This class - https://github.com/checkstyle/checkstyle/blob/master/src/main/java/com/puppycrawl/tools/checkstyle/checks/FileContentsHolder.java

The only reason of FileContentsHolder is to keep FileContents.
This is a HACK to keep file content as global variable to be accessible from all other places (class that wrap one field and provide simple getter for it).

Migration note for checkstyle users:
So FileContentsHolder is not required any more and just need to be removed from clients configurations.
In scope of #4714 , we moved FileContents object to TreeWalkerAuditEvent.