Try passing values from Java Servlet to iPhone app using JSON

While I was invited to sync my company and created an iPhone app, I was wondering how to pass values between the web server and the iPhone app. At that time, it seems that JSON is used to pass the value! I learned that. So, as a result of various investigations, I was able to pass the value using JSON safely, so I will summarize it.

I am writing with the knowledge gained from new employee training & self-learning, so if there is something wrong or a better way, please comment ...

App to create

The screen looks like this.

The flow is like this. ① Click the "Get JSON" button and pass a random number to JavaServlet. (2) JavaServlet responds with JSON according to the received numerical value. ③ Swift converts the received JSON object and outputs the value to the screen.

The JSON obtained from JavaServlet is output in the part surrounded by the red frame. Also, implement everything in the local environment.

environment

Developed using Xcode and Eclipse. 　 When using JSON, I was at a loss with GSON or Jackson, but this time I used Jackson. I referred to the following for how to set up Jackson in Eclipse. 　https://tech.pjin.jp/blog/2020/03/09/jackson_setup/

Actual code

Xcode Screen to get JSON

ViewController.swift

import UIKit

class ViewController: UIViewController {
    //Display wording
    var textId = ""
    var textName = ""
    
    //Declaration of tuple array
    var studentList:[(id:String , name:String)] = []
    
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }
    @IBAction func getJson(_ sender: Any) {
        self.performSegue(withIdentifier: "goResultVC", sender: nil)
    }
    
    override func prepare(for segue: UIStoryboardSegue, sender: Any?) {   
        //Request URL Specify a servlet that returns JSON
        guard let req_url = URL(string: "http://localhost:8080/servlet_test/JsonServlet")
            else{return}
        //Generate the information required for the request
        var req = URLRequest(url: req_url)
        //0~Get a random number of 2
        let id = Int.random(in: 0...2)
        //Create a session to manage data transfers
        let session = URLSession(configuration: .default, delegate: nil, delegateQueue: OperationQueue.main)
        //Set the information (ID) to be passed to JavaServlet in Body
        req.httpMethod = "POST"
        req.httpBody = "id=\(id)".data(using: .utf8)
        //Register request as a task
        let task = session.dataTask(with: req, completionHandler: {
            (data, response ,error) in
            //End of session
            session.finishTasksAndInvalidate()
            do{
                //Convert the retrieved JSON
                let decoder = JSONDecoder()
                let json = try decoder.decode(StudentJson.self, from: data!)

                self.textId = json.id!
                self.textName = json.name!
                
                //Segue linked to the "Get JSON" button
                if segue.identifier == "goResultVC" {
                    let nextVC = segue.destination as! ResultViewVontroller
                    nextVC.jsonId = self.textId
                    nextVC.jsonName = self.textName
                }
                
            }catch{
                print(error)
                print("I got an error")
            }
        })
        //Download started
        task.resume()
    }
    //JSON data structure
    struct  StudentJson: Codable {
        let id: String?
        let name: String?
    }
}

Screen to output JSON

ResultViewVontroller.swift

import UIKit

class ResultViewVontroller: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
    }
    //Output message field
    var jsonId = ""
    var jsonName = ""
    //Output label
    @IBOutlet weak var resultJsonId: UILabel!
    @IBOutlet weak var resultJsonName: UILabel!
    
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(true)
        resultJsonId.text = jsonId
        resultJsonName.text = jsonName
    }   
}

Eclipse

Java Servlet

JsonServlet.java

 package servlet;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.ObjectMapper;

import bean.JsonBean;

/**
 * Servlet implementation class JsonServlet
 */
@WebServlet("/JsonServlet")
public class JsonServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

    /**
     * @see HttpServlet#HttpServlet()
     */
    public JsonServlet() {
        super();
        // TODO Auto-generated constructor stub
    }

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		JsonBean jsonBeanList[] = new JsonBean[3];
		//Set a value in a Java object
        JsonBean jsonBean = new JsonBean();
        jsonBean.setId("101");
        jsonBean.setName("tanaka");
        JsonBean jsonBean2 = new JsonBean();
        jsonBean2.setId("102");
        jsonBean2.setName("yamada");
        JsonBean jsonBean3 = new JsonBean();
        jsonBean3.setId("103");
        jsonBean3.setName("satou");
        jsonBeanList[0] = jsonBean;
        jsonBeanList[1] = jsonBean2;
        jsonBeanList[2] = jsonBean3;

        //
        String str = request.getParameter("id");
        int requestId = Integer.parseInt(str);

        System.out.println(requestId);

        ObjectMapper mapper = new ObjectMapper();
        try {
            //Convert from Java object to JSON
            String testJson = mapper.writeValueAsString(jsonBeanList[requestId]);

            //JSON output
            response.getWriter().write(testJson);
            //Check the output JSON
            System.out.println(testJson);
        } catch (JsonProcessingException e) {
            e.printStackTrace();
        }
	}
}

Bean

JsonBean.java

package bean;
import java.util.List;

import com.fasterxml.jackson.annotation.JsonProperty;

public class JsonBean {

    @JsonProperty("id")
    private String id;

    @JsonProperty("name")
    private String name;

    public void setId(String id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

}

To see how JsonServlet works, we recommend the Talend API Tester, a Chrome extension! Talend API Tester

Summary

You can now communicate between the front end and the back end. Next, let's get on AWS.

In addition, I referred to many Qiita articles and books for this implementation.

It was a poor content, but thank you for reading!

Markdown notation is really easy to write.