Add ensime support to java

New variable `java-backend' controls whether to use eclim, by default,
or ensime.

The setup is identical to scala and as such there's some functionality
included that still isn't supported in java.

layers/+lang/java/README.org

@@ -9,6 +9,10 @@
    - [[#eclim][Eclim]]
      - [[#installation][Installation]]
      - [[#usage][Usage]]
+   - [[#ensime][Ensime]]
+     - [[#installation-1][Installation]]
+     - [[#usage-1][Usage]]
+     - [[#issues][Issues]]
  - [[#key-bindings][Key bindings]]
    - [[#java-mode][Java-mode]]
      - [[#project-management][Project management]]
@@ -23,13 +27,17 @@
    - [[#projects-buffer][Projects buffer]]
 
 * Description
-This layer adds support for the Java language using the [[http://eclim.org][Eclim]] client/server.
+This layer adds support for the Java language using the [[http://eclim.org][Eclim]] client/server by
+default, or alternatively the [[https://ensime.github.io/][Ensime]] client/server.
 
 * Layer Installation
 ** Layer
 To use this configuration layer, add it to your =~/.spacemacs=. You will need to
 add =java= to the existing =dotspacemacs-configuration-layers= list in this
-file.
+file. To choose a backend explicitly,
+#+BEGIN_SRC elips
+java :variables java-backend 'eclim
+#+END_SRC
 
 ** Eclim
 Eclim provides the ability to access Eclipse features such as code completion,
@@ -62,6 +70,33 @@ You can also configure other options, as shown below:
 Start the eclim daemon with ~SPC m d s~ and stop it with ~SPC m d k~. (See
 below.)
 
+** Ensime
+Another backend option is the Ensime server, which is a lot more responsive than
+Eclim. It uses the same keybinding setup as the scala layer.
+
+*** Installation
+Find it with your favourite package manager, eg:
+#+BEGIN_SRC shell
+  nix-env --install sbt
+#+END_SRC
+or refer to [[http://www.scala-sbt.org/download.html][the sbt installation instructions]].
+
+You also need to follow [[https://ensime.github.io/build_tools/sbt/][the Ensime configuration
+instructions]] to run Ensime.
+
+To use the build functions under ~SPC m b~ you need to use version =0.13.5= or
+newer of =sbt=, and specify that in your project's =project/build.properties=.
+For example,
+#+BEGIN_SRC scala
+  sbt.version=0.13.11
+#+END_SRC
+
+*** Usage
+Start the ensime server by running ~SPC SPC ensime~ or ~M-x ensime~.
+*** Issues
+Ensime is originally built for Scala, so support for java is not complete, in
+particular refactoring doesn't work.
+
 * Key bindings
 ** Java-mode
 *** Project management

layers/+lang/java/config.el

@@ -11,3 +11,7 @@
 ;; variables
 
 (spacemacs|define-jump-handlers java-mode)
+
+(defvar java-backend 'eclim
+  "The backend to use for IDE features. Possibly values are `eclim' and
+  `ensime'.")

layers/+lang/java/packages.el

@@ -14,10 +14,13 @@
         (company-emacs-eclim :toggle
                              (configuration-layer/package-usedp 'company))
         eclim
+        ;; Only use one backend
+        (eclim :toggle (eq 'eclim java-backend))
+        (ensime :toggle (or (eq 'ensime java-backend)
+                            (configuration-layer/layer-usedp 'ensime)))
         ggtags
         helm-gtags
-        (java-mode :location built-in)
-        ))
+        (java-mode :location built-in)))
 
 (defun java/init-company-emacs-eclim ()
   (use-package company-emacs-eclim
@@ -27,6 +30,18 @@
       :backends company-emacs-eclim
       :modes java-mode)))
 
+(defun java/post-init-ensime ()
+  (when (eq 'ensime java-backend)
+    (use-package ensime
+      :defer t
+      :init
+      (ensime/init 'java-mode t nil)
+      :config
+      (progn
+        (when (configuration-layer/package-usedp 'company)
+          (ensime-company-enable))
+        (ensime/configure-keybindings 'java-mode)))))
+
 (defun java/init-eclim ()
   (use-package eclim
     :defer t