@Nullable
    public static String getXID() {
        return (String) CONTEXT_HOLDER.get(KEY_XID);
    }
@Test
    public void testGetXID() {
        RootContext.bind(DEFAULT_XID);
        assertThat(RootContext.getXID()).isEqualTo(DEFAULT_XID);
        assertThat(RootContext.unbind()).isEqualTo(DEFAULT_XID);
        assertThat(RootContext.getXID()).isNull();
    }