If your talking about the Java JDK source, I download that via my operating system (Ubuntu package) and have a :lib/java17-source alias in my user wide deps.edn to include the .zip archive of the source in the classpath when starting a REPL https://github.com/practicalli/clojure-deps-edn#java-sources

Thanks. What about other 3rd part jars? Possible to automatically download the sources? I am aware intellj can do that.

Unfortunately, we don't have find to definition for java classes on clojure-lsl yet, we would need to make some changes on clj-kondo probably. Meanwhile you need the repl to find definition of those

How to perform that through the repl?

I think calva does that automatically when you connect the repl

I guess the first step is to download the jar sources.

when you jack in the repl it already download jars which contains the source, right?

I feel the JAVA jars only contain .class files, not java files.

AFAIK most java jars have the sources by default unless you tell it remove it