America’s movie stars were conspicuous by their absence at this year’s just-concluded Cannes Film Festival. But some did manage to show up, among them Grace Jones, Joan Collins, Eric Roberts and Loretta Swit. In light of many stars’ fears over terrorism, the presence of those who did attend was noted in trade papers as barely shy of heroic. Sylvester Stallone, one who changed plans about going and stayed home, had posters all about the festival hawking his new movie, ‘‘Cobra.’’ But a graffiti artist had scrawled on almost every one: “Why go to this junk? It only teaches to kill.” Michael Caine was the most visible star on celluloid, appearing in four films screened at the festival — ‘‘Mona Lisa,” ‘Hannah and Her Sisters,”’ “‘Half-Moon Street’’ and ‘‘Whistleblower.’’ But he wasn’t there in person. He was in England making yet another movie, ‘‘The Fourth Protocol.”