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I had the same question, but the answer was too obvious. The source code is actually in the JAR with the .class files, so there's nothing special you need to do. In Eclipse, you can just step into Vaadin code, and for client-side code (like VButton...) you can do the same by setting breakpoints (like select the class name on the import statement where VButton is located and press F3 (Open Declaration) on Eclipse for Windows).