【Mybatis】02：映射文件深入

• 第二章：MyBatis映射文件深入
• • • 一、Mybatis的Dao层实现
    • 二、动态SQL
    • • 1.环境搭建
      • 2.条件查询if
      • 3.复杂查询foreach
      • 4.Sql片段抽取

传统开发方式是手动对dao层进行实现，

而企业级开发的主流，采用的是Mybatis的代理开发方式实现dao层的开发。

Mapper接口开发方法只需要编写Mapper接口（类似于dao层接口），

由Mybatis框架根据接口定义创建接口的动态代理对象，代理对象的方法体同dao接口实现类的方法。

Mapper接口开发需要遵循以下规范：

1. Mapper.xml文件中的namespace与mapper接口的全限定名相同
2. Mapper接口方法名和Mapper.xml中定义的每个statement的id相同
3. Mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同
4. Mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同

Mybaits的映射文件中，有时候业务逻辑复杂sql语句是动态变化的，就难以满足要求了：

package com.itcast.domain;

public class User {
    private int id;
    private String username;
    private String password;

    public int getId() { return id; }

    public void setId(int id) { this.id = id; }

    public String getUsername() { return username; }

    public void setUsername(String username) { this.username = username; }

    public String getPassward() { return password; }

    public void setPassward(String passward) { this.password = passward; }

    
    @Override
    public java.lang.String toString() {
        return "User{" + "id=" + id + ", username=" + username + ", passward=" + password + '}';
    }
}

package com.itcast.mapper;

import com.itcast.domain.User;

import java.util.List;

public interface UserMapper {
    public List findByCondition(User user);
}

    
    
        select * from user where id = #{id} and username = #{username} and password = #{password}
    

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### direct messages to file mylog.log ###
log4j.appender.file=org.apache.log4j.FileAppender
log4j.appender.file.File=c:/mylog.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

### set log levels - for more verbose logging change 'info' to 'debug' ###

log4j.rootLogger=debug, stdout

package com.itcast.test;

import com.itcast.domain.User;
import com.itcast.mapper.UserMapper;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import org.junit.Test;

import java.io.IOException;
import java.io.InputStream;
import java.util.List;

public class MapperTest {
    @Test
    public void test1() throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        //mybatis实现dao层
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        //模拟user
        User condition = new User();
        condition.setId(1);
        condition.setUsername("zhangsan");
        condition.setPassward("123");

        //执行操作
        List userList = mapper.findByCondition(condition);
        System.out.println(userList);
    }
}

当部分查询条件缺失时，可以在UserMapper.xml文件中使用if标签拼接sql语句，修改文件如下：

    
    
        select * from user
        
            
                and id = #{id}
            
            
                and username = #{username}
            
            
                and password = #{password}
            
        
    

 
当MapperTest中给出的查询条件变化时，sql可以进行动态查询：
 
缺失所有查询条件，sql默认查询所有记录：
 
 
给出id的情况下，则查询id为1的所有记录：
 
 
3.复杂查询foreach 
Mybatis提供了用于数组和集合循环遍历的元素，实现复杂的查询操作，修改UserMapper.xml文件如下：
 



    
    
        select * from user
        
            
                and id = #{id}
            
            
                and username = #{username}
            
            
                and password = #{password}
            
        
    
    
        select * from user
        
            
                #{id}
            
        
    

 
package com.itcast.mapper;

import com.itcast.domain.User;

import java.util.List;

public interface UserMapper {
    public List findByCondition(User user);
    public List findByids(List ids);
}
 
 
4.Sql片段抽取 
Sql中可以将重复的sql提取出来，使用时用include应用即可最终达到sql重用的目的：
 



    
    select * from user
    
    
        
        
            
                and id = #{id}
            
            
                and username = #{username}
            
            
                and password = #{password}
            
        
    
    
        
        
            
                #{id}