what makes Joyce's ' Portrait of the Artist as a young man' a modernist novel?

The most overwhelming modernist tendency is the use of language. While the novel is definitely structured as a coming-of-age tale, the language is often freewheeling and seems to be driven by a logic of a particular subconscious rather than well-sculpted language chosen for its exactitude. This tendency would get more used in Ulysses. But also is the more explicit quality and honesty. Criticized for being scandalous, the book seems a bit tame in this regard now but it, for the time, really delved into the uglier (and sexual) qualities of Stephen's personality and desires. In that way too did it usher in modernism.

Literary modernism shouldn't be confused with what we would consider "modern." It is a literary movement, and in fact goes far past that into everything from science to philosophy. Modernism is based on the shared characteristics that define it, even though they're widespread. James Joyce was the epitomy of British literary modernism.

British literary modernism was a direct result of WWI. The horrors of war left people feeling betrayed, and caused a breach wherein the British citizens felt that they'd been let down or abandoned by the institutions they once believed in. Authors like James Joyce used literature to question those institutions and to look inside of themselves for the answers they couldn't find.

Portait is semi-autobiographical; it is modernist because Joyce's character, loosly based on himself, becomes an in-depth study of his consciousness, and through that we learn about the world. This is what makes it modernist, and the following quote by Joyce, explains the feeling behind this literary movement:

“I will not serve that in which I no longer believe whether it call itself home, my fatherland or my church: and I will try to express myself in some mode of life or art as freely as I can and as wholly as I can, using for my defense the only arms I allow myself to use, silence, exile, and cunning.”