The purpose of this exception is to encapsulate the same stack depth information provided by TestFailedException, which is used when running with ScalaTest, but be reported as a failure not an error when running with JUnit. The stack depth information indicates which line of test code failed, so that when running with ScalaTest information can be presented to the user that makes it quick to find the failing line of test code. (In other words, when running with ScalaTest the user need not scan through the stack trace to find the correct filename and line number of the failing test.)

JUnit distinguishes between failures and errors. If a test fails because of a failed assertion, that is considered a failure in JUnit. If a test fails for any other reason, either the test code or the application being tested threw an unexpected exception, that is considered an error in JUnit. This class differs from TestFailedException in that it extends junit.framework.AssertionFailedError. Instances of this class are thrown by the assertions provided by AssertionsForJUnit, and matcher expressions provided by ShouldMatchersForJUnit, and MustMatchersForJUnit.

The way JUnit 3 (JUnit 3.8 and earlier releases) decided whether an exception represented a failure or error is that only thrown junit.framework.AssertionFailedErrors were considered failures. Any other exception type was considered an error. The exception type thrown by the JUnit 3 assertion methods declared in junit.framework.Assert (such as assertEquals, assertTrue, and fail) was, therefore, AssertionFailedError. In JUnit 4, AssertionFailedError was made to extend java.lang.AssertionError, and the distinction between failures and errors was essentially dropped. However, some tools that integrate with JUnit carry on this distinction, so even if you are using JUnit 4 you may want to use the "ForJUnit" of ScalaTest assertions and matchers.