### Question:
TransactionManagerHolder { public static TransactionManager get() { if (SingletonHolder.INSTANCE == null) { throw new ShouldNeverHappenException("TransactionManager is NOT ready!"); } return SingletonHolder.INSTANCE; } private  TransactionManagerHolder(); static TransactionManager get(); static void set(TransactionManager mock);  }### Answer:
@Test void getTest() { Assertions.assertThrows(ShouldNeverHappenException.class, () -> { TransactionManagerHolder.set(null); TransactionManagerHolder.get();}); }