There's quite a bit of docs on the Java DSL here -> https://docs.spring.io/spring-integration/docs/5.3.1.RELEASE/reference/html/dsl.html#java-dsl
You can also find the JDBC related docs here, although the examples on this page are using XML -> https://docs.spring.io/spring-integration/docs/5.3.1.RELEASE/reference/html/jdbc.html#jdbc
If you want to see how to do something with the Java DSL, you just need to cross reference. For example, if I want to see how to use the JDBC inbound channel adapter, you can look at the Java DSL instructions for defining an inbound channel adapter and apply that to your particular channel adapter (applying usually means just creating an instance of the right object).
https://docs.spring.io/spring-integration/docs/5.3.1.RELEASE/reference/html/dsl.html#java-dsl-inbound-adapters
-------
I think the tricky part about your question is the large number of data sources that you're going to need to define. My understanding is that the JDBC related SI components are based around working with a single data source. In theory, you could create 50 data sources and associated SI components, but I'm guessing that probably wouldn't be too efficient. 50 data source pools along is going to be a lot of overhead. I think you're going to need to think through how you want that to work exactly.
Hope that helps!