Avoid submitting form more than once

Question

I would like to display an error if the user clicks twice on a submit button (payment is involved). The project requirements say no javascript. Any easy way to implement this in java ?

Answer 1

A few observations :
1)The solution(s) proposed are both valid, but may not work at all if the access to the session is not synchronized.
2)Even if you sync, the sync block only prevents the code from being executed by multiple threads within the same JVM, therefore your servlet might not operate properly if it's marked as distributed(Distributed servlets can run in multiple JVMs, so you're still exposed to race conditions), or if the servlet implements the now deprecated SingleThreadModel.
3)Keeping the tokens in session scope works well for small applications with low load, that only run on one machine, which does not require distributed session state.
4)It would be a better idea to use JSPs and custom tags, so you can re-use the solution quite easily in other apps.
5) While being overkill in most regular-sized apps, the use of entity beans with exclusive locking would be appropriate in big sites...