The J&J vacc is using a genetically modified adenovirus to get inside host cells to release a payload of genetic material (DNA) that uses the host cell machinery to generate the spike protein.
Like mRNA vaccines, experimental viral vector vaccines have led to several spectacular/tragic bad outcomes during experimental trials for other diseases.
Up until now, no viral vector vaccines have been approved for general/wide-scale use in humans. Though, China has approved a viral vector vaccine for emergency use against Ebola.
Whole virus vaccines (live attenuated virus or inactivated virus) are the gold standard. They produce antibodies based on the whole pathogen, not just a piece of it. I am no expert, but it seems like most of the whole virus SARS-2 vaccines are being developed in China. E.g., Sinovac is a whole virus vaccine.
“Viral vector vaccines use a modified version of a different virus (the vector) to deliver important instructions to our cells. For COVID-19 viral vector vaccines, the vector (not the virus that causes COVID-19, but a different, harmless virus) will enter a cell in our body and then use the cell’s machinery to produce a harmless piece of the virus that causes COVID-19. This piece is known as a spike protein and it is only found on the surface of the virus that causes COVID-19.” https://www.cdc.gov/vaccines/covid-19/hcp/viral-vector-vaccine-basics.html
Interesting paper that surveys the different types of SARS2 vaccines types:
https://www.nature.com/articles/s41418-020-00720-9#Sec5
Thanks for the detailed info, very helpful!