Is Java Regex Thread Safe?

Question

I have a function that uses Pattern#compile and a Matcher to search a list of strings for a pattern.

This function is used in multiple th

Answer 1

A quick look at the code for Matcher.java shows a bunch of member variables including the text that is being matched, arrays for groups, a few indexes for maintain location and a few booleans for other state. This all points to a stateful Matcher that would not behave well if accessed by multiple Threads. So does the JavaDoc:

Instances of this class are not safe for use by multiple concurrent threads.

This is only an issue if, as @Bob Cross points out, you go out of your way to allow use of your Matcher in separate Threads. If you need to do this, and you think that synchronization will be an issue for your code, an option you have is to use a ThreadLocal storage object to maintain a Matcher per working thread.