GAE: Java compiler level does not match the version of the installed Java project facet


The Problem
I am creating a new GAE(1.9.1) project today. There is no place to set JDK version during creating Google Web Application Project, and the default JRE in Eclipse is 1.6, and I don't want to change the default JRE to JDK 7 as most of our projects still uses Java 6.

So after creating the GAE project, I want to change GAE project to use JDK 7, so I can use new features in JDK 7.
I right click on the project -> Properties -> Java Build Path -> Libaries, remove JDK 6 runtime and add JDK 7 runtime, also in Java Compiler tab, change Complier compliance level to 1.7.

This will rebuild the project. After it's done, in eclipse Problems views, it shows error:
Java compiler level does not match the version of the installed Java project facet. HelloGAE Unknown Faceted Project Problem (Java Version Mismatch)

We can still write code, run this GAE project, but if there is syntax(commpiler level) error in our code, it will not show in the Problems view. This is kind of annoying.
The Solution
This is because when we create GAE project, it uses JDK 6, thus sets the Java Project Facet level to 1.6.
The value can alos be seen in .settings/org.eclipse.wst.common.project.facet.core.xml

The fix is to change Java Project Facet level to 1.7: right click on the GAE project -> Properties -> Project Facet, in the right panel, change the Java version to 1.7.

It will rebuild the project, now we can see the errors, warnings in Problems view.

Happy coding :)

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