So, I'm setting up a solr server (version 4.1), using Tomcat6, for development. Not a problem, until I try including the DataImportHandler, where everything just breaks.
If you're not familiar with the DataImportHandler, you should have a look. It's a very nice looking piece of code that helps you import data into Solr easily, keeping track of changed data and all. I'm looking forward to using it for a project which doesn't use the usual apache solr Drupal modules.
Alright, so the problem had to do with including the jar files, which for me means including solr-dataimporthandler-4.1.0.jar. The obvious way from looking at the solrconfig.xml is
<lib dir="../../../dist/" regex="solr-dataimporthandler-.*\.jar" />. Looking at my setup, I saw that the solrconfig.xml was two directories below the directory containing dist, so I shortened the dir path to ../../dist/, and it failed. And it kept failing.
Here's where I'm posting the various files that I promised to deliver - these are the files I used in my DrupalCamp Colorado 2010 presenation on extending Solr.