Discuss the elements of Modernism, Realism, and Naturalism in Ernest Hemingway's novel The Sun Also Rises.

Ernest Hemingway's first major novel, published in 1925, The Sun Also Rises, is a novel from the Modernist period. However, it incorporates elements from previous literary movements.
Realism is the predecessor to Naturalism. Jake Barnes's reluctance to consummate his relationship with Lady Brett Ashley is due to a war injury. Prior to the Realist movement, in both literature and visual art, such unpleasant aspects of life were not discussed. Hemingway's willingness to address war's ravages on one's sex life and self-esteem is an aspect of realism.
Naturalism, which came into vogue in the late nineteenth and early twentieth centuries, borrowed from Social Darwinism. Naturalism accepted a determinist's view of nature -- that is, one's behavior or character were ingrained due to genetic inheritance and the circumstances of one's upbringing. The attitude that some of the characters express toward Robert Cohn is anti-Semitic. They deride his character, not on an individual basis, but on the basis of his being Jewish. At the beginning of the novel, when he and Jake are talking about travel, Cohn insists on going to South America, while Jake thinks that East Africa would be a better choice. When Cohn still insists on South America, Jake narrates: "He had a hard, Jewish, stubborn streak." 
Supposedly, Cohn is based on the publisher Harold Loeb who, like Cohn, was very wealthy and in love with a noblewoman, Lady Duff Twysden -- the inspiration for Brett Ashley. While we must not confuse Barnes's attitude with that of Hemingway, the author does illustrate the anti-Semitism that was prevalent during this time. The connection he makes between Jewishness and character, as something fixed, or "stubborn," is a Naturalist motif.
Two things make the novel strikingly modern. Firstly, it is clearly exploring the disillusionment of American and British expatriates, still reeling from the trauma of the First World War. That trauma is physically evident in Jake. In a fundamental way, his life is stalled due to a war injury: he cannot make love. Moreover, the characters spend most of their time drinking, which is a method of desensitizing themselves from their realities.
The second aspect of the novel's modernity is the prose style. Hemingway differed from his contemporaries -- and certainly from his predecessors -- with his use of spare, journalistic prose. While this might have been a habit that he maintained from his years as a newsman, it is also possible that he realized that more could be understood about his characters from what he did not write, as opposed to what he did write.
Moreover, the dialogue is very fluid. The narrator seldom comes in to remind the reader of who is speaking. Finally, the incorporation of language that is deemed vulgar -- and is used for that purpose -- is another aspect of Modernist literature.