Rules are feature of JUnit, which allow to execute code befor, after or instead of a test.
Rules among other things can be used to
- setup and tear down resources
- change the way a test executes (like running it multiple times or in a special thread)
- perform additional checks on the test like checking execution doesn't take longer than a certain threshold or that it doesn't write to System.out.
A Rule gets implemented by implementing the org.junit.rules.TestRule interface and adding the Rule as a public field with the @Rule annotation to the test class.
In former version of JUnit the interface for Rules was org.junit.rules.MethodRule
For examples how to implement a TestRule, you might check out the various rules that JUnit provides out of the box: https://github.com/KentBeck/junit/tree/master/src/main/java/org/junit/rules
