Configure the debugger to catch the java.lang.AssertionError exception and suspend the execution.

Call the TestRunner to run the test under the debugger.

When the test fails again, The debugger will suspend the execution and start the debug mode.

Now you are ready to use debugger commands to find the code issue that causing the fail.

How you configure this depends on the debugger you prefer to use.

Most Java debuggers provide support to stop the program when a specific exception is raised.