CODEWITHSUNDEEP
javaconcurrencyenumssingleton
Andreas Gelever
Andreas Gelever

Reputation: 2016

Java singleton on enum and ConcurrentHashMap. Thread-safety issues?

I want to keep temporarily blocked users in a HashMap located in a singleton:

public enum BlockedUsersRepository {

    REPOSITORY;

    private final ConcurrentMap<String, User> blockedUsers = new ConcurrentHashMap<String, User>();

    /* Static 'instance' method */
    public static BlockedUsersRepository getInstance( ) {
            return REPOSITORY;
    }}

A user is getting into the map after a number of unsuccessful attempts to enter card number. I do it by launching the following methods from User class:

    public void addBlockedUser(User user) {
            blockedUsers.put(user.getEmail(),user);
    }

    public void removeBlockedUser(User user) {
            blockedUsers.remove(user.getEmail());
    }

The question is: do I undertake ENOUGH measures for thread-safety? How can I model concurrent access to the map in Terminal to make sure it really works or fails? Is it an appropriate approach at all, because I am afraid it's lame because the singletone plays a role of kinda a global variable accessible/modifiable by a number of User instances. I am new to the concurrency in Java and ask you to be lenient. Thank you.

Upvotes: 4

Views: 1443

Answers (1)

k5_
k5_

Reputation: 5558

The enum as singleton is a threadsafe way of creating one. You have final on the ConcurrentMap meaning no issues at that point.

ConcurrentHashMap can not be broken by concurrent writes.

A concurrent addBlockedUser/removeBlockedUser may lead to the user being blocked or unblocked. But imo both results are ok as you dont have a constraint saying a user has to be blocked for at least x seconds to be unblockable.

Problems that might be present but not directly threading issues.

  • Did you make sure that the email is unique? If not, only one of theses users can be blocked at the same time.
  • Did you make sure that email is not null? ConcurrentHashMap can not handle the case, and would violate uniqueness.
  • How do you handle the case somebody changes his email?

Upvotes: 2

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