It happened to me two times over the last couple of months. In the morning, when I fired up JDeveloper to work on the ADF project at my current client, JDeveloper suddenly “forgot” which project workspaces were open and lots of other settings were lost. Apparently, some settings files are corrupted for whatever reason. In this state, it is not possible to do my normal work with JDeveloper. As I don’t want to spend half the day with re-installing JDeveloper and all plugins and re-doing all my settings, I figured out a way to “repair” the JDeveloper settings. I thought I’d share this, so if you ever encounter a similar situation, you can safe yourself a lot of work by just repeating what I did.