Niels Berglund has an update on writing Java code in SQL Server 2019:
In CTP 2.5 and onwards when you write Java code for SQL Server you implement your code using the Microsoft Extensibility SDK for Java, (SDK). The SDK acts sort of like an interface as it exposes abstract classes that your code need to extend/target, (more about that later).
The SDK comes in the form of a
.jarfile, and you download the SDK from here.
Niels dives deep into the topic, so set aside a bit of time to read through this one.