Template Method

模板方法模式

Template method pattern defines the sequential steps to execute a multi-step algorithm and optionally can provide a default implementation as well (based on requirements).

特征

• 一系列固定步骤的算法
• 一些步骤是不变的，一些步骤是可变的
• 不变的步骤放在抽象基类实现，可变步骤在具体实现类中实现，步骤的方法声明都在基类

UML 类图

image.png

场景

建造一个房子，一般都是有固定步骤的，一些步骤在抽象基类中实现，一些特殊的步骤在具体的特殊房子中实现。

不变步骤

• 房子打地基
• 房子建造屋顶

可变步骤

• 建造房子的墙
• 建造窗户和门
• 房子粉刷
• 内部装饰

举例

建造一个玻璃墙的房子和一个混凝土墙的房子

抽象基类

/**
 * abstract class House containing template method buildHouse and implementation
 * of steps which is same for all types of houses. Those implementations have
 * been marked as final.
 */
 
public abstract class House {
    /**
     * This is the template method we are discussing. This method should be
     * final so that other class can't re-implement and change the order of the
     * steps.
     */
    public final void buildhouse() {
        constructBase();
        constructRoof();
        constructWalls();
        constructWindows();
        constructDoors();
        paintHouse();
        decorateHouse();
    }
 
    public abstract void decorateHouse();
 
    public abstract void paintHouse();
 
    public abstract void constructDoors();
 
    public abstract void constructWindows();
 
    public abstract void constructWalls();
 
    /**
     * final implementation of constructing roof - final as all type of house
     * Should build roof in same manner.
     */
    private final void constructRoof() {
        System.out.println("Roof has been constructed.");
    }
 
    /**
     * final implementation of constructing base - final as all type of house
     * Should build base/foundation in same manner.
     */
    private final void constructBase() {
        System.out.println("Base has been constructed.");
    }
}

混凝土墙房子

public class ConcreteWallHouse extends House {
      @Override
      public void decorateHouse() {
            System.out.println(“Decorating Concrete Wall House”);
      }
      @Override
      public void paintHouse() {
            System.out.println(“Painting Concrete Wall House”);
      }
      @Override
      public void constructDoors() {
            System.out.println(“Constructing Doors for Concrete Wall House”);
      }
      @Override
      public void constructWindows() {
            System.out.println(“Constructing Windows for Concrete Wall House”);
      }
      @Override
      public void constructWalls() {
            System.out.println(“Constructing Concrete Wall for my House”);
      }
}

玻璃墙的房子

public class GlassWallHouse extends House {
    @Override
    public void decorateHouse() {
        System.out.println("Decorating Glass Wall House");
    }
 
    @Override
    public void paintHouse() {
        System.out.println("Painting Glass Wall House");
    }
 
    @Override
    public void constructDoors() {
        System.out.println("Constructing Doors for Glass Wall House");
    }
 
    @Override
    public void constructWindows() {
        System.out.println("Constructing Windows for Glass Wall House");
    }
 
    @Override
    public void constructWalls() {
        System.out.println("Constructing Glass Wall for my House");
    }
}

使用样例

public class Demo {
      public static void main(String[] args) {
 
            System.out.println(“Going to build Concrete Wall House”);
 
            House house = new ConcreteWallHouse();
            house.buildhouse();
 
            System.out.println(“Concrete Wall House constructed successfully”);
 
            System.out.println(“********************”);
 
            System.out.println(“Going to build Glass Wall House”);
 
            house = new GlassWallHouse();
            house.buildhouse();
 
            System.out.println(“Glass Wall House constructed successfully”);
      }
}

设计关键点

• 被 final 修饰的模板方法，实现类不能覆写和改变它的执行顺序
• 在抽象基类中，实现了所有基类固定不变的默认的方法，具体实现类中，不需要实现固定不变的抽象基类的方法
• 所有抽象基类的抽象方法，在具体实现类中，必须实现

普遍应用

• Non-abstract methods of InputStream, OutputStream, Reader, Writer from Java IO.
• Non-abstract methods of some abstract collection classes like AbstractList, AbstractSet, AbstractMap etc.
• BaseExecutor Class of Mybatis ，基于缓存和事务组件，提供了查询数据库的模板方法。