You need to define a Remote Java Application in the Eclipse debug configurations:
Open the debug configurations (select project, then open from menu run/debug configurations) Select Remote Java Application in the left tree and press "New" button On the right panel select your web app project and enter 8787 in the port field. Here is a link to a detailed description of this process.
When you start the remote debug configuration Eclipse will attach to the JBoss process. If successful the debug view will show the JBoss threads. There is also a disconnect icon in the toolbar/menu to stop remote debugging.