In Spring MVC, can I have a stateful dropdown with

问题:

In Spring MVC, I want to have a form with an html drop down which is backed by a list of domain objects, but only displays one field from the objects. When the form is submitted, I want to be able to retrieve the entire object. Can I do this?

回答1:

It's obviously possible, if I have understood you correctly...

Model

public class Foo() {
    private String result;
    public String getResult() { return result; }
    public void setResult(String result) { this.result = result; }
}

Controller

This is using annotations. If you don't understand what this is doing you should probably check out the Spring documentation. The @ModelAttribute("fooResults") will be available to your view to use for your drop down elements. The @ModelAttribute("command") Foo foo will automatically "suck up" whatever you selected in the drop down.

@Controller
public class FooController() {

    @ModelAttribute("fooResults")
    public List<String> fooResults() {
        // return a list of string
    }

    @RequestMapping(method = RequestMethod.GET)
    public String get(@ModelAttribute("command") Foo foo) {
        return "fooView";
    }

    @RequestMapping(method = RequestMethod.POST)
    public String post(@ModelAttribute("command") Foo foo) {
        // do something with foo
    }

View

Using the magic of the form tag library, you can bind a drop down (the form:select) to the result property of the model, and populate the items with the fooResults.

<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form"%>

<form:form commandName="command">
    <form:select path="result">
        <form:options items="${fooResults}" itemLabel="result" itemValue="result"/>
    </form:select>
    <input type="submit" value="submit"/>
</form>

This all assumes you kind of know what you're doing :) If you don't, check out http://static.springsource.org/docs/Spring-MVC-step-by-step/

回答2:

This post explains exactly what you want to do: https://www.credera.com/blog/technology-insights/java/spring-mvc-custom-property-editors/. I spend a long time looking for the exact same question, and AJ Angus on Credera has the best explanation I saw on the web.

To summarize, you must tell Spring how to convert the string-form option values on the select tag back into an object. This is done by putting the item values as IDs of the object: So spring now has the ID of the employee, but how does Spring change an ID back into an employee when the user clicks submit? This is by means of a PropertyEditor, which the Spring documentation doesn't explain well:

public class EmployeeNamePropertyEditor extends PropertyEditorSupport {
EmployeeDAO employeeDAO;

public EmployeeNamePropertyEditor(EmployeeDAO employeeDAO)   {
    this.employeeDAO = employeeDAO;
}

public void setAsText(String text)   {
    Employee employee = new Employee();
    employee = employeeDAO.getEmployee(Long.parseLong(text));
    setValue(employee);
}
}

Then you use initBinder to let the controller know that the propertyEditor exists:

@InitBinder
public void initBinder(WebDataBinder binder)    {
    binder.registerCustomEditor(Employee.class, new        
    EmployeeNamePropertyEditor(employeeDAO));
}

Then you are all set! Check out the link for a better and more detailed explanation.