checkstyle/checkstyle — pull request #184 by kutzi (2014-07-13 09:54:33+00:00)
Title: Use a thread-safe map implementation to keep the compiled patterns.

This is a 1st step to make https://jira.codehaus.org/browse/MCHECKSTYLE-138 possible. There is surely a lot more to do.

Note that the old comment about Pattern itself not being thread-safe was
wrong, only the Matcher instances created from a Pattern are not
thread-safe.