LeBron James, left, as himself, and Bill Hader, as his best friend, Aaron, star in “Trainwreck,” the new comedy from director/producer Judd Apatow. Amy Schumer wrote the screenplay and also stars in the film.

Women can be jerks, too. Everyone knows that.

And yet in the movies, the female schmuck is generally relegated to side characters. She’s a friend. She’s a villain. She’s never the heroine.

Thank goodness for Amy Schumer, the paradigm-busting, enfant terrible who’s there to snap us out of our malaise with one rowdy gal: Amy.

Her vehicle, “Trainwreck,” written by and starring Schumer, luxuriates in the idea of selfishness while shifting our antiquated ideas of what we want from a female lead, because, most importantly, it’s very funny.

As a young girl, her good time Charlie father (Colin Quinn) instills in her the idea that monogamy is impossible. Or “monogamy isn’t realistic” as he makes Amy and her even younger sister recite over and over after a conversation comparing partners to dolls.

It becomes her mantra. We meet up with her 23 years later, boozing and sleeping her way through Manhattan. Well, not sleeping. She never sleeps over at a man’s place or lets them stay at hers.

Amy is pro-pleasure, anti-romance and just a “modern chick who does what she wants” in gold lamé miniskirts. She’s a self-satisfied mess.

Her sister Kim (Brie Larson) has chosen a different route, marrying a sweet, dorky guy (Mike Birbiglia) with a sweet dorky son (Evan Brinkman). Amy pokes fun at them with caustic abandon.

Amy is a true jerk. She’s selfish. She’s rude. And she knows it. It’s an interesting line the movie skirts for the duration and a testament to Schumer that this mostly unlikable character remains enjoyable to watch.

She’s fun, but she’s not nice or thoughtful. Nor does she want to be.

All this makes it particularly odd that for about 75 percent of the movie this freewheeling character is not only in a relationship, but she’s in a relationship with a really, truly nice guy.

Amy’s editor (an excellent, unrecognizable Tilda Swinton) assigns her to profile successful sports doctor Aaron Conners (Bill Hader) for “S’nuff,” a men’s magazine with “Vice” bite.

They meet, click and get down to business after a drunken night out.

Pretty soon, and without much of a fight, they’re subway kissing, Brooklyn Bridge-gazing, Central Park carriage-riding and engaging in other cheesy rom-com tropes that are overlaid with a snarky, self-conscious voiceover from Amy.

The biggest surprise about “Trainwreck” is how conventional Amy’s arc actually is. It’s an eyebrow-raiser that might seem like even more of a cop-out if it weren’t for the magnificent Bill Hader, charming and hilarious even in this straightforward role.

LeBron James is a comedic revelation, too, playing himself as a romantic-at-heart best friend to Dr. Conners.

With “Trainwreck,” director Judd Apatow has triumphantly exited the depressive bubble that he found himself in with the earnest but flawed “Funny People” and “This is 40.”

But, much like those films, “Trainwreck” feels overlong. The meandering third act brings the energy to a near halt, as the film veers into self-reflection territory. It also has one of the limpest movies within a movie ever.

And then Apatow sticks the landing with one of the most delightful, side-splitting closings since he “Let the Sunshine In” in “The 40-Year-Old Virgin.”

If you’re looking for something radical, you’d be best to stick with Schumer’s television show.

“Trainwreck” is just good fun, and a lot of it at that.