for(int i = 0; i < MAX_NUMBER; i++) {
list.add(i * 2);
}
long end = System.nanoTime();
System.out.println(((end - start) / Math.pow(10, 9)));
}
}
OUTPUT
6.861539857
It adds element quite slowly compared to ArrayList which took approximately 0.004690843 . I came to know the reason in the documentation,
A thread-safe variant of ArrayList in which all mutative operations (add, set, and so on) are implemented by making a fresh copy of the underlying array.
So, my understanding is, whenever I add new element in this list it will create new fresh array and add element at last index of this array. I find a lock in add method and apart from that that method is actually creating new array every time.
When I increased MAX_NUMBER to 10_00_000 my programs keeps on running and never end (it would but I can't wait for so long).
I think Collections.synchronizedList is the better choice when you want thread safety with speed. I used it and it took about 0.007673728 .
My questions :
MAX_NUMBER = 10_00_000 ? (as it took about 6 seconds with MAX_NUMBER = 10_00_00 ) Is this happening because mutative operation creates new array every time ? CopyOnWriteArrayList has performance drawback when you have huge number of elements and better to choose something else (ie Collections.synchronizedList )? CopyOnWriteArrayList in public APIs ? Is there any drawbacks other than this ? CopyOnWriteArrayList是首选选项，只有少量写入和大量读取（如果多个线程正在访问此列表）