How to populate Form from Spring dropdown when form is submitted - spring

Question

My code lists the data that I would like to see, but when I hit submit it fails to populate the backing form with exception below. How can I make the bindings work? The exception I get is of Mismatch type, trying to insert a String in List when expecting Objects. Which makes sense.
Example
<tr>
<td><form:select id="myTypes" path="myTypes" multiple="false">
<form:option value="NONE" label="--- Select ---" />
<form:options items="${form.myTypes}" itemValue="id" itemLabel="label"/>
</form:select>
</td>
<td><form:errors path="myTypes" cssClass="error" /></td>
This is how form looks like
public class MyForm {
List<MyType> myTypes;
public List<MyType> getMyTypes() {
return myTypes;
}
public void setMyTypes(List<MyType> myTypes) {
this.myTypes = myTypes;
}
}
And of course MyType has id and label.
Link to above sample code and exception below
HTTP Status 500 - Request processing failed; nested exception is org.springframework.validation.BindException: org.springframework.validation.BeanPropertyBindingResult: 1 errors
Field error in object 'Form' on field 'myTypes': rejected value [8768743658734587345]; codes [typeMismatch.Form.myTypes,typeMismatch.myTypes,typeMismatch.java.util.List,typeMismatch]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [myForm.myTypes,myTypes]; arguments []; default message [myTypes]]; default message [Failed to convert property value of type 'java.lang.String' to required type 'java.util.List' for property 'myTypes'; nested exception is java.lang.IllegalStateException: Cannot convert value of type [java.lang.String] to required type [com.x.x.MyTypeEntity] for property 'myTypes[0]': no matching editors or conversion strategy found]
Solution:
Make sure you are mapping to Single element and not to the list :)
It should be something like
<form:select path="someEntity[${status.index}].myType.id">
HTH

Answer 1

I think the problem is that Spring does not know how to convert the selected option value (which is a String posted towards your app as an HTTP parameter named "myTypes" when you submit the form) to a MyType object. You should configure a Formatter< MyType > and register it to the Spring FormatterRegistry (see Spring doc) to let Spring know how to convert the incoming String to a MyType object.
public class MyTypeFormatter implements Formatter<MyType> {
#Override
public MyType parse(String text, Locale locale) throws ParseException {
return myTypeService.getType(text); // for example
}
public String print(MyType t, Locale locale) {
return t.getId();// for example
};
}
By the way, if I may, since your dropdown list is not multiple, it means that you are going to select just one of the available MyType options. The path of the < form:select > should be named "myType" instead of "myTypes" and especially, it should refer to a MyType attribute within your Form object and not to a List< MyType > attribute. Maybe you should name your first list of available MyType objects "availableTypes" and create a second attribute named "selectedType" to bind the MyType object corresponding to the selected option on the GUI.