Spring Boot - JpaRepository not initializing when using multiple DataSources

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06-10-2024

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Spring Boot: Why JpaRepository Fails to Initialize with Multiple DataSources

Have you ever encountered the frustrating issue where your JpaRepository refuses to initialize properly when using multiple DataSources in your Spring Boot application? This problem often arises from the default behavior of Spring Data JPA, and understanding the underlying cause is crucial for finding the right solution.

The Problem: Missing Configuration

Let's imagine you're building an application with two separate databases, one for user data and another for product information. You might create two different DataSource beans, each configured to connect to their respective databases. However, if you're using a single EntityManagerFactory and TransactionManager without proper configuration, your JpaRepository will likely fail to recognize and interact with the correct database.

Here's an example of the problem:

@Configuration
public class DatabaseConfig {

    @Bean
    @ConfigurationProperties("spring.datasource.user")
    public DataSource userDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean
    @ConfigurationProperties("spring.datasource.product")
    public DataSource productDataSource() {
        return DataSourceBuilder.create().build();
    }

    // Missing configuration for EntityManagerFactory and TransactionManager
}

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // ...
}

@Entity
@Table(name = "users", schema = "user_db")
public class User {
    // ...
}

The Error: When trying to save a User entity, you might receive an error message like "Entity 'User' does not belong to entity manager" or "No identifier specified for entity." This signals that the UserRepository is not correctly linked to the userDataSource.

The Solution: Multi-Datasource Configuration

To overcome this challenge, you need to explicitly configure multiple EntityManagerFactory and TransactionManager instances, each tied to a specific DataSource. Spring Boot provides a convenient way to achieve this using the @Primary annotation, which designates the default EntityManagerFactory and TransactionManager.

Here's how to fix the previous example:

@Configuration
public class DatabaseConfig {

    @Primary
    @Bean
    @ConfigurationProperties("spring.datasource.user")
    public DataSource userDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Bean
    @ConfigurationProperties("spring.datasource.product")
    public DataSource productDataSource() {
        return DataSourceBuilder.create().build();
    }

    @Primary
    @Bean
    public LocalContainerEntityManagerFactoryBean entityManagerFactory(EntityManagerFactoryBuilder builder) {
        return builder
                .dataSource(userDataSource())
                .packages("com.example.user")
                .persistenceUnit("user")
                .build();
    }

    @Primary
    @Bean
    public PlatformTransactionManager transactionManager(EntityManagerFactory entityManagerFactory) {
        return new JpaTransactionManager(entityManagerFactory);
    }

    @Bean
    public LocalContainerEntityManagerFactoryBean productEntityManagerFactory(EntityManagerFactoryBuilder builder) {
        return builder
                .dataSource(productDataSource())
                .packages("com.example.product")
                .persistenceUnit("product")
                .build();
    }

    @Bean
    public PlatformTransactionManager productTransactionManager(EntityManagerFactory productEntityManagerFactory) {
        return new JpaTransactionManager(productEntityManagerFactory);
    }
}

@Repository
@Transactional(value = "productTransactionManager")
public interface ProductRepository extends JpaRepository<Product, Long> {
    // ...
}

@Entity
@Table(name = "products", schema = "product_db")
public class Product {
    // ...
}

Key points:

• Each DataSource is configured with a corresponding EntityManagerFactory and TransactionManager.
• The @Primary annotation is applied to the default EntityManagerFactory and TransactionManager, which will be used by default for repositories not explicitly configured.
• The persistenceUnit property in EntityManagerFactoryBuilder allows specifying the unique name for each persistence unit.
• The @Transactional annotation in ProductRepository explicitly specifies the transaction manager to use for its operations.

Additional Considerations:

• Data Source Qualifier: You can use the @Qualifier annotation to specify the exact DataSource to use in your repository, avoiding the need for @Primary and providing clearer intent.
• Persistence Unit Names: Make sure to provide unique persistence unit names for each EntityManagerFactory to prevent naming conflicts.
• Dynamic Data Source Routing: For more complex scenarios involving multiple databases and dynamic routing decisions, consider using libraries like Spring Data Multi-Tenancy or Apache ShardingSphere.

By understanding the intricacies of multi-datasource configuration in Spring Boot and implementing these best practices, you can ensure smooth operation of your JpaRepository and other data access components.