For a while now, I've used H2 as the in-memory database for a bunch of Selenium and Spring Boot integration tests. The primary advantages are 1. speed and 2. simplicity.

Simplicity is the big thing here; I can configure Spring to launch an in-memory H2 database and not have to worry about state, clearing and reseeding, or any of that stuff. The integration tests can just run.

I've been so happy with it that I'm pushing a customer to use H2 in production. I would normally use Postgres, but my customer doesn't really have an IT staff, and an embedded database will be a lot easier for them to keep up and running.