When I compile the GroupDocs.Viewer_2.3.0_java_samples\GroupDocs.Viewer_2.3.0_java_samples\samples\servlet,too many errs occured as the picture you gave me(comPileErr).
We are sorry you get a problem like this. It seems that there are code and library version mismatch. Please, review carefully the "GroupDocs.Viewer_2.3.0_java_samples\samples\servlet\build.properies" file and make sure that it uses the v2.3.0 for the compilation, because the default specified there was v.2.2.0. So, you have to change the line #3 from:
Also, make sure you correctly installed the GroupDocs.Viewer v2.3.0 library into your local Maven repository, like it is described here: http://groupdocs.com/docs/display/viewerjava/GroupDocs.Viewer+for+Java+-+Installation