Spring MVC: Error 400 The request sent by the client was syntactically incorrect

I also had this issue and my solution was different, so adding here for any who have similar problem.

My controller had:

@RequestMapping(value = "/setPassword", method = RequestMethod.POST)
public String setPassword(Model model, @RequestParameter SetPassword setPassword) {
    ...
}

The issue was that this should be @ModelAttribute for the object, not @RequestParameter. The error message for this is the same as you describe in your question.

@RequestMapping(value = "/setPassword", method = RequestMethod.POST)
public String setPassword(Model model, @ModelAttribute SetPassword setPassword) {
    ...
}

The @RequestParam String action suggests there is a parameter present within the request with the name action which is absent in your form. You must either:

1. Submit a parameter named value e.g. <input name="action" />
2. Set the required parameter to false within the @RequestParam e.g. @RequestParam(required=false)