标签:mybatis
一、数据库字段名与实体类属性名不相同问题
1.1 准备数据表和数据
CREATE TABLE orders( order_id INT PRIMARY KEY AUTO_INCREMENT, order_no VARCHAR(20), order_price FLOAT ); INSERT INTO orders(order_no, order_price) VALUES('aaaa', 23); INSERT INTO orders(order_no, order_price) VALUES('bbbb', 33); INSERT INTO orders(order_no, order_price) VALUES('cccc', 22);
public class Order { private int id; private String orderNo; private float price; }
方式一: 通过在sql语句中定义别名 <select id="selectOrder" parameterType="int" resultType="_Order"> select order_id id, order_no orderNo,order_price price from orders where order_id=#{id} </select> 方式二: 通过<resultMap> <select id="selectOrderResultMap" parameterType="int" resultMap="orderResultMap"> select * from orders where order_id=#{id} </select> <resultMap type="_Order" id="orderResultMap"> <id property="id" column="order_id"/> <result property="orderNo" column="order_no"/> <result property="price" column="order_price"/> </resultMap>
2.1 创建表和数据
CREATE TABLE teacher( t_id INT PRIMARY KEY AUTO_INCREMENT, t_name VARCHAR(20) ); CREATE TABLE class( c_id INT PRIMARY KEY AUTO_INCREMENT, c_name VARCHAR(20), teacher_id INT ); ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id); INSERT INTO teacher(t_name) VALUES('LS1'); INSERT INTO teacher(t_name) VALUES('LS2'); INSERT INTO class(c_name, teacher_id) VALUES('bj_a', 1); INSERT INTO class(c_name, teacher_id) VALUES('bj_b', 2);
提出需求:根据班级id查询班级信息(带老师的信息)
public class Teacher { private int id; private String name; }
public class Classes { private int id; private String name; private Teacher teacher; }
<!-- 方式一:嵌套结果:使用嵌套结果映射来处理重复的联合结果的子集 封装联表查询的数据(去除重复的数据) select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=1 --> <select id="getClass" parameterType="int" resultMap="ClassResultMap"> select * from class c, teacher t where c.teacher_id=t.t_id and c.c_id=#{id} </select> <resultMap type="_Classes" id="ClassResultMap"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" javaType="_Teacher"> <id property="id" column="t_id"/> <result property="name" column="t_name"/> </association> </resultMap> <!-- 方式二:嵌套查询:通过执行另外一个SQL映射语句来返回预期的复杂类型 SELECT * FROM class WHERE c_id=1; SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值 --> <select id="getClass2" parameterType="int" resultMap="ClassResultMap2"> select * from class where c_id=#{id} </select> <resultMap type="_Classes" id="ClassResultMap2"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" select="getTeacher"> </association> </resultMap> <select id="getTeacher" parameterType="int" resultType="_Teacher"> SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} </select>
三、一对多的关联查询
3.1 增加学生表
CREATE TABLE student( s_id INT PRIMARY KEY AUTO_INCREMENT, s_name VARCHAR(20), class_id INT ); INSERT INTO student(s_name, class_id) VALUES('xs_A', 1); INSERT INTO student(s_name, class_id) VALUES('xs_B', 1); INSERT INTO student(s_name, class_id) VALUES('xs_C', 1); INSERT INTO student(s_name, class_id) VALUES('xs_D', 2); INSERT INTO student(s_name, class_id) VALUES('xs_E', 2); INSERT INTO student(s_name, class_id) VALUES('xs_F', 2);
public class Student { private int id; private String name; } public class Classes { private int id; private String name; private Teacher teacher; private List<Student> students; }
<!-- 方式一: 嵌套结果: 使用嵌套结果映射来处理重复的联合结果的子集 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=1 --> <select id="getClass3" parameterType="int" resultMap="ClassResultMap3"> 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} </select> <resultMap type="_Classes" id="ClassResultMap3"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" javaType="_Teacher"> <id property="id" column="t_id"/> <result property="name" column="t_name"/> </association> <!-- ofType指定students集合中的对象类型 --> <collection property="students" ofType="_Student"> <id property="id" column="s_id"/> <result property="name" column="s_name"/> </collection> </resultMap> <!-- 方式二:嵌套查询:通过执行另外一个SQL映射语句来返回预期的复杂类型 SELECT * FROM class WHERE c_id=1; SELECT * FROM teacher WHERE t_id=1 //1 是上一个查询得到的teacher_id的值 SELECT * FROM student WHERE class_id=1 //1是第一个查询得到的c_id字段的值 --> <select id="getClass4" parameterType="int" resultMap="ClassResultMap4"> select * from class where c_id=#{id} </select> <resultMap type="_Classes" id="ClassResultMap4"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <association property="teacher" column="teacher_id" javaType="_Teacher" select="getTeacher2"></association> <collection property="students" ofType="_Student" column="c_id" select="getStudent"></collection> </resultMap> <select id="getTeacher2" parameterType="int" resultType="_Teacher"> SELECT t_id id, t_name name FROM teacher WHERE t_id=#{id} </select> <select id="getStudent" parameterType="int" resultType="_Student"> SELECT s_id id, s_name name FROM student WHERE class_id=#{id} </select>
四、动态SQL与模糊查询
4.1 准备数据表
create table d_user( id int primary key auto_increment, name varchar(10), age int(3) ); insert into d_user(name,age) values('Tom',12); insert into d_user(name,age) values('Bob',13); insert into d_user(name,age) values('Jack',18);
4.2 查询条件实体类ConditionUser
private String name; private int minAge; private int maxAge;
private int id; private String name; private int age;
<?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="com.atguigu.day03_mybatis.test6.userMapper"> <select id="getUser" parameterType="com.atguigu.day03_mybatis.test6.ConditionUser" resultType="com.atguigu.day03_mybatis.test6.User"> select * from d_user where age>=#{minAge} and age<=#{maxAge} <if test='name!="%null%"'>and name like #{name}</if> </select> </mapper>
String statement = "com.atguigu.day03_mybatis.test6.userMapper.getUser"; List<User> list = sqlSession.selectList(statement, new ConditionUser("%a%", 1, 12)); System.out.println(list);
五、Mybatis缓存
正如大多数持久层框架一样,MyBatis 同样提供了一级缓存和二级缓存的支持
1. 一级缓存: 基于PerpetualCache 的 HashMap本地缓存,其存储作用域为 Session,当 Session flush 或 close 之后,该Session中的所有 Cache 就将清空。
2. 二级缓存与一级缓存其机制相同,默认也是采用 PerpetualCache,HashMap存储,不同在于其存储作用域为 Mapper(Namespace),并且可自定义存储源,如 Ehcache。
3. 对于缓存数据更新机制,当某一个作用域(一级缓存Session/二级缓存Namespaces)的进行了 C/U/D 操作后,默认该作用域下所有 select 中的缓存将被clear。
5.1 一级缓存
一级缓存: 也就Session级的缓存(默认开启)
a. 一级缓存: 也就Session级的缓存(默认开启)
b. 查询条件是一样的
c. 没有执行过session.clearCache()清理缓存或者session.close()
d. 没有执行过增删改的操作(这些操作都会清理缓存)
5.2 二级缓存
只需要在userMapper.xml文件中添加一个标签,即启用二级缓存:
<cache/> <!—映射文件级别的缓存 -->
a. 映射语句文件中的所有select语句将会被缓存。
b. 映射语句文件中的所有insert,update和delete语句会刷新缓存。
c. 缓存会使用Least Recently Used(LRU,最近最少使用的)算法来收回。
d. 缓存会根据指定的时间间隔来刷新。
e. 缓存会存储1024个对象
<cache eviction="FIFO" //回收策略为先进先出 flushInterval="60000" //自动刷新时间60s size="512" //最多缓存512个引用对象 readOnly="true"/> //只读
标签:mybatis
原文地址:http://blog.csdn.net/huhui_cs/article/details/44946175