1. Singleton
…there can be only one…

2. 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 …
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3. 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…
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4. 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
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5. 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…?
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6. The Singleton pattern public class DBManager { public DBManager() {} …
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7. The Singleton pattern public class DBManager { private DBManager() {}… }
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8. 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…
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9. The Singleton pattern public class DBManager { private DBManager() {}
public DBManager createInstance()
return new DBManager(); }
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10. 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; }
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11. 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
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12. 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; }
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13. 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() {}
... }
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14. Singleton vs global variables
Issue
Singleton
Global variable
Creation time
On demand
Initial
Access control
Possible
None
Pseudo-uniqueness
Destruction order
Unpredictable
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15. 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
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16. 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!
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17. 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
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18. 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…
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19. 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…?
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