Swift4_JSON_01.swift

import Foundation

let json = """
{
    "user_id": 1,
    "user_name": "magicien",
    "url": "http://darkhorse2.0spec.jp"
}
""".data(using: .utf8)!

struct UserInfo: Codable {
    let user_id: Int
    let user_name: String
    let url: URL
}

let decoder = JSONDecoder()
do {
    let userInfo = try decoder.decode(UserInfo.self, from: json)
    print(userInfo)  // UserInfo(user_id: 1, user_name: "magicien", url: http://darkhorse2.0spec.jp)
} catch DecodingError.keyNotFound(let key, let context) {
    print("keyNotFound: \(key): \(context)")
} catch {
    print("\(error.localizedDescription)")
}