Best Practice: passing properties to a Singleton

Question

Hi,
I have an object in my app that must only exist once. It is a DatabasConnectionPool. I would like to implement it as a Singleton and access it from anywhere in the app with
DatabaseConnectionPool dbcp = DatabaseConnectionPool.getInstance();
But the thing is that I would like to pass it some settings: number of connections to the database, username and password, ...
If I had a constructor for the class I would pass a Properties object with the constructor containig all the values but how do I do it with a Singleton?
Right now I have:
class Singleton {
private static Properties _appProps = null;
private static Singleton _instance = null;
static public void setProperties(Properties appProps) {
_appProps = appProps;
static public Singleton getInstance() {
if (null == _instance) {
instance = new Singleton(appProps);
private Singleton(Properties props) {
So, the Launch code for the application reads the .properties files and calls:
DatabaseConnectionPool.setProperties(props);
WebServer.setProperties(props);
LDAPServer.setProperties(props);
Thanks a lot for your time guys!
Rafa

Answer 1

You don't need synchronization at all if you do it the way a previous post suggested:
private static Singleton instance = new Singleton();
public Singleton getInstance(){
    return instance;
}This is lazy initialization by default because the singleton instance will only be created when the class is loaded into memory. It will be loaded into memory when you first call getInstance() so it's already using lazy initialization.
The properties question seems to have several options
1. Use your setProperties method to pass in dynamic properties. This will potentially make your singleton behave differently between uses if you change its properties while it's running.
2. Hardcode the properties inside the singleton. This doesn't allow dynamic changes.
3. Your connection pool is probably dishing out Connection objects and creating new ones when needed right? So, put the Connection creation in a different class called ConnectionFactory and pass in the factory to the singleton. This way you can customize the kind of connections to make in the factory without changing the singleton code. All the singleton cares about is saying factory.createConnection() and returning it.