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

Answer 1

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