This common wide ranging species is found from India to East Asia as far as southern Japan. The large white patch on the hind-wings are concealed when perched. Numerous subspecies have been described. Based on range, this is presumably P. h. nicconicolens characterized by its large size. This one is nectaring on Glory Bower (Clerodendrum trichotomum) for which it is one of the main pollinators. Canon PowerShot SX60 HS..
Reference:
Suzuki, N. , Yamashita, K. , Niizuma, A. and Kiritani, K. (1987), Studies on ecology and behavior of Japanese black swallowtail butterflies. 6. Nectar feeding of Papilio helenus nicconicolens Butler and P. protenor demetrius Cramer as main pollinators of glory bower, Clerodendron trichotomum, Thunb. Ecol. Res., 2: 41-52. doi:10.1007/BF02348618