如何在Ajax中获得错误响应？

Question

I am trying to add data to a cart(using) Ajax jQuery, now I have added a primary key constraint and I want when the constraint is violated in get the error message in jQuery:

 function add()
       {
        $(document).ready(function() 
        {
        $('#addtocart').submit(function() {
           //$('#add-button').prop('disabled',true);
    var user = $('#user').val();
           var pid = $('#pid').val();
        $.ajax({
        type:       "post",
        url:        "/devilmaycry/register?action=addtocart",
        data:       {pid:pid ,user:user},
        success:    
                   function() 
                    {
                          alert("Item has been added to cart");
                },
                  error:
                     function() 
                    {
                          alert("Item already present in the cart");
                } 
              });
     return false;
   });

        });
       }   

The error function never runs, the functionality in the DB is running fine but I don't see the error message in Ajax.

Here is Java code:

public int addintocart(String user,int pid)
{
 try
  {
   conn = obj.connect();
   String sql="insert into cart(userid,product_id,quantity)  values(?,?,?)"; 
   ps1 = conn.prepareStatement(sql);
        ps1.setString(1,user);
        ps1.setInt(2,pid);
        ps1.setInt(3,1);

         ps1.executeUpdate();

  }
    catch(SQLException e)
    {
        e.printStackTrace();
    }
    catch(Exception k)
    {
        k.printStackTrace();
    }

   return x;
 }

What is going wrong?

Answer 1

You have to send the error back as a servlet response from Java. Here is an example: How to return Java Exception info to jQuery.ajax REST call?

Also, you are missing some parameters in the ajax error function. It might be another problem. Check out http://api.jquery.com/jquery.ajax/ You should evaluate the error and present the appropriate message.

Answer 2

That error code wwill never work. the onerror event of AJAX only runs when connection cannot be established or URL does not exist. You have to return something from your the server indicating success or faliure.

I would do it this way.

PHP(convert this code to JAVA):

if(success){
    die(1);
} else {
    if(notyetincart){
       die(0);
    } else {
        die(2);
    }
}

Then Jquery coded:

function add()
       {
        $(document).ready(function() 
        {
        $('#addtocart').submit(function() {
           //$('#add-button').prop('disabled',true);
    var user = $('#user').val();
           var pid = $('#pid').val();
        $.ajax({
        type:       "post",
        url:        "/devilmaycry/register?action=addtocart",
        data:       {pid:pid ,user:user},
        success:    
                   function(response) 
                    {
                          if(reponse==="1")
                          alert("Item has been added to cart");
                          else if(reponse==="0")
                          alert("Item could not be added to cart");
                          else alert("Item already present in the cart");

                },
                  error:
                     function() 
                    {
                          alert("Item could not be added to cart due to poor network connection");
                } 
              });
     return false;
   });

        });
       }

Hope this helps