This is something bothering me for a long time. I was working in a project dealing with several dependencies. After some reboots I got an exclamation mark next to the java project, like this one. There are no notes or suggestions on what is going wrong. Cleaning the projects won’t work either. After some research, I have figured out that this kind of problem is related to the working code, which means it is something going wrong on the build directory. Whenever you got exclamation mark error, you first better try to clean your project, if no success, try deleting the build directory and any other build related material in your project code.
Here it says
The red exclamation mark means Eclipse is having trouble finding a required library or source.