How do you configure Spring Data JPA to use multiple data sources in a Spring Boot application?

Spring Boot, Spring Data JPA with Multiple DataSources

In Spring Boot, using multiple data sources allows for connecting different repositories to separate data sources. To achieve this, we use annotations and configurations to specify the data source and transaction manager for each repository.

Configuration

CustomerDataSourceConfiguration.java (First Data Source)

<code class="java">@Configuration
@EnableJpaRepositories(
        entityManagerFactoryRef = "customerEntityManager",
        transactionManagerRef = "customerTransactionManager",
        basePackages = {"com.mm.repository.customer"})
public class CustomerDataSourceConfiguration {

    @Bean(name = "customerDataSource")
    public DataSource dataSource() {
        return DataSourceBuilder.create()
                .url("jdbc:h2:mem:customer:H2")
                .driverClassName("org.h2.Driver")
                .username("sa")
                .password("")
                .build();
    }

    @Bean(name = "customerEntityManager")
    public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
        LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
        em.setDataSource(dataSource());
        em.setPackagesToScan(new String[] {"com.mm.domain.customer"});

        JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        em.setJpaVendorAdapter(vendorAdapter);
        em.setJpaProperties(additionalJpaProperties());
        em.setPersistenceUnitName("customerPersistence");
        em.setPackagesToScan("com.mm.domain.customer");

        return em;
    }

    Properties additionalJpaProperties() {
        Properties properties = new Properties();
        properties.setProperty("hibernate.hbm2ddl.auto", "create-drop");
        properties.setProperty("hibernate.dialect", "org.hibernate.dialect.H2Dialect");
        properties.setProperty("hibernate.show_sql", "true");

        return properties;
    }

    @Bean(name = "customerTransactionManager")
    public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
        JpaTransactionManager transactionManager = new JpaTransactionManager();
        transactionManager.setEntityManagerFactory(emf);

        return transactionManager;
    }
}</code>

GenericDataSourceConfiguration.java (Second Data Source)

<code class="java">@Configuration
@EnableJpaRepositories(
        entityManagerFactoryRef = "genericEntityManager",
        transactionManagerRef = "genericTransactionManager",
        basePackages = {"com.mm.repository.generic"})
public class GenericDataSourceConfiguration {

    @Bean(name = "genericDataSource")
    public DataSource dataSource() {
        return DataSourceBuilder.create()
                .url("jdbc:h2:mem:generic:H2")
                .driverClassName("org.h2.Driver")
                .username("sa")
                .password("")
                .build();
    }

    @Bean(name = "genericEntityManager")
    public LocalContainerEntityManagerFactoryBean entityManagerFactory() {
        LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
        em.setDataSource(dataSource());
        em.setPackagesToScan(new String[] {"com.mm.domain.generic"});

        JpaVendorAdapter vendorAdapter = new HibernateJpaVendorAdapter();
        em.setJpaVendorAdapter(vendorAdapter);
        em.setJpaProperties(additionalJpaProperties());
        em.setPersistenceUnitName("genericPersistence");
        em.setPackagesToScan("com.mm.domain.generic");

        return em;
    }

    Properties additionalJpaProperties() {
        Properties properties = new Properties();
        properties.setProperty("hibernate.hbm2ddl.auto", "create-drop");
        properties.setProperty("hibernate.dialect", "org.hibernate.dialect.H2Dialect");
        properties.setProperty("hibernate.show_sql", "true");

        return properties;
    }

    @Bean(name = "genericTransactionManager")
    public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
        JpaTransactionManager transactionManager = new JpaTransactionManager();
        transactionManager.setEntityManagerFactory(emf);

        return transactionManager;
    }
}</code>

Application.java

<code class="java">@Configuration
@ComponentScan
@EnableAutoConfiguration
public class Application extends SpringApplication {

    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }

    @Bean
    public ServletRegistrationBean h2Console() {
        ServletRegistrationBean reg = new ServletRegistrationBean(new WebServlet(), "/console/*");
        reg.setLoadOnStartup(1);
        return reg;
    }
}</code>

Entities and Repositories

Customer.java (Entity)

<code class="java">@Entity
@Table(name = "customer")
@Data
@EqualsAndHashCode(exclude = {"id"})
public class Customer {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @Column(name = "name", nullable = false)
    private String name;

    @Column(name = "age", nullable = false)
    private Integer age;

    ....
}</code>

Order.java (Entity)

<code class="java">@Entity
@Table(name = "order")
@Data
@EqualsAndHashCode(exclude = {"id"})
public class Order {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @Column(name = "code", nullable = false)
    private Integer code;

    @Column(name = "quality", nullable = false)
    private Integer quality;
}</code>

CustomerRepository.java (Repository for First Data Source)

<code class="java">public interface CustomerRepository extends JpaRepository<Customer, Integer> {}</code>

GenericRepository.java (Repository for Second Data Source)

<code class="java">public interface GenericRepository extends JpaRepository<Generic, Integer> {}</code>

By using this approach, Spring Data JPA will create separate EntityManagerFactories and TransactionManagers for each data source, enabling seamless integration with multiple databases in a Spring Boot application.

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