行为型模式-模板方法模式(Template Method)

一：定义:

Template Method:Define the skeleton of an algorithm in an operation, deferring some steps to subclasses. Template Method lets subclasses redefine certain steps of an algorithm without changing the algorithm's structure.

二：引入

一个咖啡店，卖咖啡和茶。

制作咖啡的步骤是：

1. 把水煮开。
2. 放入咖啡。
3. 把煮好的咖啡倒进杯子。
4. 加糖或奶。 

制作茶的步骤是：

1. 把水煮开。
2. 放入茶叶。
3. 把茶倒进茶杯。
4. 加柠檬或其他调料。

public class Coffee  ...{
    public  void prepareRecipe()
    ...{
        boilWater();
        brew();
        pourInCup();
        addCondiments();
    }
    void boilWater()
    ...{
        System.out.println("boil water");
    }
    public void brew()
    ...{
        System.out.println("brew coffee");
    }

    void pourInCup()
    ...{
        System.out.println("pour in cup");
    }
    public void addCondiments()
    ...{
        System.out.println("add sugar");
    }
}

public class Tea  ...{
    public  void prepareRecipe()
    ...{
        boilWater();
        brew();
        pourInCup();
        addCondiments();
    }
    void boilWater()
    ...{
        System.out.println("boil water");
    }
    public void brew()
    ...{
        System.out.println("brew tea");
    }
    void pourInCup()
    ...{
        System.out.println("pour in cup");
    }
    public void addCondiments()
    ...{
        System.out.println("add lemon");
    }
}

我们发现，制作两种饮料的方法有些地方是类似的，我们可以对这两个类进行抽象。

public abstract class CaffeineBeverage ...{
    public final void prepareRecipe()
    ...{
        boilWater();
        brew();
        pourInCup();
        addCondiments();
    }
    void boilWater()
    ...{
        System.out.println("boil water");
    }
    
    void pourInCup()
    ...{
        System.out.println("pour in cup");
    }
    
     abstract void brew();
     abstract void addCondiments();
}

public class Coffee extends CaffeineBeverage ...{
    public void brew()
    ...{
        System.out.println("brew coffee");
    }

    public void addCondiments()
    ...{
        System.out.println("add sugar");
    }
}

public class Client ...{
    public static void main(String[] args) ...{
        CaffeineBeverage beverage=new Coffee();
        beverage.prepareRecipe();

    }
}

父类提供了一个按步骤执行方法的模板，公共部分已经在父类实现，其它由具体子类自己实现。 

三：结构

    

四：实际应用

1.   javx.servlet.http.HttpServlet

HttpServlet代码,doGet等方法由子类实现。

public abstract class HttpServlet
extends GenericServlet
implements Serializable
...{

protected void service(HttpServletRequest request,HttpServletResponse response)
    throws ServletException, IOException
  ...{
    if(request.getMethod().equals("GET") )
      ...{
    if(testConditional(request,response))
      doGet(request,response);
      }
    else if (request.getMethod().equals("HEAD") )
      ...{
    if(testConditional(request,response))
      doHead(request,response);
      }
    else if (request.getMethod().equals("POST") )
      ...{
    doPost(request,response);
      }
    else if (request.getMethod().equals("DELETE") )
      ...{
    doDelete(request,response);
      }
    else if (request.getMethod().equals("OPTIONS") )
      ...{
    doOptions(request,response);
      }
    else if (request.getMethod().equals("PUT") )
      ...{
    doPut(request,response);
      }
    else if (request.getMethod().equals("TRACE") )
      ...{
    doTrace(request,response);
      }
    else
      ...{
    response.sendError(HttpServletResponse.SC_BAD_REQUEST,"Method ""
               +request.getMethod()+"" is not supported by this servlet");
      }
  }
  
   protected void doGet(HttpServletRequest request,HttpServletResponse response)
    throws ServletException, IOException
  ...{
    response.sendError(HttpServletResponse.SC_BAD_REQUEST,
               "Method "GET" is not supported by this servlet");
  }

 
}





  

一个例子:

public class HelloServlet extends HttpServlet ...{
   public void doGet(HttpServletRequest request,
                  HttpServletResponse response)
     throws ServletException, IOException
   ...{
     response.setContentType("text/html");
     PrintWriter out = response.getWriter();
     out.println("Hello, World");
     out.close();
   }
 }

 

 五：适用情形

The Template Method pattern should be used

• to implement the invariant parts of an algorithm once and leave it up to subclasses to implement the behavior that can vary.

   

• when common behavior among subclasses should be factored and localized in a common class to avoid code duplication. This is a good example of "refactoring to generalize" as described by Opdyke and Johnson [OJ93]. You first identify the differences in the existing code and then separate the differences into new operations. Finally, you replace the differing code with a template method that calls one of these new operations.

   

• to control subclasses extensions. You can define a template method that calls "hook" operations (see Consequences) at specific points, thereby permitting extensions only at those points.

参考文献：

1:阎宏，《Java与模式》，电子工业出版社
2:Eric Freeman & Elisabeth Freeman，《Head First Design Pattern》，O'REILLY
3:GOF,《designpatterns-elements.of.reuseable.object-oriented.software》