Java locking of elements inside array

Question

Hello

I have dynamic array of hashtables

Can I use synchronized for each of them separately? Like synchronized(array[1]) { code .. }, synchronized(array[2]) { code .. }

Thanks

Answer 1

Sure, but it's better to use a concurrent map os a concurrent skip list for throughput concerns, if you can.

BTW, if you provide us a bit of context, we can suggest you a (maybe) better data organization and structure.

Answer 2

You can certainly synchronize on the object in a particular position in the array writing:

synchronized (arr[x]) {
  ...
}

However, just be careful to make sure you understand whether this is doing what you want it to do.

This will lock on the particular object referenced by arr[x]. However, it won't buy you any thread-safety in terms of accesses to the array itself-- in other words, for example:

• while you're locking on the object at arr[x], another thread could still potentially change which object is at arr[x];
• if two threads simultaneously access the same position of arr (either to read which hash map/object is there, or to set a new one), there'll be a race condition.

I also tend to agree with akappa -- what you're doing sounds slightly unusual, and it might be better to rephrase your question as "what data structure do I need in order to do X" rather than assuming that an array of hashmaps is the appropriate one from the outset?