This Jazoon conference session takes a tour of the Java class loading mechanism, both from JVM and developer point of view. We’ll see how different delegation systems are built, how synchronization works, what is the difference between finding classes and resources, what wrong assumptions has been made and are now supported. Next we will look at typical problems that you get with class loading and how to solve them. ClassNoDefError, IncompatibleClassChangeError, LinkageError and many others are symptoms of specific things going wrong that you can usually find and fix. For each problem we’ll go through a hands on demo with a corresponding solution. We’ll also take a look at how and why classloaders leak and how can you remedy that. Finally we’ll take a look at the complicated class loading mechanisms like the ones used in OSGi and Tapestry 5. We’ll look in detail at the benefits they have and problems they might cause.