Simple Java name based locks?

Asked 2011-04-12T18:26:31.593

MySQL has a handy function:

SELECT GET_LOCK("SomeName")

This can be used to create simple, but very specific, name-based locks for an application. However, it requires a database connection.

I have many situations like:

someMethod() {
    // do stuff to user A for their data for feature X
}

It doesn't make sense to simply synchronize this method, because, for example, if this method is called for user B in the meantime, user B does not need to wait for user A to finish before it starts, only operations for the user A and feature X combination need to wait.

With the MySql lock I could do something like:

someMethod() {
    executeQuery("SELECT GET_LOCK('userA-featureX')")
    // only locked for user A for their data for feature X
    executeQuery("SELECT RELEASE_LOCK('userA-featureX')")
}

Since Java locking is based on objects, it seems like I would need to create a new object to represent the situation for this lock and then put it in a static cache somewhere so all the threads can see it. Subsequent requests to lock for that situation would then locate the lock object in the cache and acquire its lock. I tried to create something like this, but then the lock cache itself needs synchronization. Also, it is difficult to detect when a lock object is no longer being used so that it can be removed from the cache.

I have looked at the Java concurrent packages, but nothing stands out as being able to handle something like this. Is there an easy way to implement this, or am I looking at this from the wrong perspective?

Edit:

To clarify, I am not looking to create a predefined pool of locks ahead of time, I would like to create them on demand. Some pseudo-code for what I am thinking of is:

LockManager.acquireLock(String name) {
    Lock lock;  

    synchronized (map) {
        lock = map.get(name);



        
            2 Answers
            
            
                
                    
                        
                            
                            
                        
                        18
                        
                            
                            
                        
                        
                    
                    
                        
                        maybe this is useful for you: jkeylockmanager

Edit:

My initial response was probably a bit short. I am the author and was faced with this problem several times and could not find an existing solution. That's why I made this small library on Google Code.

                        
                        
                            answered 2012-10-21T15:05:38.037
                            
                        
                        
                            
                                
                                
                            
                        
                        
                            
                            
                            
                                
                                
                            
                            
                        
                    
                
            
            
            
                
                    
                        
                            
                            
                        
                        -2
                        
                            
                            
                        
                        
                    
                    
                        
                        TreeMap because in HashMap size of inner array can only increase 

public class Locker<T> {
    private final Object lock = new Object();
    private final Map<T, Value> map = new TreeMap<T, Value>();

    public Value<T> lock(T id) {
        Value r;
        synchronized (lock) {
            if (!map.containsKey(id)) {
                Value value = new Value();
                value.id = id;
                value.count = 0;
                value.lock = new ReentrantLock();
                map.put(id, value);
            }
            r = map.get(id);
            r.count++;
        }
        r.lock.lock();
        return r;
    }

    public void unlock(Value<T> r) {
        r.lock.unlock();
        synchronized (lock) {
            r.count--;
            if (r.count == 0)
                map.remove(r.id);
        }
    }

    public static class Value<T> {

        private Lock lock;
        private long count;
        private T id;
    }
}


                        
                        
                            answered 2012-11-21T19:45:54.943
                            
                        
                        
                            
                                
                                
                            
                        
                        
                            
                            
                            
                                
                                
                            
                            
                        
                    
                
            
            
        

        
        
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