Spring Hateoas xml序列化有关使用ResourceAssemblerSupport构建的资源列表

Question

I am trying to support XML responses for my Spring HATEOAS based application. JSON responses work fine as well as XML for a single resource. The problem starts with the list of the resources. Spring MVC controller cannot serialize the list built with help of ResourceAssemblerSupport derived class. The controller throws "org.springframework.web.HttpMediaTypeNotAcceptableException: Could not find acceptable representation" for the curl command

curl -k -i -H "Accept:application/xml" -H "Media-Type:application/xml" -X GET http://127.0.0.1:8080/admin/roles*

My HATEOAS resource is a wrapper around entity class:

@XmlRootElement
@XmlSeeAlso(RoleModel.class)
public class RoleResource  extends ResourceSupport {
    public RoleModel role;

}

The controller is simple:

@RequestMapping(method = RequestMethod.GET)
   public @ResponseBody HttpEntity<List<RoleResource>> getAllRoles() 
        throws ObjectAccessException, ObjectNotFoundException {
    List<RoleModel> resp = rolesManagement.getRoles();

    return new ResponseEntity<List<RoleResource>>(roleResourceAssembler.toResources(resp),
            HttpStatus.OK);
}

Resource assembler class:

@Configuration
public class RoleResourceAssembler extends ResourceAssemblerSupport<RoleModel, RoleResource> {

public RoleResourceAssembler() {
    super(RolesRestController.class, RoleResource.class);
}

@Bean 
public RoleResourceAssembler roleResourceAssembler(){
   return new RoleResourceAssembler();
}

@Override
public RoleResource toResource(RoleModel role) {
    RoleResource res = instantiateResource(role); 
    res.role = role;
    try {
        res.add(linkTo(methodOn(RolesRestController.class).getRole(role.getRoleId())).withSelfRel());
    } catch (ObjectAccessException | ObjectNotFoundException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }
    return res;
}

}

When I avoid ResourceAssemblerSupport and build my resources manually like this:

@XmlRootElement
@XmlSeeAlso(RoleModel.class)
public class RolesList {
    private List<Resource<RoleModel>> roles;

...
}
@RequestMapping(method = RequestMethod.GET)
public @ResponseBody HttpEntity<RolesList> getAllRoles() 
        throws ObjectAccessException, ObjectNotFoundException {
    List<RoleModel> resp = rolesManagement.getRoles();

    List<Resource<RoleModel>> roles =new ArrayList<>(); 
    for (RoleModel model: resp) {
        Resource<RoleModel> res =  new Resource<RoleModel>(model);
        res.add(linkTo(methodOn(RolesRestController.class).getRole(model.getRoleId())).withSelfRel());
        roles.add(res);
    }
    RolesList list = new RolesList();
    list.setRoles(roles);

    return new ResponseEntity<RolesList>(list,
            HttpStatus.OK);
}

XML serialization works. I guess I could avoid using resource assembler and build my resources manually, but that makes the code not as clean and modular. I wonder if it is still possible to use ResourceAssemblerSupport as resource builder and return the list of resources as XML

Answer 1

A resource assembler is for converting one pojo/entity/whatever to 1 hateoas resource. You are trying to convert a list to a list.

If you assembler was

public class RoleResourceAssembler extends ResourceAssemblerSupport<List<RoleModel>, RolesResource> {

and RolesResource was something that you wanted...it should work.

However i would suggest you look at using the PagedResourceAssembler which takes a page of "things" and uses an assembler to create a page of resources. The page can be the full collection or just a page in the collection. Here's a simple one for categories:

public HttpEntity<PagedResources<CategoryResource>> categories(
    PagedResourcesAssembler<Category> pageAssembler,
    @PageableDefault(size = 50, page = 0) Pageable pageable
){

    Page<Category> shopByCategories = categoryService.findCategories(pageable);        

    PagedResources<CategoryResource> r = pageAssembler.toResource(shopByCategories,
            this.categoryAssembler);

    return new ResponseEntity<PagedResources<CategoryResource>>(r, HttpStatus.OK);
}

But..i've had some problems with jackson marshaling the PagedResources as XML...works fine for JSON.

Answer 2

It seems that with ResourceAssemblerSupport it is impossible to marshal list of HATEOAS resources to XML. The reason is because the list of resources returned by toResources() method of the class extending ResourceAssemblerSupport has no @XmlRootElement and JAXB fails to marshal it. I had to create classes like

 @XmlRootElement
    public class Roles {    
    private List<RoleResource> roleResource;
    ....
}

@XmlRootElement
public class RoleResource  extends ResourceSupport {
    private RoleModel role;
    ...
}

and manually build my resource list. The same problem occurred when I tried to use to use Spring HATEOAS resource wrapper like

Resource<RoleModel> resource = new Resource<>();

Since Spring's Resource class is not annotated with @XmlRootElement REST controller is unable to marshal it to XML

Answer 3

If your only requirement is to generate links with org.springframework.hateoas.Link and marshall as XML, the following may help.

Add a Link item to your model class.

@XmlRootElement(name="Role")
public class Roles
{
   Link link;
   <your Roles content>
   ...
}

Wrap the base class in a list in order to provide a base tag which supports XML marshalling.

@XmlRootElement(name="Roles")
public class RolesList
{
   private List<Roles> rolesList;
   ...
   <constructors>
   ...
   @XmlElement(name="Role")
   public List<Roles> getRolesList()
   {
      return rolesList;
   }

   <set/get/add methods>
}

Your controller code becomes (roughly):

@RequestMapping(method = RequestMethod.GET)
public @ResponseBody HttpEntity<RolesList> getAllRoles() 
   throws ObjectAccessException, ObjectNotFoundException
{
   RolesList resp = new RolesList(rolesManagement.getRoles());
   for (Roles r: resp)
   {
       r.setLink(linkTo(methodOn(RolesRestController.class).getRole(model.getRoleId())).withSelfRel());
   }
   return new ResponseEntity<RolesList>(resp,HttpStatus.OK);
}