Jenkinsfile syntax highlighting in Java project using Intellij Idea
If you want IDEA to recognize a Jenkinsfile as a Groovy file, then you can add the String "Jenkinsfile" as a valid file name pattern (normally contains file endings) for Groovy files. This is supported "out of the box" without requiring any additional Plugin (except the "Groovy" Plugin, but that is already part of IDEA).
To do that go to the settings menu, open the "Editor" item and then "File Types". Now select "Groovy" in the upper list and add "Jenkinsfile". You can also use a regex like "Jenkinsfile*" if you want to be more flexible regarding an optional file ending for the Jenkinsfile.
The setting should now look like this:
Your example now looks like this in IDEA (with the Dracula theme):
So IDEA now provides syntax highlighting and auto completion as far as I can tell. It suggests existing function/method names while writing, but I'm not a Groovy developer, thus I can't tell if some suggestions are missing.
At long last we found a solution that works for us and provides syntax highlighting and code completion for the Jenkinsfile present in an otherwise normal Java project in Idea. The solution is taken from here, here (and from additional personal experiments / research)
Download the Groovy SDK (if you did not do so already) from the Groovy Page and configure it on your Java project. For help on this see here
Download the pipeline GDSL file from your Jenkins instance which should be available under a link like https://yourJenkinsInstance.tld/pipeline-syntax/gdsl, and add it to the classpath of your Java project. E.g. by creating a new folder src/main/jenkins, putting the pipeline gdsl file there and marking the folder as source root in IntelliJ Idea
Add "Jenkinsfile" as a valid file name pattern for groovy files as described here
To avoid the error message
'node' cannot be applied to '(groovy.lang.Closure<java.lang.Object>), you can add this line at the top of your Jenkinsfile:// noinspection GroovyAssignabilityCheck