lsp-javacomp is a lsp-mode client that provides Java code-completion and other IDE features for Emacs. It's backed by JavaComp.

lsp-javacomp is available on MELPA. To install it, first setup MELPA, then M-x package-install RET lsp-javacomp RET.

After installing it, add the snippet below to your .emacs file to enable it for java-mode:

(require 'lsp-javacomp)
(add-hook 'java-mode-hook #'lsp-javacomp-enable)

I recommend using lsp-javacomp in with [company-lsp][compan-lsp]. If you are lazy-loading company-lsp, make sure it's loaded before calling lsp-javacomp-enable. You can lazily load lsp-javacomp and company-lsp with use-package:

(use-package lsp-javacomp
  :init
  (add-hook 'java-mode-hook
            (lambda ()
              ;; Load lsp-javacomp before enabling lsp, so that javacomp client
              ;; is registed.
              (require 'lsp-javacomp')
              (lsp)
              ;; Use company-lsp as the company completion backend
              (set (make-variable-buffer-local 'company-backends) '(company-lsp))
              ;; Optional company-mode settings
              (set (make-variable-buffer-local 'company-idle-delay) 0.1)
              (set (make-variable-buffer-local 'company-minimum-prefix-length) 1)))
  ;; Optional, make sure JavaComp is installed. See below.
  :config
  (lsp-javacomp-install-server))

After installing lsp-javacomp to your Emacs, you can use M-x lsp-javacomp-install-server to install the JavaComp server.

If you want to update to the latest version of JavaComp, you can use M-x lsp-javacomp-update-server.

Alternatively, you can build or download the jar file manually. You must rename the jar file to javacomp.jar and put it into the directory specified by the lsp-javacomp-server-install-dir variable. The default value of lsp-javacomp-server-install-dir is ~/.emacs.d/javacomp/.

If you don't use company-lsp, you can use the vanilla completion-at-point implemented by lsp-mode.

Run M-x completion-at-point in a .java file buffer to get completion candidates. The default key binding is C-M-i.

You can provide initialization options to JavaComp by setting the following variables:

• lsp-javacomp-server-log-path
• lsp-javacomp-server-log-level
• lsp-javacomp-server-ignore-paths
• lsp-javacomp-server-type-index-files

If there is javacomp.json file in the project root, the corresponding options will be overridden if specified in the javacomp.json file.