Spring InitBinder如何工作？

Question

I read about InitBinder on net but not very clear how it works. As per my understanding it can be used to perform cross cutting concern like setting validator, conversion of request parameter to some custom object etc

Came across below example on net

@InitBinder
    public void initBinder(WebDataBinder binder) {
        SimpleDateFormat dateFormat = new SimpleDateFormat("yyyy-MM-dd");

        binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, true));
    }

Handler method is

 public void handlerMethod(@RequestParam("date") Date date) {
 }

The advantage is before DispatcherServlet calls the handlerMethod it converts the request parameter in to Date object (otherwise developer has to do it handleMethod). Right?

My question how spring knows which request parameter needs to be converted to Date object?

Say my request string is /someHandler/name?user=Brian&userCreatedDate=2011-01-01&code=aaaa-bb-cc

So how spring knows it has to convert userCreatedDate not other two parameters ie code/user?

Answer 1

It knows which request parameters to apply the conversion to based on their datatype.

By doing this:

binder.registerCustomEditor(Date.class, new CustomDateEditor(dateFormat, true));

You are registering the editor for the Date type.

So if you have

@RequestMapping("/foo")
public String foo(@RequestParam("date") Date date,
                  @RequestParam("name") String name) {
    // ...
}

Then the editor will be applied only to the first parameter, because the second one is String not Date .