Spring5设计模式 - 策略模式

冷不防 2022-10-25 11:29 210阅读 0赞

# 设计模式 - 策略模式 #

### \-----------目录直通车----------- ###

*  设计模式 - 策略模式
 *   *  一、用策略模式实现选择支付方式的业务场景
     *  二、支付案例
     *  三、优缺点
     *  四、简单实现Spring中的 DispatcherServlet

**策略模式**：它定义了算法的家族，分别封装起来，让他们之前可以相互替换，此模式让算法的改变，不会影响到算法的客户。

适用场景：

1.  假如系统中有很多类,而他们的区别仅仅在于他们的行为不同.
2.  一个系统需要动态地在几种算法中选择一种.

## 一、用策略模式实现选择支付方式的业务场景 ##

在商城系统中，常常存在多种策略的优惠活动，比如拼团、返现促销、优惠券抵扣。下面是一个模拟这个场景的demo：

促销策略的接口 PromotionStrategy：

public interface PromotionStrategy { 
        void doPromotion();
    }

优惠券抵扣策略 CouponStrategy：

public class CouponStrategy implements PromotionStrategy { 
        @Override
        public void doPromotion() { 
            System.out.println("领取优惠券，课程价格直接抵扣优惠券面值");
        }
    }

返现促销策略 CashbackStrategy：

public class CashbackStrategy implements PromotionStrategy { 
        @Override
        public void doPromotion() { 
            System.out.println("返现促销，返回现金到支付宝账户");
        }
    }

拼团策略 GroupBuyStrategy：

public class GroupBuyStrategy implements PromotionStrategy { 
        @Override
        public void doPromotion() { 
            System.out.println("拼团，满20人，全团享受团价");
        }
    }

当然还有一个无优惠活动策略 EmptyStrategy：

public class EmptyStrategy implements PromotionStrategy { 
        @Override
        public void doPromotion() { 
            System.out.println("无促销活动");
        }
    }

促销活动方案类 PromotionActivity：

public class PromotionActivity { 
        private PromotionStrategy promotionStrategy;
    
        public PromotionActivity(PromotionStrategy promotionStrategy) { 
            this.promotionStrategy = promotionStrategy;
        }
    
        public void execute() { 
            promotionStrategy.doPromotion();
        }
    }

测试类：

public class PromotionTest { 
        public static void main(String[] args) { 
            PromotionActivity activity618 = new PromotionActivity(new CashbackStrategy());
            PromotionActivity activity1111 = new PromotionActivity(new CouponStrategy());
            activity618.execute();
            activity1111.execute();
        }
    }

![优惠策略UML][UML]

从测试类来看，一次开启了多项优惠活动，但我们通常并不会一次性执行多种优惠，所以进行改造：

public class PromotionTest { 
        public static void main(String[] args) { 
            PromotionActivity activity = null;
            String promotionKey = "COUPON";
            if ("COUPON".equals(promotionKey)) { 
                activity = new PromotionActivity(new CouponStrategy());
            } else if ("CASHBACK".equals(promotionKey)) { 
                activity = new PromotionActivity(new CashbackStrategy());
            } else { 
                activity = new PromotionActivity(new EmptyStrategy());
            }
            activity.execute();
        }
    }

这样就满足了业务需求，但是随着优惠活动的增多，判断逻辑越来越复杂，编写好活动代码后需要重复测试，会大大增加工作量。那么我们可以结合工厂模式和单例模式来对此进行优化：

新增活动策略工厂类 PromotionStrategyFactory ：

public class PromotionStrategyFactory { 
        private static final Map<String, PromotionStrategy> PROMOTION_STRATEGY_MAP = new HashMap<>();
    
        private static final PromotionStrategy NON_PROMOTION = new EmptyStrategy();
    
        static { 
            PROMOTION_STRATEGY_MAP.put(PromotionKey.COUPON, new CouponStrategy());
            PROMOTION_STRATEGY_MAP.put(PromotionKey.CASHBACK, new CashbackStrategy());
            PROMOTION_STRATEGY_MAP.put(PromotionKey.GROUPBUY, new GroupBuyStrategy());
        }
    
        private interface PromotionKey { 
            String COUPON = "COUPON";
            String CASHBACK = "CASHBACK";
            String GROUPBUY = "GROUPBUY";
        }
    
        public PromotionStrategyFactory() { 
        }
    
        public static PromotionStrategy getPromotionStrategy(String promotionKey) { 
            PromotionStrategy promotionStrategy = PROMOTION_STRATEGY_MAP.get(promotionKey);
            return promotionStrategy == null ? NON_PROMOTION : promotionStrategy;
        }
    }

测试类

public class PromotionTest { 
        public static void main(String[] args) { 
            String promotionKey = "COUPON";
            PromotionStrategy strategy = PromotionStrategyFactory.getPromotionStrategy(promotionKey);
            PromotionActivity activity = new PromotionActivity(strategy);
            activity.execute();
        }
    }

如此一优化，直接删除了逻辑判断，每次新增活动时都会轻松很多。再举一个支付的案例，如果对接过银联、百度收银台、京东、QQ、支付宝、微信等支付的朋友看了下面这个案例，绝对深有体会。

## 二、支付案例 ##

所有的支付方式都有 名称、支付、查询余额等行为，抽取成一个支付方式的抽象类 Payment：

public abstract class Payment { 
    
        public abstract String getName();
    
        //通用逻辑放到抽象类里面实现
        public R pay(String uid, double amount) { 
            //余额是否足够
            if (queryBalance(uid) < amount) { 
                return new R(500, "支付失败", "余额不足");
            }
            return new R(200, "支付成功", "支付金额" + amount);
        }
    
        protected abstract double queryBalance(String uid);
    }

R 是一个统一返回类

public class R { 
        private int code;
        private Object data;
        private String msg;
    
        public R(int code, String msg, Object data) { 
            this.code = code;
            this.data = data;
            this.msg = msg;
        }
    
        @Override
        public String toString() { 
            return "{" +
                    "code=" + code +
                    ", data=" + data +
                    ", msg='" + msg + '\'' +
                    '}';
        }
    }

AliPay

public class AliPay extends Payment { 
        public String getName() { 
            return "支付宝";
        }
    
        protected double queryBalance(String uid) { 
            return 900;
        }
    }

JDPay

public class JDPay extends Payment { 
        public String getName() { 
            return "京东白条";
        }
        protected double queryBalance(String uid) { 
            return 500;
        }
    }

UnionPay

public class UnionPay extends Payment { 
        @Override
        public String getName() { 
            return "银联支付";
        }
        @Override
        protected double queryBalance(String uid) { 
            return 120;
        }
    }

WeChatPay

public class WeChatPay extends Payment { 
        @Override
        public String getName() { 
            return "微信支付";
        }
        @Override
        protected double queryBalance(String uid) { 
            return 263;
        }
    }

订单类 Order

public class Order { 
        private String uid;
        private String orderId;
        private double amount;
    
        public Order(String uid, String orderId, double amount) { 
            this.uid = uid;
            this.orderId = orderId;
            this.amount = amount;
        }
    
        public R pay() { 
            return pay(PayStrategy.DEFAULT_PAY);
        }
    
        // 完美解决 if...else 或者 switch
        public R pay(String payKey) { 
            Payment payment = PayStrategy.get(payKey);
            System.out.println("欢迎使用" + payment.getName());
            System.out.println("本次交易金额为" + amount + "，开始扣款");
            return payment.pay(uid, amount);
        }
    }

PayStrategy

import java.util.HashMap;
    import java.util.Map;
    
    public class PayStrategy { 
        public static final String ALI_PAY = "AliPay";
        public static final String JD_PAY = "JdPay";
        public static final String WECHAT_PAY = "WeChatPay";
        public static final String UNION_PAY = "UnionPay";
        public static final String DEFAULT_PAY = ALI_PAY;
    
        private static Map<String, Payment> strategy = new HashMap<String, Payment>();
    
        static { 
            strategy.put(ALI_PAY, new AliPay());
            strategy.put(JD_PAY, new JDPay());
            strategy.put(WECHAT_PAY, new WeChatPay());
            strategy.put(UNION_PAY, new UnionPay());
        }
    
        public static Payment get(String payKey) { 
            if (!strategy.containsKey(payKey)) { 
                return strategy.get(DEFAULT_PAY);
            }
            return strategy.get(payKey);
        }
    }

测试类 Test

public class PayTest { 
        public static void main(String[] args) { 
            Order order = new Order("1", "123456789123", 325);
            R result = order.pay(PayStrategy.ALI_PAY);
    
            System.out.println(result);
        }
    }

## 三、优缺点 ##

优点：

1、策略模式符合开闭原则。

2、避免使用多重条件转移语句，如if…else…语句、switch 语句

3、使用策略模式可以提高算法的保密性和安全性。

缺点：

1、客户端必须知道所有的策略，并且自行决定使用哪一个策略类。

2、代码中会产生非常多策略类，增加维护难度。

## 四、简单实现Spring中的 DispatcherServlet ##

我们常用的两种架构风格：REST和RPC，REST API 的每个URI对应的都是某一份资源，在我们通过Spring的Controller的URI进行访问资源时，会经过DispatchServlet分发给对应的Servlet。在J2EE的标准中，每一个URL对应一个Servlet。

**会员控制器**

public class MemberController { 
        public Object getMemberById(int id) { 
            return null;
        }
    }

**订单控制器：**

public class OrderController {
        public Object getOrderById(int id) {
            return null;
        }
    }

**DispatcherServlet**

/** * SpringMVC的委派模式就是通过DispatcherServlet将请求的URI转为不同的java代码 */
    public class DispatcherServlet extends HttpServlet { 
    
        private List<Handler> handlerMapping = new LinkedList<>();
    
        @Override
        public void init() throws ServletException { 
            try { 
                handlerMapping.add(new Handler()
                .setUri("/web/getMemberById")
                .setController(MemberController.class.newInstance())
                .setMethod(MemberController.class.getMethod("getMemberById", int.class)));
               
                handlerMapping.add(new Handler()
                .setUri("/web/getOrderById")
                .setController(OrderController.class.newInstance())
                .setMethod(OrderController.class.getMethod("getOrderById", int.class)));
            } catch (Exception e) { 
                e.printStackTrace();
            }
        }
    
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { 
            doDispatcher(req, resp);
        }
    
        /** * 使用策略模式去掉多个if-else判断 */
        private void doDispatcher(HttpServletRequest req, HttpServletResponse resp) { 
            String requestURI = req.getRequestURI();
            Handler h = null;
            for (Handler handler : handlerMapping) { 
                if (Objects.equals(handler.getUri(), requestURI)) { 
                    h = handler;
                    break;
                }
            }
            try { 
                h.getMethod().invoke(h.getController(), Integer.parseInt(req.getParameter("mid")));
            } catch (IllegalAccessException e) { 
                e.printStackTrace();
            } catch (InvocationTargetException e) { 
                e.printStackTrace();
            }
        }
    
        class Handler { 
            String uri;
            Object controller;
            Method method;
    
            public String getUri() { 
                return uri;
            }
    		
            // 返回this是函数式编程中的链式
            public Handler setUri(String uri) { 
                this.uri = uri;
                return this;
            }
    
            public Object getController() { 
                return controller;
            }
    
            public Handler setController(Object controller) { 
                this.controller = controller;
                return this;
            }
    
            public Method getMethod() { 
                return method;
            }
    
            public Handler setMethod(Method method) { 
                this.method = method;
                return this;
            }
        }
    }

[UML]: /images/20221024/754b711389d649ffb66e88902913ff51.png