一、动态SQL【重点】
1.1 < sql >
<mapper namespace="com.qf.dao.UserDao">
<sql id="BaseSql">
select id,name,password
</sql>
<!--配置查询所有-->
<select id="findAll" resultType="user">
<!-- 引入sql片段 -->
<include refid="BaseSql"></include>
FROM t_user
</select>
</mapper>
1.2 < if >
<select id="findByUser" resultType="com.qf.entity.User">
<include refid="BaseSql"></include>
FROM t_user
where
<if test="name!=null">
name=#{name}
</if>
<if test="password!=null">
password=#{password}
</if>
</select>
1.3 < where >
<select id="findByUser" resultType="com.qf.entity.User">
<include refid="BaseSql"></include>
FROM t_user
<where> <!-- WHERE，会自动忽略前后缀（如：and | or） -->
<if test="name!=null">
name=#{name}
</if>
<if test="password!=null">
and password=#{password}
</if>
</where>
</select>
1.4 < set >
<update id="UpdateByUser">
update t_user
<set>
<if test="name!=null">
name=#{name},
</if>
<if test="password!=null">
password=#{password},
</if>
</set>
<where>
<if test="id!=null">
id=#{id}
</if>
</where>
</update>
1.5 < trim >
<select id="findByUser" resultType="com.qf.entity.User">
<include refid="BaseSql"></include>
FROM t_user
<trim prefix="WHERE" prefixOverrides="AND|OR"> <!-- 增加WHERE前缀，自动忽略前缀 -->
<if test="name!=null">
and name=#{name}
</if>
<if test="password!=null">
and password=#{password}
</if>
</trim>
</select>
<update id="UpdateByUser">
update t_user
<trim prefix="SET" suffixOverrides=","> <!-- 增加SET前缀，自动忽略后缀 -->
<if test="name!=null">
name=#{name},
</if>
<if test="password!=null">
password=#{password},
</if>
</trim>
<where>
<if test="id!=null">
id=#{id}
</if>
</where>
</update>
1.6 < foreach >
<!-- 多个id查询 -->
<!--collection="list"-->
<select id="findUserByIds" resultType="user">
<include refid="BaseSql"></include>
from t_user
<where>
id in
<foreach collection="array" open="(" close=")" separator="," item="id">
#{id}
</foreach>
</where>
</select>
| 参数 |
描述 |
取值 |
| collection |
容器类型 |
list、array、map |
| open |
起始符 |
( |
| close |
结束符 |
) |
| separator |
分隔符 |
, |
| index |
下标号 |
从0开始，依次递增 |
| item |
当前项 |
任意名称（循环中通过 #{任意名称} 表达式访问） |
二、缓存（Cache）【重点】
| 无缓存：用户在访问相同数据时，需要发起多次对数据库的直接访问，导致产生大量IO、读写硬盘的操作，效率低下 |
|---|
|
|
| 有缓存：首次访问时，查询数据库，将数据存储到缓存中；再次访问时，直接访问缓存，减少IO、硬盘读写次数、提高效率 |
|---|
|
|
2.1 一级缓存
@Test
public void testfindById()throws Exception {
InputStream in = Resources.getResourceAsStream("mybatis-config.xml");
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
SqlSession session = factory.openSession();
//---------------------------------------------
UserDao userDao = session.getMapper(UserDao.class);
//证明SqlSession级别的一级缓存存在
User user1 = userDao.findById(1);
//当调用SqlSession的修改，添加，删除，commit()，close()等方法时也会清空一级缓存。
//session.clearCache();//清空缓存
//session.close();
//session = factory.openSession();
//userDao = session.getMapper(UserDao.class);
User user2 = userDao.findById(1);
System.out.println(user1 == user2);
//---------------------------------------------
session.close();
in.close();
}
2.2 二级缓存
- 注意：在sqlSession.commit()或者sqlSession.close()之后生效。
2.2.1 开启全局缓存
<configuration>
<properties .../>
<!-- 注意书写位置 -->
<settings>
<setting name="cacheEnabled" value="true"/> <!-- mybaits-config.xml中开启全局缓存（默认开启） -->
</settings>
<typeAliases></typeAliases>
</configuration>
2.2.2 指定Mapper缓存
<mapper namespace="com.qf.dao.UserDao">
<cache /> <!-- 指定缓存 -->
<!-- 查询单个对象 -->
<select id="findById" resultType="com.qf.entity.User">
select id,name,password FROM t_user where id=#{id}
</select>
</mapper>
@Test
public void testfindById2() throws Exception {
//1.读取配置文件
InputStream in = Resources.getResourceAsStream("mybatis-config.xml");
//2.创建SqlSessionFactory工厂
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
//3.使用工厂生产SqlSession对象
SqlSession sqlSession1 = factory.openSession();
UserDao dao1 = sqlSession1.getMapper(UserDao.class);
User user1 = dao1.findById(1);
System.out.println(user1);
sqlSession1.close();//一级缓存消失
SqlSession sqlSession2 = factory.openSession();
UserDao dao2 = sqlSession2.getMapper(UserDao.class);
User user2 = dao2.findById(1);
System.out.println(user2);
sqlSession2.close();
in.close();
}
2.3 延迟加载
延迟加载
MyBatis中的延迟加载，也称为懒加载，是指在进行表的关联查询时，按照设置延迟规则推迟对关联对象的select查询。例如在进行一对多查询的时候，只查询出一方，当程序中需要多方的数据时，mybatis再发出sql语句进行查询，这样子延迟加载就可以的减少数据库压力。MyBatis的延迟加载只是对关联对象的查询有迟延设置，对于主加载对象都是直接执行查询语句的。
三、注解
3.1 MyBatis注解操作
@Select("select * from user where id = #{id}")
public User findById(Integer id);
@Select("select * from user")
public List<User> findAll();
@Insert("insert into user(username) values( #{username} )")
public void add(User user);
@Update("update user set username = #{username} where id = #{id}")
public void update(User user);
@Delete("delete from user where id = #{id}")
public void delete(Integer id);
//获取总记录数
@Select("select count(id) from user")
public Integer getTotalCount();
//获取分页数据
@Select("select * from user limit #{first},#{second}")
public List<User> findPageData(@Param("first") Integer first,@Param("second") Integer second);
