Only letting an external file be accessed by one testcase

So when I kick off the testsuite to run the tests in parallel I would have a groovy script at the start of each testcase which would access the file and get a value for the test but because the tests all kick off so quickly the external file is being read by multiple tests at the exact same time resulting in the same value being read and used in multiple tests. Is there any way to prevent the testcases accessing the file at the same time? Some kind of lock?