(面向对象的设计)Part4Elaboration Iteration 2 More Patterns.ppt

26.4. Factory Data-driven Design 26.5. Singleton (GoF) Who creates the factory itself, and how is it accessed? Only one instance of the factory is needed within the process. The methods of this factory may need to be called from various places in the code, Different places need access to the adapters for calling on the external services. How to get visibility to this single ServicesFactory instance? 26.5. Singleton (GoF) Problem: Exactly one instance of a class is allowed (singleton). Objects need a global and single point of access. Solution: Define a static method of the class that returns the singleton. 26.5. Singleton (GoF) 26.5. Singleton (GoF) Implementation and Design Issues Lazy initialization public static synchronized ServicesFactory getInstance() { if ( instance == null ) { // critical section if multithreaded application instance = new ServicesFactory(); } return instance; } 26.5. Singleton (GoF) Eager initialization public class ServicesFactory { // eager initialization private static ServicesFactory instance = new ServicesFactory(); public static ServicesFactory getInstance() { return instance; } // other methods... } 26.5. Singleton (GoF) 26.5. Singleton (GoF) Why not make all the service methods static methods of the class itself Instance-side methods permit subclassing and refinement of the singleton class into subclasses; Most object-oriented remote communication mechanisms (for example, Javas RMI) only support remote-enabling of instance methods. The instance-side solution offers flexibility. A class is not always a singleton in all application contexts. It is common to start off a design thinking the object will be a singleton, and then discovering a need for multiple instances in the same process. 26.6. Conclusion of the External Services with Varying Interfaces Proble