Creating a body from a file can be accomplished using BodyPublishers.ofFile(), as shown in the following snippet of code:

HttpRequest requestBodyOfFile = HttpRequest.newBuilder()
  .header("Content-Type", "application/json")
  .POST(HttpRequest.BodyPublishers.ofFile(Path.of("user.json")))
  .uri(URI.create("https://reqres.in/api/users"))
  .build();