定位sql需要很长的字符串,namespace+id的使用方式很容易出错，因为是string类型的，编译时没发检查，写错运行时才会抛出异常。所以，mybatis提供了一种非常好的设计方式来避免这种问题，即Mapper接口。namespace的值统一为接口的全路径。
接口开发步骤：[]里是你自己需要的名字
1.修改映射文件[User]Mapper.xml
映射文件中的namspace的值是接口的包名+接口的类名
dao.[User]MapperDao(绑定Mapper文件和接口的关系)
(Dao层就是和数据库交互的)
2.创建[User]Mapper接口(interface)
2.1接口所在包的包名+接口名=[User]Mapper.xml中namespace的值
(绑定接口和映射文件的关系)
2.2接口中的方法名=[User]Mapper.xml中定义SQL的id值
(绑定了将要执行的SQL的id)
2.3接口中的方法的返回值类型和参数类型要和UserMapper.xml
映射文件中resultType的类型一致
3.在核心配置文件sqlMapConfig.xml的<mappers>中引入
<mappr resource="[mapper]/[User]Mapper.xml">

<mapper namespace="dao.UserMapperDao"></mapper>

获取接口实现类对象，getMapper方法能基于类全名找到对应名称空间，然后基于名称空间找到映射文件，最后基于你的方法在映射文件找到与元素id相同的方法。

UserMapperDao dao = session.getMapper(UserMapperDao.class);

1、在mysql中执行下面的sql

CREATE DATABASE mybatisdb;
USE mybatisdb;
CREATE TABLE USER(id INT PRIMARY KEY AUTO_INCREMENT,NAME VARCHAR(100),addr VARCHAR(100),age INT);
INSERT INTO USER VALUES(NULL,'张三','北京',28);
INSERT INTO USER VALUES(NULL,'李四','上海',30);
INSERT INTO USER VALUES(NULL,'王五','广州',19);

2、创建maven工程

pom.xml输入如下内容，保存更新项目

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <groupId>com.company</groupId>
    <artifactId>mybatisv2</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>
    <dependencies>
        <!--添加mysql驱动程序依赖 -->
        <dependency>
            <groupId>mysql</groupId>
            <artifactId>mysql-connector-java</artifactId>
            <version>5.1.40</version>
        </dependency>
        <dependency>
            <groupId>junit</groupId>
            <artifactId>junit</artifactId>
            <version>4.11</version>
        </dependency>
        <dependency>
            <groupId>org.mybatis</groupId>
            <artifactId>mybatis</artifactId>
            <version>3.2.8</version>
        </dependency>
        <!--日志包依赖 -->
        <dependency>
            <groupId>log4j</groupId>
            <artifactId>log4j</artifactId>
            <version>1.2.17</version>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <version>3.5.1</version>
                <configuration>
                    <source>1.8</source>
                    <target>1.8</target>
                    <encoding>UTF-8</encoding>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

3像下面的方式创建对应的文件,在对应的文件添加代码

UserMapperDao接口

/usermapper/src/main/java/dao/UserMapperDao.java

注意：
1、 包名 + 类名 = UserMapper.xml中namespace的值
2、 接口中方法名 = UserMapper.xml中定义的SQL的id值
3、 方法的返回值类型(resultType)和参数类型(parameterType或sql中的参数(parameterType可以不传))要和UserMapper.xml映射文件中类型一致

package dao;

import java.util.List;

import pojo.User;

/**
 * 这个类用来完成用户表的业务
 * 
 * @author Administrator
 *
 */
public interface UserMapperDao {
    // <select id="findAll" resultType="User">
    // select * from user
    // </select>
    // 查询所有表的所有数据
    public List<User> findAll();

    // <select id="count" resultType="int">
    // select count(*) from user
    // </select>
    public int count();

    // <select id="findOne" parameterType="int" resultType="User">
    // select * from user where id=#{id}
    // </select>
    public User findOne(int id);

    // <insert id="save" parameterType="User">
    // insert into user values(null,#{name},#{addr},#{age})
    // </insert>
    public void save(User user);

    // <update id="updateUser" parameterType="User" >
    // update user <set> age=#{age}</set> <where>name=#{name}</where>
    // </update>
    public void updateUser(User user);

    // <delete id="deleteUser" parameterType="User">
    // delete from user where name=#{name}
    // </delete>
    public void deleteUser(User user);
}

User

/usermapper/src/main/java/pojo/User.java

package pojo;
/**
 * 这个类用来和user表做映射关系
 * 要求:属性名必须和表字段名保持一致
 */
public class User {
    //id
    private int id;
    //用户名
    private String name;
    //地址
    private String addr;
    //年龄
    private int age;
    public int getId() {
        return id;
    }
    public void setId(int id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getAddr() {
        return addr;
    }
    public void setAddr(String addr) {
        this.addr = addr;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    @Override
    public String toString() {
        return "User [id=" + id + ", name=" + name + ", addr=" + addr + ", age=" + age + "]";
    }
    
}

UserMapper

/usermapper/src/main/resources/mapper/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">
<!--映射文件，写SQL，namespace唯一标识  -->
<mapper namespace="dao.UserMapperDao">
    <!--查询所有记录
    id唯一标识
    resultType把结果封装在对象上，要写类的全路径-->
    <select id="findAll" resultType="User">
        select * from user
    </select>
    <select id="count" resultType="int">
        select count(*) from user
    </select>
    <!--根据id查询记录-->
    <select id="findOne" parameterType="int" resultType="User">
        select * from user where id=#{id}
    </select>
    <!--新增一条记录  -->
    <insert id="save" parameterType="User">
        insert into user values(null,#{name},#{addr},#{age})
    </insert>
    <!--修改指定记录
    动态取值:#{age}
    age就是user对象的age属性
    -->
    <update id="updateUser"  parameterType="User" >
        update user <set> age=#{age}</set> <where>name=#{name}</where>  
    </update>
    <delete id="deleteUser" parameterType="User">
        delete from user where name = #{name}
    </delete>
</mapper>

db.properties

/usermapper/src/main/resources/db.properties

driver:com.mysql.jdbc.Driver
url:jdbc:mysql:///mybatisdb
username:root
password:123456

log4j.properties

/usermapper/src/main/resources/log4j.properties

# Global logging configuration
log4j.rootLogger=DEBUG, stdout
# Console output...
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

sqlMapConfig.xml

/usermapper/src/main/resources/sqlMapConfig.xml

<?xml version="1.0" encoding="UTF-8"?>
<!--xml约束信息-->
<!DOCTYPE configuration
  PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 "http://mybatis.org/dtd/mybatis-3-config.dtd">
 <configuration>
    <!--引入数据源的配置文件
    resource指定文件的位置-->
    <properties resource="db.properties"></properties>
    <!-- 设置别名 -->
    <typeAliases>
        <typeAlias type="pojo.User" alias="User"/>
    </typeAliases>
    <!-- 配置开发环境 ，default用来设置默认的开发环境-->
    <environments default="test">
        <!--测试环境-->
        <environment id="test">
            <!--事务管理器:mybatis的事务交个jdbc处理-->
            <transactionManager type="jdbc"></transactionManager>
            <!--连接池:配置数据源，连接池  -->
            <dataSource type="pooled">
            <!--数据库驱动的名字-->
                <property name="driver" value="${driver}"/>
                <!-- 数据库的url地址-->
                <property name="url" value="${url}"/>
                <!-- 数据库的用户名-->
                <property name="username" value="${username}"/>
                <!-- 数据库的密码-->
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
     <!--引入映射文件 -->
    <mappers>
        <!--引入UserMapper文件
        resource指定文件的位置
         -->
        <mapper resource="mapper/UserMapper.xml"/>
    </mappers>
 </configuration>

InterfaceTest.java

/usermapper/src/test/java/test/InterfaceTest.java

package test;

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

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.Before;
import org.junit.Test;

import dao.UserMapperDao;
import pojo.User;

/**
 * 这个类用来测试Mapper接口
 * @author Administrator
 *
 */
public class InterfaceTest {
    SqlSessionFactory ssf=null;
    @Before
    public void init(){
        //获取会话工厂
        try {
            InputStream in = Resources.getResourceAsStream("sqlMapConfig.xml");
            ssf = new SqlSessionFactoryBuilder().build(in);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //查询user表的所有数据
    @Test
    public void findAll(){
        //获取sqlsession，执行SQL
        SqlSession session = ssf.openSession();
        //利用接口方法
        UserMapperDao dao = session.getMapper(UserMapperDao.class);
        List<User> list = dao.findAll();
        //处理结果
        for (User user : list) {
            System.out.println(user);
        }
        //释放资源
        session.close();
    }
    @Test
    public void count() {
        // 2.创建sqlsession对象,执行sql
        SqlSession session = ssf.openSession();
        UserMapperDao dao = session.getMapper(UserMapperDao.class);
        int i = dao.count();
        // 3.处理结果
        System.out.println("count=" + i);
        // 4.释放资源
        session.close();
    }
    
    //根据id查询记录
        @Test
        public void findOne(){
            // 2.创建sqlsession对象,执行sql
            SqlSession session = ssf.openSession();
            UserMapperDao dao = session.getMapper(UserMapperDao.class);
            User user=dao.findOne(1);
            System.out.println(user);
            // 4.释放资源
            session.close();
        }
        //新增一条记录
        @Test
        public void save(){
            //创建sqlsession,执行sql
            SqlSession session = ssf.openSession();
            UserMapperDao dao = session.getMapper(UserMapperDao.class);
            //利用sqlsession执行插入的sql
            User user=new User();
            user.setName("张三");
            user.setAge(28);
            user.setAddr("河北");
            dao.save(user);
            session.commit();
            //释放资源
            session.close();
        }
        //修改指定记录
        @Test
        public void updateUser(){
            //创建sqlsession,执行sql
            SqlSession session = ssf.openSession();
            UserMapperDao dao = session.getMapper(UserMapperDao.class);
            User user=new User();
            user.setAge(9);
            user.setName("张三");
            dao.updateUser(user);
            session.commit();
            //释放资源
            session.close();
        }
        //删除指定记录
        @Test
        public void deleteUser(){
            //创建sqlsession,执行sql
            SqlSession session = ssf.openSession(true);
            UserMapperDao dao = session.getMapper(UserMapperDao.class);
            User user=new User();
            user.setName("张三");
            dao.deleteUser(user);
            session.close();
        }
}

如果在User类中加入输出System.out.println("User.setId()");

    public void setId(int id) {
        System.out.println("User.setId()");
        this.id = id;
    }

DEBUG [main] - ==>  Preparing: select * from user 
DEBUG [main] - ==> Parameters: 
User.setId()
User.setId()
User.setId()
User.setId()
User.setId()
DEBUG [main] - <==      Total: 5
User [id=1, name=张三, addr=北京, age=28]
User [id=2, name=李四, addr=上海, age=6]
User [id=3, name=王五, addr=广州, age=19]
User [id=4, name=张三, addr=河北, age=28]
User [id=5, name=张三, addr=河北, age=28]
DEBUG [main] - Resetting autocommit to true on JDBC Connection [com.mysql.jdbc.JDBC4Connection@4973813a]
DEBUG [main] - Closing JDBC Connection [com.mysql.jdbc.JDBC4Connection@4973813a]
DEBUG [main] - Returned connection 1232306490 to pool.

会发现底层会调用类的set方法,不过当set方法不存在是，底层会直接通过反射为私有属性赋值