MemoryLeak: The Spring RDF4JTemplate does not close connections

[jwesselink]

Current Behavior

I'm encountering a memory leak issue caused by RDF4JTemplate not properly closing connections.

The RDF4JTemplate implementation retrieves a connection from the repositoryConnectionFactory but fails to close it. Since a new connection is created for each request, memory usage increases rapidly, leading to a significant memory leak.

According to the Javadoc for Repository, it’s explicitly stated that connections must be closed after use.

I attempted to resolve the issue by overriding consumeConnection and applyToConnection, wrapping the logic in a try-with-resources block. However, this led to problems in my unit tests using MemoryStore, where I started encountering “Connection is already closed” exceptions.

What I don’t understand is why nobody else seems to have run into this issue. For instance, connection pooling also doesn’t work as expected—because connections are never closed, they’re not returned to the pool, eventually exhausting it.

Expected Behavior

The RDF4JTemplate should close the connection and not cause memory leaks.

Steps To Reproduce

Execute the RDF4JTemplate.update query inside a loop, repeating the call 500 times. You'll observe a steady increase in memory usage, or—depending on your JVM memory settings—encounter a java.lang.OutOfMemoryError: Java heap space exception.

Version

5.1.3

Are you interested in contributing a solution yourself?

None

Anything else?

No response

[fkleedorfer]

Could you share your spring configuration? The RDF4JTemplate integrates with spring's transaction system and should close connections (unless you are also using the transaction pool)

[fkleedorfer]

The RDF4JTemplate is not responsible for opening or closing connections - It would also be helpful to see the code that causes the OOME.

[jwesselink]

Thanks for taking the time to respond!

I quickly extracted some code from an existing project into a new demo project.
You can check it out here: https://github.com/jwesselink/spring-rdf4j-issue

Take a look at the DemoServiceTest. I’ve added a test that demonstrates all the open connections after the code has run.

[fkleedorfer]

Cannot test your code right now but there are a few things that I notice:

• In the spring config, a Repository bean is defined. That should not be necessary. The system should instantiate it.
• In the application.yml file, the manager-url is specified but not the repo name (rdf4j.spring.repository.remote.name, I believe)

With the second point it makes sense that you had to instantiate your own repo, which may trip things up (but it might be just fine)

You are disabling spring-tx, and that may be the culprit. It is true that there is no mechanism other than spring transaction integration that would cause connections to close. If you need that, I'd have to look into providing a fallback mechanism.

If you want to test this suspicion, try

• adding the 'name' property, making sure that the manager-url points to the manager (server root, I think), not the repo.
• removing your repository bean
• annotate your service methods with @transactional and activate spring-tx in application.yml

[aschwarte10]

Hi @jwesselink

is this issue still relevant? Did you have a chance to try out the suggestions from the previous comment?

We are currently cleaning up our backlog and I propose to close this issue for now. If you think there is a bug in RDF4J, please reopen.

Thanks,
Andreas