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);
        }
    }
}

CodePudding user response：

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.

CodePudding user response：

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

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

1. methodA and methodB are calling each other, seems like the infinite loop problem
2. 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