After Android Studio sync my gradle project, I see the following message in the event log:
Android Studio is using this JDK location:
Set your JAVA_HOME environnement variable to point to your installed jdk. In my case that was C:\Program Files\Java\jdk-9.0.1
Then, In Android studio, click on Select a JDK from the File System from your Even Log or go to project struture then sdk location.
Set the jdk location as indicate in the above image and click on apply.