优化if...else...语句

写代码的时候经常遇到这样的场景：根据某个字段值来进行不同的逻辑处理。例如，不同的会员等级在购物时有不同的折扣力度。如果会员的等级很多，那么代码中与之相关的if...elseif...else...会特别长，而且每新增一种等级时需要修改原先的代码。可以用策略模式来优化，消除这种场景下的if...elseif...else...，使代码看起来更优雅。

首先，定义一个接口

/**
 * 会员服务
 */
public interface VipService {
    void handle();
}

然后，定义实现类

/**
 * 白银会员
 */
public class SilverVipService implements VipService {
    @Override
    public void handle() {
        System.out.println("白银");
    }
}

/**
 * 黄金会员
 */
public class GoldVipService implements VipService {
    @Override
    public void handle() {
        System.out.println("黄金");
    }
}

最后，定义一个工厂类，目的是当传入一个会员等级后，返回其对应的处理类

public class VipServiceFactory {
    private static Map<String, VipService> vipMap = new ConcurrentHashMap<>();

    public static void register(String type, VipService service) {
        vipMap.put(type, service);
    }

    public static VipService getService(String type) {
        return vipMap.get(type);
    }
}

为了建立会员等级和与之对应的处理类之间的映射关系，这里通常可以有这么几种处理方式：

方式一：实现类手动注册

可以实现InitializingBean接口，或者在某个方法上加@PostConstruct注解

import org.springframework.beans.factory.InitializingBean;
import org.springframework.stereotype.Component;

/**
 * 白银会员
 */
@Component
public class SilverVipService implements VipService, InitializingBean {
    @Override
    public void handle() {
        System.out.println("白银");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        VipServiceFactory.register("silver", this);
    }
}

/**
 * 黄金会员
 */
@Component
public class GoldVipService implements VipService, InitializingBean {
    @Override
    public void handle() {
        System.out.println("黄金");
    }

    @Override
    public void afterPropertiesSet() throws Exception {
        VipServiceFactory.register("gold", this);
    }
}

方式二：从Spring容器中直接获取Bean

public interface VipService {
    void handle();
    String getType();
}

/**
 * 白银会员
 */
@Component
public class SilverVipService implements VipService {
    @Override
    public void handle() {
        System.out.println("白银");
    }
    @Override
    public String getType() {
        return "silver";
    }
}

/**
 * 黄金会员
 */
@Component
public class GoldVipService implements VipService {
    @Override
    public void handle() {
        System.out.println("黄金");
    }
    @Override
    public String getType() {
        return "gold";
    }
}

/**
 * 上下文
 */
@Component
public class VipServiceFactory implements ApplicationContextAware {
    private static Map<String, VipService> vipMap = new ConcurrentHashMap<>();

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        Map<String, VipService> map = applicationContext.getBeansOfType(VipService.class);
        map.values().forEach(service -> vipMap.put(service.getType(), service));
    }

    public static VipService getService(String type) {
        return vipMap.get(type);
    }
}

/**
 * 测试
 */
@SpringBootTest
class DemoApplicationTests {
    @Test
    void contextLoads() {
        VipServiceFactory.getService("silver").handle();
    }
}

方式三：反射+自定义注解

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
public @interface MemberLevel {
    String value();
}

@MemberLevel("silver")
@Component
public class SilverVipService implements VipService {
    @Override
    public void handle() {
        System.out.println("白银");
    }
}

@MemberLevel("gold")
@Component
public class GoldVipService implements VipService {
    @Override
    public void handle() {
        System.out.println("黄金");
    }
}

/**
 * 上下文
 */
@Component
public class VipServiceFactory implements ApplicationContextAware {
    private static Map<String, VipService> vipMap = new ConcurrentHashMap<>();

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
//        Map<String, VipService> map = applicationContext.getBeansOfType(VipService.class);
        Map<String, Object> map = applicationContext.getBeansWithAnnotation(MemberLevel.class);
        for (Object bean : map.values()) {
            if (bean instanceof VipService) {
                String type = bean.getClass().getAnnotation(MemberLevel.class).value();
                vipMap.put(type, (VipService) bean);
            }
        }
    }

    public static VipService getService(String type) {
        return vipMap.get(type);
    }
}

完整示例代码

/**
 * 结算业务种类
 * @Author: ChengJianSheng
 * @Date: 2023/1/16
 */
@Getter
public enum SettlementBusiType {
    RE1011("RE1011", "转贴现"),
    RE4011("RE4011", "买断式贴现"),
    RE4021("RE4021", "回购式贴现"),
    RE4022("RE4022", "回购式贴现赎回");
//    ......

    private String code;
    private String name;

    SettlementBusiType(String code, String name) {
        this.code = code;
        this.name = name;
    }
}


/**
 * 结算处理器
 * @Author: ChengJianSheng
 * @Date: 2023/1/16
 */
public interface SettlementHandler {
    /**
     * 清算
     */
    void handle();

    /**
     * 获取业务种类
     */
    SettlementBusiType getBusiType();
}


/**
 * 转贴现结算处理
 */
@Component
public class RediscountSettlementHandler implements SettlementHandler {
    @Override
    public void handle() {
        System.out.println("转贴现");
    }

    @Override
    public SettlementBusiType getBusiType() {
        return SettlementBusiType.RE1011;
    }
}


/**
 * 买断式贴现结算处理
 */
@Component
public class BuyoutDiscountSettlementHandler implements SettlementHandler {
    @Override
    public void handle() {
        System.out.println("买断式贴现");
    }

    @Override
    public SettlementBusiType getBusiType() {
        return SettlementBusiType.RE4011;
    }
}


/**
 * 默认处理器
 * @Author: ChengJianSheng
 * @Date: 2023/1/16
 */
@Component
public class DefaultSettlementHandler implements /*SettlementHandler,*/ ApplicationContextAware {
    private static Map<SettlementBusiType, SettlementHandler> allHandlerMap = new ConcurrentHashMap<>();

    public static SettlementHandler getHandler(SettlementBusiType busiType) {
        return allHandlerMap.get(busiType);
    }

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        Map<String, SettlementHandler> map = applicationContext.getBeansOfType(SettlementHandler.class);
        map.values().forEach(e -> allHandlerMap.put(e.getBusiType(), e));
    }
}


@SpringBootTest
class Demo2023ApplicationTests {
    @Test
    void contextLoads() {
        SettlementHandler handler = DefaultSettlementHandler.getHandler(SettlementBusiType.RE1011);
        if (null != handler) {
            handler.handle();
        }
    }
}