Reading and writing JAX-RS Link objects

JAX-RS contains a rather nice handy representation of the a Link that can be serialised with and adapter into XML and JSON, unfortunately there is a bug in the JAX-RS spec that means that the standard adapter provided is missing the setters required to deserialise the Link later on.

This isn't a problem if you are using the JAX-B RI as it appears to be more relaxed than the standard; but it will be a problem for other implementations. There is a further bug if you are using MOXy, aka EclipseLink, to produce either JSON or XML that it will fail and just call toString() because it doesn't like the type adapter being an inner class of the class that is being adapted. (Bug TBC)

Direction	JAXB RI + JAX-RS Adapter	MOXy + JAX-RS Adapter	MOXy + Outer Adapter	MOXy + Outer Adapter + Setters	MOXy + Inner Adapter (Different class)
Marshalling	Yes	No	Yes	Yes	Yes
Unmarshalling	Yes	No	No	Yes	No

The workaround is to create a new adapter as a top level class in order to replace the one provided by the standard. Just quickly it would look something like this:

import java.util.HashMap;
import java.util.Map;

import javax.ws.rs.core.Link;

import javax.xml.bind.annotation.adapters.XmlAdapter;
import javax.xml.namespace.QName;

public class LinkAdapter
    extends XmlAdapter {

    public LinkAdapter() {
    }

    public Link unmarshal(LinkJaxb p1) {
        
        Link.Builder builder = Link.fromUri(p1.getUri());
        for (Map.Entry<QName, Object> entry : p1.getParams().entrySet()) {
            builder.param(entry.getKey().getLocalPart(), entry.getValue().toString());
        }
        return builder.build();
    }

    public LinkJaxb marshal(Link p1) {
        
        Map<QName, Object> params = new HashMap<>();
        for (Map.Entry<String,String> entry : p1.getParams().entrySet()) {
            params.put(new QName("", entry.getKey()), entry.getValue());
        }
        return new LinkJaxb(p1.getUri(), params);
    }
}

With a simple POJO to go with it.

import java.net.URI;

import java.util.HashMap;
import java.util.Map;

import javax.xml.bind.annotation.XmlAnyAttribute;
import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.namespace.QName;

public class LinkJaxb  {

    private URI uri;
    private Map<QName, Object> params;


    public LinkJaxb() {
        this (null, null);
    }

    public LinkJaxb(URI uri) {
        this(uri, null);
    }

    public LinkJaxb(URI uri, Map<QName, Object> map) {
        
        this.uri = uri;
        this.params = map!=null ? map : new HashMap<QName, Object>();
        
    }



    @XmlAttribute(name = "href")
    public URI getUri() { 
        return uri;
    }

    @XmlAnyAttribute
    public Map<QName, Object> getParams() { 
        return params;
    }

    public void setUri(URI uri) {
        this.uri = uri;
    }

    public void setParams(Map<QName, Object> params) {
        this.params = params;
    }

}

With this you can read and write the Link objects to and from XML, and JSON with MOXy, to your hearts content.

Declarative Linking in Jersey 2.9 and up

A couple of weeks ago A couple of months ago I was looking how I was going to engineers new REST API for an Oracle Cloud project. Once of the things I had planned to do was to use the declarative link injection created in Jersey 1.x by Marc Hadley. Sadly this hadn't been forwarded ported yet, so a quick chat to the project lead and I took on the small medium sized job of bringing the code up to date.

One of the things that has changed in the new version is that in JAX-RS 2.0 there is a Link object so rather than being able to only inject String and URI you can also inject the correct rel attributes. This has means that the existing annotations coded by Marc have been merged into once simple set of annotations for both Link headers and for injected properties.

This functionality is available now, along with a simple example. The original version of the feature that I committed has some serious limitations that are described later, you will need a version of Jersey post 2.8 or you can build a 2.9-SNAPSHOT image that contains my changes currently to implement the example in this blog.

This blog looks at using this new API to provide simple injection for a collections API. One of the common patterns in RESTful services, in particular those based on JSON, is to have an array of structural links at the top level of the structure. For the purposes of this blog I am going to follow the form of the Collection+JSON hypermedia type.

{ "collection" :
  {
    "version" : "1.0",
    "href" : "http://example.org/friends/?offset=10&limit=10",
    
    "links" : [
      {"rel" : "create", "href" : "http://example.org/friends/"}
      {"rel" : "next", "href" : "http://example.org/friends/?offset=20&limit=10"}
      {"rel" : "previous", "href" : "http://example.org/friends/?offset=0&limit=10"}
    ],
   
    "items" : [
       ...
    ]
  }
}

So I can inject the links in the following form, not there is a bunch of boiler plate missing here for clarity. This is not the tidiest code; but in a later cycle it should be possible to simply them somewhat. The design currently uses EL to access properties - this has the advantage of making it possible to write back values as you can represent properties. I can understand it is disliked by some; but I am not sure if I see any value in moving to JavaScript at the moment. Also don't be put of by the @Xml annotations, I am using MOXy for JSON generation - this isn't an XML only thing.

{


  @XmlTransient
  private int limit, offset; // Getters for these

  @XmlTransient
  private int modelLimit; // Getters for these


  @InjectLink(
            resource = ItemsResource.class,
            method = "query",
            style = Style.ABSOLUTE,
            bindings = {@Binding(name = "offset", value="${instance.offset}"),
                @Binding(name = "limit", value="${instance.limit}")
            },
            rel = "self"
  )
  @XmlElement(name="link")
  private String href;

  @InjectLinks({
    @InjectLink(
          resource = ItemsResource.class,
          style = Style.ABSOLUTE,
          method = "query",
          condition = "${instance.offset + instance.limit < instance.modelLimit}",
          bindings = {
            @Binding(name = "offset", value = "${instance.offset + instance.limit}"),
            @Binding(name = "limit", value = "${instance.limit}")
          },
          rel = "next"
    ),
    @InjectLink(
          resource = ItemsResource.class,
          style = Style.ABSOLUTE,
          method = "query",
          condition = "${instance.offset - instance.limit >= 0}",
          bindings = {
            @Binding(name = "offset", value = "${instance.offset - instance.limit}"),
            @Binding(name = "limit", value = "${instance.limit}")
          },
          rel = "prev"
  )})
  @XmlElement(name="link")
  @XmlElementWrapper(name = "links")
  @XmlJavaTypeAdapter(Link.JaxbAdapter.class)
  List<Link> links;

  ....
}

The original porting of the declarative linking code that exists in version Jersey before 2.8 had very naive code with regards working out what the URI should be for a particular resource, it couldn't deal with any resources that were not at the root of the application, nor would it cope with query parameters which are so important when dealing with collections.

In theory there could be more than one URI for a particular resource class; but this code does need to assume a 1:1 mapping, the current implementation contains a simple algorithm that walks the Jersey meta-model to try to work out the structure, is this doesn't work in your can you can simple provide another implementation of ResourceMappingContext.

Some may question why should I use these ugly annotations when it might be easier just to inject the URI myself? Well the reason is to provide metadata that other tools can use. One of my next jobs is to extend this work to generate the hypermedia extensions and for this I need the above metadata. (Waiting on a pull request being approved before I can really get into it).

Finally it is worth noting that the paging model has its own problems which become apparent if you think of a REST collection as some kind of array that you can safely page over. Concurrent updates along with the lack of state mean that the client can never be sure they have the complete model and should expect to see some items more than once as the model is updated. Cursor or linking based schemes should be considered instead which is yet another good reminder as to why you should always treat the URI as opaque - the sever might need to changes its structure in the future. But that is a entirely different blog for another day.....

Update: See the following blog for some workarounds when reading and writing Link objects.