Java 7 runtime support on App Engine standard was deprecated on December 13, 2017 and will be shut down entirely on January 16, 2019. It is replaced by the Java 8 Runtime Environment.
Samples for the Java 8 runtime can be found here.
An App Engine guestbook using Java, Maven, and Objectify.
Data access using Objectify
Please ask questions on Stackoverflow
How do I, as a developer, start working on the project?
mvn clean appengine:devserver
mvn clean appengine:update -Dappengine.appId=PROJECT -Dappengine.version=VERSION