Plugins will be able to start using Java 8 once they upgraded their dependency to SonarQube 5.6. This means that at any point of time, if someone runs SQ 5.6 on Java 7 without noticing this, there's one day when the instance will be broken once a Java8-based plugin will be installed.
For this reason, it's best to fail fast and:
- Make sure that the SonarQube instance fails with a gentle message at startup if not running on Java 8+
- Same on scanner side