Class loaders are at the core of the Java language. Java EE containers, OSGi, NetBeans modules, Grails and many others use class loaders heavily. Yet when something goes wrong, do you know how to solve it? In this session we'll take a tour of the Java class loading mechanism. We will look at typical problems that you get with class loading and how to solve them. ClassNoDefError, IncompatibleClassChangeError, LinkageError and others are symptoms of specific things going wrong that you can usually find and fix. We'll go through a hands on demos of problems with corresponding solutions.