Wednesday, October 18, 2006

Duplicate Form Submissions


In web applications, sometimes when a user submits a form and it takes very long for the form to be submitted, a user will tend to click on the submit button again. This can cause for dupes to appear in the application, e.g. submitting the same payment twice (sorry for my financial background...), uploading the same file twice, etc.

There are two ways of avoiding this. The first involves writing server side code that spawns threads that check whether the exact same request is submitted again. The problem with this approach is that it involves spawning threads inside the server, an undesirable and sometimes even disallowed practice (e.g. J2EE application server security policy disallows spawning threads).

The other solution, found on the web, is to use JavaScript to disable the buttons on submit and reenable them after e.g. 60 seconds using the setTimeout function. It is then impossible for the user to click on the button twice. I saw a problem with this however. What if a user wants to stop submitting by clicking the browser stop button? Obviously, after 60 seconds, the buttons will be reenabled again. But why wait for 60 seconds? The user clicked stop and now the submit button is disabled for a long period of time.

A solution would be to keep the button disabled as long as the page is loading. If the user clicked the browser stop button, reenable the submit button again. Variations on this could include showing message boxes warning the user.

The Client

Testing revealed that using the onstop and onreadystatechanged events don't work, as they only fire from a page that has loaded, they do not fire when another page is loading. For this reason, the setInterval method is used to start a "checker process" that checks if the browser stopped loading the other page. Below is a simple HTML page that includes the basics of reenabling as soon as the user clicks stop:

<script language="JavaScript">
traps = 0;
timerId = 0;

function catchDC() {
if (traps > 0) {
return false;
} else {
timerId = setInterval("handleStop();", 100);

function handleStop() {
if (document.readyState == 'complete') {
traps = 0;

<body onload="javascript: trap = 0;">
<form method="POST" action="">
<div onclick="javascript: return catchDC();">Click</div>

In the above code, a fictional server (sample code below) is called when the "Click" div is clicked. Function catchDC checks if the user had already clicked the "Click" button by checking if steps variable is greater than zero. If this is the case, the event is cancelled and the submit is not executed twice. One could also display a message box to the user when this happens, warning him not to do this.

If the form was not submitting, a timer is started with an interval of 100 msecs. This timer calls handleStop every 100 msecs, and this function checks the document.readyState to see if it is still loading or if the user had clicked the browser stop button (which would cause the readyState to be set back to complete).

If the user clicks the browser stop button, in the handleStop function called every 100 msecs the variable steps is set back to zero, essentially reenabling the "Click" div again. One could also set steps to a different value, which could then be used to determine if the user had clicked the div before and if so, issue a warning explaining the risk of duplicates and asking the user if he/ she really wants to submit again (are you sure? yes/ no...).

The Server

Below is a simple sample server that accepts a socket and then waits for ten seconds before closing the socket again. This will cause the web browser to stall, giving you enough time to test the double click (or triple or etc.) and to click the stop button and find out that this causes the "Click" button to work again. The server writes "Ding dong!" to stdout for every call made from the browser to the server:

package tst;


public class Servertje extends Thread {

private Socket s = null;

public Servertje(Socket s) {
this.s = s;

* Main
* @param args argh!
* @throws IOException on error
public static void main(String[] args) throws IOException {
ServerSocket ss = new ServerSocket(9999);
while (true) {
Socket s = ss.accept();
Servertje servert = new Servertje(s);

public void run() {
try {
System.out.println("Ding dong!");
} catch (InterruptedException e) {
} catch (IOException e) {