Cache the results of two methods in a single cache in Spring

Question

I have two methods, the first returning a list of elements and the second returning a single element:

List<User> getUsersFromExternalSystem(List<Integer> userIds);
User getUserFromExternalSystem(Integer userId);

I would like Spring to cache the results of these two methods, so that when the list of elements method ( getUsersFromExternalSystem() ) is called it caches the results for the provided ids ( userIds ) and when the single element method ( getUserFromExternalSystem() ) is called with the id previously provided to the list of elements method it uses the cache.

I can simply apply @Cacheable to these methods, then (if I understand correctly) when I call:

getUsersFromExternalSystem(Arrays.asList(1, 2))

the results will be cached but when I call

getUserFromExternalSystem(1);

the cache will not be used. How this be done in Spring?

Answer 1

You can use following approach. Only first method getUser(Integer id) is cacheable, and second method just combines the results of getUser invocations.

@Service
@Scope(proxyMode = ScopedProxyMode.TARGET_CLASS)
class UserService {

    private final UserService self;

    @Autowired
    public UserService(UserService userService) {
        self = userService;
    }

    @Cacheable(cacheNames = "users", key = "id") // assuming that you've already initialized Cache named "users"
    public User getUser(Integer id) {
        return new User(); // ... or call to some DataSource
    }

    public List<User> getUsers(List<Integer> ids) {
        List<User> list = new ArrayList<>();
        for (Integer id : ids) {
            list.add(self.getUser(id));
        }
        return list;
    }
}

The trick with injecting a bean into himself and calling

self.getUser(id) instead of this.getUser(id)

is required because @Cacheable will be actually invoked only when used on a Spring proxied bean, and this is not a proxy. More details here Transactions, Caching and AOP: understanding proxy usage in Spring