为什么request.getParameter（）方法返回null？

Question

I have a static HTML document that posts a form to a Java servlet. 我有一个静态HTML文档，该文档将表单发布到Java servlet。 The servlet then takes the values of the form and forwards them to a SQL database. 然后，该Servlet接受表单的值并将其转发到SQL数据库。 However, the problem is that the database claims a value to be null when I am certain that the value is not null. 但是，问题在于，当我确定该值不为空时，数据库声称该值为空。

Here is the form: 形式如下：

<form method="post" action="Handler" target="_blank" enctype="multipart/form-data">
  <div class="row uniform 50%">
    <div class="6u 12u(mobilep)">
      <input type="text" name="name" id="name" placeholder="Username" />
    </div>
    <div class="6u 12u(mobilep)">
      <input type="email" name="email" id="email" placeholder="Email" />
    </div>
  </div>
  <div class="row uniform 50%">
    <div class="6u 12u(mobilep)">
      <input type="password" name="password" id="password" placeholder="Password" />
    </div>
  </div>
  <div class="row uniform 50%">
    <div class="12u">
      <textarea name="bio" id="bio" placeholder="Describe yourself" maxlength="200" rows="3"></textarea>
    </div>
  </div>
  <div class="row uniform">
    <div class="12u">
      <ul class="actions align-center">
        <li><input type="submit" value="Send Message" /></li>
      </ul>
    </div>
  </div>
</form>

Here is the servlet code: 这是servlet代码：

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    // TODO Auto-generated method stub
    Connection c = null;
    PrintWriter op = response.getWriter();
    response.setContentType("text/html");

    try{
        Class.forName("com.mysql.jdbc.Driver");
        c = DriverManager.getConnection(DB_URL, "root", "root");
    }catch(Exception s){
        op.println("<html>");
        op.println("<body><h1><strong>"+ "Error: "+ s +"</strong></h1></body>");
        op.println("</html>");
    }

    String username = request.getParameter("name");
    String email = request.getParameter("email");
    String pass = request.getParameter("password");
    String bio = request.getParameter("bio");
    String proPicName = "false";
    Long time = System.currentTimeMillis();
    String sysTime = time.toString();

    Statement stmt = null;

    try{
        stmt = c.createStatement();
        stmt.execute("USE dvlpr;");
        stmt.execute("INSERT INTO user_tbl (username, email, password, bio, pro_pic, last_on, date_created)" + " VALUES ("+username+", "+email+", "+pass+", "+bio+", "+proPicName+", "+sysTime+", "+sysTime+");");


        op.println("<html>");
        op.println("<body><h1><strong>Connection made! Username: " + username+ " Email: " + email+ " Your account has been created. We'll keep your password private, too. Thanks!</strong></h1>");
        op.println("</body>");
        op.println("</html>");
    }catch(Exception s){
        op.println("<html>");
        op.println("<body><h1><strong>"+ "Error: "+ s +"</strong></h1></body>");
        op.println("</html>");
    }

}

The error code is below: 错误代码如下：

Error:com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException: Column 'username' cannot be null 错误：com.mysql.jdbc.exceptions.jdbc4.MySQLIntegrityConstraintViolationException：列'username'不能为null

Why is this returning null? 为什么返回的是null？

Answer 1

The parameters are fine. 参数很好。 The query isn't being executed properly because of syntax. 由于语法原因，查询未正确执行。 The values themselves need to be surrounded by quotes so it should look like: 值本身需要用引号引起来，因此应类似于：

VALUES('"+username+"',....

As you can see I added one ' before the double quotation mark and one after so the ' will be part of the resulting String. 如您所见，我在双引号之前添加了一个' ，在双引号之后添加了一个，因此'将成为结果字符串的一部分。 Just like in any other SQL insert you would do VALUES('MyUsername', 'MyPassword',...); 就像在其他任何SQL插入中一样，您将执行VALUES('MyUsername', 'MyPassword',...); . 。

Also you might want to use one execute method instead of 2 so it would be: 另外，您可能希望使用一种execute方法而不是2种，因此它将是：

stmt.execute("USE dvlpr; INSERT INTO....");

And also there is no need for the + before " VALUES 而且也不需要在" VALUES前加上+

为什么request.getParameter（）方法返回null？

问题描述

1 个解决方案

解决方案1
1 已采纳 2015-07-20 20:30:38

为什么request.getParameter（）方法返回null？

问题描述

1 个解决方案

解决方案1 1 已采纳 2015-07-20 20:30:38

解决方案1
1 已采纳 2015-07-20 20:30:38