in-memory-store

name	in-memory-store
description	ConcurrentHashMap CRUD pattern for the Spring MVC demo. Use when adding a new resource type with in-memory CRUD.

Skill: In-Memory Store (spring-mvc-demo)

Pattern — ConcurrentHashMap in the service

@Service
public class [Resource]Service {
    private final Map<UUID, [Resource]> store = new ConcurrentHashMap<>();

    public [Resource] create(Create[Resource]Request request) {
        UUID id = UUID.randomUUID();
        [Resource] item = new [Resource](id, request.name(), ...);
        store.put(id, item);
        return item;
    }

    public [Resource] getById(UUID id) {
        [Resource] item = store.get(id);
        if (item == null) throw new [Resource]NotFoundException(id);
        return item;
    }

    public void delete(UUID id) {
        if (store.remove(id) == null) throw new [Resource]NotFoundException(id);
    }
}

Rules

• ConcurrentHashMap — thread-safe without external locking.
• UUID.randomUUID() for every new ID — never accept IDs from the client.
• store.remove(id) == null is the idiomatic not-found check for delete.
• new ArrayList<>(store.values()) for list — returns a snapshot, safe to return.

Design Constraints

• Do not accept IDs from the request body — always generate with UUID.randomUUID()
• Do not add a ProductRepository interface — the map is the store
• Do not expose the map directly — always through service methods