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How do you create a Java long running scheduled task (like every hour) on app engine flexible environment?
The only docs I can find are for standard environment:
https://cloud.google.com/appengine/docs/standard/java/config/cron
Is there anything in the platform?

Yes, it does. Documentation is here for Flexible/Java.
To upload your cron jobs, you must specify the cron.yaml as a parameter to the following gcloud command:
gcloud app deploy cron.yaml
To delete all cron jobs, change the cron.yaml file to just contain:
cron:
The cron.yaml file lives in the WEB-INF directory of your application (alongside app.yaml). The following is an example cron.yaml file:
cron:
- description: daily summary job
url: /tasks/summary
schedule: every 24 hours