【设计模式】装饰器模式

使用频率：★★★☆☆

一、什么是装饰模式

通过关联机制给类增加行为，其行为的扩展由修饰对象来决定；

如JAVA IO流里的以下形式，BufferedReader为装饰类，其关联了一个具体对象（new FileReader(new File("test.txt"))），并对其进行装饰，装饰后拥有readLine行为（方法）：

new BufferedReader(new FileReader(new File("test.txt")));

二、补充说明

与继承相似，不同点在于继承是在编译期间扩展父类，而装饰器模式在运行期间动态扩展原有对象；

或者说，继承是对类进行扩展，装饰模式是对对象进行扩展；

三、角色

抽象构件

具体构件

抽象装饰类

具体装饰类

说明：具体构件、抽象装饰类、具体装饰类的共同父类是抽象构件，具体装饰类继承抽象装饰类并在运行期间装饰具体构件；

四、例子

例子说明：

画家接口Painter，为抽象构件，有两个方法，获取画家描述信息及绘画；

PaintBeginner实现Painter接口，为具体构件；

PainterDecorator实现Painter接口，为抽象装饰类，其内部关联一个Painter对象，通过构造函数获取；

HillPainterDecorator、RiverPainterDecorator、TreePainterDecorator为具体装饰类，表明被装饰的画家能够绘画Hill、River、Tree；

类图：

代码实现：

Painter.java

package com.pichen.dp.decorator;

public interface Painter {


    public abstract String getDescription();
    
    public abstract String painting();
    
}

PaintBeginner.java

package com.pichen.dp.decorator;

public class PaintBeginner implements Painter{

    @Override
    public String getDescription() {

        return "";
    }

    @Override
    public String painting() {
        /* do nothing */
        return "";
    }

}

PainterDecorator.java

package com.pichen.dp.decorator;

public abstract class PainterDecorator implements Painter{

    private Painter decoratedPainter;
    public PainterDecorator(Painter decoratedPainter) {
        this.decoratedPainter = decoratedPainter;
    }
    
    public Painter getPainter(){
        return this.decoratedPainter;
    }
}

HillPainterDecorator.java

package com.pichen.dp.decorator;

public class HillPainterDecorator extends PainterDecorator{

    public HillPainterDecorator(Painter paper) {
        super(paper);
    }

    
    
    @Override
    public String getDescription() {
        return this.getPainter().getDescription() + "can paint hill, ";
    }

    @Override
    public String painting() {
        /* painting the hill */
        return this.getPainter().painting() + paintingHill();
    }
    
    public String paintingHill(){
        return "Hill, ";
    }

}

RiverPainterDecorator.java

package com.pichen.dp.decorator;

public class RiverPainterDecorator extends PainterDecorator{

    public RiverPainterDecorator(Painter paper) {
        super(paper);
    }

    @Override
    public String getDescription() {
        return this.getPainter().getDescription() + "can paint river, ";
    }

    @Override
    public String painting() {

        /* painting the river */
        return this.getPainter().painting() + paintingRiver();
    }
    
    public String paintingRiver(){
        return "River, ";
    }

}

TreePainterDecorator.java

package com.pichen.dp.decorator;

public class TreePainterDecorator extends PainterDecorator{

    public TreePainterDecorator(Painter paper) {
        super(paper);
    }

    @Override
    public String getDescription() {
        return this.getPainter().getDescription() + "can paint tree, ";
    }

    @Override
    public String painting() {
        /* painting the tree */
        return this.getPainter().painting() + paintingTree();
    }
    
    public String paintingTree(){
        return "Tree, ";
    }

}

Main.java

package com.pichen.dp.decorator;

public class Main {

    public static void main(String[] args) {
        
        Painter p0 = new PaintBeginner();
        System.out.println("Painter description:" + p0.getDescription());
        System.out.println("Painting:" + p0.painting() + "\n");

        HillPainterDecorator p2 = new HillPainterDecorator(new PaintBeginner());
        System.out.println("Painter description:" + p2.getDescription());
        System.out.println("Painting:" + p2.painting());
        System.out.println("Painting:" + p2.paintingHill() + "\n"); //新增的行为
        
        RiverPainterDecorator p3 = new RiverPainterDecorator(new PaintBeginner());
        System.out.println("Painter description:" + p3.getDescription());
        System.out.println("Painting:" + p3.painting());
        System.out.println("Painting:" + p3.paintingRiver() + "\n"); //新增的行为
        
        HillPainterDecorator p4 = new HillPainterDecorator(new RiverPainterDecorator(new TreePainterDecorator(new PaintBeginner())));
        System.out.println("Painter description:" + p4.getDescription());
        System.out.println("Painting:" + p4.painting());
        System.out.println("Painting:" + p4.paintingHill() + "\n"); //新增的行为

        
        

    }

}

执行结果如下，PaintBeginner类的对象未装饰前，无行为；在被装饰器装饰后，行为可以改变：

五、JAVA IO流与装饰模式

这里简单的以Reader、BufferedReader、FileReader举个例子，如下代码：

        BufferedReader br = new BufferedReader(new FileReader(new File("test.txt")));
        br.readLine();

说明：

其中BufferedReader与FileReader有一个共同抽象父类Reader，Reader为抽象构件；

new FileReader(new File("test.txt"))为具体构件，运行期间被修饰的对象；

BufferedReader为具体修饰类，运行期间修饰具体构件；

装饰后，被修饰的对象新增的行为是拥有readLine方法；

ps：查看源码，没发现BufferedReader对应的抽象装饰类，个人觉得没有抽象装饰类，装饰模式也是可以正常工作的，抽象构件（Reader）可以由具体修饰类关联；

另外，具体修饰类也可以作为基类，被其它类继承的，继承后的类同样也是具体修饰类，如LineNumberReader就是继承BufferedReader；

所以，上面语句还可以这样写（ps：只是举例，其实没必要用BufferedReader修饰，直接LineNumberReader装饰下就可以）：

        BufferedReader br = new LineNumberReader(new BufferedReader(new FileReader(new File("test.txt"))));
        br.readLine();

links：

装饰器模式★★★☆☆

生成器or建造者模式★★☆☆☆

抽象工厂模式★★★★★

工厂方法模式★★★★★

简单工厂模式★★★★☆