Sunday, December 12, 2010

Automating Closure Compiler with Ant

In a previous post I talked about how to use Google's Closure Compiler to optimize and minify JavaScript files. Now I have shifted to a dual Ubuntu and Mac OS X development environment, I am determined to invest in a strong automatic build process. This is just as well, since compiled JavaScript is impossible to maintain or develop. Compiling JavaScript needs to be performed automatically and transparently.

I am building out my build scripts using Apache Ant. Ant is cross-platform compatible and is well supported in the Eclipse-based Aptana and Pydev development environments. Automating the JavaScript compilation using Ant proved a bit troublesome.

In Ant you use the Apply Task to run executable programs. Running Closure Compiler using the Apply task for each JavaScript file was the first hurdle, but Apply has obvious support for passing a FileSet to determine what those files would be. The bigger challenge was working out how to specify the input and output files using redirection.

My task was made a lot easier when I found a useful example of using Ant with JSMin as the last example on the documentation page, which is invoked in similar fashion to Closure Compiler. Modifying the example I came up with this Ant Target:

  <target name="-compilejs" depends="-copybuild"
description="(runs the closure js compiler against all javascript files to replace the files in the build folder">

<!-- modeled off example found at -->
<apply executable="java" parallel="false" failonerror="true" addsourcefile="false">
<arg value="-jar" />
<arg value="${path.assets.bin.closurecompiler}" />
<arg line="--warning_level QUIET" />
<arg line="--compilation_level SIMPLE_OPTIMIZATIONS" />
<fileset dir="${path.assets.src}" includes="**/*.js" />
<inputmapper type="glob" from="*.js" to="${path.assets.src}/*.js" />
<outputmapper type="glob" from="*.js" to="${}/*.js" />
This isn't perfect. This approach always compiles all of the JavaScript files, even when they have not been altered. and JavaScript compilation with Closure Compiler is currently the longest running activity of my build. I will be trying to tweak Apply invocation to get this fixed.

The other problem is the suppression of warnings. Warnings appear inline in the output JavaScript, which breaks the JavaScript file but does not fail the build, which is what I would prefer. I am wondering if I redirect the error output and check its contents and manually fail the build whether that will catch the problem, at which point I can remove the (dangerous) suppression of warnings.

I will let you know when I surmount these problems, but if you have any thoughts let me know. I will also go into more detail about the rest of my build process - specifically as it applies to Google App Engine, which is probably the more unique aspect, and therefore, most useful.


  1. Thanks Ben Davies, your blog is very interesting and useful, helped us very much on Google App Engine knowledges. I write a blog post for your blog, I hope you like it.

  2. I have created maven plugin to listen for any file changes and generate minifed js automatically.

  3. This one is an excellent way to improve and minify JavaScript files. Above example is very effective to understand the process of function. event planning applications for ipad

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