Test setup methods are defined in a test class, take no arguments, and return no value. The following is the syntax of a test setup method.
@testSetup static void methodName() { }
If a test class contains a test setup method, the testing framework executes the test setup method first, before any test method in the class. Records that are created in a test setup method are available to all test methods in the test class and are rolled back at the end of test class execution. If a test method changes those records, such as record field updates or record deletions, those changes are rolled back after each test method finishes execution. The next executing test method gets access to the original unmodified state of those records.
Test setup methods are supported only with the default data isolation mode for a test class. If the test class or a test method has access to organization data by using the @isTest(SeeAllData=true) annotation, test setup methods aren’t supported in this class. Because data isolation for tests is available for API versions 24.0 and later, test setup methods are also available for those versions only.
For more information, see Using Test Setup Methods.