import java.io.IOException;
import java.io.PrintWriter;
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 java.sql.*;
import java.util.*;
import com.google.gson.Gson;

@WebServlet(urlPatterns = {"/getter"})

class Command {
    public String command;
    public int qr_desktop;
};

public class getter extends HttpServlet {

    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {

        String host_name = "localhost:5432/postgres/";
        String s = request.getParameter ("data");
        Gson gson = new Gson();
        Command command = gson.fromJson(s, Command.class);
        response.setContentType("text/html;charset=UTF-8");

        try {
            Class.forName("org.postgresql.Driver");
            Connection connection = DriverManager.getConnection(
                "jdbc:postgresql://" + host_name, "postgres", "admin");
            Statement statement = connection.createStatement ();
            ResultSet resultset = statement.executeQuery(
                    "select * from qrcode where qr_desktop = " + command.qr_desktop);
            List<String> list = new ArrayList<>();
            while (resultset.next ()) {
                list.add (resultset.getString (1));
            }
            response.getWriter().write (gson.toJson (list));
        }
        catch (Exception e) {
            e.printStackTrace();
            System.err.println (e.getClass().getName() + ": " + e.getMessage());
        }
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>

}