@Override
    public String toString() {
        return StringUtil.simpleClassName(this) + '(' + contentToString() + ')';
    }
@Test
    public void testToString() {
        ByteBufHolder holder = new DefaultByteBufHolder(Unpooled.buffer());
        assertEquals(1, holder.refCnt());
        assertNotNull(holder.toString());
        assertTrue(holder.release());
        assertNotNull(holder.toString());
    }