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Learn how to resolve connection limit issues with HikariCP when running integration tests in Spring using Testcontainers.
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How to Fix HikariCP Connection Pool Issues in Spring with Testcontainers

When performing integration tests with Testcontainers in a Spring application, developers sometimes encounter the frustrating issue where the connection pool limit is reached. Specifically, if you’re running multiple tests that each require a new database connection, you might hit a wall after a certain number of tests, usually around the 11th test, causing an exception.

Understanding the Problem

The root of the problem lies in the way database connections are managed in your application code. In typical scenarios, Spring’s context is designed to manage the lifecycle of beans, including closing resources such as database connections. However, when you're testing with Testcontainers and constantly creating new contexts with @ DirtiesContext, some connections might not get closed properly. This leads to an “exceeded connection limit” error:

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Why This Occurs

Bean Lifecycles: The database connections, created by the Hikari connection pool, are not treated as Spring-managed beans due to not being declared properly. Therefore, their lifecycle is not managed, leading to them not being closed when the tests finish.

Connection Overload: Each new connection that is created takes up a slot in the pool, and if they aren't cleaned up because they are not managed, the pool reaches its limit.

Solution: Declare Your DataSource as a Bean

To resolve this issue, it's crucial to ensure that your DataSource is declared as a Spring bean. By doing so, you leverage Spring's management of the lifecycle, which includes automatically closing connections after tests complete.

Steps to Implement the Solution

Modify the DataSource Declaration:
Ensure that your DataSource is defined as a Spring bean in your configuration class.

Here’s an updated version of your ApplicationConfigurationTest class:

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Verify EntityManagerFactory Uses the Bean:
Make sure that your entityManagerFactory method is well-configured to use the defined DataSource bean. It looks like you already have this set up correctly, but ensure that there are no conflicts:

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Lifecycle Management:
Ensure that the HikariDataSource implements AutoCloseable. In your case, it does, and Spring will take care of closing it when the application context is destroyed.

Best Practices

Declare Dependencies as Beans: It’s generally a good practice to declare dependencies that implement AutoCloseable or those that rely on lifecycle callbacks as beans to make sure Spring manages them properly.

Monitor Connection Usage: Use monitoring tools or logs to ensure your connections are being used properly and discharged when no longer needed.

Conclusion

By declaring your DataSource as a Spring bean and ensuring it is managed by Spring’s lifecycle, you will effectively mitigate the issue of hitting connection limits while running integration tests with Testcontainers and HikariCP. This not only simplifies connection management but also improves the overall robustness of your application tests.

Implement these changes, and you should notice a significant reduction in connection-related errors during your tests!