CODEWITHSUNDEEP
javamultithreadingconcurrencyreentrantreadwritelock
Sprint21
Sprint21

Reputation: 89

ReentrantReadWriteLock blocking me from act on an object?

I have the following object:

    public class DataStructures {
    public Map<String, User> registeredUser;
    private ReadWriteLock readWriteLock;

    public DataStructures() {
        this.registeredUser = new ConcurrentHashMap<>();
        readWriteLock = new ReentrantReadWriteLock(true);
    }

    public ReadWriteLock getReadWriteLock() {
        return readWriteLock;
    }
}

I Have the following block:

    if(user != null && user.status()) {
            String userNameList = "";
            dataStructures.getReadWriteLock().readLock().lock();//ReentrantReadWriteLock
            List<User> userList = new ArrayList<>(dataStructures.registeredUser.values());
            //Will perform a sort on the userlist according their initial registration number
            Collections.sort(userList, Comparator.comparing(User::getRegistrationId));
                for (User tmpUser : userList) {
                    userNameList += tmpUser.getUsername() + " ";
                }
                connections.send(connectionId, "10 " + "7 " + userList.size() + " " + userNameList);
                dataStructures.getReadWriteLock().readLock().unlock();//ReentrantReadWriteLock
}

As you can see I locked part of this code in order to let thread read from this code at the time that no one try to write something.

also, I have the following code:

dataStructures.getReadWriteLock().writeLock().lock();//ReentrantReadWriteLock
    if(dataStructures.registeredUser.putIfAbsent(username, new User(username, password, dataStructures.registraionId)) == null) {
            connections.send(connectionId, "10 " + "01");
            dataStructures.registraionId++;
    }
    else
        connections.send(connectionId, "11 " + "01");
    dataStructures.getReadWriteLock().writeLock().unlock();//ReentrantReadWriteLock

Now, I know that when the first block of code will happen the second will wait for the first to finish, my question is if in the next block of code I will be able to read from the dataStructure object at the same time that someone will write or read from him despite I didn't mentioned any of the ReentrantReadWriteLock locks:

String username = seperate[1];
    String password = seperate[2];
    User user = dataStructures.registeredUser.get(username);

Thank you in advance!

Upvotes: 0

Views: 49

Answers (1)

HPCS
HPCS

Reputation: 1454

Yes, you will be able to read from the dataStructure object at the same time someone will write or read holding the lock, because you are using thread-safe ConcurrentHashMap for the registeredUser.

If you would use normal HashMap for the registeredUser, you would be still allowed to read from the registeredUser but you would see incorect/stale data because normal HashMap is not thread-safe and you are accessing it without the lock.

Upvotes: 1

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