Can't edit/navigate standard Java class library files

[simonis]

This extension doesn't seem to support editing the Java class library files themselves (e.g. java/lang/String.java). When I open such a file, I get a lot of errors starting from the package/import declarations ("The package java.lang conflicts with a package accessible from another module: java.base") up to all the other standard classes which are referenced in the sources ("DeflaterOutputStream cannot be resolved to a typeJava(16777218)").

Is there a simple way to configure this extension in such a way that it allows editing/navigating the standard Java class library files?

Environment

• Operating System: Ubuntu 18.0.4
• JDK version: 8/11
• Visual Studio Code version: 1.39.2
• Java extension version: Java Extension Pack 0.8.0, Language Support for Java(TM) by Red Hat 0.52.0

Steps To Reproduce

1. open a directory with standard Java class library files (e.g. "jdk/src/share/classes" after cloning jdk from http://hg.openjdk.java.net/jdk8u/jdk8u/jdk

[fbricon]

Wow I never even thought about opening the JDK sources directly from vscode-java.
Have you tried using Eclipse? If it works there, then there might be a way to do it from vscode-java too (same underlying engine)

[simonis]

Well, it somehow works in Eclipse, but it requires a considerable amount of manual configuration. See for example:

https://www.eclipsecon.org/europe2019/sessions/developing-openjdk-java-libraries-eclipse-jdt
https://www.eclipsecon.org/sites/default/files/slides/OpenJDKJavaDevWithEclipse.pdf
https://github.com/RealCLanger/OpenJDKEclipseProjects

I thought vscode-java only used the Eclipse Java Compiler to implement a language server for Java. If it uses a whole Eclipse project under the hood, how can I configure that? I.e. how can I set the classpath for my project. Maybe I have to set the bootclasspath for working on system classes (or use patchModule for Java 9 and higher) but I have no idea where I could do such configurations in VSC?