Singleton …there can be only one…

The Singleton pattern In some cases, we only want one instance of a class to be created…ever! Could be a class which manages access to some critical resource A database manager A device driver A manager of application preferences … RHS – SWC

The Singleton pattern How can we implement such a restriction? Should we throw an exception, if someone tries to create a second instance…? DBManager dbm1 = new DBManager(); // OK ... DBManager dbm2 = new DBManager(); // Error! Not really what we want… RHS – SWC

The Singleton pattern Clients Creator DBManager Please give me a reference to the (only) DBManager object OK OK (hey, I already have it! DBManager Please give me a reference to the (only) DBManager object RHS – SWC

The Singleton pattern A client should not be able to create an object directly (new …) – he must ask a creator for it Creator creates an object (unless it has already been created), and hands a reference back to the client How do we prevent clients from creating objects themselves…? RHS – SWC

The Singleton pattern public class DBManager { public DBManager() {} … RHS – SWC

The Singleton pattern public class DBManager { private DBManager() {} … } RHS – SWC

The Singleton pattern Private methods can only be invoked by other methods in the same class The class itself becomes respon-sible for creating objects of itself… RHS – SWC

The Singleton pattern public class DBManager { private DBManager() {} public DBManager createInstance() return new DBManager(); } RHS – SWC

The Singleton pattern public class DBManager { private static DBManager theInstance = null; private DBManager() {} public static DBManager getInstance() if (theInstance == null) theInstance = new DBManager(); return theInstance; } RHS – SWC

The Singleton pattern Turning a class T into a Singleton: Make the constructor for T private Add a static variable to T, storing a reference to an object of class T Add a static method to T, which returns a reference to the object pointed to by the static variable. If the reference is null, the method should create an object of type T first, and set the static variable to refer to this object RHS – SWC

The Singleton pattern public class T // T is a Singleton { private static T theInstance = null; private T() {} public static T getInstance() if (theInstance == null) theInstance = new T(); return theInstance; } RHS – SWC

Singleton vs global variables But wait – haven’t we just made a nice wrapping for a global variable? Why not just do like this: public class T { public static T theInstance = new T(); private T() {} ... } RHS – SWC

Singleton vs global variables Issue Singleton Global variable Creation time On demand Initial Access control Possible None Pseudo-uniqueness Destruction order Unpredictable RHS – SWC

Singleton vs global variables A global variable is created when the program starts (eager initialisation) A Singleton is created when somebody asks for it (lazy initialisation) If we only need the object sometimes, it is wasteful to create it unconditionally – in particular if it is resource-intensive RHS – SWC

Singleton destruction We can control when a Singleton is created, but not when it is destroyed Garbage collector should claim it when noone refers to it anymore Order of destruction might matter! RHS – SWC

Singleton destruction Suppose two singletons DataManager and DBManager are dependent: DataManager DBManager Creation Load app data, using DBManager Connect to DB Destruction Save app data, using DBManager Disconnect from DB RHS – SWC

Singleton destruction If the DBManager is destroyed before the DataManager, there might be a problem! Somewhat difficult to manage the dependen-cies between singletons, but it can be done… RHS – SWC

Singleton– final remarks Singleton is a useful – but also debated design pattern (see the Net) Using Singletons in multi-threaded scenarios requires some enhancements You cannot make a ”generic” Singleton class, and let your class inherit from it – who ”owns” the static object reference…? RHS – SWC