This post is a general start guidance of Mybatis.

1. Introduction

What is MyBatis?

MyBatis is a first class persistence framework with support for custom SQL, stored procedures and advanced mappings. MyBatis eliminates almost all of the JDBC code and manual setting of parameters and retrieval of results. MyBatis can use simple XML or Annotations for configuration and map primitives, Map interfaces and Java POJOs (Plain Old Java Objects) to database records.

2. Getting Started

2.1 Installation

To use MyBatis you just need to include the mybatis-x.x.x.jar file in the classpath.

If you are using Maven just add the following dependency to your pom.xml:

<dependency>
  <groupId>org.mybatis</groupId>
  <artifactId>mybatis</artifactId>
  <version>x.x.x</version>
</dependency>

2.2 Building SqlSessionFactory from XML

Every MyBatis application centers around an instance of SqlSessionFactory. A SqlSessionFactory instance can be acquired by using the SqlSessionFactoryBuilder. SqlSessionFactoryBuilder can build a SqlSessionFactory instance from an XML configuration file, or from a custom prepared instance of the Configuration class.

Building a SqlSessionFactory instance from an XML file is very simple. It is recommended that you use a classpath resource for this configuration, but you could use any InputStream instance, including one created from a literal file path or a file:// URL. MyBatis includes a utility class, called Resources, that contains a number of methods that make it simpler to load resources from the classpath and other locations.

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory =
  new SqlSessionFactoryBuilder().build(inputStream);

The configuration XML file contains settings for the core of the MyBatis system, including a DataSource for acquiring database Connection instances, as well as a TransactionManager for determining how transactions should be scoped and controlled. The full details of the XML configuration file can be found later in this document, but here is a simple example:

<?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="${driver}"/>
        <property name="url" value="${url}"/>
        <property name="username" value="${username}"/>
        <property name="password" value="${password}"/>
      </dataSource>
    </environment>
  </environments>
  <mappers>
     <!-- Every Mapper.XML needs to be stated here-->
    <mapper resource="org/mybatis/example/BlogMapper.xml"/>
  </mappers>
</configuration>

While there is a lot more to the XML configuration file, the above example points out the most critical parts. Notice the XML header, required to validate the XML document. The body of the environment element contains the environment configuration for transaction management and connection pooling. The mappers element contains a list of mappers – the XML files and/or annotated Java interface classes that contain the SQL code and mapping definitions.

## 2.3 mybatis Utils Class

Now that you have a SqlSessionFactory, as the name suggests, you can acquire an instance of SqlSession. The SqlSession contains absolutely every method needed to execute SQL commands against the database. You can execute mapped SQL statements directly against the SqlSession instance.

package com.danny.utils;


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 java.io.IOException;
import java.io.InputStream;

public class MybatisUtils {

    //sqlSessionFactory --> sqlSession
    private static SqlSessionFactory sqlSessionFactory;

    static {
        try{
            // The first step: Obtain a sqlSession object
            String resource = "mybatis-config.xml";
            InputStream inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        }catch(IOException e){
            e.printStackTrace();
        }
    }

    //Since we have sqlSessionFactory,we can instantiate sqlSession Instances
    // SqlSession contains all the functions that execute SQL commands
    
    public static SqlSession getSqlSession(){
        return sqlSessionFactory.openSession();
    }

    public static void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
        MybatisUtils.sqlSessionFactory = sqlSessionFactory;
    }
}

2.4 Writing Entity Class

• pojo directory:

package com.danny.pojo;

public class User {
    private int id;
    private String name;
    private String pwd;

    public User() {
    }

    public User(int id, String name, String pwd) {
        this.id = id;
        this.name = name;
        this.pwd = pwd;
    }

    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    public String getPwd() {
        return pwd;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

    public void setPwd(String pwd) {
        this.pwd = pwd;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", pwd='" + pwd + '\'' +
                '}';
    }

}

• Dao interface:

  package com.danny.dao;
    
  import com.danny.pojo.User;
    
  import java.util.List;
    
  public interface UserDao {
      List<User> getUserList();
  }
  

2.5 Exploring Mapped SQL Statements

​ At this point you may be wondering what exactly is being executed by the SqlSession or Mapper class. The topic of Mapped SQL Statements is a big one, and that topic will likely dominate the majority of this documentation. But to give you an idea of what exactly is being run, here are a couple of examples.

​ In either of the examples above, the statements could have been defined by either XML or Annotations. Let’s take a look at XML first. The full set of features provided by MyBatis can be realized by using the XML based mapping language that has made MyBatis popular over the years. If you’ve used MyBatis before, the concept will be familiar to you, but there have been numerous improvements to the XML mapping documents that will become clear later. Here is an example of an XML based mapped statement that would satisfy the above SqlSession calls.

<?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">
<!--namespace=combine a corresponding Dao/Mapper interface-->
<mapper namespace="com.danny.dao.UserDao">

    <!-- SQL statements -->
   <select id="getUserList" resultType="com.danny.pojo.User">
       select * from mybatis.user
   </select>

</mapper>

2.5 Test

Notice that due to the fact that the convention of Maven is greater than its configuration, we may encounter the problem that our configuration file cannot be exported, the solution:

Add the following statements at the end of pom.XML

<build>
    <resources>
        <resource>
            <directory>src/main/resources</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
        <resource>
            <directory>src/main/java</directory>
            <includes>
                <include>**/*.properties</include>
                <include>**/*.xml</include>
            </includes>
            <filtering>true</filtering>
        </resource>
    </resources>
</build>

• The following is the testing code:

package com.danny.dao;

import com.danny.pojo.User;
import com.danny.utils.MybatisUtils;
import org.apache.ibatis.session.SqlSession;
import org.junit.Test;

import java.util.List;

public class UserDaoTest {

    @Test
    public void test(){

        //obtain sqlSession Object
        SqlSession sqlSession = MybatisUtils.getSqlSession();

        try{
            //getMapper
            UserDao userDao = sqlSession.getMapper(UserDao.class);
            List<User> userList = userDao.getUserList();

            for (User user : userList) {
                System.out.println(user);
            }


        }catch (Exception e){
            e.printStackTrace();
        }finally {
            sqlSession.close();
        }
    }
}

2.6 CRUD

• The package name in namespace has to be accordance with the interfaces in Dao/mapper.
• Select: SQL query statement.
  • id: Corresponding function name in namesopace.
  • resultType: Returning value of SQL statement.
  • parameterType: Corresponding type of the passing parameter.

1. Write interfaces in Dao/userMapper:

   package com.danny.dao;
      
   import com.danny.pojo.User;
      
   import java.util.List;
      
   public interface UserMapper {
       // Query all the users
       List<User> getUserList();
       // Query users according to ID
       User getUserId(int id);
       // Insert one user
       int addUser(User user);
       // Modify a user
       int updateUser(User user);
   }
   

2. Write the corresponding SQL statements in Mapper.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">
   <!--namespace=combine a corresponding Dao/Mapper interface-->
   <mapper namespace="com.danny.dao.UserMapper">
      
       <!-- SQL statements -->
       <!--Selection-->
      <select id="getUserList" resultType="com.danny.pojo.User">
          select * from mybatis.user
      </select>
      
       <select id="getuserId" parameterType="int" resultType="com.danny.pojo.User">
           select * from mybatis.user where id = #{id}
       </select>
      
       <!--Insertion: the properties in the object can be pulled out-->
       <insert id="addUser" parameterType="com.danny.pojo.User">
           insert into mybatis.user (id, name, pwd) values (#{id},#{name},#{pwd});
       </insert>
      
       <update id="updateUser" parameterType="com.danny.pojo.User">
           update mybatis.user set name=#{name},pwd=#{pwd} where id =#{id} ;
       </update>
   </mapper>
   

3. Test with Junit (Insertion, Deletion, Modification need to be committed ):

   package com.danny.dao;
      
   import com.danny.pojo.User;
   import com.danny.utils.MybatisUtils;
   import org.apache.ibatis.session.SqlSession;
   import org.junit.Test;
      
   import java.util.List;
      
   public class UserDaoTest {
      
       // Test selection
       @Test
       public void test(){
      
           //obtain sqlSession Object
           SqlSession sqlSession = MybatisUtils.getSqlSession();
      
           try{
               //getMapper
               UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
               List<User> userList = userMapper.getUserList();
      
               for (User user : userList) {
                   System.out.println(user);
               }
      
      
           }catch (Exception e){
               e.printStackTrace();
           }finally {
               sqlSession.close();
           }
       }
      
       // Test getter
       @Test
       public void getUser(){
           SqlSession sqlsession = MybatisUtils.getSqlSession();
           UserMapper mapper = sqlsession.getMapper(UserMapper.class);
           User user =mapper.getUserId(1);
           System.out.println(user);
           sqlsession.close();
       }
      
       // Test add user
       @Test
       public void addUser(){
           SqlSession sqlSession = MybatisUtils.getSqlSession();
      
           UserMapper mapper = sqlSession.getMapper(UserMapper.class);
      
           int res = mapper.addUser(new User(3,"hhh","123456"));
           if (res > 0){
               System.out.println("Insertion succeeded");
           }
      
           // Commit changes
           sqlSession.commit();
      
           sqlSession.close();
       }
      
       // Test modify user
       @Test
       public void updateUser(){
           SqlSession sqlSession = MybatisUtils.getSqlSession();
           UserMapper mapper = sqlSession.getMapper(UserMapper.class);
      
          mapper.updateUser(new User(1,"hehe","123123"));
      
          sqlSession.commit();
          sqlSession.close();
       }
   }