Emacs Javadoc Lookups Get a Facelift

Ever since I started using the Emacs package archive, specifically MELPA, I’d been wanting to tidy up my Emacs Java extensions, java-mode-plus, into a nice, official package. Observing my own attitude after the switch, I noticed that if a package isn’t available on ELPA or MELPA, it practically doesn’t exist for me. Manually installing anything now seems like so much trouble in comparison, and getting a package on MELPA is so easy that there’s little excuse for package authors not to have their package in at least one of the three major Elisp archives. This is exactly the attitude my own un-archived package would be facing from other people, and rightfully so.

Before I dive in, this is what the user configuration now looks like,

(javadoc-add-artifacts [org.lwjgl.lwjg lwjgl "2.8.2"]
                       [com.nullprogram native-guide "0.2"]
                       [org.apache.commons commons-math3 "3.0"])

That’s right: it knows how to find, fetch, and index documentation on its own. Keep reading if this sounds useful to you.

The Problem

The problem was that java-mode-plus was doing two unrelated things:

I also didn’t like the names I had picked. java-mode-plus wasn’t even a mode until recently and its name isn’t conventional. And “java-docs” is just stupid. I recently solved all this by splitting the java-mode-plus into two new packages,


This is used like java-docs before it, just under a different name. The function javadoc-lookup asks for a Java class for documentation. I like to bind this to C-h j.

The function javadoc-add-roots provides filesystem paths to be indexed for lookup.

(javadoc-add-roots "/usr/share/doc/openjdk-6-jdk/api"

Also, as before, if you don’t provide a root for the core Java API, it will automatically load an index of the official Javadoc hosted online. This means it can be installed from MELPA and used immediately without any configuration. Good defaults and minimal required configuration is something I highly value.

Back in the java-docs days, when I started using a new library I’d track down the Javadoc jar, unzip it somewhere on my machine, and add it to be indexed. I regularly do development on four different computers, so this gets tedious fast. Since the Javadoc jars are easily available from the Maven repository, I maintained a small Ant project within my .emacs.d for awhile just to do this fetching, but it was a dirty hack.

Finally, the Goodies

Here’s the cool new part: I built this functionality into javadoc-lookup. It can fetch all your documentation for you! Instead of providing a path on your filesystem, you name an artifact that Maven can find. javadoc-lookup will call Maven to fetch the Javadoc jar, unzip it into a cache directory, and index it for lookups. You will need Maven installed either on your $PATH or at maven-program-name (Elisp variable).

Here’s a sample configuration. It’s group, artifact, version provided as a sequence. I say “sequence” because it can be either a list or a vector and those names can be either strings or symbols. I prefer the vector/symbol method because it requires the least quoting, plus it looks Clojure-ish.

(javadoc-add-artifacts [org.lwjgl.lwjg lwjgl "2.8.2"]
                       [com.nullprogram native-guide "0.2"]
                       [org.apache.commons commons-math3 "3.0"])

Put that in your initialization and all this documentation will appear in the lookup index. It only needs to fetch from Maven once per artifact per system — a very very slow process. After that it operates entirely from its own cache which is very fast, so it won’t slow down your startup.

This has been extremely convenient for me so I hope other people find it useful, too.

As a final note, javadoc-lookup also exploits structural sharing in its tables, using a lot less memory than java-docs. Not that it was a problem before; it’s a feel-good feature.

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Chris Wellons