What can you say about Holden's relations with his friends? Is there any difference in his attitude to Ackley and Stradlater? Were they real friends?

I would add that Holden wanted both Ackley and Stradlater to care about him; he wanted to be real friends with both of them. Holden willingly writes Stradlater's composition for him even though he is going out on a date with Jane Gallagher, Holden's dream girl. Now you could say, well Stradlater is his roommate so he just wants to keep the peace, but its more than that, he also lends him his hound's-tooth jacket.

Holden tries to get Stradlater to like him by lending him things and doing his homework, anything to be accepted.  Then when Stradlater won't share details about his date with Jane, Holden gets into a fist fight with him, after which time, he runs to Ackley for sympathy and comfort.

Holden longs to sleep in Ely's bed for the night, but Ackley won't give Holden permission to sleep in his roommate's empty bed.  Holden feels rejected and isolated by Ackley's rejection,

"You're a prince, Ackley kid, I said.  You know that. Your a gentleman and a scholar, kid." (Salinger)

Holden feels so awful that the can't get Ackley to care about him while he stands in front of him bleeding after his fight with Stradlater that he says:

"I felt so lonesome, all of a sudden, I wished I was dead." (Salinger)

Another good example of Holden reaching out for friendship in the novel is when he contacts Carl Luce, an older boy that he knows from the Whooten School.  Holden is desperate to talk to someone about flunking out of Pencey, so he arranged to meet Luce at the Wicker Bar.  But once he gets there, Luce doesn't want to hear about Holden's problems, he is bored with him even before they are together for five minutes.

"Listen, hey Luce, you're one of these intellectual guys.  I need your advice, I'm in a terrific, he let out this big groan on me.  "Listen Caulfield, if you want to sit here and have a quiet peaceful drink and a quiet peaceful conver."  (Salinger)

At the end of their brief get together, Luce recommends that Holden seek psychoanalysis.

Holden is desperate for a friend, but he seems to get rejected over and over again.