While using dev mode of GWT in Jetbrains IDEA, which runs jetty, I need to use spring instrumentation.
Default parameter -javaagent:spring-instrumentation.jar didn't work for me.
To run it wit Jetty you are to add additional parameter -Xbootclasspath. The final string:
-javaagent:/spring-instrument-3.2.0.RELEASE.jar -Xbootclasspath/a:spring-instrument-3.2.0.RELEASE.jar


Thanks to this:
http://gradle.1045684.n5.nabble.com/Java-Agent-LTW-problem-with-Gradle-and-Jetty-td4938600.html