Confgiuring multiple databases in a single Spring Boot application with MyBatis requires defining separate DataSource, SqlSessionFactory, and SqlSessionTemplate beans per database, and mapping each mapper package to the appropriate factory/template.

Maven dependencies

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <dependency>
        <groupId>org.mybatis.spring.boot</groupId>
        <artifactId>mybatis-spring-boot-starter</artifactId>
        <version>2.2.2</version>
    </dependency>

    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>8.0.33</version>
        <scope>runtime</scope>
    </dependency>

    <dependency>
        <groupId>com.alibaba</groupId>
        <artifactId>druid-spring-boot-starter</artifactId>
        <version>1.2.16</version>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

application.properties

# DataSource A
spring.datasource.alpha.url=jdbc:mysql://localhost:3306/db_alpha?useUnicode=true&characterEncoding=utf-8&serverTimezone=UTC
spring.datasource.alpha.username=root
spring.datasource.alpha.password=123456
spring.datasource.alpha.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.alpha.type=com.alibaba.druid.pool.DruidDataSource

# DataSource B
spring.datasource.beta.url=jdbc:mysql://localhost:3306/db_beta?useUnicode=true&characterEncoding=utf-8&serverTimezone=UTC
spring.datasource.beta.username=root
spring.datasource.beta.password=123456
spring.datasource.beta.driver-class-name=com.mysql.cj.jdbc.Driver
spring.datasource.beta.type=com.alibaba.druid.pool.DruidDataSource

DataSource beans

import com.alibaba.druid.spring.boot.autoconfigure.DruidDataSourceBuilder;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;

import javax.sql.DataSource;

@Configuration
public class DataSourceConfig {

    @Bean(name = "alphaDataSource")
    @Primary
    @ConfigurationProperties(prefix = "spring.datasource.alpha")
    public DataSource alphaDataSource() {
        return DruidDataSourceBuilder.create().build();
    }

    @Bean(name = "betaDataSource")
    @ConfigurationProperties(prefix = "spring.datasource.beta")
    public DataSource betaDataSource() {
        return DruidDataSourceBuilder.create().build();
    }
}

MyBatis wiring for DataSource A

import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.mybatis.spring.SqlSessionTemplate;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.core.io.support.PathMatchingResourcePatternResolver;

import javax.sql.DataSource;

@Configuration
@MapperScan(
    basePackages = "com.example.multids.alpha.mapper",
    sqlSessionFactoryRef = "alphaSqlSessionFactory",
    sqlSessionTemplateRef = "alphaSqlSessionTemplate"
)
public class MybatisAlphaConfig {

    @Bean(name = "alphaSqlSessionFactory")
    @Primary
    public SqlSessionFactory alphaSqlSessionFactory(@Qualifier("alphaDataSource") DataSource ds) throws Exception {
        SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
        factory.setDataSource(ds);
        factory.setMapperLocations(
            new PathMatchingResourcePatternResolver().getResources("classpath*:mappers/alpha/*.xml")
        );
        return factory.getObject();
    }

    @Bean(name = "alphaSqlSessionTemplate")
    @Primary
    public SqlSessionTemplate alphaSqlSessionTemplate(
        @Qualifier("alphaSqlSessionFactory") SqlSessionFactory sf
    ) {
        return new SqlSessionTemplate(sf);
    }
}

MyBatis wiring for DataSource B

import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionFactoryBean;
import org.mybatis.spring.SqlSessionTemplate;
import org.mybatis.spring.annotation.MapperScan;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.beans.factory.annotation.Qualifier;
import org.springframework.core.io.support.PathMatchingResourcePatternResolver;

import javax.sql.DataSource;

@Configuration
@MapperScan(
    basePackages = "com.example.multids.beta.mapper",
    sqlSessionFactoryRef = "betaSqlSessionFactory",
    sqlSessionTemplateRef = "betaSqlSessionTemplate"
)
public class MybatisBetaConfig {

    @Bean(name = "betaSqlSessionFactory")
    public SqlSessionFactory betaSqlSessionFactory(@Qualifier("betaDataSource") DataSource ds) throws Exception {
        SqlSessionFactoryBean factory = new SqlSessionFactoryBean();
        factory.setDataSource(ds);
        factory.setMapperLocations(
            new PathMatchingResourcePatternResolver().getResources("classpath*:mappers/beta/*.xml")
        );
        return factory.getObject();
    }

    @Bean(name = "betaSqlSessionTemplate")
    public SqlSessionTemplate betaSqlSessionTemplate(
        @Qualifier("betaSqlSessionFactory") SqlSessionFactory sf
    ) {
        return new SqlSessionTemplate(sf);
    }
}

Mapper packages and XML placement

• Java interfaces
  • com.example.multids.alpha.mapper for DataSource A
  • com.example.multids.beta.mapper for DataSource B
• Mapper XML
  • src/main/resources/mappers/alpha/*.xml
  • src/main/resources/mappers/beta/*.xml

If mapper XML is stored under src/main/java instead of resources, include them during packaging by adding resources configuration to the build section of pom.xml:

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

Verification test

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;

import java.util.List;

@SpringBootTest
class MultiDsTests {

    @Autowired
    private com.example.multids.alpha.mapper.BookMapper alphaBookMapper;

    @Autowired
    private com.example.multids.beta.mapper.BookMapper betaBookMapper;

    @Test
    void readFromBothDatabases() {
        List<?> a = alphaBookMapper.selectAll();
        List<?> b = betaBookMapper.selectAll();
        System.out.println("alpha => " + a);
        System.out.println("beta  => " + b);
    }
}