GWT Compile Issue

I have included org.apache.common.lang.StrinUtils  in my Entry Point java file . I have included common-lang.jar in my web-app library .

The Java Compilation goes through . but if i do a GWT compile i get the following error 

[ERROR] Line 32: No source code is available for type org.apache.commons.lang.StringUtils; did you forget to inherit a required module?

I use Eclipse , GWT plug in . 

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