Dropdown value binding in Spring MVC

Question

I am new to Spring MVC. I am writing an app that uses Spring, Spring MVC and JPA/Hibernate I don't know how to make Spring MVC set a value coming from a dropdown into a model object. I can imagine this a very common scenario

Here is the code:

Invoice.java

@Entity
public class Invoice{    
    @Id
    @GeneratedValue
    private Integer id;

    private double amount;

    @ManyToOne(targetEntity=Customer.class, fetch=FetchType.EAGER)
    private Customer customer;

    //Getters and setters
}

Customer.java

@Entity
public class Customer {
    @Id
    @GeneratedValue
    private Integer id;

    private String name;
    private String address;
    private String phoneNumber;

    //Getters and setters
}

invoice.jsp

<form:form method="post" action="add" commandName="invoice">
    <form:label path="amount">amount</form:label>
    <form:input path="amount" />
    <form:label path="customer">Customer</form:label>
    <form:select path="customer" items="${customers}" required="true" itemLabel="name" itemValue="id"/>                
    <input type="submit" value="Add Invoice"/>
</form:form>

InvoiceController.java

@Controller
public class InvoiceController {

    @Autowired
    private InvoiceService InvoiceService;

    @RequestMapping(value = "/add", method = RequestMethod.POST)
    public String addInvoice(@ModelAttribute("invoice") Invoice invoice, BindingResult result) {
        invoiceService.addInvoice(invoice);
        return "invoiceAdded";
    }
}

When InvoiceControler.addInvoice() is invoked, an Invoice instance received as a parameter. The invoice has an amount as expected, but the customer instance attribute is null. This is because the http post submits the customer id and the Invoice class expects a Customer object. I don't know what is the standard way to convert that.

I have read about Controller.initBinder(), about Spring Type conversion (in http://static.springsource.org/spring/docs/current/spring-framework-reference/html/validation.html) but I don't know if that is the solution for this problem.

Any ideas?

Answer 1

The trick as you have already noted is to register a custom converter which will convert the id from the drop down into a Custom instance.

You can write a custom converter this way:

public class IdToCustomerConverter implements Converter<String, Customer>{
    @Autowired CustomerRepository customerRepository;
    public Customer convert(String id) {
        return this.customerRepository.findOne(Long.valueOf(id));
    }
}

Now register this converter with Spring MVC:

<mvc:annotation-driven conversion-service="conversionService"/>

<bean id="conversionService"
    class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
    <property name="converters">
       <list>
          <bean class="IdToCustomerConverter"/>
       </list>
    </property>
</bean>