原文链接：blog.csdn.net/hncu1306602liuqiang

看到crossover Jie的文章《利用策略模式优化过多if else 代码》后受到启发，可以利用策略模式简化过多的if else代码。

需求

这里虚拟一个业务需求，让大家容易理解。假设有一个订单系统，里面的一个概念是根据订单的不同类型做出不同的处理。

项目结构

订单实体

/*** 订单实体*/
public class OrderDTO {private String code;private BigDecimal price;/** 订单类型：* 1：普通订单* 2：团购订单* 3：促销订单*/private String type;
//getter,setter自己实现
}

service接口

/*** 订单处理*/
public interface IOrderService {/*** 根据订单的不同类型做出不同的处理** @param dto 订单实体* @return 为了简单，返回字符串*/String orderHandler(OrderDTO dto);}
//实现类1
@Component
public class OrderServiceImpl implements IOrderService {@Overridepublic String orderHandler(OrderDTO dto) {if ("1".equals(dto.getType())) {//普通订单处理} else if ("2".equals(dto.getType())) {//团购订单处理} else if ("3".equals(dto.getType())) {//促销订单处理}//未来订单类型增加}}
//实现类二
@Component
public class OrderServiceImpl implements IOrderService {//使用策略模式实现@Autowiredprivate HandlerContext handlerContext;@Overridepublic String orderHandler(OrderDTO dto) {/** 1：使用if..else实现* 2：使用策略模式实现*/AOrderTypeHandler instance = handlerContext.getInstance(dto.getType());return instance.handler(dto);}}

利用策略模式只需要2行代码就可以搞定（也可以用工厂）

HandlerContext和HandlerProccessor
/*** 订单策略模式环境* 这个类的注入由HandlerProccessor实现*/
public class HandlerContext {private Map<String, AOrderTypeHandler> handlerMap;/*** 构造传参不能直接使用注解扫入*/public HandlerContext(Map<String, AOrderTypeHandler> handlerMap) {this.handlerMap = handlerMap;}/*** 获得实例** @param type* @return*/public AOrderTypeHandler getInstance(String type) {if (type == null) {throw new IllegalArgumentException("type参数不能为空");}AOrderTypeHandler clazz = handlerMap.get(type);if (clazz == null) {throw new IllegalArgumentException("该类型没有在枚举OrderTypeHandlerAnno中定义，请定义：" + type);}return clazz;}}

/*** 策略模式，处理type与实现类的映射关系*/
@Component
public class HandlerProccessor implements BeanFactoryPostProcessor {/*** 扫描@OrderTypeHandlerAnno注解，初始化HandlerContext，将其注册到spring容器** @param beanFactory bean工厂* @throws BeansException*/@Overridepublic void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {Map<String, AOrderTypeHandler> handlerMap = new HashMap<>();for (OrderTypeEnum temp : OrderTypeEnum.values()) {AOrderTypeHandler beanInstacle = getBeansWithAnnotation(beanFactory, AOrderTypeHandler.class, OrderTypeHandlerAnno.class, temp.getCode());handlerMap.put(temp.getCode(), beanInstacle);}HandlerContext context = new HandlerContext(handlerMap);//单例注入beanFactory.registerSingleton(HandlerContext.class.getName(), context);}/** 通过父类+注解找到实体类*/private <T> T getBeansWithAnnotation(ConfigurableListableBeanFactory beanFactory, Class<T> manager, Class<? extends OrderTypeHandlerAnno> annotation, String code) throws BeansException {if (ObjectUtils.isEmpty(code)) {throw new RuntimeException("code is null ");}Collection<T> tCollection = beanFactory.getBeansOfType(manager).values();for (T t : tCollection) {OrderTypeHandlerAnno orderTypeHandlerAnno = t.getClass().getAnnotation(annotation);if (ObjectUtils.isEmpty(orderTypeHandlerAnno)) {throw new RuntimeException("该注解没有写入值 :" + code);}//注解值是否与code相等if (code.equals(orderTypeHandlerAnno.value().getCode())) {return t;}}throw new RuntimeException("通过code没有找到该注解对应的实体类 :" + code);}}

父抽象类+注解+枚举

/*** 订单类型处理定义* 使用抽象类，那么子类就只有一个继承了*/
public abstract class AOrderTypeHandler {/*** 一个订单类型做一件事** @param dto 订单实体* @return 为了简单，返回字符串*/abstract public String handler(OrderDTO dto);}/*** 订单类型注解* 使用方式：* 1：普通订单 @OrderTypeHandlerAnno("1")* 2：团购订单 @OrderTypeHandlerAnno("2")* 3：促销订单 @OrderTypeHandlerAnno("3")*/
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
public @interface OrderTypeHandlerAnno {OrderTypeEnum value();}/*** 订单类型枚举*/
public enum OrderTypeEnum {Normal("1", "普通"),Group("2", "团队"),Promotion("3", "促销");private String code;    //代码private String name;    //名称，描述OrderTypeEnum(String code, String name) {this.code = code;this.name = name;}public String getCode() {return code;}public void setCode(String code) {this.code = code;}public String getName() {return name;}public void setName(String name) {this.name = name;}/*** 根据code属性获取name属性** @param code* @return*/public static String getNameByCode(String code) {for (OrderTypeEnum temp : OrderTypeEnum.values()) {if (temp.getCode().equals(code)) {return temp.getName();}}return null;}}

业务人员实现类

//业务代码
/*** 普通订单处理*/
@Component
@OrderTypeHandlerAnno(OrderTypeEnum.Normal)
public class NormalOrderHandler extends AOrderTypeHandler {@Overridepublic String handler(OrderDTO dto) {return "处理普通订单";}}/*** 团队订单处理*/
@Component
@OrderTypeHandlerAnno(OrderTypeEnum.Group)
public class GroupOrderHandler extends AOrderTypeHandler {@Overridepublic String handler(OrderDTO dto) {return "处理团队订单";}}/*** 促销订单处理*/
@Component
@OrderTypeHandlerAnno(OrderTypeEnum.Promotion)
public class PromotionOrderHandler extends AOrderTypeHandler {@Overridepublic String handler(OrderDTO dto) {return "处理促销订单";}}

测试结果(使用chrome浏览器测试结果)

如果类型不存在

controller/*** 策略模式*/
@RestController
public class StrategyController {@Resource(name = "orderServiceImpl")private IOrderService orderService;@GetMapping("/api/order")@ResponseBodypublic String orderSave(OrderDTO dto) {String str = orderService.orderHandler(dto);return "{\"status\":1,\"msg\":\"保存成功\",\"data\":\"" + str + "\"}";}}
pom.xml文档<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd"><modelVersion>4.0.0</modelVersion><parent><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-parent</artifactId><version>2.2.1.RELEASE</version><relativePath/> <!-- lookup parent from repository --></parent><groupId>com.kayak</groupId><artifactId>study-design</artifactId><version>0.0.1-SNAPSHOT</version><name>study-design</name><description>Demo project for Spring Boot</description><properties><java.version>1.8</java.version></properties><dependencies><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-web</artifactId></dependency><dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-test</artifactId><scope>test</scope><exclusions><exclusion><groupId>org.junit.vintage</groupId><artifactId>junit-vintage-engine</artifactId></exclusion></exclusions></dependency></dependencies><build><plugins><plugin><groupId>org.springframework.boot</groupId><artifactId>spring-boot-maven-plugin</artifactId></plugin></plugins></build></project>

总结：

利用策略模式可以简化复杂的if else代码，方便维护，而利用自定义注解和自注册的方式，可以方便应对需求的变更。