Clubby, convivial and absent of self-importance, “Jersey Boys” is the first Clint Eastwood film since, I would imagine, “Blood Work,” that doesn’t seem to be angling for an Oscar.

It’s a musical of sorts, based on the Broadway smash about Frankie Valli and the Four Seasons, and it’s a genre Eastwood hasn’t really explored behind the camera. You wouldn’t know it while watching this alternately foul-mouthed and corny adaptation, which screenwriters Marshall Brickman and Rick Elice scripted from their own musical. It’s both involving and silly, a rags-to-riches story chockablock with grandiose, hindsight-aided proclamations about the inevitable greatness of its vocal quartet. Only minutes of screen time have passed when Valli (John Lloyd Young, reprising his Broadway role) then an aspiring hairdresser in 1951 New Jersey, is told, “The world is gonna hear that voice.” Thrown out of one of their earliest gigs, at a local bowling alley, due to criminal allegations against one of its members, Four Season Tommy DeVito comments, “One day we’re gonna be on that jukebox.” But of course.

Sometimes it’s hard to tell if Eastwood and his screenwriters are taking the material seriously. And if they aren’t, it’s a more interesting film—perhaps the most postmodern picture Eastwood has directed. A young Eastwood himself appears on a black-and-white television at a party in which the Four Seasons are invited; it’s as much a cheeky inside joke as an earlier scene in which a young Joe Pesci (Joey Russo), a real-life player in the Four Seasons’ origin story, references his future “Goodfellas” monologue by confronting someone with “You think I’m funny?”

But these references have nothing on the movie’s revelatory koans—the groan-worthy eureka moments that are the stock in trade of the clichéd biopic. A band viewing of Billy Wilder’s “Ace in the Hole,” in which Four Seasons producer Bob Crewe (Mike Doyle) comments about actress Jan Sterling that “big girls don’t cry,” cuts to a scene of the Seasons crooning, um, “Big Girls Don’t Cry.” And wait until you see how the “Four Lovers” become the Four Seasons—a name change triggered, literally, by a signpost.

Moments like these, self-conscious and bordering on parody, carry the movie’s tone for most of the running time, and they make its maudlin third-act sentimentality involving Valli and his estranged daughter Francine (Freya Tingley) all the more inappropriate. These syrupy scenes of sadness and reconciliation are scored and directed in a manner befitting a lesser filmmaker than Eastwood.

Elsewhere, though, Eastwood’s authorial signature is apparent, albeit faint, on the movie’s beigy canvas; it’s there in the expert use of light and shadow, the construction of historical sets that look more like a noir director’s playground than a lived-in New Jersey, and the stellar CinemaScope compositions. In the movie’s best shot, Eastwood starts at street level and cranes his camera up several floors of the Brill Building, capturing brief glimpses of bands auditioning through windows on each floor. It’s hard to imagine the movie’s originally slated director, Jon Favreau, orchestrating such an ambitious shot.

You might recall that one of Eastwood’s finest directorial efforts was also a musical biopic of sorts: “Bird,” his mercurial movie about Charlie Parker. Eastwood is a longtime jazz devotee, and that project was clearly a labor of love, leading to his exploring new directions as both a visual stylist and storyteller. It was a jazzy movie for a jazz legend. “Jersey Boys” is part nonthreatening gangster film, part proudly conventional “Behind the Music” narrative: a square movie for a square band. While it’s easy to dismiss, it’s much too fun to hate.