Downloading JAR files at runtime in Java

I am wondering if it is possible to have a Java desktop application, on startup, look to some URL, to see if it needs an update, and if so download necessary JAR files, and add them to classpath for the running program. If the old jars are there, they shouldn't have been loaded into the classloader yet, at this point should they? Is it possible to swap them out before they are loaded w/out restarting the application?