gradle appRun
Then open http://localhost:8080/gradle-gretty-demo/
appRun is provided by gretty plugin, replace the jettyRun from gradle jetty plugin.
Gradle jetty plugin uses jetty:6.1.25 which is pretty old and doesn't support servlet 3.x.
If we want to use newer version of jetty, we have to use another plugin like gretty here.
For now, gretty is using jetty 9 by default, but we can change it in gretty block in build.gradle file. In this example, we change it to use jetty 8.