trouble having ultiple LockManager instances

[Holmes Wong]

Hi,
I declare and initialize a Singleton LockManager
instance in DataInterface, RemoteDataAccess and LocalDataAcccess implement DataInterface. Since
I use Singleton to control LockManager instance
creation, and variables in an interface should be public, static, and final. So, I suppose there should be one LockManager instance shared by all remote accesses. Somehow, in the client, every time I get a remote connection (which is DataInterface since RemoteDataAccess sits in server, and that for hiding the distinction of local/remote connections), a new LockManager is created. In lock/unclock methods of RemoteDataAccess, I only use the LockManager instance created in DataInterface. I am confused about this. I believe something is wrong with my design. Any suggestions appreciated.

[Sai Prasad]

Holmes,
I don't know the reason behind keeping an instance of the LockManager in the DataInterface. All you have to do is whenever you need a LockManager instance from the RemoteDataAccess, you make a call like:
LockManager.getInstance();

[Holmes Wong]

Thanks, Sai:
I did exactly like what you mentioned. Somehow,
I did some checking inside getInstance() function:
every time a client window starts up, (private static) LockManager variable got initialized. I am wondering if a Singleton instance only applied to a single VM, not for multiple VM's. Or somethig
else plays a role in this thing. Here is my code:
In LockManager class:

private static LockManager instance = null;
public static LockManager getInstance()
{
  if(instance==null) {
      System.out.println("Create new manager.")
      instance=new LockManager();
   }
   System.out.println("Old manager.")
   return instance;
}
protected LockManager(){}

And in RemoteDataAccess class, I used LockManager.getInstance() to access lock and unlock methods. Everytime a remote connection is
established, "Create new manager" will appear.
Right now I am using multiple Data instances for
multiple remote connections, but I don't think this has anything to do with it.
[ June 19, 2002: Message edited by: Holmes Wong ]

[Holmes Wong]

Could anyone tell what is wrong with my implementation or give an pointer? Thanks.

[John Smith]

Somehow, in the client, every time I get a remote connection (which is DataInterface since RemoteDataAccess sits in server, and that for hiding the distinction of local/remote connections), a new LockManager is created.

Sounds like your singleton is a singleton for a given connection only. If you want a single object to be shared by all connections, you need to create it on the server and pass a reference to it to all the connections.
In my implementation, I actually instantiate LockManager from the Data constructor.
Eugene.

[Sai Prasad]

Holmes,
The value of instance is obviously null when you call getInstance() every time from the Connection object. Check for places where you might set the instance variable to null.

[Holmes Wong]

Guys, thank you for giving pointers. I will try out these ideas.

[arun s kumar]

Holmes:
getInstance method should be synchronized.
If Lockmanager.getInstance is called in 3
different threads simultaneously there is
chance that all 3 see instance initialized to
null and all 3 can create new objects.
This is called "Lazy initialization" problem.