When you generate a module (or modules) from an open model within Gen Studio, targeting Java Webview, this will generated the necessary .html files within the '\WebViewUI' directory from within the model. For example.....'C:\...\<local_model>.ief\WebViewUI'. Once you generated and then select to launch the Buildtool from within Gen Studio, it will launch the Buildtool and build the module. Once the build is successful, you would then assemble the module in order to create a <module>.ear file for deployment to a web server, such as jboss, Websphere or Weblogic.
To view/prototype the application module prior to deploying it, you would need to open the Gen Toolset and go to the 'Navigational Diagram' and select the application module window. Once selected, change the Menu Item 'Mode' to 'HTML' and then from the same Menu Item 'Mode', select 'Prototype'. This will launch the application html in a browser for the selected window, such that you can see a prototype of the application module. The html that is being referenced for this prototype is from the 'C:\...\<local_model>.ief\html' directory which will be created on the fly when you select to prototype.