用于获取响应并根据 servlet 的响应进行操作的 JavaScript 代码

Question

我制作了一个登录表单，它会比较从文本文件输入的用户名和密码，然后显示用户名/密码是否不正确或是否允许访问。

    <form role="form" action="/traveositelogin" method="post">
                        <div class="form-group">
                            <label class="sr-only" for="form-username">Username</label>
                            <input type="text" placeholder="Username..." class="form-username form-control" id="username">
                            <p id="UsernameIncorrect" hidden>Incorrect Username. Try Again</p>
                        </div>
                        <div class="form-group">
                            <label class="sr-only" for="form-password">Password</label>
                            <input type="password" placeholder="Password..." class="form-password form-control" id="password">
                            <P id="PasswordIncorrect" hidden>Incorrect Password. Try Again.</p>
                        </div>
                              <button type="submit" class="btn" id="Login">Login</button>
                                  <p><a href="contact.html"> Forgot Password?</a></p>

                    <div class="form-group">
                </div>

这是我的 servlet 代码:

package com.traveosoft;

import java.io.IOException;
import java.util.Properties;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.Scanner;
import java.io.FileInputStream;
import java.io.InputStreamReader;

public class TraveoWebsiteLoginServlet extends HttpServlet {
  @Override
  public void doGet(HttpServletRequest req, HttpServletResponse resp)
      throws IOException {
      resp.setContentType("text/plain");
      resp.getWriter().println("Hello, this is a testing servlet. \n\n");
      Properties p = System.getProperties();
      p.list(resp.getWriter());

     }

     public void doPost(HttpServletRequest req, HttpServletResponse resp) 
          throws IOException {
          String filePath = "{login.txt}";
          String username = null;
          String password = null;

      //resp.setContentType("text/plain");
      //resp.getWriter().println("Hello, this is a testing servlet. Post method \n\n");
      //Properties p = System.getProperties();
      //p.list(resp.getWriter());

      if (req.getParameter("Login")!= null )
      {
          username = req.getParameter("username");
          password = req.getParameter("password");
          //resp.sendRedirect("index.html");
      }
  try
            {
                FileInputStream fstream = new FileInputStream("login.txt");
                // Use DataInputStream to read binary NOT text.
                BufferedReader br = new BufferedReader(new InputStreamReader(fstream));
                String strLine;
                int count = 0;

                while((strLine = br.readLine())!= null)
                {
                    //Enter userName
                    String[] creds = strLine.split(",");
                    System.out.println("creds[0], creds[1]:" + creds[0] + "," +creds[1] );
                    //System.out.println("User name from the file is:"+ creds[0]);
                    if (creds[0].equals(username))
                    {
                        //user name matches. So we need to check for password now
                        count++;                        
                        if (creds[1].equals (password))
                        {
                            //Great... password also matches. Allow the user access to our repository
                            System.out.println("Allow access");
                        }
                        else
                        {
                            // Password didn't match. Ask the user to reenter the password
                            System.out.println("Wrong password. Try Again.");
                        }
                        break;
                    }

                    //strLine = br.readLine();
                    //count++;
                    //Enter Password
                    //System.out.println("Password from the file is:"+ creds[1]);
                }
                if (count == 0 )
                {
                    // No user name matched with the user name entered by the user. So user name itself is wrong
                    System.out.println("Your user name is wrong. Try Again.");
                }
                fstream.close();
                br.close();
            }
            catch (Exception e)
            {
                System.err.println("Error: " + e.getMessage());
            }
      }

} 

我怎样才能编写一个javascript代码，当用户名错误时，打印“用户名错误”；密码错误，则打印“密码错误”；如果两者都正确，则允许我将超链接引用放置到保存资源的页面。

我希望使用 javascript 示例代码(而不是 jsp 文件)。我仍在学习这一切，任何帮助将不胜感激。提前致谢。

Answer 1

如果我理解正确的话，你想要一个响应对象(或者说消息)从服务器端直接发送到客户端到 JS，而不是通过 JSP 发送。
为了做到这一点，你需要付出更多的努力......
您可以按照以下步骤操作:
1. 创建响应对象。
2. 将该对象转换为 JSON 格式。
3. 将响应内容类型设置为text/json并设置响应。

//要发送到 JavaScript 的响应对象

            public class ResponseMessage {
                private boolean isLoginSuccess;
                private String errorMessage;

                // Getters and setters ...
            }

            // Servlet code 

            ResponseMessage message = new ResponseMessage();
            message.setError(true);
            message.setErrorMessage("Your error message");
            Gson gson = new Gson(); // GSON: library to convert Java object to JSON
            String content = gson.toJson(message);
            response.setContentType("text/json");
            response.getWriter().print(content);

JS代码

function processRequest() {
var result = jQuery.ajax ({
    url: "your.url",
    method: "post",
    async: false,
    data: ""
});

// Create JS object from JSON
var message = jQuery.parseJSON(result.responseText);

// Now you can show your message
if(message.isLoginSuccess) {
    alert("Welcome");
}else{
    alert("Login Failed");
}
}