无法编组类型，用于弹簧控制器的XML输出

Question

I am working in an environment where all the common dependent jars are residing in tomcat/lib folder, and application specific jars in war file.

I have a simple controller and using spring-hateoas

@RestController
@ExposesResourceFor(AccountResource.class)
@RequestMapping("/accounts")
public class AccountController {

   @RequestMapping(method = { RequestMethod.GET })
   public ResponseEntity<Resources<AccountResource>> getAccounts() {
       List<Account> accounts = //get list of accounts;
       return new ResponseEntity<Resources<AccountResource>>(
            this.accountResourceAssembler.toEmbeddedList(accounts),
            HttpStatus.OK);
   }
}


@XmlRootElement(name = "account")
@Relation(value = "account", collectionRelation = "accounts")
public class AccountResource extends ResourceWithEmbeddeds {
  private Account account;

    //getters
}

As the spring hateoas jar is in tomcat/lib, the XML marshalling of Resources class does not work, throws error as mentioned at the end.

Is it possible to set the child classloader to the Jaxb converter in spring configuration so that this error can be avoided?

com.sun.istack.internal.SAXException2: unable to marshal type "package.AccountResource" as an element because it is not known to this context.
    com.sun.xml.internal.bind.v2.runtime.XMLSerializer.reportError(XMLSerializer.java:234)
    com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:323)
    com.sun.xml.internal.bind.v2.runtime.property.ArrayReferenceNodeProperty.serializeListBody(ArrayReferenceNodeProperty.java:103)
    com.sun.xml.internal.bind.v2.runtime.property.ArrayERProperty.serializeBody(ArrayERProperty.java:144)
    com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeBody(ClassBeanInfoImpl.java:345)
    com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsSoleContent(XMLSerializer.java:578)
    com.sun.xml.internal.bind.v2.runtime.ClassBeanInfoImpl.serializeRoot(ClassBeanInfoImpl.java:326)
    com.sun.xml.internal.bind.v2.runtime.XMLSerializer.childAsRoot(XMLSerializer.java:479)
    com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.write(MarshallerImpl.java:308)
    com.sun.xml.internal.bind.v2.runtime.MarshallerImpl.marshal(MarshallerImpl.java:236)
    org.springframework.http.converter.xml.Jaxb2RootElementHttpMessageConverter.writeToResult(Jaxb2RootElementHttpMessageConverter.java:187)
    org.springframework.http.converter.xml.AbstractXmlHttpMessageConverter.writeInternal(AbstractXmlHttpMessageConverter.java:66)
    org.springframework.http.converter.AbstractHttpMessageConverter.write(AbstractHttpMessageConverter.java:195)
    org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodProcessor.writeWithMessageConverters(AbstractMessageConverterMethodProcessor.java:239)
    org.springframework.web.servlet.mvc.method.annotation.HttpEntityMethodProcessor.handleReturnValue(HttpEntityMethodProcessor.java:183)
    org.springframework.web.method.support.HandlerMethodReturnValueHandlerComposite.handleReturnValue(HandlerMethodReturnValueHandlerComposite.java:81)
    org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:126)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:832)
    org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:743)
    org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)
    org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:961)
    org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:895)
    org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:967)
    org.springframework.web.servlet.FrameworkServlet.doGet(FrameworkServlet.java:858)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:687)

I won't be able to move the jars, so need the fix on the spring side. BTW, JSON response works just fine, issue is only with the XML response for list.

Answer 1

I have reproduced the error with a small Spring boot example, so I'm pretty sure it is not a classpath issue.

The problem is that when the JAXBContext for the Hateoas Resources class is being constructed, there is no reference to your AccountResource class. This means that when Spring ask JAXB to serialize your ResponseEntity it breaks when it encounters AccountResource, because this class is not registered in the JAXBContext used for serialization.

If you make a method in you controller that returns ResponseEntity directly, you can see that this works fine.

The JAXBContext is immutable, and as far as I can see there is no way to affect the construction of JAXBContext because AbstractJaxb2HttpMessageConverter.getJaxbContext() is final.

I'm not an expert in JAXB, but from the documentation, it looks like Resource.getContent() is correctly annotated with @XmlAnyElement, but for some reason AccountResource is not serialized inside Resource.

If my analysis is correct this would be a problem for everyone using Hateoas with XML, so either no one is doing that, or I'm wrong. Do you really need it to produce XML ?

If I had to debug this any further I would start by checking out the Hateoas source, and see if they have any test that verifies that XML serialization actually works, if there is no test there is a chance it is completely broken.

Edit If you can live without the namespaces* I believe I have found a solution.

If I replace the default Jaxb2RootElementHttpMessageConverter with MappingJackson2XmlHttpMessageConverter, and use @JacksonXmlRootElement, I can get the following output (*It may be possible to add the namespaces using MixIns, but I have not checked).

<Resources xmlns="">
<links></links>
<content>
    <content>
        <account>
            ....
        </account>
        <links></links>
    </content>
</content>

In order to modify the HttpMessageConverters after construction you need Spring 4.1.3 or newer, and make you configuration extend WebMvcConfigurationSupport, this allows you to do the following:

@Override
protected void extendMessageConverters(List<HttpMessageConverter<?>> converters) {
    for (Iterator<HttpMessageConverter<?>> iterator = converters.iterator(); iterator.hasNext(); ) {
        HttpMessageConverter<?> converter = iterator.next();
        if (converter instanceof Jaxb2RootElementHttpMessageConverter) {
            iterator.remove();
        }
    }

    ObjectMapper objectMapper = Jackson2ObjectMapperBuilder.xml().applicationContext(this.getApplicationContext()).build();
    converters.add(new MappingJackson2XmlHttpMessageConverter(objectMapper));
}