Java JNI (JPype) Wrapper

[adriangonz]

What this PR does / why we need it:
Add an experimental option to wrap Java models which leverages the Python inference server under the hood. The communication between the Python and Java side is managed through JNI using JPype.

To make the interop more efficient, this PR also introduces a method to disable the payload serialisation / deserialisation on the Python side (so that is managed by Java). This is controlled through the PAYLOAD_PASSTHROUGH env variable.

To Do:

• Add some testing for JNI wrapper
• Publish 0.3.0 version of Java wrapper
• Add example (moved to Add example for Java wrapper using JNI #2458)

Which issue(s) this PR fixes:

Work towards #1344

Special notes for your reviewer:
This works is considered as an incubating project.

Does this PR introduce a user-facing change?:

Added incubating approach to wrap Java models, leveraging the Python inference server under the hood.

[seldondev]

Thu Sep 3 13:27:26 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
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[seldondev]

Thu Sep 3 13:27:32 UTC 2020
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[adriangonz]

While this is still a WIP PR, it would be great to hear your thoughts about the changes introduced in this PR @cliveseldon @axsaucedo @RafalSkolasinski .

[axsaucedo]

This is great @adriangonz ! I added a couple of comments, would be great to hear your thoughts

[seldondev]

Fri Sep 18 17:10:16 UTC 2020
The logs for [pr-build] [9] will show after the pipeline context has finished.
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[seldondev]

Fri Sep 18 17:10:23 UTC 2020
The logs for [lint] [10] will show after the pipeline context has finished.
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[adriangonz]

/cc @RafalSkolasinski @axsaucedo

I just added some tests to this PR using the new JNI wrapper. I tried to also add some for the pure Java wrapper but there was some issue there. This can be tackled on a separate PR though.

This should now be ready for review 👍

[axsaucedo]

THis is great! Nice one @adriangonz - should be good to go once integration passes (excluding known tests with issues)
/approve

[seldondev]

New changes are detected. LGTM label has been removed.

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

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[seldondev]

Mon Sep 21 07:53:08 UTC 2020
The logs for [pr-build] [11] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2374/11.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2374 --build=11

[seldondev]

Mon Sep 21 07:53:17 UTC 2020
The logs for [lint] [12] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2374/12.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2374 --build=12