Why @Rule annotated fields in JUnit has to be public?

Question

In JUnit test case, a field annotated by @Rule must be public. It breaks a common Java coding convention (all class member variables should not be public). Why does JUnit require this?

Documentation for @Rule: https://github.com/junit-team/junit/blob/master/src/main/java/org/junit/Rule.java

K Erlandsson · Answer 1 · 2013-01-15T11:39:19.483

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The JUnit runner will need to access the field reflectively to run the rule. If the field was private the access would throw IllegalAccessException.

Another option would have been to have the runner modify the access from private to public before running the rule. However that could cause problems in case a security manager is enabled.

If you want to avoid having public fields in your test class you can from JUnit 4.11 annotate methods that return a Rule with @Rule or @ClassRule.

edited Jan 15 '13 at 11:39

answered Jan 15 '13 at 10:24

K Erlandsson

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+1 You can also annotate a method with @Rule, so this would avoid the problem. You should add this to your answer. – Matthew Farwell Jan 15 '13 at 11:12
Thanks Matthew, I did not know that. Apparently new functionality in 4.11. – K Erlandsson Jan 15 '13 at 11:40
@KErlandsson - are You completely sure about that? This is what the JUnit API documentation states: "A field must be public, not static, and a subtype of TestRule (preferred) or MethodRule" – hc0re Nov 18 '15 at 09:01
@dziki What do you refer to? Am I sure about what exactly? – K Erlandsson Nov 18 '15 at 12:00

Why @Rule annotated fields in JUnit has to be public?

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