From checkstyle/checkstyle#8987:

I have noticed this failure several times before, should we consider switching to one of the mirrors on github?

yes, better to use git version of code (github).

Originally posted by @romani in checkstyle/checkstyle#8987 (comment)

We have experienced numerous failures with the openjdk mercurial repositories, we should change all to GitHub mirrors so that they will be more reliable in the future.

OpenJDK7: https://github.com/openjdk-mirror/jdk7u-jdk
OpenJDK8: https://github.com/AdoptOpenJDK/openjdk-jdk8u
OpenJDK9: https://github.com/AdoptOpenJDK/openjdk-jdk9u
OpenJDK10: https://github.com/AdoptOpenJDK/openjdk-jdk10u
OpenJDK11: https://github.com/AdoptOpenJDK/openjdk-jdk11u