Data Binding

So, the Spring framework is a wonderful help for programmers, but sometimes it needs a bit of help. It does a good job of trying to match fields on a webpage to Java objects (aka Java Beans), but sometimes when the “mapping” is complex, Spring needs our help.

Consider this situation:

Date of Birth:

You've got two fields here that aren't out of the ordinary username and password, these would both be represented as Strings on the server side. But! We have a dateOfBirth field here that should be interpreted as a Date object on the server side (as opposed to a String). Unfortunately, Spring isn't smart enough to translate text into a Date object so it's going to need some help.

@InitBinder to the Rescue

Spring provides a mechanism that will allow you to specify your own custom binding of objects. So for the scenario that we have found ourselves in above, we'll need to specify how we would like Spring to bind this “Date of Birth” text to an actual Date object. Let's take a look at that code:

  import org.springframework.web.bind.annotation.InitBinder;
  import org.springframework.web.bind.WebDataBinder;
  import java.text.ParseException;
  import java.text.SimpleDateFormat;

  protected class myInitBinderMethod (WebDataBinder binder)
    binder.registerCustomEditor(Date.class, new PropertyEditorSupport() {
      public void setAsText(String text) {
        try {
          Calendar calendar = Calendar.getInstance();
          SimpleDateFormat format = new SimpleDateFormat("MM/dd/yyyy");
          Date dateObj = format.parse(text);
        } catch (ParseException e1) {
          // handle error parsing text

There's a fair bit going on here, so let's break it down.

@InitBinder – This is what we use to tell Spring that this method should be used to implement our custom binding for our fields. Without this line of code, Spring would just ignore this method and it would never get called.

WebDataBinder binder – Once you've labelled a method as the @InitBinder method, you must then pass in the WebDataBinder object. This will get populated by Spring automatically, and it's the object that you'll use to make the custom binding happen. If you don't understand what this means, then be sure to listen to the podcast by clicking the “Play” button at the top of this post.

setAsText(String text) – This is a method that belongs to the PropertyEditorSupport object and we are overriding it here. We override it because we want to tell it exactly how we want to bind this dateOfBirth field from our webpage. Spring will automatically populate the text variable with the text from the webpage, so if we were to type in “07/01/2000” as the date, that's exactly the String that will be passed to the text variable.

setValue(dateObj) – Once we have taken our text and converted it into a Date object, we'll need to be sure and SET (or assign) that object for Spring to use and bind to our command object. I haven't shown you any code that relates to a command object, but in the example I'm talking about in the podcast, I mention that there's a User command object that we would use. Once Spring finishes running through this code, the Users dateOfBirth property will be set with the Date object that we just created!


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