JDK 7 introduced NIO.2 and the java.nio.file package, within it came the Files featuring a very interesting method : probeContentType(Path path). Mime type detection is always a pain, thus having a simple way to do it in the JDK is a very interesting feature indeed. Unfortunately, on Mac OS X, this feature is broken (see my gist for a test program).

As the javadoc for probeContentType explains, mime type detection is based on having a FileTypeDetector installed. The default one provided in JDK 7 is the GnomeFileTypeDetector class and for some reason it won’t pick up libgio even if it is installed on the system. Or at least, I haven’t managed to get it to detect the lib, if you do I would love to hear from you in the comments. A bug was opened on this subject at Oracle but they decided to fix it only in jdk8 which is not yet released. I tried submitting a new bug report hoping to prompt Oracle to backport the JDK8’s file detector to JDK7. I have little hope, so you can forget about probeContentTypeon OSX, unless …

The FileTypeDetector mechanism uses SPI (Service Provider Interface) to allow for loading additional detection providers. I hacked JDK 8’s default provider and created a small maven project to generate a jar which will register the provider with the JDK. Since this is platform specific issue and some of my coworkers use linux I didn’t add the jar to my project dependencies. Instead I dropped it into my JDK7’s jre/lib/ext folder. This way the jar is registered automatically whenever I use that JDK.

The code can be found on github. Hopefully I have respected the requirements of the JDK licensing by reproducing both the license and the copyright header.

For the impatients, I made a branch with a binary of the jar and a sample mime.types file lifted from some apache source repository and ready to be copied to $HOME/.mime.types.