Dropdown value binding in Spring MVC

spring-mvc

Solution

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>

Problem

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?

Original source