Modifying List contained in Object inside ConcurrentHashMap

Question

I am using a Map to store customer's basket items in-memory. My map has the following signature Map<Integer, Basket> customerBaskets = new ConcurrentHashMap<>();

Each Basket class has a list of items defined as Collection<Item> items = new ArrayList<>().

During the app startup all the customers loaded into the map and an empty basket is assigned to them. Now I would like to add items to the customer's basket in a thread-safe manner. Since ConcurrentHashMap reads are not locking, I could get the Basket based on customerId and freely add items to the items list, this doesn't guarantee me thread safety. How can I ensure thread-safety for writes to this list and reads have the latest written value? Please advise.

Thank You.

Answer 1

You need either to synchronize the read-write operations on the ArrayList using something like ReentrantLock or use CopyOnWriteArrayList implementation for this underlying array.