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Template Method Design Pattern

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Template method design pattern is quite useful for framework designers as it provides a mechanism for users of framework to extend the framework.

Template method design pattern is quite useful for framework designers as it provides a mechanism for users of framework to extend the framework.

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  • 1. Template Method Pattern ~ Mother of all frameworks ~ Srikanth P Shreenivas [email_address] http://www.srikanthps.com
  • 2. Game Console for playing Pool Slide public class Pool { aTypicalGamingSession() { startup(); retrieveLastSavedState(); displayPoolTable(); while (not exit) { acceptUserInput(); redrawTheTable(); } printWinner() saveUserScore; updateHighScores; } }
  • 3. Game Console – More games Slide
    • public class Game {
    • aTypicalGamingSession() {
    • startup();
    • retrieveLastSavedState();
    • if (game == pool);
    • displayPoolTable();
    • if (game == chess);
    • displayChessBoard();
    • while (not exit) {
    • acceptUserInput();
        • if (game == pool);
        • reCalculatePositionOfBalls();
        • if (game == chess);
        • reCalculatePositionsOfPawns();
    • }
    • printWinner()
    • saveUserScore;
    • updateHighScores;
    • }
    • }
  • 4. Game Console – What’s common? Slide
    • Every game has following sequence
      • User login
      • Retrieve last saved state.
      • Display the game.
      • React to user controls.
      • Save the game state.
      • Print winner
    Game startGame() { retrieveLastSavedState(); displayGame(); while (not exit) { reactToUserControl(); } saveGame(); printWinner(); } Abstract displayGame(); Abstract reactToUserControl(); Delegate game specific behavior to derived classes, while controlling the main game behavior in base class. Common Behavior Game-specific Behavior Chess displayGame() { displayChessBoard(); } reactToUserControl() { moveThePiece(); } Pool displayGame() { displayPoolTable(); } reactToUserControl() { strikeWithCueBall(); }
  • 5. Template Method Pattern Slide
    • Template method defines the steps of an algorithm, and allows subclasses to provide an implementation of one or more steps.
    • The template method is used for:
    • letting subclasses implement behavior that can vary
    • avoiding duplication in the code: you look for the general code in the algorithm, and implement the variants in the subclasses
    • controlling at what point(s) sub-classing is allowed.
    ConcreteClass 2 step1() { } step3OptionalStep() { //diff. implementation } Algorithm() { step1(); step2MustAlwaysDoThis(); step3OptionalStep(); step4finalStep(); } Abstract step1(); Final step2MustAlwaysDoThis() { } step3OptionalStep() { //Default code } Final step4FinalStep() { } Base Class ConcreteClass 1 step1() { }
  • 6. Inversion of control
    • Game class calls the method of sub-classes, thus, telling the sub-classes – “Don’t call me, I will call you”
    • Template method enforces “Open Closed Principle” – A class should be open for extension, but closed for modification”
    Slide Hollywood Principle
    • Inversion of Control
    • A key differentiator between frameworks and libraries
    • Framework invokes your code.
    • You invoke library’s code.
  • 7. Servlets – A case study for IoC
    • Life Cycle
      • init()
      • service()
      • destroy()
    • service() method is an abstract method in “GenericServlet” class.
    • HttpServlet is a sub-class of “GenericServlet”.
    • HttpServlet implements “service()” method.
    • Based on HTTP request type, HttpServlet invokes
      • doGet, doPost, doHead, doPut, doDelete(), doOptions(), doTrace().
    • HttpServlet’s service method defines a template for handling HTTP requests, it calls the right methods at right time.
    • Developers extend HttpServlet and write meaninful implementation of doGet, doPost, etc.
    Slide
  • 8. Servlets – IoC and Template Method Pattern
    • We don’t control when our code is invoked.
    • Servlet container invokes our servlet’s code.
    • HttpServlet defines a template method , service(), which takes care of general purpose handling of HTTP requests by calling doGet, doPost…
    • We can extend HttpServlet by overriding the steps of the algorithm, doGet and doPost methods, to provide meaningful results.
    Slide Servlet Container’s Hollywood Principle: Don’t call me, I will call you (servlet) !!! (whenever I hear from a browser) Servlet’s Template Method: Let me have the control of algorithm and let me deal with HTTP, you (Developer) just respond with some meaningful action when I call your methods.
  • 9. “ Mother of all frameworks”….How?
    • All frameworks implement some kind of “Inversion of Control”.
      • Examples: Servets, EJBs, Web Service Frameworks, Spring
    • Frameworks, which depend on application to extend it by sub-classing some of its classes, uses template method pattern.
      • Servlet’s doGet and doPost methods
    • Frameworks which allow applications to get involved at critical points (starting of transaction, object initialization) provide hooks which can be implemented by application. This again requires use template method pattern.
      • MDB’s onMessage method
    Slide
  • 10. Ways of Extending Classes (and Frameworks)
    • Inheritance
      • Use template method design pattern when you need to extend using inheritance
    • Composition
      • Use Strategy design pattern when you want to extend using composition
    Slide
  • 11. Example of Strategy Pattern Slide Use Strategy to plug-in a different implementation. CRT print() { print on console’s monitor } PaperPrinter print() { printOnLaserPrinter(); } Game void startGame() { … printer.printWinner(); … } Printer printer; Printer print()
  • 12. Thanks