Question

I wrote a sample java application which allocates memory and then running forever.
why is the memory used by the survivor space 0kbytes ?!

    List<String> stringlist = new ArrayList<String>();

    while (true) {
        stringlist.add("test");
        if (stringlist.size() >= 5000000)
            break;
    }

    while (true)
        for (String s : stringlist);

Answer 1

Because “test” is a String literal it will end up in permanent memory not heap.
Memory size of objects you create is 5000000 + 4*2 ~ 5MB which will easily fit into Eden space.

Modify

stringlist.add("test");

to

stringlist.add(new String("test"));

and you will get 5000000 * 4 * 2 =38MB which most probably will still fit into Eden. You can either increase your list size or String length to make sure you have survivors.