Using jdtls without maven, gradle, ant, ...?

[jreicher]

Is there documentation on how jdtls identifies the files in projects?

I have found partly through trial and error that if it has the project root, and there is a "src" directory with a .java file that is not in a package, then everything that .java file references (including other files in the same project) work.

I'm not sure if this is by design though, or if there is a better way to use jdtls without the typical project build tools.

[mickaelistria]

Attempting to detect a source folder and a project root is by design.
There are various strategies to achieve that: some rely on the build tools, the "native" one (since JDT-LS uses the Eclipse model under the hood) is usage of typical eclipse files (.project, .classpath) that will also be used to load a project. Maybe those last ones actually exist in your filesystem and get used to configure the project?
In case no suitable configuration file is found at all, JDT-LS tries to infer a usable enough project structure by looking at the tree:

eclipse.jdt.ls/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/managers/InvisibleProjectImporter.java

Line 196 in 26983b0

private static Collection<IPath> collectSourcePaths(IPath triggerFilePath, IFolder triggerFolder,

[jreicher]

I know the workspace root is being found because of .git, but none of those other files exist so I'm most interested in the inference logic. Is it documented somewhere, and if not, shouldn't it be?

Obviously it's happy with what I've given it, because everything is working, but as I said it was slightly trial and error. For example I don't know if I actually had to use a "src" dir, but I could see that was mentioned in the jdtls code so I tried it.

[mickaelistria]

Is it documented somewhere, and if not, shouldn't it be?

It's documented in the source code ;) (see link above)
I personally don't think it's worth documenting it further as it's more an internal implementation detail (that is likely to change over time),, but it's just a personal opinion.