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