Relation between runtimes and language server's JDT compiler

[tsaarni]

What are the use cases for runtimes configured in java.configuration.runtimes property?

Language server itself runs on JRE 17 embedded in the extension. The project README says

Project JDKs
If you need to compile your projects against a different JDK version, it's recommended you configure the java.configuration.runtimes property in your user settings

but as far as I know, the extension uses JDT Eclipse compier for Java. Not JDK java compiler.

Some guesses:

Is the runtime used when launching the (already compiled) application?
Is it used to open JDK javadocs and sources when navigating to JDK classes and methods?

[rgrunber]

Yes, the specified runtime from java.configuration.runtimes can be used when launching the application. It is basically a mapping between the execution environment and a JDK location that can deal with that environment. From there, if (for example), a Maven project is configured for Java 11, then JDT-LS will attempt to locate a JDK that matches that (eg. under the JavaSE-11 entry).

It does more than just allow you to link to the sources/javadocs. java.configuration.runtimes is meant for those wishing to ensure the Java code they write is compatible with a particular version of Java. Imagine using a language feature (eg. Text Blocks, new in Java 15) or API that is from a version of Java that is newer than the version(s) you wish to support. In these cases you will get warnings/errors.

As to the relation with the JDT-LS runtime. We require Java 17 to run JDT-LS, because certain underlying dependencies increased their minimum required version. Not everyone may have Java >= 17 on their system, so it became important to embed a version of Java that could meet this requirement with minimal burden on users. Another advantage of this approach is it can also be used in the event a user has no Java whatsoever. Makes it easier to get started.