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8.3 Java Configuration and Multi-DataSource Setup

Learn how to manually configure a DataSource using Java code when bypassing Spring Boot's Auto-Configuration, and how to connect to two or more databases within a single application.

1. Manual DataSource Configuration via Java​

Using Java configuration classes is useful for compatibility with specific libraries or when settings need to be changed dynamically at runtime.

@Configuration
public class DataSourceConfig {

    @Bean
    @ConfigurationProperties("spring.datasource.hikari") // Bind settings from yml
    public DataSource dataSource() {
        // Create manually using DataSourceBuilder
        return DataSourceBuilder.create()
                .type(HikariDataSource.class)
                .build();
    }
}

2. Multi-DataSource Configuration​

This is used when connecting to different databases simultaneously (e.g., MySQL and PostgreSQL) or when separating Read-only (Slave) and Write-only (Master) databases.

2.1 application.yml Setup​

Define unique prefixes to distinguish between the data sources.

spring:
  datasource:
    # Primary Database (Main)
    main:
      jdbc-url: jdbc:mysql://localhost:3306/main_db
      username: root
      password: root
      driver-class-name: com.mysql.cj.jdbc.Driver
    
    # Secondary Database (Sub)
    sub:
      jdbc-url: jdbc:postgresql://localhost:5432/sub_db
      username: user
      password: password
      driver-class-name: org.postgresql.Driver

2.2 Java Configuration Implementation​

Use @Primary and @Qualifier properly to prevent bean conflicts.

@Configuration
public class MultiDataSourceConfig {

    // Main DataSource
    @Primary
    @Bean(name = "mainDataSource")
    @ConfigurationProperties("spring.datasource.main")
    public DataSource mainDataSource() {
        return DataSourceBuilder.create().type(HikariDataSource.class).build();
    }

    // Sub DataSource
    @Bean(name = "subDataSource")
    @ConfigurationProperties("spring.datasource.sub")
    public DataSource subDataSource() {
        return DataSourceBuilder.create().type(HikariDataSource.class).build();
    }

    // Specify Qualifier when using JdbcTemplate
    @Bean
    public JdbcTemplate mainJdbcTemplate(@Qualifier("mainDataSource") DataSource dataSource) {
        return new JdbcTemplate(dataSource);
    }

    @Bean
    public JdbcTemplate subJdbcTemplate(@Qualifier("subDataSource") DataSource dataSource) {
        return new JdbcTemplate(dataSource);
    }
}

3. Practical Tips and Considerations​

  1. Mandatory @Primary: If multiple beans of the same type exist, you must specify which one Spring should inject by default to avoid errors.
  2. ConfigurationProperties Note: In Spring Boot 2.x and later, when binding Hikari settings directly, you often need to use jdbc-url instead of url in your property names.
  3. Transaction Managers: When using multi-DB setups, you should also register separate Transaction Managers (PlatformTransactionManager) and specify them where needed (e.g., @Transactional("mainTransactionManager")).
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