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Discover why your h2 database is empty upon starting your Spring Boot application and learn simple steps to resolve this issue effectively.
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Understanding the Issue: Why is My H2 Database Empty on Startup?

As a developer using Spring Boot with an H2 database, you may encounter a common issue: the database appears empty each time you start the application. This problem can be frustrating, especially if you expect your data to persist between application runs. In this guide, we will delve into the reasons behind this issue and guide you on how to address it effectively.

The Problem

You are working on a Spring Boot application and have configured it to use H2 as an in-memory database. Despite successfully adding entries during your previous runs, upon restarting the application, your userRepository retrieves no users. You notice that the data.mv.db and data.trace.db files are created, indicating that the database is indeed instantiated, but frustratingly, it remains empty.

Here's a quick glance at the relevant configuration and code snippet you have:

Your Database Configuration in application.yml

[[See Video to Reveal this Text or Code Snippet]]

Your DataPopulation Class

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Identifying the Cause

The default behavior of Spring Boot with Hibernate is crucial to understand in this scenario. By default, Hibernate uses the create-drop setting for the ddl-auto property. This means that every time your application starts, Hibernate will create a new database and drop it when the application shuts down. Hence, any data you previously populated is lost on restarting the application.

What are Hibernate DDL Modes?

Hibernate DDL (Data Definition Language) modes determine how the database schema is handled at startup. The modes can be set as follows:

create: Creates the schema, destroying existing data.

create-drop: Similar to create, but drops the schema when the session ends.

update: Updates the schema without losing existing data.

validate: Validates the schema without making any changes.

The Solution: Change the DDL Mode

To resolve the empty database issue, you need to switch from the default create-drop mode to update mode. This allows Hibernate to create the necessary schema on the first run but prevents it from dropping the data upon restarting the application.

Steps to Implement the Solution

Open your application.yml file.

Add the following line to configure the Hibernate DDL mode:

[[See Video to Reveal this Text or Code Snippet]]

What Does spring.jpa.hibernate.ddl-auto=update Do?

It instructs Hibernate to automatically update the database schema with any changes without dropping existing data.

This ensures that your data persists between application restarts, resolving your issue of the empty database.

Conclusion

By changing the Hibernate DDL mode to update, you can ensure that your H2 database retains its data across application restarts. This change alleviates the problem you're experiencing and provides a stable environment to develop your Spring Boot applications.

If you encounter any further issues or need clarification regarding this solution or other Spring Boot configurations, feel free to reach out or drop a comment below! Happy coding!