Mybatis入门级教程（一）

一、Mybatis介绍

MyBatis是支持普通SQL查询，存储过程和高级映射的优秀持久层框架。MyBatis消除了几乎所有的JDBC代码和参数的手工设置以及对结果集的检索封装。MyBatis可以使用简单的XML或注解用于配置和原始映射，将接口和Java的POJO（Plain Old Java Objects，普通的Java对象）映射成数据库中的记录.
JDBC-dbutils(自动封装)-MyBatis-Hibernate
http://blog.csdn.net/softwave/article/details/18604565

为什么要学Mybatis?而不使用传统的jdbc?
Mybatis是一个持久层框架，使用Mybatis代替传统的jdbc，代替vo+dao持久层。持久化：把内存中的数据保存到可掉电的存储设备中（硬盘、u盘）。
传统的jdbc存在缺点，操作起来相当繁琐，过多的set和get也相当繁琐。因此使用Mybatis简化操作。

二、如何使用Mybatis?

从一个简单的用户管理例子学起，使用步骤：

1. 建库+表（使用mysql需要建库，oracle不用）
2. 定义表所对应的实体类PO
3. 添加jar包
4. 添加Mybatis的配置文件conf.xml
5. 定义操作user表的sql映射文件userMapper.xml
6. 在conf.xml文件中注册userMapper.xml文件
7. 编写测试代码，执行定义的select语句

1.建库和表create database mybatis;use mybatis;CREATE TABLE user(id INT PRIMARY KEY AUTO_INCREMENT, NAME VARCHAR(20), age INT);INSERT INTO user(NAME, age) VALUES('小明', 22);INSERT INTO user(NAME, age) VALUES('小红', 23);2.定义表所对应的实体类 PO持久化对象public class User {    private int id;    private String name;    private int age;    //get,set方法 和toString()}3.添加jar包【mybatis】    mybatis-3.1.1.jar【MYSQL驱动包】mysql-connector-java-5.1.7-bin.jar4.添加Mybatis的配置文件conf.xml放到src文件夹下<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration>    <environments default="development">        <environment id="development">            <transactionManager type="JDBC" />            <dataSource type="POOLED">                <property name="driver" value="com.mysql.jdbc.Driver" />                <property name="url" value="jdbc:mysql://localhost:3306/mybatis" />                <property name="username" value="root" />                <property name="password" value="root" />            </dataSource>        </environment>    </environments></configuration>environments  表示程序的运行模式 有两个选项development:开发模式work:工作模式 发布以后写成它 效率高environments下environment可以写多个,但一般写一个就可以id中"development" 要和 default的development保持一致<dataSource type="POOLED">数据源类型:POOLED:使用连接池的数据源UNPOOLED:不使用连接池的数据源JDNI：使用JNDI实现的数据源5.定义操作user表的sql映射文件userMapper.xml<?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"><!-- 定义sql文件的唯一标示  通过namespace这个标示 找到这个xml文件 --><mapper namespace="com.vo.userMapper">    <select id="getUser" parameterType="int"         resultType="com.vo.User">        select * from users where id=#{id}    </select></mapper>6.在conf.xml中注册userMapper.xml文件在environments 下面加入以下代码<mappers>    <mapper resource="com/vo/userMapper.xml"/></mappers>7.编写测试代码：执行定义的select语句public class Test {    public static void main(String[] args) throws IOException {        String resource = "conf.xml";         //加载mybatis的配置文件（它也加载关联的映射文件）        Reader reader = Resources.getResourceAsReader(resource);         //构建sqlSession的工厂        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(reader);        //创建能执行映射文件中sql的sqlSession        SqlSession session = sessionFactory.openSession();        //映射sql的标识字符串        String statement = "com.vo.userMapper"+".getUser";        //执行查询返回一个唯一user对象的sql        User user = session.selectOne(statement, 1);        System.out.println(user);    }}或者用类加载器也可以演示代码如下:package com.vo;import java.io.InputStream;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;public class Test {    public static void main(String[] args) {        String resource = "conf.xml";        InputStream is = Test.class.getClassLoader().getResourceAsStream(resource);        SqlSessionFactory sf = new SqlSessionFactoryBuilder().build(is);        SqlSession session = sf.openSession();        User user =  session.selectOne("com.vo.userMapper.getUser",1);        System.out.println(user);if(session!=null){            session.close();        }    }}

三、操作user表的CRUD

一）

XML的实现1).创建SqlSessionFactory的工具类package com.util;import java.io.InputStream;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;public class SessionFactoryUtil {private static SqlSessionFactory factory;    static {        String resource = "conf.xml";        InputStream is = SessionFactoryUtil.class.getClassLoader().getResourceAsStream(resource);        factory = new SqlSessionFactoryBuilder().build(is);    }public static SqlSessionFactory getFactory(){        return factory;    }    public static SqlSession getSqlSession(){        SqlSession session = null;        //默认是false 事务手工提交 即开启事务,true 事务是自动提交,即关闭事务        session = factory.openSession(true);        return session;    }public static void closeSqlSession(SqlSession session){        if(session!=null){            session.close();    }}    public static void main(String[] args) {        System.out.println(SessionFactoryUtil.getSqlSession());    }}2). 定义sql映射xml文件：<?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"><!-- 定义sql文件的唯一标示  通过namespace这个标示 找到这个xml文件 --><mapper namespace="com.vo.userMapper">    <!-- 演示CRUD -->    <!-- 1.增加 -->    <insert id="addUser" parameterType="com.vo.User">        insert into user values(null,#{name},#{age})    </insert>    <!-- 2.删除 -->    <delete id="deleteUser" parameterType="com.vo.User">        delete from user where id=#{id}    </delete>    <!-- 3.修改 -->    <update id="updateUser" parameterType="com.vo.User">        update user set name=#{name},age=#{age} where id=#{id}    </update>    <!-- 4.查1 -->    <select id="getUserById" parameterType="java.lang.Integer" resultType="com.vo.User">        select * from user where id=#{id}    </select>    <!-- 5.按sql查询 -->    <select id="getPageByQuery" parameterType="String" resultType="com.vo.User">        ${value}    </select>    <!-- 6.按总共有多少条记录 -->    <select id="getTotalRecordSum" parameterType="String" resultType="int">        ${value}    </select></mapper>2). 在conf.xml中注册这个映射文件<mappers>    <mapper resource="com/vo/userMapper.xml" /></mappers>3). 在dao中调用：接口package com.dao.inter;import java.util.List;import com.vo.User;public interface UserDao {    //一.增加    public void addUser(User user);    //二.删除    public void deleteUser(User user);    //三.修改    public void updateUser(User user);    //四.查1    public User getUserById(Integer id);    //五.按SQL语句查    public List<User> getPageByQuery(String sql);    //六.查询总共有多少条记录    public int getTotalRecordSum(String sql);}实现类package com.dao.impl;import java.util.List;import org.apache.ibatis.session.SqlSession;import com.dao.inter.UserDao;import com.po.User;import com.util.SessionFactoryUtil;public class UserDaoImpl implements UserDao{    private SqlSession session;    public UserDaoImpl(){        session = SessionFactoryUtil.getSqlSession();    }    @Override    public int addUser(User user){        int count;        String statement = "com.po.userMapper.addUser";        try{            count = session.insert(statement, user);        }finally{            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    @Override    public int deleteUser(User user) {        int count;        String statement = "com.po.userMapper.deleteUser";        try{            count = session.delete(statement, user);        }finally{            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    @Override    public int updateUser(User user){        int count;        String statement = "com.po.userMapper.updateUser";        try{            count = session.update(statement, user);        }finally{            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    @Override    public User getUserById(Integer userid){        User user = null;        String statement = "com.po.userMapper.getUser";        try {            user = session.selectOne(statement, userid);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return user;    }    @Override    public List<User> getPageByQuery(String sql){        List<User> list = null;        String statement = "com.po.userMapper.getPageByQuery";        try{            list = session.selectList(statement, sql);        }finally {            SessionFactoryUtil.closeSqlSession(session);        }        return list;    }    @Override    public int getTotalRecordSum(String sql){        int count = 0;        String statement = "com.po.userMapper.getTotalRecordSum";        try{            count = session.selectOne(statement, sql);        }finally{            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }}二）. 注解的实现1). 定义sql映射的接口package com.dao.inter;import java.util.List;import org.apache.ibatis.annotations.*;import com.po.User;public interface UserMapper {    //一.增加    @Insert("insert into user values(null,#{name},#{age})")    public int addUser(User user);    //二.删除    @Delete("delete from user where id=#{id}")    public int deleteUser(User user);     //三.修改    @Update("update user set name=#{name},age=#{age} where id=#{id}")    public int updateUser(User user);    //四.查1    @Select("select * from user where id=#{id}")    public User getUserById(Integer userid);    //五.按sql查询    @Select("${value}")    public List<User> getPageByQuery(String sql);    //六.查询总共有多少条记录    @Select("${value}")    public int getTotalRecordSum(String sql);}2). 在conf.xml中注册这个映射接口<mappers>    <mapper class="com.dao.inter.UserMapper"/></mappers>3). 在dao类中调用package com.dao.impl;import java.util.List;import org.apache.ibatis.session.SqlSession;import com.dao.inter.UserMapper;import com.po.User;import com.util.SessionFactoryUtil;public class UserMapperImpl implements UserMapper {    private SqlSession session;    public UserMapperImpl() {        session = SessionFactoryUtil.getSqlSession();    }    @Override    public int addUser(User user) {        int count;        try {            UserMapper userMapper = session.getMapper(UserMapper.class);            count = userMapper.addUser(user);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    @Override    public int deleteUser(User user) {        int count;        try {            UserMapper userMapper = session.getMapper(UserMapper.class);            count = userMapper.deleteUser(user);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    @Override    public int updateUser(User user) {        int count;        try {            UserMapper userMapper = session.getMapper(UserMapper.class);            count = userMapper.updateUser(user);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    @Override    public User getUserById(Integer userid) {        User user = null;        try {            UserMapper userMapper = session.getMapper(UserMapper.class);            user = userMapper.getUserById(userid);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return user;    }    @Override    public List<User> getPageByQuery(String sql) {        List<User> list = null;        try {            UserMapper userMapper = session.getMapper(UserMapper.class);            list = userMapper.getPageByQuery(sql);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return list;    }    @Override    public int getTotalRecordSum(String sql) {        int count = 0;        try {            UserMapper userMapper = session.getMapper(UserMapper.class);            count = userMapper.getTotalRecordSum(sql);        } finally {            SessionFactoryUtil.closeSqlSession(session);        }        return count;    }    public static void main(String[] args) {        UserMapperImpl dao = new UserMapperImpl();        User user = new User();        user.setName("小刚");        user.setAge(21);        int count = dao.addUser(user);        System.out.println(count);    }}强烈推荐使用XML配置方式，原因如下:1.Java注解是受限的,功能较少，而MyBatis的Mapper内容相当多,相当复杂,功能强大, 使用XML文件方式可以带来更为灵活的空间，显示出MyBatis功能的强大和灵活2.如果你的SQL很复杂,条件很多,尤其是存在动态SQL的时候,写在Java文件里面可读性较差,增加了维护成本.总结:SqlSessionFactoryBuilderSqlSessionFactorySqlSessionMapper 实例

四、几个可以优化的地方

4.1. 连接数据库的配置单独放在一个properties文件中db.propertiesdriver=com.mysql.jdbc.Driverurl=jdbc:mysql://localhost:3306/mybatisusername=rootpassword=root修改conf.xml<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd"><configuration>    <properties resource="db.properties"/>    <environments default="development">        <environment id="development">            <transactionManager type="JDBC" />            <dataSource type="POOLED">                  <property name="driver" value="${driver}" />                <property name="url" value="${url}" />                <property name="username" value="${username}" />                <property name="password" value="${password}" />            </dataSource>        </environment>    </environments>    <mappers>        <mapper resource="com/vo/userMapper.xml" />    </mappers></configuration>4.2. 为实体类定义别名,简化sql映射xml文件中的引用在conf.xml中,environments节点前,加入如下代码即可<typeAliases>    <typeAlias type="com.vo.User" alias="_User"/></typeAliases>注意:conf.xml配置文件中的节点位置有一定的顺序,如果位置颠倒,会报错Caused by: org.xml.sax.SAXParseException; lineNumber: 30; columnNumber: 17; 元素类型为 "configuration" 的内容必须匹配 "(properties?,settings?,typeAliases?,typeHandlers?,objectFactory?,objectWrapperFactory?,plugins?,environments?,databaseIdProvider?,mappers?)"。或者直接写上<typeAliases>        <package name="com.vo"/></typeAliases>在userMapper.xml中就可以不需要写类全名com.vo.User了,只需要写简单类名User即可4.3. 可以在src下加入log4j的配置文件,打印日志信息1. 添加jar:     log4j-1.2.16.jar2.1. log4j.properties(方式一)    log4j.properties，    log4j.rootLogger=DEBUG, Console    #Console    log4j.appender.Console=org.apache.log4j.ConsoleAppender    log4j.appender.Console.layout=org.apache.log4j.PatternLayout    log4j.appender.Console.layout.ConversionPattern=%d [%t] %-5p [%c] - %m%n    log4j.logger.java.sql.ResultSet=INFO    log4j.logger.org.apache=INFO    log4j.logger.java.sql.Connection=DEBUG    log4j.logger.java.sql.Statement=DEBUG    log4j.logger.java.sql.PreparedStatement=DEBUG2.2. log4j.xml(方式二)<?xml version="1.0" encoding="UTF-8" ?><!DOCTYPE log4j:configuration SYSTEM "log4j.dtd"><log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">    <appender name="STDOUT" class="org.apache.log4j.ConsoleAppender">        <layout class="org.apache.log4j.PatternLayout">            <param name="ConversionPattern"                 value="%-5p %d{MM-dd HH:mm:ss,SSS} %m  (%F:%L) \n" />        </layout>    </appender>    <logger name="java.sql">        <level value="debug" />    </logger>    <logger name="org.apache.ibatis">        <level value="debug" />    </logger>    <root>        <level value="debug" />        <appender-ref ref="STDOUT" />    </root></log4j:configuration>

五、解决字段名和实体类属性名不同的冲突

5. 解决字段名与实体类属性名不相同的冲突5.1. 准备表和数据：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);5.2. 定义实体类：public class Orders {    private int orderId;    private String orderNo;    private float orderPrice;}5.3. 实现getOrderById(id)的查询：方式一: 通过在sql语句中定义别名<select id="selectOrder" parameterType="int" resultType="_Order">    select order_id orderId, order_no orderNo,order_price orderPrice 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="orderId" column="order_id"/>    <result property="orderNo" column="order_no"/>    <result property="orderPrice" column="order_price"/></resultMap>方式三:  mybatis开启数据库字段自动映射驼峰命名规则java属性  在conf.xml中 <typeAliases> 节点前加入setting节点<settings>        <setting name="mapUnderscoreToCamelCase" value="true"/></settings>

六、实现关联表查询和懒加载

6.1. 一对一关联1). 提出需求根据班级id查询班级信息(带老师的信息)2). 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 UNIQUE);ALTER TABLE class ADD CONSTRAINT fk_teacher_id FOREIGN KEY (teacher_id) REFERENCES teacher(t_id);   INSERT INTO teacher(t_name) VALUES('zhangzy');INSERT INTO teacher(t_name) VALUES('liusp');INSERT INTO class(c_name, teacher_id) VALUES('基地0620班', 1);INSERT INTO class(c_name, teacher_id) VALUES('基地16班', 2);3). 定义实体类：public class Teacher {    private int id;    private String name;}public class Classes {    private int id;    private String name;    private Teacher teacher;}4). 定义sql映射文件ClassMapper.xml<!-- 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 id="getClass" parameterType="int" resultMap="ClassResultMap">    select * from teacher t inner join class c on(t.t_id=c.teacher_id)     where c_id=#{c_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>5). 测试public Classes getClassesById(Integer id) {        Classes classes = null;        String statement = "com.vo.classMapper.getClass";            classes = session.selectOne(statement, id);         }finally{            session.close();        }        return classes;    }public Classes getClassesById2(Integer id) {        Classes classes = null;        String statement = "com.vo.classMapper.getClass2";        try{            classes = session.selectOne(statement, id);        }finally{            session.close();        }        return classes;    }6.2.延迟加载(懒加载)依赖两个jar包  cglib.jar、asm.jar,Cglib依赖于asm.jar<settings>        <!--打开延迟加载的开关  -->        <setting name="lazyLoadingEnabled" value="true"/>            <!--将积极加载改为消极加载及按需加载  -->        <setting name="aggressiveLazyLoading" value="false"/></settings>6.3. 一对多关联1). 提出需求根据班级id查询对应的班级信息,包括学生,老师2). 创建表和数据：CREATE TABLE student(    s_id INT PRIMARY KEY AUTO_INCREMENT,     s_name VARCHAR(20),     class_id INT);ALTER TABLE student ADD CONSTRAINT fk_class_id FOREIGN KEY (class_id) REFERENCES class(c_id);   INSERT INTO student(s_name, class_id) VALUES('赵英龙', 1);INSERT INTO student(s_name, class_id) VALUES('杨帆', 1);INSERT INTO student(s_name, class_id) VALUES('王爽', 1);INSERT INTO student(s_name, class_id) VALUES(‘吴亚明’', 2);INSERT INTO student(s_name, class_id) VALUES('赵文宇', 2);3). 定义实体类：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;}4). 定义sql映射文件ClassMapper.xml<!-- 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 teacher t inner join class c  on(t.t_id=c.teacher_id)     inner join student s on(c.c_id=s.class_id) where c.c_id=1</select><resultMap type="Classes" id="ClassResultMap3">    <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>    <!-- 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="getStudents"></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="getStudents" parameterType="int" resultType="Student">    SELECT s_id id, s_name name FROM student WHERE class_id=#{id}</select>5). 测试：public Classes getClassesById3(Integer id) {        Classes classes = null;        String statement = "com.vo.classMapper.getClass3";        try{            classes = session.selectOne(statement, id);        }finally{            session.close();        }        return classes;    }public Classes getClassesById4(Integer id) {        Classes classes = null;        String statement = "com.vo.classMapper.getClass4";        try{            classes = session.selectOne(statement, id);        }finally{            session.close();        }        return classes;    }

七、动态SQL与模糊查询

7.1. 提出需求: 实现多条件查询用户(姓名模糊匹配, 年龄在指定的最小值到最大值之间)7.2. 准备数据表和数据:create table d_user(      id int primary key auto_increment,      name varchar(10),    age int(3)); insert into d_user(name,age) values('小明1',20);insert into d_user(name,age) values('小明2',21);  insert into d_user(name,age) values('小明3',22);  insert into d_user(name,age) values('小明4',28);  insert into d_user(name,age) values('小红',23);  insert into d_user(name,age) values('小红',18);7.3. User(表实体类)private int id;private String name;private int age;7.4. ConditionUser(查询条件实体类)private String name;private int minAge;private int maxAge;7.5. userMapper.xml(映射文件)<?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.po.userMapper">    <select id="getUsers" parameterType="com.vo.ConditionUser"        resultType="User">        select * from d_user where 1=1        <if test='name!=null and name!=""'> and name like CONCAT('%','${name}','%')</if>        <if test='minAge!=null'> and age>=#{minAge}</if>        <if test='maxAge!=null'> and age&lt;=#{maxAge}</if>    </select></mapper>7.6. UserTest(测试)    public List<User> getUsers(ConditionUser conditionUser) {        List<User> list;        try {            String statement = "com.po.userMapper.getUsers";            list = sqlSession.selectList(statement, conditionUser);        } finally {            SessionFactoryUtil.closeSqlSession(sqlSession);        }        return list;    }public static void main(String[] args) {        UserDaoImpl dao = new UserDaoImpl();        ConditionUser conditionUser = new ConditionUser();        conditionUser.setName("小明");        conditionUser.setMinAge(21);        conditionUser.setMaxAge(28);        List<User> users = dao.getUsers(conditionUser);        System.out.println(users);    }       }