006. MyBatis基本使用---03MyBatis数据查询

1.MyBatis数据查询步骤

1.1 创建实体类entity

1.2 创建mapper  xml文件来说明当前sql语句是什么并编写<select > sql语句

1.3 在mybatis-config.xml开启驼峰映射

1.4 在mybatis-config.xml新增mapper  xml的声明

1.5 SqlSession执行select语句

2.具体实现操作

2.1 创建entity包用来存放实体类

package com.imooc.mybatis.entity;

public class Category {
    private Integer categoryId;
    private String categoryName;
    private Integer parentId;
    private Integer categoryLevel;
    private Integer categoryOrder;

    public Integer getCategoryId() {
        return categoryId;
    }

    public void setCategoryId(Integer categoryId) {
        this.categoryId = categoryId;
    }

    public String getCategoryName() {
        return categoryName;
    }

    public void setCategoryName(String categoryName) {
        this.categoryName = categoryName;
    }

    public Integer getParentId() {
        return parentId;
    }

    public void setParentId(Integer parentId) {
        this.parentId = parentId;
    }

    public Integer getCategoryLevel() {
        return categoryLevel;
    }

    public void setCategoryLevel(Integer categoryLevel) {
        this.categoryLevel = categoryLevel;
    }

    public Integer getCategoryOrder() {
        return categoryOrder;
    }

    public void setCategoryOrder(Integer categoryOrder) {
        this.categoryOrder = categoryOrder;
    }
}

2.2 在resource目录下创建新的子目录mappers（映射器）存放xml文件，创建goods.xml，用来说明实体类和哪一个表对应

      resultType代表将sql语句返回的结果包装成对应的goods对象

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
        PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="goods">

　　<select id="selectAll" resultType="com.imooc.mybatis.entity.Goods" >
　　　　select * from t_goods order by goods_id desc limit 10
　　</select>

</mapper>

2.3 在mybatis-config.xml开启驼峰命名转换 

    <settings>
        <!-- goods_id ==> goodsId 驼峰命名转换 -->
        <setting name="mapUnderscoreToCamelCase" value="true"/>
    </settings>

2.4 在mybatis-config.xml声明mapper的配置文件

<!--声明mapper的配置文件-->
    <mappers>
        <mapper resource="mappers/goods.xml"/>
    </mappers>

2.5 测试，查询常用方法selectOne() 、selectList（）

 /**
     * select查询语句执行
     */
    @Test
    public void testSelectAll() throws Exception
    {
        SqlSession session = null;
        try
        {
            session = MyBatisUtils.openSession();
            List<Goods> list = session.selectList("goods.selectAll");//goods.selectAll    命名空间.（SQL语句的ID）
            for (Goods g : list)
            {
                System.out.println(g.getTitle());
            }
        }
        catch (Exception e)
        {
            throw e;
        }
        finally
        {
            MyBatisUtils.closeSession(session);
        }
    }