Pattern Reminder: Singleton

Definition: The pattern is used to ensure that only one instance of an class is used. It offers a global point of access to this object. Before Java5 consider double checked locking to be thread-safe.

Example: Lets think of using only one connection to whatever throughout the program.

Connection:

public class Connection {
//volatile !!!!
private static volatile Connection connection;
//private Constructor
private Connection(){}
public static Connection getInstance(){
if (connection == null)//check first
{
synchronized(Connection.class) //one access per thread. sync only once
{
if (connection == null) //check again
return new Connection();
}
}
return connection;
}
}