Tweaking Emacs for Ant and Java

Update: This is now part of my java-mode-plus Emacs extension.

Developing C in Emacs is a real joy, and it's mostly thanks to the compile command. Once you have your Makefile — or SConstruct or whatever build system you like — setup and you want to compile your latest changes, just run M-x compile, which will run your build system in a buffer. You can then step through the errors and warnings with C-x `, and Emacs will take you to them. It's a very nice way to write code.

I use the compile command so much that I bound it to C-x C-k (C-k tends to be part of compile key bindings),

(global-set-key "\C-x\C-k" 'compile)

Until recently, I didn't have as nice of a setup for Java. Since they generally force offensive IDEs onto me at work this wasn't something I needed yet anyway, but I get to choose my environment on a new project this time. If you're using Makefiles for some reason when building your Java project, it still works out fairly well because they're usually called recursively. It gets more complicated with Ant, where there is only one top-level build file. Emacs' compile command only runs the build command in the buffer's current directory.

I know three solutions to this problem. One is to provide the build file's absolute path when compile asks for the command with the -buildfile (-f) option. You only need to type it once per Emacs session, so that's not too bad.

ant -emacs -buildfile /path/to/build.xml

It's not well documented, but there is a -find option that can be given to Ant that will cause it to search for the build file itself. This is even nicer than the previous solution. Just remember to place it last, unless you give it the build filename too. For example, if you wanted to run the clean target,

ant -emacs clean -find

To keep the actual call as simple as possible, I wrote a wrapper for compile, and put a hook in java-mode to change the local binding. The wrapper, ant-compile, searches for the build file the same way -find would do.

(defun ant-compile ()
  "Traveling up the path, find build.xml file and run compile."
    (while (and (not (file-exists-p "build.xml"))
                (not (equal "/" default-directory)))
      (cd ".."))
    (call-interactively 'compile)))

So I can transparently keep using my muscle memory compile binding, I set up the key binding in a hook,

(add-hook 'java-mode-hook
          (lambda () (local-set-key "\C-x\C-k" 'ant-compile)))

Voila! Java works looks a little bit more like C.

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Chris Wellons