While developing an Eclipse plug-in for a small spare-time project of mine I wondered where to find the sources of the Java Development Tools (JDT). They used to be part of the Eclipse RCP/plug-in developers package in previous releases, but this isn't the case anymore with Eclipse 3.6 (Helios).
After some googling I found the reason: as of release 3.6 the RCP distribution (now named "Eclipse for RCP and RAP Developers") no longer contains any sources (except those for the actual platform plug-ins) in order to reduce the size of the download package.
If you want to get the sources for plug-ins such as JDT which are not part of the core platform you have to retrieve them separately. To do so you can use the new plug-in import wizard which allows to fetch source projects corresponding to plug-ins of the target platform directly from the Eclipse CVS.
Just open the "Plug-ins" view, right-click on the plug-in you want to retrieve (e.g.
org.eclipse.jdt.ui) and select "Import as" > "Project from a Repository ...". Confirm the next dialog by clicking "Finish", and the check-out starts. Afterwards the new project will automatically replace the plug-in JAR as dependency in any dependent projects within your workspace.