I added a new fieldType to the solr schema.xml file like this:
fieldType class="myJavaClass" name="myClass"
But when I run solr it throws an error saying that it is undefined – where else does this new class have to be defined for solr to know about it?
(copied from comments as it was found to be a solution)
The problem could have a number of causes – the class may not be found, it might not have extend the right class, it might not extend org.apache.solr.schema.FieldType, it might not have a public zero-argument constructor. I suggest posting the full error message (if there is a stack trace all the better) and maybe some code from your field type class.