When you add Jdk9 to eclipse installed JREs, you are prompted with "Target is not a JDK root. System Library is not found. "

 Enter eclipse->help->install new software-> input URL: http://download.eclipse.org/eclipse/updates/4.7-U-builds

Remove the group item by category and then enter in the address bar to find the Eclipse SDK tick in the displayed content and click Next:

You will then download this JDK for a period of time, and when you are done, click Install. After the installation is completed, enter Window->preferences->java->compiler to modify the JDK build version, you can choose 9, then you can add JDK9 environment in the installed JREs.