Ability to atomically compute a data loader if absent

[tinnou]

Description

I use java-dataloader in coordination of graphql-java. I'm running into a use case where I need to lazily construct and register data loaders only if they don't exist already in the registry.

Currently, there is no exposed way to perform this action atomically through the DataLoaderRegistry since it rightfully encapsulates its concurrent hash map.

This PR exposes this capability.

Testing

unit tests covering the new method.
Note: Any thoughts about adding Spock to the mix later?

[tinnou]

Looks like the CI build fails when trying to install the OracleJDK8.

From this link, a possible workaround is to add dist: trusty to the travis.yml file or to use openjdk8
https://travis-ci.community/t/error-installing-oraclejdk8-expected-feature-release-number-in-range-of-9-to-14-but-got-8/3766/6

[bbakerman]

Note: Any thoughts about adding Spock to the mix later?

No objection at all. I love groovy testing via Spock. I am a convert.

This was a port of an existing project and hence Junit was used as it was previously used.

I would be happy for a future with new tests being written in Spock / Groovy

[bbakerman]

Looks like the CI build fails when trying to install the OracleJDK8.
From this link, a possible workaround is to add dist: trusty to the travis.yml file or to use openjdk8

Would you be willing to put that config into this PR and try to get it working. That would be really helpful?

[bbakerman]

Thanks for this improvement

[tinnou]

Looks like the CI build fails when trying to install the OracleJDK8.
From this link, a possible workaround is to add dist: trusty to the travis.yml file or to use openjdk8

Would you be willing to put that config into this PR and try to get it working. That would be really helpful?

Oh yea absolutely, I wasn't sure what approach you wanted to go with. We can either add this dist: trusty (it looks like it did the trick) or we could switch to using OpenJDK as well. It might be a bigger endeavor so I opted for the former.

[bbakerman]

@tinnou - 2.2.3 has been released with these changes