Ignore missing JAR in native installations (allows installation of jdtls through NixOS)

[ibbem]

Do not check for the existence of a JAR if a native command is preferred and there is a suitable executable.

I was hit by this issue on NixOS which puts the JAR at a different location than the auto detection of this package expects it. The alternative, setting lsp-java-server-install-dir such that the JAR instead of the executable is found, leads to a failure. When the JAR is launched directly, jdtls tries to write to the Nix Store which is read-only. Launching the jdtls executable provided by nixpkgs works perfectly though.

The error I encountered was exactly the same as described in #421. Hence, I believe this PR fixes that issue without overriding any private functions in the user configuration (see below).
As I understand it, this is also the alternative solution for #487 as suggested by @LuigiPiucco in #487 (comment)

In case this is desired, I could also add a default fallback in lsp-java--locate-server-command if lsp-java-jdt-ls-prefer-native-command is t and lsp-java-jdt-ls-command is found in exec-path.

Working NixOS configuration

When installing jdtls globally, my working configuration after this PR looks like this:

(setq lsp-java-jdt-ls-prefer-native-command t)
(setq lsp-java-server-install-dir (f-dirname (f-dirname (executable-find lsp-java-jdt-ls-command))))

Note that jdt-language-server was renamed to jdtls in nixpkgs. In case you are using some old nixpkgs version, you also need

(setq lsp-java-jdt-ls-command "jdt-language-server")

before the first two lines.

If you want to use nix shells with direnv, you can follow the guide by @dschrempf and use the following Emacs configuration:

(defun my-set-lsp-path ()
  (let ((jdtls-exe (executable-find lsp-java-jdt-ls-command)))
    (when jdtls-exe
      (setq lsp-java-jdt-ls-prefer-native-command t)
      (setq lsp-java-server-install-dir (f-dirname (f-dirname jdtls-exe))))))
(after! cc-mode
  (add-hook 'java-mode-hook #'my-set-lsp-path))