Java9 released on September 21st, 2017.
This tutorial shows you how to download latest Java9 SDK and install and then how to setup your eclipse to start writing Java9 programs.

1. Download & Install Java9 SDK

First step you need to do is download the Java9 Software Development Kit. You can download it from following link http://www.oracle.com/technetwork/java/javase/downloads/index.html
Once you downloaded the installer then start installing it. This will take you through the following steps

Once the installation is complete cross check the installation by running the following command on terminal

java -version

Then you should see that the version is '9'

2. Install Java 9 support plugin in Eclipse

First download and install Latest Eclipse version ( Oxygen is the latest version at the time of writing this Tutorial )
One the eclipse is ready download and install the following plugin from Eclipse market place
https://marketplace.eclipse.org/content/java-9-support-beta-oxygen
Install all the features as shown in below image

Once the plugin is installed it asks you to restart your eclipse, do that and you will be ready to write your first program using Java9

3 Update the eclipse.ini file

Update the vm path to java9, your eclipse.ini vm section will look something like below after the update

-vm
/Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home/bin

Also add the following to vmargs in eclipse.ini

--add-modules=ALL-SYSTEM

Now your eclipse.ini file should look like below

-vm
/Library/Java/JavaVirtualMachines/jdk-9.jdk/Contents/Home/bin
-vmargs
-Dosgi.requiredJavaVersion=1.8
--add-modules=ALL-SYSTEM

4. Add Java 9 to Installed JREs

Goto Eclipse > Preferences > Java > Installed JREs and add Java9
You can also simply hit the search button, eclipse automatically search and add the JRE for you

5. Create your first Java9 project

Now you are all set to write your first Java9 project, Just go and create a new Java project and you will see Java9 selected by default in JRE section