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How Spring Boot Injects `SqlSessionTemplate` for `@Mapper` Interfaces

February 17, 2025 by walter

In Spring Boot with MyBatis, the @Mapper annotation is used to mark an interface as a MyBatis Mapper. Spring Boot automatically injects SqlSessionTemplate behind the scenes to enable SQL execution for these mapper interfaces.

How Spring Boot Injects SqlSessionTemplate for @Mapper Interfaces

Spring Boot and MyBatis integrate through MyBatis-Spring-Boot-Starter, which:

1. Scans for @Mapper interfaces and registers them as Spring beans.
2. Automatically configures a SqlSessionFactory and SqlSessionTemplate.
3. Injects SqlSessionTemplate behind the scenes into each @Mapper interface.

Example Setup

1. Add Dependencies

If using Maven, add:

<dependency>
    <groupId>org.mybatis.spring.boot</groupId>
    <artifactId>mybatis-spring-boot-starter</artifactId>
    <version>3.0.3</version>
</dependency>

2. Define the MyBatis @Mapper Interface

@Mapper
public interface UserMapper {
    @Select("SELECT * FROM users WHERE id = #{id}")
    User findById(@Param("id") Long id);
}

3. Configure MyBatis in application.properties

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=root
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver
mybatis.mapper-locations=classpath:mappers/*.xml

4. Enable @MapperScan in the @SpringBootApplication

@SpringBootApplication
@MapperScan("com.example.mappers")  // Automatically scans @Mapper interfaces
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

5. Use the UserMapper in a Service

@Service
public class UserService {

    @Autowired
    private UserMapper userMapper;

    public User getUserById(Long id) {
        return userMapper.findById(id);
    }
}

How SqlSessionTemplate Works Behind the Scenes

• Spring Boot auto-configures SqlSessionFactory and SqlSessionTemplate.
• MyBatis-Spring framework generates a dynamic proxy for UserMapper and internally uses SqlSessionTemplate.
• No need to manually inject SqlSessionTemplate. It is injected by Spring Boot through SqlSessionFactoryBean.

Manually Defining SqlSessionTemplate (If Needed)

If you want explicit control over SqlSessionTemplate, you can configure it manually:

@Configuration
public class MyBatisConfig {

    @Bean
    public SqlSessionFactory sqlSessionFactory(DataSource dataSource) throws Exception {
        SqlSessionFactoryBean factoryBean = new SqlSessionFactoryBean();
        factoryBean.setDataSource(dataSource);
        return factoryBean.getObject();
    }

    @Bean
    public SqlSessionTemplate sqlSessionTemplate(SqlSessionFactory sqlSessionFactory) {
        return new SqlSessionTemplate(sqlSessionFactory);
    }
}

Summary

✅ Spring Boot automatically injects SqlSessionTemplate into @Mapper interfaces.
✅ No need to manually define SqlSessionTemplate unless you have custom configurations.
✅ @MapperScan allows Spring Boot to detect and register @Mapper interfaces automatically.
✅ UserMapper gets proxied using SqlSessionTemplate, handling database interactions seamlessly.

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