Spring ClassCastException with Generic Form Bean Class

Question

I'm trying to use a generic class as a Spring form backing bean, but end up with a ClassCastException when the Spring framework attempts to cast the Object into the actual type.

On submission of the form, the following error occurs when attempting to call a method on the SrvRecord object (line 105, marked with comment):

java.lang.ClassCastException: java.lang.Object cannot be cast to com.[...].portal.entity.SrvRecord
        at com.[...].portal.controller.SrvController.add(SrvController.java:105)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
        at java.lang.reflect.Method.invoke(Method.java:597)
        at org.springframework.web.bind.annotation.support.HandlerMethodInvoker.invokeHandlerMethod(HandlerMethodInvoker.java:176)
        at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.invokeHandlerMethod(AnnotationMethodHandlerAdapter.java:436)
        at org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter.handle(AnnotationMethodHandlerAdapter.java:424)
        at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:790)
        at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:719)
        at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:669)
        at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:585)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)
        at javax.servlet.http.HttpServlet.service(HttpServlet.java:820)
        at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
        at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:390)
        at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
        at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
        at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
        at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:440)
        at org.mortbay.jetty.handler.ContextHandlerCollection.handle(ContextHandlerCollection.java:230)
        at org.mortbay.jetty.handler.HandlerCollection.handle(HandlerCollection.java:114)
        at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
        at org.mortbay.jetty.Server.handle(Server.java:326)
        at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
        at org.mortbay.jetty.HttpConnection$RequestHandler.content(HttpConnection.java:943)
        at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:756)
        at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:218)
        at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
        at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:410)
        at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

Form Bean:

public class RecordBean<T>
{

    private T original;
    private T modified;

    public RecordBean()
    {
        super();
    }

    public RecordBean(T original)
    {
        this.original = original;
        this.modified = original;
    }

    public T getOriginal()
    {
        return original;
    }

    public void setOriginal(T original)
    {
        this.original = original;
    }

    public T getModified()
    {
        return modified;
    }

    public void setModified(T modified)
    {
        this.modified = modified;
    }

}

Controller methods:

@RequestMapping(value = "new", method = RequestMethod.GET)
public String add(Model model)
{
    SrvRecord srvRecord = getSrvRecord();

    RecordBean<SrvRecord> record = new RecordBean<SrvRecord>(srvRecord);
    model.addAttribute("record", record);

    return "generic/new";
}

@RequestMapping(value = "new", method = RequestMethod.POST)
public String add(Model model, @ModelAttribute("record") RecordBean<SrvRecord> record)
{
    // Call a method on the SrvRecord object
    doSomething(record.getModified().getZone().getName());  // line 105
    doSomething(record.getOriginal().getZone().getName());

    // ...
}

View:

<c:url value="/edit" var="formUrl" />
<form:form commandName="record" action="${formUrl}">
    <form:input type="hidden" path="original.zone" />
    <form:input type="hidden" path="original.name" />    
    <!-- ... -->

    <form:input path="modified.zone" /><br />
    <form:input path="modified.name" /><br />
    <!-- ... -->
</form:form>

Any thoughts or suggestions would be great. Being able to work with the generic form bean will eliminate a large amount of unnecessary code from the baseline.

Just for reference, the Spring version being used is 3.0.6.RELEASE.

Thanks, Beau

Apparently getSrvRecord() is generic as well and it is returning the wrong type... Right? Which line is SrvController.java:105? — Saintali
– Saintali, Commented Nov 16, 2011 at 15:53
getSrvRecord() is not generic, and returns an SrvRecord object. Line 105 is marked with a comment in the modify method. — Beau Grantham
– Beau Grantham, Commented Nov 16, 2011 at 16:04
But the stack trace says that line 105 is located in SrvController.add. Maybe a compilation issue, I feel its good to try recompiling and and running again. — Saintali
– Saintali, Commented Nov 16, 2011 at 16:27
Sorry, you are right. I copied the wrong method. The content however, is the same. I will update the original post. — Beau Grantham
– Beau Grantham, Commented Nov 16, 2011 at 16:31

Peter Szanto · Accepted Answer · 2011-12-13 21:02:23Z

You can file a bug at Spring. The problem is that Spring is using reflection to determine the parameter types and they are ignoring the generics therefore just instantiating a plain RecordBean object without the generic. As a result the objects inside RecordBean are just created as Object and there is no way to cast it to SrvRecord. The only workaround is to not to use generics.

BACKGROUND

Spring internally uses the MethodParameter class to read out the method parameters. There is a method called

public Class<?> getParameterType()

that calls :

this.method.getParameterTypes()[this.parameterIndex]

this code reads out the parameters without generics. They would need to call this to handle it properly

this.method.getGenericParameterTypes()[this.parameterIndex]

Later the method

 HandlerMethodInvoker.resolveModelAttribute()

is called to instantiate the command class by doing

 bindObject = BeanUtils.instantiateClass(paramType);

but the value of paramType is "com.test.RecordBean" and not "com.test.RecordBean<SrvRecord>" as it would be expected

Devon Moss · Accepted Answer · 2011-11-16 15:48:30Z

1

You might try making your generic more specific.

public class RecordBean<T extends interfaceOrSuperclassOfSrvRecord>

answered Nov 16, 2011 at 15:48

Devon Moss

335 bronze badges

5 Comments

Beau Grantham Over a year ago

Thanks so much for the suggestion, this is a best practice and I have updated my code accordingly. public class RecordBean<T extends DnsRecord> But I continue to have a ClassCastException as follows: java.lang.ClassCastException: com.[...].portal.entity.DnsRecord cannot be cast to com.[...].portal.entity.SrvRecord

Devon Moss Over a year ago

Perhaps your DnsRecord and SrvRecord should implement or extend an interface or class named Record. Then you could do class RecordBean<T extends Record>

Beau Grantham Over a year ago

SrvRecord extends DnsRecord, and DnsRecord is the lowest level class from which all other entities are derived. The RecordBean should perhaps be named DnsRecordBean for clarity, and will only ever be used for objects which extend DnsRecord. Regardless, adding another Record class doesn't seem like it will solve anything.

Devon Moss Over a year ago

If your project is structured as you say then you are correct, adding a Record class wouldn't solve anything. Maybe something like this will help: if (dnsRecord instanceof SrvRecord) { SrvRecord srvRecord = (SrvRecord)dnsRecord; }

Beau Grantham Over a year ago

It looks like it is not an instance of SrvRecord, which is the crux of the matter. Spring should be constructing as SrvRecord, but instead creates the superclass (either Object or DnsRecord, depending on the definition.

Saintali · Accepted Answer · 2011-11-16 16:51:54Z

1

You may be able to find the source of the bug by implementing checked generics for your class, that is, holding a Class reference in your objects and using explicit casts when necessary:

public class RecordBean<T>
{
    private Class<T> clazz;
    private T original;
    private T modified;

    public RecordBean(Class<T> clazz)
    {
        super();
        this.clazz = clazz;
    }

    public RecordBean(T original, Class<T> clazz)
    {
        this.clazz = clazz;
        this.original = original;
        this.modified = original;
    }

    public RecordBean(T original)
    {
        this(original, (Class<T>) original.getClass());
    }

    public T getOriginal()
    {
        return original;
    }

    public void setOriginal(T original) 
    {
        this.original = clazz.cast(original);
    }

    public T getModified()
    {
        return modified;
    }

    public void setModified(T modified) 
    {
        this.modified = clazz.cast(modified);
    }

}

answered Nov 16, 2011 at 16:51

Saintali

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1 Comment

Beau Grantham Over a year ago

No luck with the new class.. it's good for debugging though. Everything looks good when building the object in code, but when Spring constructs the object on submission of the form, it always seems to be a instance of the superclass (Object in the case of <T>, or DnsRecord in the case of <T extends DnsRecord>). I'm starting to wonder if this is just a limitation of Spring.

barrenaedu · Accepted Answer · 2019-08-10 15:10:08Z

Same problem here, the only solution I found was creating an additional class (FormModelObjectCity) per controller, so my classes look like this:

public class CityDTO {
    private Long id;
    private String name;

    public CityDTO() {

    }

    public Long getId() {
    return id;
    }

    public void setId(Long id) {
    this.id = id;
    }

    public String getName() {
    return name;
    }

    public void setName(String name) {
    this.name = name;
    }

}

...

public class FormModelObjectCity {
    private CityDTO data;
    private String action;

    public CityDTO getData() {
    return data;
    }

    public void setData(CityDTO data) {
    this.data = data;
    }

    public String getAction() {
    return action;
    }

    public void setAction(String action) {
    this.action = action;
    }

}

...

@Controller
public class CitiesController {

    @RequestMapping(value = "/cities/city", method = RequestMethod.GET)
    public String cityForm(Model model, FormModelObjectCity command) {
        ...
    }

    @RequestMapping(value = "/cities/city", method = RequestMethod.POST)
    public String cityFormSubmit(@ModelAttribute("command") FormModelObjectCity command, BindingResult errors, Model model) {
        ...
    }
}

form.html, replace [] per <>

[form action="#" th:action="@{/cities/city}" th:object="${command}" method="post"]
   [input type="hidden" th:field="*{action}"]
   [input type="hidden" th:field="*{data.id}"]
   [div class="form-group" style="width: 40%;"]
      [label th:text="#{city.name}"][/label]
      [input type="text" class="form-control" th:field="*{data.name}"]
      [small th:if="${#fields.hasErrors('data.name')}" th:errors="*{data.name}" class="form-text" style="color: red;"]&nbsp;[/small]
   [/div]
[/form]

...

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