This plugin uses PlantUML: free diagram generator, to generate class diagram from the java classes in the workspace. Diagram is generated after drag-and-drop of class, package or the whole java project.
You need these things to use PlantUML:
Use the Eclipse update manager via Help -> Install New Software... to install this new feature into your Eclipse IDE. Add new repository by clicking Add... -> Archive... and point to plugin.zip. Then select and install the Class Diagram feature. (If you don't see Class Diagram, uncheck Group items by category checkbox.)
The plugin is available via Window -> Show View -> Class Diagram.
This plugin uses BSD 3-Clause License.
There might be some issues with generating the class diagram from big amount of classes.