How to create a Log4J Logger for unit testing Java code that logs

LoggerTest.java

    LoggingEvent loggedEvent = null;

    private Logger capturingLogger() {
        return new Logger("testing") {
            {
                level = Level.ALL;
                repository = mock(LoggerRepository.class);
                when(repository.isDisabled(anyInt())).thenReturn(false);
            }

            @Override
            public void callAppenders(LoggingEvent event) {
                assertNull("expected only one event", loggedEvent);
                loggedEvent = event;
            }
        };
    }