As a follow up to the previous article on binding entities and their children to form objects in Spring Framework it’s important to know how to submit the values of form objects in a standard form back to the controller. In the previous example we had a “Parent” entity, with a dropdown “select” on the form where you could choose one of the available “Children” objects. We also had a “Description” textbox that allowed the user to type in a suitable description.
The actual form, a very simple interface with both these elements and a submit button looks like:
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ taglib uri="http://www.springframework.org/tags/form" prefix="form" %>
<c:url var="saveParentUrl" value="/parent/save" />
<form:form modelAttribute="parent" method="POST" action="${saveParentUrl}">
Description: <form:input path="description"/>
Child: <form:select path="child" id="child" items="${children}" itemValue="id" itemLabel="name"/>
<input type="submit" value="Save Parent" />
</form:form>
In this example, when the user clicks on the “Save Parent” button the form will be posted to the “parent” controller action “saveParent” at “/parent/save” where we can save the updated “Parent” object.
Now the problem is that the POST header only contains text strings, not a full description of the actual “Child” object we have chosen for “Parent”. When you try to save the “Parent” object in the controller action the result is an exception, ”org.springframework.validation.BeanPropertyBindingResult”. The error message itself tells you exactly what is happening:
default message [Failed to convert property value of type 'java.lang.String[]‘ to required type ‘models.Child’ for property ‘Child’; nested exception is java.lang.IllegalStateException: Cannot convert value of type ] to required type [models.Child] for property ‘Child’: no matching editors or conversion strategy found]
The message tells you that Spring can’t automatically convert from the POST string value of the “child” select option (actually “id” in our case) to an actual “Child” object. We need to convert this string to a “Child” object in order to save the updated “Parent” entity. This is easily achieved using an “InitBinder” method within the controller class and a “@Validated” annotation on the “save” action’s “Parent” “@ModelAttribute” argument.
The code for the controller action looks like the following. Note that in this example I am using a “parentService” to pull the protocol to be updated from a “Parent” stored in session. It is likely that you will have something different or even pass in the id as part of the POST:
// ParentController.java
@RequestMapping(value = "/save", method=RequestMethod.GET)
public ModelAndView saveParent (@Validated @ModelAttribute("parent") Parent formparent, HttpServletRequest request )
{
...
// find parent to update in database using parent service
Parent parent = parentService.findById(sessionparent.getId());
// set parent description and Child based on user input and save
parent.setDescription(formparent.getDescription());
parent.setChild(formparent.getChild());
parentService.saveParent(parent);
...
}
The “formparent” is the “Parent” object passed in POST and mapped by Spring using the “@ModelAttribute” annotation. The “InitBinder” required to map the “Child” id passed in as part of POST to an actual “Child” object is:
@InitBinder
protected void initBinder(HttpServletRequest request, ServletRequestDataBinder binder) throws Exception {
binder.registerCustomEditor(Child.class, "child", new PropertyEditorSupport() {
@Override
public void setAsText(String text) {
Child ch = childService.findById(Long.parseLong(text));
setValue(ch);
}
});
}
What this does is register a binder for the “child” object when it is passed from the form to Spring that converts the value to an actual object. By overriding the “SetAsText” function I use a “childService” service to retrieve the correct “Child” entity based on the text value passed in POST. Spring can now understand and convert the POST values to actual objects and the “Parent” entity can be safely modified by the “saveParent” controller action mapped to “/parent/save”. More information and some alternative methods are available at Develop and Conquer and Empire5.