Why Java keeps List values for each request even using concurrent list?

Question

In the following methods, I have a list to keep the data and I can populate all the data into this menuDTOList. However, when a send a new request after getting all the data from the first request, the menuDTOList still keeps the data of the first request.

As there are 2 methods recursively calls each other, I cannot make the list to be cleared properly. I tried Collections.synchronizedList() for thread-safety, but it does not make any sense. Also tried to clear the list at different steps, but due to recursive method calls, it did not work correctly.

So, how can I clear the list values on each request while keeping them recursive calls between methodA and methodB?

public class Demo {

    // private List<MenuDTO> menuDTOList  = new ArrayList<>();
    List<MenuDTO> menuDTOList = 
        Collections.synchronizedList(new ArrayList<MenuDTO>());

    protected Iterable<MenuDTO> getData() {
        for (...) {
            methodA();
        }
                
        return menuDTOList;
    }

    private void methodA(final MenuItemExpandedDTO menuItemExpandedDTO) {
            
        menuDTOList.add(dto);

        for (...) {
            methodB();
        }
    }

    private void methodB() {
        for (...) {
            methodA(dto);
        }
    }
}

From the code you have shown perhaps `menuDTOList` shouldn't be a field of `Demo`. Instead, create it in `getData` and pass it to `methodA` and `methodB`? — tgdavies, Nov 03 '21 at 11:10
@tgdavies Thanks a lot for your valuable helps, yes when create my list value to the `getData()` method, it seems to be fixed. **However**, I think when there is concurrent requests, then a new instance is created that is not wanted our singleton approach. So, how can I test this? And how can I fix that problem (maybe I need to use local variables for this list instead of global one). Any idea? — , Nov 03 '21 at 11:37
I don't understand the semantics of your class -- are you saying that there can be two concurrent calls to `getData` that you *want* to use the same `menuDTOList` instance? — tgdavies, Nov 03 '21 at 11:47

score 0 · Answer 1 · answered Nov 03 '21 at 11:35

0

Could you please add more details, what is the condition of for loops?

In the current state, I see 2 different problems here.

methodA and methodB are calling each other, seems like the infinite loop problem
Also if you want to make the list just thread-safe for setters, you can create a plain volatile list. Something like volatile List<MenuDTO> menuDTOList = new ArrayList<>() or if you're looking for out of the box data structure, you could use ConcurrentLinkedQueue. Or best of all, provide list as method param and avoid class level variable altogether

answered Nov 03 '21 at 11:35

Kiran A B

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1
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Thanks a lot for your nice explanations. For **#1**, there is no infinity problem and I retrieve the list properly. So, there is no need to share the loop parameters (I just omitted). For **#2**, could you please post these options in your answer so that I can apply without any mistake? – Nov 03 '21 at 11:40
And also how can I test if concurrent requests affect list value? – Nov 03 '21 at 11:41

score 0 · Accepted Answer · answered Nov 03 '21 at 11:54

If you do:

public class Demo {

    protected Iterable<MenuDTO> getData() {
        List<MenuDTO> menuDTOList = new ArrayList<>();
        for (...) {
            methodA(menuDTOList);
        }
                
        return menuDTOList;
    }

    private void methodA(final MenuItemExpandedDTO menuItemExpandedDTO, final List<MenuDTO> menuDTOList) {
            
        menuDTOList.add(dto);

        for (...) {
            methodB(menuDTOList);
        }
    }

    private void methodB(List<MenuDTO> menuDTOList) {
        for (...) {
            methodA(dto, menuDTOList);
        }
    }
}

Then each list instance is only used by a single thread, and the list will be available to be garbage collected as soon as the caller of getData discards the reference.

That's also good, I think now it is ok. Thanks a lot – Nov 03 '21 at 13:27 — , Nov 03 '21 at 13:27

Why Java keeps List values for each request even using concurrent list?

2 Answers2