Those are open source, which means that the sources are available - download the sources and put them on your PC. And Eclipse let's you attach the sources to the JARs - in your project's properties, under Java Build Path, Libraries tab, expand the desired JAR and you will see a Source Attachment entry, click on that click the Edit button and browse to where the sources are located.
By the way, if you are using Maven you can do this automatically by configuring the Maven setting in Eclipse to download the source and doc files automatically. Then as long as the source and doc jars are available in the Maven repository, they will be used.
Once you do all of that, you should be able to step into the methods used in that JAR.