Properly set up Java 9 as SDK in Intellij IDEA on

Question

Before you write me off, please consider that neither of these are answers to my question:

• How to setup SDK in IntelliJ IDEA?
• How do I change the IntelliJ IDEA default JDK?

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In IntelliJ IDEA 2017.2.6, attempting to add JDK9 as an SDK passes but does not work as the classpaths end up empty. Steps to reproduce:

1. Open 'Add new SDK dialog'
   (go to Project Settings > Project > Project SDK > New > JDK)
2. Select JDK 9
   
3. Passes, but if you look under SDK > JDK9 classpaths are empty and your code errors out due to base classes not being found. See image:
   

In IntelliJ IDEA 2017.3 EAP you get an error about JDK classes not being found. See images below:

1. Open 'Add new SDK dialog'
   (go to Project Settings > Project > Project SDK > New > JDK)
2. Select JDK 9
   
3. Should pass, but produces error popup
   

Command line compilation of HelloWorld example with jdk9 works as expected.

EDIT: Found an almost-duplicate: Intellij IDEA 2017.2 can't add openjk 9 on Linux Mint 18. Key differences:

• Linux version: they're using Mint 18, and I'm using Debian Stretch.
• OpenJDK is the latest from the repo at the moment of writing: 9~b181-4~bpo9+1. Even though is the same version, it still does not work in my case.

EDIT: Another possible duplicate: intellij idea does not see java 9 standard classes

I did not understand the answer from the comments though. Tried setting different names for JDK (9 and 1.9) but it still did not show modules instead of classpaths and classpaths remained empty.

Answer 1

Current debian binary package openjdk-9-jre-headless 9~b181-4 contains incorrectly compiled lib/jrt-fs.jar file.
There are 2 filed issues separately on both idea youtrack and also ubuntu launchpad.

As it is indicated here:

Probable reason:
Classes in lib/jrt-fs.jar were compiled by Java 9 with options "-source 8"/"target 8". They should be compiled with "--release 8" option instead (or by Java 8)

A temporary workaround may be replacing /usr/lib/jvm/java-9-openjdk-amd64/lib/jrt-fs.jar with the one from Oracle JDK.
You may also try to recompile the openjdk-9 source using the advised option "--release 8".

Anyway I advice to vote up this issue on the above link to attract more attention by dev team.

Answer 2

Use Oracle Java instead of OpenJDK for now. You can pull that in through WebUpd8's repository.

sudo add-apt-repository ppa:webupd8team/java &&
sudo apt-get update &&
sudo apt-get install oracle-java9-installer

It doesn't look like this is going to work with OpenJDK, so if you want to play with the latest and greatest Java, this is going to be the way to do it for now. Otherwise, from what I've seen, you've done this correctly and once this gets actually fixed, it will work just fine.