Passing initialization parameters to a singleton

Question

So here's the classic Singleton code:
public class MySingleton {
private static MySingleton _instance;
private MySingleton() {
// construct object . . .
public static synchronized MySingleton getInstance() {
if (_instance==null) {
_instance = new MySingleton();
return _instance;
What I can't find any discussion of is: what do you do if that "// construct object . . ." bit requires parameters (e.g. the username and password for the DB connection)?
Obviously, adding those parameters to the getInstance() method would be a bad idea so some kind of init() method is probably required. I've used that approach in simple cases so the calling sequence is
MySingleton foo = MySingleton.getInstance();
foo.init(parameterObject);
That's fine so long as the initialization is simple and I can keep track of where the singleton gets used and just be careful not to use it before init() gets called. But if the initialization involves setting up a DB connection, how do I arrange the code so (a) most of my application can just call getInstance() without worrying about error situations (b) the code that does set up the connection can do so reliably and provide detailed analysis when something does go wrong and (c) the app doesn't just crash when some other idiot calls getInstance() without first calling init(), or calls init() multiple times with different parameters? Or does that last possibility mess up the singleton so badly I should reject the init() idea and take a different approach entirely?
Thanks,
Graeme

Answer 1

Hi Graeme! I guess I misunderstood what you wanted to do. I was under the impression that the necessary data (eg. DB connection parms) was not dependent upon user input.
Based on what you've described, are you sure you need a singleton? Here's a possible implementation. If there are items that are constant (for the most part anyway) such as database names, connection strings, etc., those can be located in a properties file which is read once, when the singleton instance is created. Now, let's say one of the variables is a user id & password, which are supplied by a given user. Then, your implementation could look somewhat like the following:
public class DBAccess
   private static DBAccess m_instance;
   private DBAccess()
      // Read properties file and store necessary data
   public static DBAccess getInstance()
      // Standard lazy initialization here
   public DBConnection getConnection( String userId, String password )
      // Get actual DB connection using the supplied arguments
   // Any other required methods
} // class DBAccessNote that the getConnection() method above is an instance method, not a class method.
The above code presents a single, well-known point of access to a database. You can add any other instance methods you deem necessary. Note that some instance methods may need to be declared with the synchronized keyword, based upon the data it handles to avoid concurrency problems.
Hope this helps!
Cheers!
-- Amol.