When you build a project, the project's JSP files are only built if you have selected the "Test compile all JSP files during builds" checkbox (in the Compiling panel of the Project Properties dialog box). (Until yesterday, I had assumed that JSP files are compiled when a project is built.) This is pretty cool. If the JSP files were automatically included in the build, you might have to wait a very long time for your builds to complete -- because each JSP file is translated to a servlet before it is compiled. With the Tomcat Web Server (not sure yet about Sun Java System Application Server), the only time when a JSP file is automatically
compiled is when it is accessed
by a user, which means that your web application could be up and running for weeks and weeks before compilation errors are discovered... So setting that checkbox (or manually compiling JSP files) is pretty useful, even though it might slow a lot of things down, because otherwise compilation errors might be discovered when it's too late.
By the way, you can also view the translated JSP file (right-click the JSP file and choose "View Servlet"). But this is only possible after you have deployed it. This, in turn, means that the servlet you see is the one that is created by the server, which could be slightly different to the one that the IDE creates.