Eclipse Web Tools Platform的Web App Libraries为空的问题

转自：

http://www.eclipsezone.com/eclipse/forums/t61235.html

问题：

When I launched Eclipse and opened my web app project, the "Web App
Libraries" Build path entry is empty. In other words, I see three
library icons (I think that's what the picture is) in the Package Explorer:
- JRE System Library
- Tomcat v5.5 runtime
- Web App Libraries
The first two have a little plus sign (disclosure control) to the left
and open up to show many .jar files. The "Web App Libraries" doesn't
have any plus sign or any .jar files associated with it.

All my JAR files haven't moved. They are still in
"WebContent\WEB-INF\lib". However, the Eclipse compiler no longer
includes them on the build path. This was working earlier today.

解决：

I had this too several times. Workaround: remove "Web App Libraries"
from the build path (right click and remove), then add it again: right
click, choose "configure build path", Button "add Library", select "Web
App Libraries" and choose your project.