How to view SQL statements in MyBatis

When using MyBatis for database operations, sometimes we want to view the generated SQL statements for debugging and optimization. This article will introduce how to achieve this in MyBatis through configuration and specific code examples. View the SQL statement.

1. Configure MyBatis to display SQL statements

To view SQL statements in MyBatis, you first need to do some configuration. Add the following configuration to the MyBatis configuration file mybatis-config.xml:

<configuration>
    <settings>
        <setting name="logImpl" value="STDOUT_LOGGING" />
    </settings>
</configuration>

This configuration tells MyBatis to use STDOUT_LOGGING as the log outputter, so that when MyBatis executes the SQL statement, it will print out the corresponding SQL statement.

2. Write a Mapper file

Assume there is an entity class corresponding to the User tableUser, let’s write a simple Mapper fileUserMapper.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">

<mapper namespace="com.example.mapper.UserMapper">
    <select id="selectUserById" resultType="com.example.model.User">
        SELECT * FROM user WHERE id = #{id}
    </select>
</mapper>

3. Write Java code

Next, we write Java code to use this Mapper file and view the generated SQL statement. Suppose there is a UserService class:

package com.example.service;

import com.example.model.User;
import com.example.mapper.UserMapper;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;

public class UserService {
    private static final Logger logger = LogManager.getLogger(UserService.class);

    private SqlSessionFactory sqlSessionFactory;

    public UserService(SqlSessionFactory sqlSessionFactory) {
        this.sqlSessionFactory = sqlSessionFactory;
    }

    public User getUserById(int id) {
        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
            UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
            User user = userMapper.selectUserById(id);
            logger.info(sqlSession.getConfiguration().getMappedStatement("com.example.mapper.UserMapper.selectUserById").getBoundSql(user).getSql());
            return user;
        }
    }
}

4. View the generated SQL statement

Finally, we can view the generated SQL statement by calling the getUserById method SQL statement. For example, in the main method:

public static void main(String[] args) {
    String resource = "mybatis-config.xml";
    try (InputStream inputStream = Resources.getResourceAsStream(resource)) {
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        UserService userService = new UserService(sqlSessionFactory);
        User user = userService.getUserById(1);

        // 这里将会输出生成的SQL语句
    } catch (IOException e) {
        e.printStackTrace();
    }
}

After running the program, you will see the SQL statement printed in the log, so you can easily view the SQL statement generated by MyBatis.

Through the above configuration and code examples, you can easily view the generated SQL statements in MyBatis to help you debug and optimize database operations. I wish you progress in your studies!

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