【策略模式】模拟Struts选择策略决策转移

目标：我们需要一个计算价格的类，有时候会打折

1.我们先设计一个接口

public interface PriceCalculationStrategy {
		
	public double calculatePrice(double price , double copies);
	
	public boolean match(double price , double copies);
}

2.我们有一些计算实现

public class DefaultDiscountPriceCalStrategy implements PriceCalculationStrategy{
	public double rate = 0.1;
	
	public double calculatePrice(double price, double copies) {
		return price * copies * rate; 
	}

	@Override
	public boolean match(double price, double copies) {
		return false;		//这是不可能的
	}

}

public class NoDiscountPriceCalStrategy implements PriceCalculationStrategy{
	
	public double calculatePrice(double price, double copies) {
		return price * copies;
	}

	public boolean match(double price, double copies) {
		return price < 300;			
	}

}

public class DiscountPriceCalStrategy implements PriceCalculationStrategy{
		
	public double rate ;
	
	public double calculatePrice(double price, double copies) {
		return price * copies * rate;
	}

	public void setDiscount(double rate) {
		this.rate = rate;
	}

	public boolean match(double price, double copies) {
		boolean match = false;		
		if(price >= 300 && price <1000){
			setDiscount(0.9);
			match = true;
		}else if( price >=1000){
			setDiscount(0.8);
			match = true;
		}		
		return match;
	}
}

3.计算工具类，发现我们根本不要自己判断，每个策略自己判断执不执行

public class PriceCalculationUtil {
			
	public static List<PriceCalculationStrategy> priceCalculationStrategys = new ArrayList<PriceCalculationStrategy>();
	
	static{ //等价于  容器内查找所有策略
		PriceCalculationStrategy pcs = new NoDiscountPriceCalStrategy();		
		PriceCalculationStrategy pcs1 = new DiscountPriceCalStrategy();		
		DefaultDiscountPriceCalStrategy defaultD =new DefaultDiscountPriceCalStrategy();  
		priceCalculationStrategys.add(pcs);
		priceCalculationStrategys.add(pcs1);
		priceCalculationStrategys.add(defaultD);
	}
	
	public static double priceCalculation(double price , double copies) {				
		
		for (PriceCalculationStrategy strategy : priceCalculationStrategys) {
			 if( strategy.match(price, copies)){
				 return strategy.calculatePrice(price, copies);
			 }				
		}
		
		//优先其他策略，我不会告诉你我打的广告“ 1折 商品  ” 是假的，你看我有这个策略的
		DefaultDiscountPriceCalStrategy defaultD = new DefaultDiscountPriceCalStrategy();  
		return defaultD.calculatePrice(price, copies);
	}			
}

4.测试

public class PriceCalculationUtilTest {

	@Test
	public void testPriceCalculation(){		
		assertEquals(200,PriceCalculationUtil.priceCalculation(100, 2),0);
		assertEquals(360,PriceCalculationUtil.priceCalculation(400, 1),0);
		assertEquals(800,PriceCalculationUtil.priceCalculation(1000, 1),0);		
	}	
}

策略模式在Struts2中主要作用位置：

1.加载多配置，默认的，插件的，应用的；

2.用于插件模式，插件就好比如其中的一个策略。每个策略选择的地方，就是Struts2的一个扩展点。