Add support for OSGI bundles

[martmoldau]

There are classes used from Gson internal API
com.google.gson.internal.Streams is used in com/microsoft/graph/serializer/ODataTypeParametrizedIJsonBackedTypedAdapter.java
com.google.gson.internal.bind.ReflectiveTypeAdapterFactory is used in com/microsoft/graph/serializer/FallbackTypeAdapterFactory.java

This is also the reason why this library cannot be used in the OSGI environment as starting from Gson 2.6, the com.google.gson.internal packages are no longer exported in the OSGi bundles.

[baywet]

Hi @martmoldau
Thanks for reaching out.
FYI we're working on an overhaul of the Java SDK by switching it to our newest generator kiota. This effort will take time, and you should see previews for that newer version in the next few months.

Part of that effort includes changes in the way serialization/deserialization is handled to avoid a strong dependency on any given serialization format/library and allow people to easily replace the implementation should they want to.

We've already had one request for OSGI in the past, could you expand on what scenario it'd unlock for you? what environments are you trying to build a solution for? Thanks!

[martmoldau]

Hi @baywet

Solution I'm working on is integration between different systems including different parts from Microsoft O365. This is based on Apache Camel which runs in OSGI environment Apache Karaf. There are ready made enterprise integration platforms also built on top of these. Red Hat Fuse or Talend for an example.
MS Graph API is a big step forward when one has to interface with different parts from MS Cloud. Right now getting it going under the OSGI is quite a hassle as OSGI's classloading is very strict. I had to customise gson package manifest to get around this problem and also a lot of guesswork to explain what packages MS Grapi API needs to import. But it is still worth it. Keep up the good work!

About the OSGI request you were referring. Actually it's still same jar packaging as now. OSGI needs more detailed manifest and that is a task all the build tools can do I guess. I would vote for it:)

[baywet]

Thanks for the additional information.
So, if I understand things properly:

• Because msgraph and msgraph-core are not OSGI bundles, you can't import them as-is in the targeted runtime?
• One reason why they are not OSGI bundles is because they are missing properties in the manifest?
• Another reason why they are not OSGI bundles is because they rely on internal APIs from GSON?
• You are able to workaround those limitations by tweaking the manifests of some packages?

Now, here come the questions where I'm still fuzzy on the details:

• Is listing our the "services" in the bundle, and deriving from BundleActivator, mandatory to be considered an OSGI bundle?
• Do all dependencies also need to be OSGI bundles to be considered an OSGI bundle?

[martmoldau]

You understood and described things in the first section quite right. Adding manifest properties is actually only reason why they are not OSGI bundles. Adding those properties to manifest will make it OSGI ready, but then it wants to import gson internal classes and this fails because gson OSGI bundle has those internal classes, but for internal use only.

It's not mandatory to create or export any services. Services can be useful when one starts to build some solution specially for OSGI environment, but for a regular java library this is usually not the case. Good enough is when there are package name, description, version, imports and exports listed in the manifest. Imports can be mandatory or optional and also version or version range can be specified e.g. gson >=2.6 and < 3.0.

Dependecies don't need to be OSGI bundles. For a user it is convinient when they are, but adding this extra info into manifest is up to each library maintainer not yours. There can be even different packaging for the same library. for example Kotlin has put together their different libraries into separately packaged kotlin-osgi-bundle.
In current ms-graph api dependencies there where only two without OSGI support; com.squareup.okhttp3 and com.squareup.okio if I recall it right. This is usually not a problem when this kind of package has only few dependencies, but can be a headache when there are lot of depencies. Bndtools can be useful in those cases, It scans the code for the imported classes, but cannot help with required library versions or when dynamic classloading is used and this is case with this library too.