Binding checkboxes to Enums and possible bug in Booking demo

[tonyclyde]

Im trying to bind a list of 5 optional checkboxes (so 0 to 5 values will be form submitted). I want these to bind into a model field of type List<MyEnumeration>, similar to what the Amenities do on the Booking application.

I have encountered a few problems while working on this, but firstly I dont think the Booking example works quite right.

Booking.java

    public void setAmenities(final List<Amenity> amenities) {
	this.amenities = amenities;
    }

bookingForm.jsp

<li><form:checkbox path="amenities" value="OCEAN_VIEW" label="Ocean View" /></li>
<li><form:checkbox path="amenities" value="LATE_CHECKOUT" label="Late Checkout" /></li>
<li><form:checkbox path="amenities" value="MINIBAR" label="Minibar" /></li>

Rendered bookingForm.jsp

<ul>
	<li>
            <input id="amenities1" name="amenities" type="checkbox" value="OCEAN_VIEW"/>
            <label for="amenities1">Ocean View</label>
            <input type="hidden" name="_amenities" value="on"/>
       </li>
	<li>
           <input id="amenities2" name="amenities" type="checkbox" value="LATE_CHECKOUT"/>
           <label for="amenities2">Late Checkout</label>
           <input type="hidden" name="_amenities" value="on"/>
       </li>
	<li>
            <input id="amenities3" name="amenities" type="checkbox" value="MINIBAR"/>
            <label for="amenities3">Minibar</label>
            <input type="hidden" name="_amenities" value="on"/>
</li>
</ul>

If you run a debugger on it, it looks like it takes the "amenities" parameter from the request as a String (if one is selected) or Array of Strings (if more than one are selected) and converts them to a List<String>. It then puts the List into setAmenities(List<Amenity>). The JVM doesnt complain because its all done in the EL with reflection and its apparently not clever enough to figure out that the type of List is wrong. (Im told this is due to Java Generics using type Erasure) If you do something like the following in the setter, you get a ClassCastException (String to Amenity) that results in an ExpressionEvaluationException in the EL:

Booking.java

    public void setAmenities(final List<Amenity> amenities) {
    	this.amenities = amenities;
        for(Amenity a : this.amenities){ //ClassCastException thrown
            System.out.println("Amenity: "+ a);
        }
    }

I dont think theres any place in the sample app where the Amenities are extracted from a saved Booking, but if they were, I think ClassCastExceptions would ensue.

So, part 2, Id like to sidestep this by doing custom conversion. The problem I have there is that when one box is selected it wants to convert String to List, but when 2 or more are selected, it wants to convert Array of String to List. However I can only associate one custom converter Id with the bound field:

webflow code

<binder>
	<binding property="amenities" required="false" converter="myAmenitiesConverter"/>
</binder>

Converter Code:

    public Class getSourceClass() {
        return String[].class;
        //or return String.class; ???
    }

    public Class getTargetClass() {
        return List.class;
    }

I tried using indexed properties to force the source type to be an array all the time, but webflow didnt seem to work with that:

<li><form:checkbox path="amenities[0]" value="X" label="X" /></li>
<li><form:checkbox path="amenities[1]" value="Y" label="Y" /></li>

Any help on what to do about mapping checkboxes into Lists of Enums is greatly appreciated but hopefully someone will at least check out the Booking example and confirm or disconfirm that its got a problem.

[Keith Donald]

This is a known limitation with today's EL-based binding systems.

The Spring BeanWrapper can do this type of conversion when binding to parameterized lists; however it's only enableable in Spring MVC view environments. To turn it on, configure the "useSpringBeanBinding" flag on a custom MvcViewFactoryCreator instance.

Spring EL will generally solve this problem in the Spring Framework 3.0 timeframe.

Keith

[tonyclyde]

Thanks for the quick reply, sorry to hear that. I guess since im using someone elses model object my best bet will just be to make a wrapper class for it that takes List<String>

I tried turning on Spring BeanMapping but it looks like that just makes things fail faster by detecting the mismatch at set-time instead of get-time.

[dreed]

Had anyone figured this out? Is there a provisional fix? We are having the same problem and any suggestions greatly appreciated.

Thanks,

Dave

[GrantGochnauer]

I'm running into the same problem

I'm trying to bind checkboxes to a List<Integer> and what I get is a List full of string values and not Integers so when I look over the List using generics, I get a class cast exception.

I'm still trying to figure out a way to write a custom converter to get this to work.

[GrantGochnauer]

I'm running into the same problem

I'm trying to bind checkboxes to a List<Integer> and what I get is a List full of string values and not Integers so when I look over the List using generics, I get a class cast exception.

I'm still trying to figure out a way to write a custom converter to get this to work.

Looks like I'm going to have to run a post-bind method that goes through every list that contains Integer objects and manually loop through and convert each value. Then I manually create a new Integer object and re-add it back to a list.

Very terrible hack. I wish there was a better way.