Eglot-java would not start when open java file

[eugeneandrienko]

Eglot and eglot-java didn't start when I open any file with java source code from my project.

Steps to reproduce:

1. Install eglot and eglot-java
2. Go to folder with java project and open any java file
3. Got message: File mode specification error: (wrong-type-argument consp nil)
4. Eclipse Java Language Server didn't installs automatically.

Emacs 28.2
Eglot 1.10
Eglot-java 20221218.1809 from MELPA

Му configuration:

(use-package eglot)

(use-package eglot-java
  :requires eglot
  :config
  (progn
    (add-hook 'java-mode-hook 'eglot-java-mode)))

I found (via M-x toggle-debug-on-error) what problem also reproduces, when I call M-x eglot-java-mode after open the java file.

Debugger entered--Lisp error: (wrong-type-argument consp nil)
  eglot-java--init()
  eglot-java-mode(toggle)
  funcall-interactively(eglot-java-mode toggle)
  call-interactively(eglot-java-mode record nil)
  command-execute(eglot-java-mode record)
  execute-extended-command(nil "eglot-java-mode" "eglot-java-m")
  funcall-interactively(execute-extended-command nil "eglot-java-mode" "eglot-java-m")
  call-interactively(execute-extended-command nil nil)
  command-execute(execute-extended-command)

[eugeneandrienko]

I've made a little research and found the roots of problem.

Don't know how, but in ELPA there is a eglot version "from future". Depending on releases, commits and issues from the joaotavora/eglot, the latest official version of eglot is 1.9.
But in ELPA there is a version 1.10!

In sources of that version, which I got when installing eglot-1.10, record about Java in eglot-server-programs is the next:

((java-mode java-ts-mode)
 "Initialize the library for use with the Eclipse JDT language server."
 "jdtls")

In sources of eglot from official GitHub-repositry that record differs:

(java-mode . ("jdtls"))

And, of course eglot-java fails when eglot-java--init tried to find 'java-mode record, which is not exists in eglot-1.10.

When I removed that version of eglot "from future" and install the normal eglot 1.9 — eglot-java works correctly!

[yveszoundi]

Thanks for reporting this issue.

I think that it's a valid concern though, as there could be several java-related modes associated to eglot and the "version of the future" will eventually become "actual" for everyone (including on MELPA).

• eglot upstream could make any changes related to those modes: a new hypothetical "java-ex-mode" could be added and then this package would just break again. I had never heard about java-ts-mode until today...
• users might have their own customization of java-related modes (known modes or homegrown modes)

[yveszoundi]

Here's what will be implemented for mitigation

• eglot-java will account for the fact that there could be multiple java-related mode associations in eglot:
  • eglot-java will manage eglot initialization for java-related modes found in eglot-server-programs
  • users are still expected to register java related modes that they care about through a eglot-java-mode-hook (java-ts-mode or anything other java editing mode).
• a new flag eglot-java-eglot-server-programs-manual-updates will allow further user control:
  • By default the flag is set to false and can be changed via M-x customize-variable
  • If one day upstream eglot changes drastically how it initializes LSP servers, users might still be able to update their configuration without waiting for this package to catch up.
  • If you have a heavily customized eglot configuration, you might not necessarily want eglot-java to "magically" update eglot-server-programs behind the scenes.