设计模式之装饰器模式

1. 装饰器模式（Decorator Pattern

1.1 基本概念

• 装饰器模式也叫作包装器模式（Wrapper Pattern），指在不改变原有对象的基础上，动态地给一个对象添加一些额外的职责
• 核心是在不改变原有类的基础上给类新增功能（类比俄罗斯套娃）
• 可以随机组合
  • 例如，通过服饰组合出一个有个性的人完全可以有无数种方案，并非固定的

1.2 四个角色

• 抽象组件（Component）是一个接口或者抽象类，定义装饰对象的行为
• 具体组件（ConcreteComponent），被装饰者
• 抽象装饰器（Decorator），持有被装饰器的引用
• 具体装饰器（ConcreteDecorator）

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1.3 适用场景

• 扩展一个类的功能
• 动态添加功能，动态撤销
• 把对象传进去，随机组合

2. 示例

2.1 装饰模式

public abstract class AbstractComponent {
    protected abstract void operation();
}

// 具体组件
public class ConcreteComponent extends AbstractComponent {
    @Override
    protected void operation() {
        System.out.println("ConcreteComponent: do something...");
    }
}

// 装饰器
public abstract class AbstractDecorator extends AbstractComponent {
    // 组件
    protected AbstractComponent component;

    public AbstractDecorator(AbstractComponent component) {
        this.component = component;
    }

    @Override
    protected void operation() {
        if (component != null) {
            component.operation();
        }
    }
}

public class ConcreteDecoratorA extends AbstractDecorator {
    public ConcreteDecoratorA(AbstractComponent component) {
        super(component);
    }

    @Override
    protected void operation() {
        System.out.println("Concrete decorator A: First");
        super.operation();
        System.out.println("Concrete decorator A: End");
    }
}

public class ConcreteDecoratorB extends AbstractDecorator {
    public ConcreteDecoratorB(AbstractComponent component) {
        super(component);
    }

    @Override
    protected void operation() {
        System.out.println("Concrete decorator B: First");
        super.operation();
        System.out.println("Concrete decorator B: End");
    }
}

@Test
public void test() {
    ConcreteComponent component = new ConcreteComponent();
    AbstractDecorator decoratorA = new ConcreteDecoratorA(component);
    decoratorA.operation();
    AbstractDecorator decoratorB = new ConcreteDecoratorB(component);
    decoratorB.operation();
}

打印结果
Concrete decorator A: First
ConcreteComponent: do something...
Concrete decorator A: End
Concrete decorator B: First
ConcreteComponent: do something...
Concrete decorator B: End

2.2 装饰器装饰具体的装饰对象

@Test
public void test2() {
    ConcreteComponent component = new ConcreteComponent();
    AbstractDecorator decoratorA = new ConcreteDecoratorA(component);
    AbstractDecorator decoratorB = new ConcreteDecoratorB(decoratorA);
    decoratorB.operation();
}

打印结果
Concrete decorator B: First
Concrete decorator A: First
ConcreteComponent: do something...
Concrete decorator A: End
Concrete decorator B: End

3. 应用

3.1 Stream IO 流

• 抽象组件：InputStream
• 被装饰者：FileInputStream, ByteArrayInputStream
• 装饰者组件：FilterInputSteam
• 具体装饰：BufferedInputStream, DataInputStream
• 示例：添加 Buffer 缓冲功能 InputStream is = new BufferedInputStream(new FileInputStream("path"));

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