1 引言

在工作中有没有遇到这种情况，需要根据某个条件的不同实例化不同的类，被实例化的类基本功能相同，都继承自同一父类。这种情况，就是典型的简单工厂模式应用之处。简单工厂模式，创建一个工厂，通过工厂来创建其他对象的实例。被创建的实例通常都有相同的父类。

2 业务举例

这里举一个我在实际工作当中遇到的例子。业务需求是审核公司线上商城每一笔订单的用户评论和商户评论（对用户评论的一个回复），因为两种评论的模型有一些属性的不同，用户评论对应一个实体（UserComment）一张数据表(user_comment)，商户回应评论对应一个实体（MerchantComment）一张数据表(merchant_comment)。两个实体，两张数据表，对应两个mapper（持久层框架用的Mybatis Plus）。现在前端调用接口对评论进行审核，返回审核的结果，审核结果中有一个参数（identity）标识是审核用户评论还是审核商户评论，依据identity的值在service层调用不同的mapper对数据进行持久化存储。

3 简单代码实现

3.1 UserComment与MerchantComment共有属性

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:28
 **/
@Data
public class CommonComment {

    private Integer id;

    private String commentContent;

    private String commentResult;
}

3.2 UserComment

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:29
 **/
@Data
public class UserComment extends CommonComment{

    private String userName;
}

3.3 merchantComment

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:29
 **/
@Data
public class MerchantComment extends CommonComment{

    private String merchantName;
}

虽然此处的MerchantComment与UserComment字段实际没什么不同，但此代码只是一个简单的抽象，和实际的业务属性相差甚远。

3.4 定义mapper接口

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:24
 **/
public interface Mapper<T> {

    /**
     * 存储实体到数据库
     * @param entity 存储的实体
     */
    void saveEntity(T entity);
}

3.5 存储商户评论审核结果的mapper

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:26
 **/
public class MerchantCommentMapper implements Mapper<MerchantComment>{

    @Override
    public void saveEntity(MerchantComment merchantComment) {
        System.out.println("存储的是审核商户回复内容，" + merchantComment.getMerchantName());
    }
}

3.6 存储用户评论审核结果的mapper

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:26
 **/
public class UserCommentMapper implements Mapper<UserComment>{

    @Override
    public void saveEntity(UserComment userComment) {
        System.out.println("存储的是审核用户评论内容，" + userComment.getUserName());
    }

3.7 简单工厂SimpleFactory

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:38
 **/
public class SimpleFactory {

    public static Mapper createMapper(String identity){
        Mapper mapper = null;
        switch (identity){
            case "merchant":
                mapper = new MerchantCommentMapper();
                break;
            case  "user":
                mapper = new UserCommentMapper();
                break;
            default:
        }
        return mapper;
    }

    public static CommonComment createCommentEntity(String identity, CommonComment commonComment){
       CommonComment comment = null;
        switch (identity){
            case "merchant":
                MerchantComment merchantComment = new MerchantComment();
                merchantComment.setId(commonComment.getId());
                merchantComment.setCommentContent(commonComment.getCommentContent());
                merchantComment.setCommentResult(commonComment.getCommentResult());
                merchantComment.setMerchantName("hello，我是商户评论回复");
                comment = merchantComment;
                break;
            case  "user":
                UserComment userComment = new UserComment();
                userComment.setId(commonComment.getId());
                userComment.setCommentContent(commonComment.getCommentContent());
                userComment.setCommentResult(commonComment.getCommentResult());
                userComment.setUserName("hello，我是用户评论");
                comment = userComment;
                break;
            default:
        }
        return comment;
    }
}

简单工厂里面相当于有两个生产线，一个生产mapper，一个生产comment。生产comment的生产线还可以再简单一些，此处没有再做封装。

3.8 主类

/**
 * @program: design-pattern-learning
 * @author: zgr
 * @create: 2021-09-01 16:23
 **/
public class MainClass {
    public static void main(String[] args) {

        boolean flag = true;
        while (flag) {
            //模拟接口参数
            Scanner sc = new Scanner(System.in);
            String identity = sc.nextLine();
            if (identity.equals("exit")){
                flag = false;
            }else {

                //模拟接口接待的审核结果body
                CommonComment comment = new CommonComment();
                comment.setId(1);
                comment.setCommentContent("审核内容");
                comment.setCommentResult("yes");

                Mapper mapper = SimpleFactory.createMapper(identity);
                if (mapper == null) {
                    System.out.println("身份标识参数输入错误");
                    continue;
                }
                mapper.saveEntity(SimpleFactory.createCommentEntity(identity, comment));
            }
        }

    }
}