servlet不执行任务

Question

您好，我创建了一个servlet来处理数据库操作，但是每当servlet加载它时，它什么都不做，仅出现白屏，这是我的servlet文件的代码。

doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    try {
        Class.forName("com.mysql.jdbc.Driver");
        System.out.println("Connection successfull");
    } catch (ClassNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    String op=request.getParameter("operation");
    if(op=="create")
    {
    try {
        String usrnm=request.getParameter("usr");
        String pswd=request.getParameter("pass");
        String tp=request.getParameter("type");
        Connection con=DriverManager.getConnection(url, host, password);
        Statement stmt=con.createStatement();
        String sql="insert into login (username,password,type) values('"+usrnm+"','"+pswd+"','"+tp+"')";
        stmt.executeUpdate(sql);
        RequestDispatcher ct=request.getRequestDispatcher("createuser.jsp");
        ct.forward(request, response);
        con.close();
    } catch (SQLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    }
    else if(op=="update")
    {
        try {
            String usrnm=request.getParameter("usr");
            String pswd=request.getParameter("pass");
            String tp=request.getParameter("type");
            Connection con=DriverManager.getConnection(url, host, password);
            Statement stmt=con.createStatement();
            String sql="update login set password='"+pswd+"' where username='"+usrnm+"'";
            stmt.executeUpdate(sql);
            RequestDispatcher rd=request.getRequestDispatcher("createuser.jsp");
            rd.forward(request, response);
            con.close();
        } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
    else if(op=="login"){
        try {
            String usrnm=request.getParameter("user");
            String pswd=request.getParameter("pass");
            String type=null;
            Connection con=DriverManager.getConnection(url, host, password);
            Statement stmt=con.createStatement();
            String sql="select * from login where username='"+usrnm+"' AND password='"+pswd+"'";
            ResultSet rs=stmt.executeQuery(sql);

            if(rs.first())
            {
                type=rs.getString(4);
                switch(type){
                case "admin":
                    RequestDispatcher admin=request.getRequestDispatcher("admin.jsp");
                    admin.forward(request, response);
                    break;
                case "selector":
                    RequestDispatcher selector=request.getRequestDispatcher("selector.jsp");
                    selector.forward(request, response);
                    break;
                case "player":
                    RequestDispatcher player=request.getRequestDispatcher("player.jsp");
                    player.forward(request, response);
                    break;
                case "comentator":
                    RequestDispatcher comentator=request.getRequestDispatcher("comentator.jsp");
                    comentator.forward(request, response);
                    break;
                }
            }
            else{
                RequestDispatcher rd=request.getRequestDispatcher("dummylogin.jsp");
                rd.forward(request, response);
            }               } catch (SQLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}`

我创建了两个jsp文件，这是login.jsp的代码

'<div class="content_item">

      <h1 align="left">Welcome user, login here</h1>
      <br/><br/><br/>
      <form action="dbcon" method="post">
      <table cellspacing="15" cellpadding="15">
      <tr><td>
    <font size="5px" color="silver"><b>Username: </b></font></td><td><input type="text" name="user" id="user" style="height: 18px; width: 200px;" ></td></tr>
    <tr><td> <font size="5px" color="silver" style="font: bolder;"><b>Password: </b></font></td><td><input type="password" name="pass" id="pass" style="height: 18px; width: 200px;" ></td></tr>
  <tr><td></td><td></td></tr>
    <tr><td><input type="submit" onclick="return validate()" value="Login" style="height: 31px; width: 85px;"></td>
      <td><input type="reset" value="Reset" style="height: 31px;width: 85px;"></td></tr>
      </table></form>
    <input type="hidden" value="login" name="operation">
     </div> '

Answer 1

在您的代码中if会执行

更改

if(op=="login")

至

if(op.equals("login"))

因为

==将比较参考

equals()将比较值