Mybatis使用

主配置文件及mapper映射文件

（1）mybatis主配置文件模板去哪找，复制过来的约束文件发红？（2）mapper映射文件

传入的参数是容器类型？返回值的分析标签

动态sql 配置过程中产生的一些问题 DAO代理

传统dao的使用 难题

一对一，一对多查询递归查询 使用json显示返回的复合结构的对象

解决办法：

模板：

为什么叫映射，就是能完全一对一的对应起来；
就是一个接口对应一个mapper；所以一给xml文件只能放一组mapper

方法名字===========》Mapper的namespace
方法返回类型=========》resultMap
方法参数类型===========》parameterType，一般自动推断，不用写
单个对象
①如果是对象，直接将属性字段一一对应#{}依次
②如果是Map，和以上相同

如果是多个对象和参数
要在接口使用@parameter表示对应的关系，
很少使用标号来映射；

 
        
            insert into users (name, age, county, date)
            values
            
                (#{user.name}, #{user.age}, #{user.county}, #{user.date})
            
        

resultMap和resultType
（1）如果我们要求的返回对象要求是基本类型的，那就很简单；+
（2）如过我们要求的返回对象是内嵌引用类型的，那怎么办啊？
使用assolation，关联；

	
	
		
		
		
		
		
		
		
		
		
		
	
原文链接：https://blog.csdn.net/lzxomg/article/details/104497127

（3）如果返回对象又内嵌的引用类型，而且还是个容器?

(4)如果返回对象内嵌引用类型，引用类型又嵌套…或者容器
这就引出了递归查询！

resultMap重点介绍
resultMap其实就是对应返回的对象的，但是可能名字不匹配，也可能查出的字段个数和需求的对象模型不匹配，这个时候resultMap就发挥作用了

一样的标签一样要支持增删改查；
#和$ 前者是占位符，后者直接一起编译禁区，可能会注入；
resultMap 用于装载返回数据

为什么需要动态sql？
因为前端传来的值是多变的，不能写死写法，比如六个参数，允许空
不可能写六个sql来面对这种情况把…所以有了动态sql
http://t.csdn.cn/IqhKT

Client does not support authentication protocol requested by server; consider upgrading MySQL client

use mysql;
alter user 'root'@'localhost' identified with mysql_native_password by '123456';
flush privileges;

Cause: java.sql.SQLException: Unknown initial character set index '255' received from server. Initial client character set can be forced via the 'characterEncoding' property.
### Cause: java.sql.SQLException: Unknown initial character set index '255' received from server. Initial client character set can be forced via the 'characterEncoding' property.

mysql-connector 版本和mysql不同，修改pom一致即可；

 Cause: org.apache.ibatis.reflection.ReflectionException: There is no getter for property named 'username' in 'class useExamp.enity.User'

类里成员变量名为:我把映射类成员字段写为userName,对应数据为驼峰原则写为user_name，所以找不到；把userName改为username即可，或者将数据库字段名字改为user_name；

XXDao dao=session.getMapper（XXDao.class）；
不再每次区实现dao接口才能使用dao；

调用session.

public List selectStudents() {
		SqlSession session = MyBatisUtil.getSqlSession();
		List studentList = session.selectList(
				"com.bjpowernode.dao.StudentDao.selectStudents");
		session.close();
		return studentList;
}

为什么要使用动态sql？

  
      
      
        select * from class c,teacher t where c.teacher_id=t.t_id and c.c_id=#{id}  
      
 
      
          
          
          
          
              
              
          
      
 
 
      
      
        select * from class c,teacher t,student s where c.teacher_id=t.t_id and c.c_id=s.class_id and c.c_id=#{id}  
      
 
      
          
          
          
              
              
          
 
          
              
              
          
      
 

模型

http://t.csdn.cn/7uJJo

        
        
        
        
        
        
        
    



    
    
    
    
    
    
    



    TENANT_ID,
    CL_ID,
    CL_P_ID,
    EP_TIZONE,
    DEP_TYPE,
    START_TIME



    SELECT
    
    FROM c_line_loss_info
    WHERe CL_P_ID = #{CL_ID}



    SELECT
    
    FROM c_line_loss_info
    WHERe CL_P_ID = #{clpId}

#为什么两个一模一样的map要取不同的id名呢？
我偏向于认为递归查询是一种特殊情况，一个特殊的内嵌对象；
事实证明，没有必要用两个resultMap和两个方法

    
        
        
        
        
        
        
    





    
        select * from places where pid=#{id}
    

校验

https://www.sojson.com/

解析

https://www.sojson.com/simple_json.html

 List list = placesMapper.getTree("昭通市");
        ObjectMapper objectMapper=new ObjectMapper();
        try {
            String json=objectMapper.writevalueAsString(list);
            System.out.println(json);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }

以上的结果：

[{"id":"微信县","pid":"昭通市","distance":0,"size":"20","peopleNum":30,"placesList":[]},{"id":"盐津县","pid":"昭通市","distance":0,"size":"10","peopleNum":100,"placesList":[{"id":"串丝","pid":"盐津县","distance":110,"size":"1","peopleNum":20,"placesList":[]},{"id":"普洱镇","pid":"盐津县","distance":100,"size":"1","peopleNum":30,"placesList":[]}]},{"id":"绥江县","pid":"昭通市","distance":0,"size":"20","peopleNum":30,"placesList":[]},{"id":"镇雄县","pid":"昭通市","distance":0,"size":"10","peopleNum":300,"placesList":[{"id":"簸箕","pid":"镇雄县","distance":200,"size":"2","peopleNum":50,"placesList":[]},{"id":"罗坎","pid":"镇雄县","distance":113,"size":"5","peopleNum":40,"placesList":[]}]}]

在线工具解析后：

[{
	"id": "微信县",
	"pid": "昭通市",
	"distance": 0,
	"size": "20",
	"peopleNum": 30,
	"placesList": []
}, {
	"id": "盐津县",
	"pid": "昭通市",
	"distance": 0,
	"size": "10",
	"peopleNum": 100,
	"placesList": [{
		"id": "串丝",
		"pid": "盐津县",
		"distance": 110,
		"size": "1",
		"peopleNum": 20,
		"placesList": []
	}, {
		"id": "普洱镇",
		"pid": "盐津县",
		"distance": 100,
		"size": "1",
		"peopleNum": 30,
		"placesList": []
	}]
}, {
	"id": "绥江县",
	"pid": "昭通市",
	"distance": 0,
	"size": "20",
	"peopleNum": 30,
	"placesList": []
}, {
	"id": "镇雄县",
	"pid": "昭通市",
	"distance": 0,
	"size": "10",
	"peopleNum": 300,
	"placesList": [{
		"id": "簸箕",
		"pid": "镇雄县",
		"distance": 200,
		"size": "2",
		"peopleNum": 50,
		"placesList": []
	}, {
		"id": "罗坎",
		"pid": "镇雄县",
		"distance": 113,
		"size": "5",
		"peopleNum": 40,
		"placesList": []
	}]
}]