The Synchronizer Token pattern addresses the problem of duplicate form submissions. A synchronizer token is set in a user's session and included with each form returned to the client. When that form is submitted, the synchronizer token in the form is compared to the synchronizer token in the session. The tokens should match the first time the form is submitted. If the tokens do not match, then the form submission may be disallowed and an error returned to the user. Token mismatch may occur when the user submits a form, then clicks the Back button in the browser and attempts to resubmit the same form.
On the other hand, if the two token values match, then we are confident that the flow of control is exactly as expected. At this point, the token value in the session is modified to a new value and the form submission is accepted.
Struts has inbuild support for the Synchronizer Token pattern. Method such as "saveToken()" and "isTokenValid()" are present in the ActionForm class.