I am working on the Google App engine basic stuff. As suggested by the Google guys, I am using the Eclipse IDE and their plugin.
Following a nice tutorial on Google Code, I created a JSP page in the war folder. And there in the project explorer I saw an error on the project and in the war folder. Apparently no detail was showed elsewhere, just a little red square with a white x in it. And when I tried to run the application I got an ominous warning: "Errors exist in required project".
What happened? And, more how to clean up the error?
First of all, I found out where to get more information on what the actual problem was. It was in the "Markers" window. There I saw this message: "Your project must be configured to use a JDK in order to use JSPs".
This made things a lot clearer. What happened was that Eclipse didn't care about the fact I actually had a JDK installed on my machine, and used instead just the JRE. This was not an issue until I tried to create this first JSP page, for which I got the warning.
To see which JREs are available to Eclipse, and which one is currently used, we could open the Window menu, select the Preferences item, go in the Java section, and see the Installed JREs.
What I did was adding, as Standard VM, my new shiny JDK 7 update 2, and ensuring that it was checked. After that the issue vanished.