Small exclamation icon over my STS project icon

I am working on Struts project using the Spring Source Toolsuite IDE.

When there are complier errors or something, a red X mark or yellow exclamation mark appear over the icon for my project.

But even after I made sure there are no complier errors, I got this exclamation mark in my project icon. My project is building and running successfully and I am getting the proper output.

Why is it there? How can I find out开发者_如何学运维 what it is complaining about?

Click Window -> Show View -> Problems and you'll see the list of errors/warnings and you can take it from there.

As ptsw pointed out, too:
If the Problems item isn't visible in Window -> Show View menu, choose Other... instead and select Problems in the General group to view the Problems pane: