JAX-RS HATEOAS 使用 Jersey，JSON 中不需要的链接属性

Question

Since Jersey 2.9, it's been possible to create link relations for hypermedia-driven REST APIs through declarative linking.

This code, for example:

@InjectLink(
    resource = ItemResource.class,
    style = Style.ABSOLUTE,
    bindings = @Binding(name = "id", value = "${instance.id}"),
    rel = "self"
)
@XmlJavaTypeAdapter(Link.JaxbAdapter.class)
@XmlElement(name="link")
Link self;

...in theory is expected to produce JSON like this:

"link" : {
    "rel" : "self",
    "href" : "http://localhost/api/resource/1"
}

However, Jersey produces different JSON with a lot of properties that I don't need:

"link" : {
   "rel" : "self",
   "uri" : "http://localhost/api/resource/1",
   "type": null,
   "uriBuilder" : null
}

Notice also that instead of href , it uses uri . I looked at Jersey's implementation of the Link object and found JerseyLink .

I want to use Jersey's declarative linking instead of rolling out my own implementation. I ended up using Jackson annotations just to ignore other JerseyLink properties.

@JsonIgnoreProperties({ "uriBuilder", "params", "type", "rels" })

Has anyone used declarative linking with Jersey and had the expected JSON output (eg, href instead of uri , without extra Jersey properties) without having to use JsonIgnoreProperties or other hacks?

Thanks.

EDIT

I resolved this using an approach which I think is a hack but works well for me and doesn't require the use of a complicated adapter.

I realized that I can actually expose a different object instead of the Link injected by Jersey.

I created a wrapper object named ResourceLink:

public class ResourceLink {
  private String rel;
  private URI href;

  //getters and setters
}

Then in my representation object I have a getter method:

public ResourceLink getLink() {
   ResourceLink link = new ResourceLink();
   link.setRel(self.getRel());
   link.setHref(self.getUri());

   return link;
}

So I used Jersey to inject the link but returned a different object in a getter method in my representation object. This would be the property that would be serialized to JSON and not the injected link object because I didn't create a getter method for it.

Answer 1

Invesitigation

Environment: Jersey 2.13 ( all provider versions are also 2.13 ).

Whether you use declarative or programmatic linking, the serialization shouldn't differ. I chose programmatic, just because I can :-)

Test classes:

@XmlRootElement
public class TestClass {
    private javax.ws.rs.core.Link link;

    public void setLink(Link link) { this.link = link; }

    @XmlElement(name = "link")
    @XmlJavaTypeAdapter(Link.JaxbAdapter.class)
    public Link getLink() { return link; }
}

@Path("/links")
public class LinkResource {  
    @GET
    @Produces(MediaType.APPLICATION_JSON)
    public Response getResponse() {
        URI uri = URI.create("https://stackoverflow.com/questions/24968448");
        Link link = Link.fromUri(uri).rel("stackoverflow").build();
        TestClass test = new TestClass();
        test.setLink(link);
        return Response.ok(test).build();
    }
}

@Test
public void testGetIt() {
    WebTarget baseTarget = target.path("links");
    String json = baseTarget.request().accept(
            MediaType.APPLICATION_JSON).get(String.class);
    System.out.println(json); 
}

---

Results with different Providers (with no extra configurations)

jersey-media-moxy

Dependency

<dependency>
    <groupId>org.glassfish.jersey.media</groupId>
    <artifactId>jersey-media-moxy</artifactId>
</dependency>

Result (weird)

{
    "link": "javax.ws.rs.core.Link$JaxbLink@cce17d1b"
}

jersey-media-json-jackson

Dependency

<dependency>
    <groupId>org.glassfish.jersey.media</groupId>
    <artifactId>jersey-media-json-jackson</artifactId>
</dependency>

Result (close, but what's with the params ?)

{
    "link": {
        "params": {
            "rel": "stackoverflow"
        },
        "href": "https://stackoverflow.com/questions/24968448"
    }
}

jackson-jaxrs-json-provider

Dependency

<dependency>
    <groupId>com.fasterxml.jackson.jaxrs</groupId>
    <artifactId>jackson-jaxrs-json-provider</artifactId>
    <version>2.4.0</version>
</dependency>

Result (Two different results, with two different JSON providers)

resourceConfig.register(JacksonJsonProvider.class);

{
    "link": {
        "uri": "https://stackoverflow.com/questions/24968448",
        "params": {
            "rel": "stackoverflow"
        },
        "type": null,
        "uriBuilder": {
            "absolute": true
        },
        "rels": ["stackoverflow"],
        "title": null,
        "rel": "stackoverflow"
    }
}

resourceConfig.register(JacksonJaxbJsonProvider.class);

{
    "link": {
        "params": {
            "rel": "stackoverflow"
        },
        "href": "https://stackoverflow.com/questions/24968448"
    }
}

---

My Conclusions

We are annotating the field with @XmlJavaTypeAdapter(Link.JaxbAdapter.class) . Let look at a snippet of this adapter

public static class JaxbAdapter extends XmlAdapter<JaxbLink, Link> {...}

So from Link , we are being marshalled to JaxbLink

public static class JaxbLink {

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

jersey-media-moxy

Seems to be a bug... See below in solutions.

The others

The other two are dependent on jackson-module-jaxb-annotations to handle marshalling using JAXB annotations. jersey-media-json-jackson will automatically register the required JaxbAnnotationModule . For jackson-jaxrs-json-provider , using JacksonJsonProvider will not support JAXB annotations (without confgiruation), and using JacksonJsonJaxbProvider will give us the JAXB annotation support.

So if we have JAXB annotation support, we will get marshalled to JaxbLink , which will give this result

{
    "link": {
        "params": {
            "rel": "stackoverflow"
        },
        "href": "https://stackoverflow.com/questions/24968448"
    }
}

The ways we can get the result with all the unwanted properties, is to 1) , use the jackson-jaxrs-json-provider 's JacksonJsonProvider or 2) , create a ContextResolver for ObjectMapper where we don't register the JaxbAnnotationModule . You seem to be doing one of those.

---

Solutions

The above still doesn't get us where we want to get to (ie no params ).

For jersey-media-json-jackson and jackson-jaxrs-json-provider ...

...which use Jackson, the only thing I can think of at this point is to create a custom serializer

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import java.io.IOException;
import javax.ws.rs.core.Link;

public class LinkSerializer extends JsonSerializer<Link>{

    @Override
    public void serialize(Link link, JsonGenerator jg, SerializerProvider sp) 
            throws IOException, JsonProcessingException {
        jg.writeStartObject();
        jg.writeStringField("rel", link.getRel());
        jg.writeStringField("href", link.getUri().toString());
        jg.writeEndObject();
    }
}

Then create a ContextResolver for the ObjectMapper , where we register the serializer

@Provider
public class ObjectMapperContextResolver 
                          implements ContextResolver<ObjectMapper> {

    private final ObjectMapper mapper;

    public ObjectMapperContextResolver() {
        mapper = new ObjectMapper();
        SimpleModule simpleModule = new SimpleModule();
        simpleModule.addSerializer(Link.class, new LinkSerializer());
        mapper.registerModule(simpleModule);
    }

    @Override
    public ObjectMapper getContext(Class<?> type) {
        return mapper;
    }
}

This is the result

{
    "link": {
        "rel": "stackoverflow",
        "href": "https://stackoverflow.com/questions/24968448"
    }
}

With jersey-media-moxy , it appears there's a Bug with missing setters in the JaxbLink class, so the marshalling reverts to calling toString , which is what's shown above. A work around, as proposed here by Garard Davidson , is just to create another adapter

import java.net.URI;
import java.util.HashMap;  
import java.util.Map;  

import javax.ws.rs.core.Link;  
import javax.xml.bind.annotation.XmlAnyAttribute;
import javax.xml.bind.annotation.XmlAttribute;

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

public class LinkAdapter  
    extends XmlAdapter<LinkJaxb, Link> {  

    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);  
    }  
}  

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;  
    }    
}

Using this adapter instead

@XmlElement(name = "link")
@XmlJavaTypeAdapter(LinkAdapter.class)
private Link link;

will give us the desired output

{
    "link": {
        "href": "https://stackoverflow.com/questions/24968448",
        "rel": "stackoverflow"
    }
}

---

UPDATE

Now that I think about it, the LinkAdapter would work with the Jackson provider also. No need to create a Jackson Serializer/Deserializer. The Jackson module should already support the JAXB annotations out the box, given the JacksonFeature is enabled. The examples above show using the JAXB/JSON providers separately, but given just the JacksonFeature is enabled, the JAXB version of the provider should be used. This may actually be the more preferred solution. No need to create an ContextResolvers for the ObjectMapper :-D

It's also possible to declare the annotation at the package level, as seen here

Answer 2

I'd like to share with my solution for serialising/deserialising Link objects using with Jackson and the mix-in annotations .

LinkMixin:

@JsonAutoDetect(
        fieldVisibility = JsonAutoDetect.Visibility.NONE,
        getterVisibility = JsonAutoDetect.Visibility.NONE,
        isGetterVisibility = JsonAutoDetect.Visibility.NONE)
@JsonDeserialize(using = LinkMixin.LinkDeserializer.class)
public abstract class LinkMixin extends Link {

    private static final String HREF = "href";

    @JsonProperty(HREF)
    @Override
    public abstract URI getUri();

    @JsonAnyGetter
    public abstract Map<String, String> getParams();

    public static class LinkDeserializer extends JsonDeserializer<Link> {

        @Override
        public Link deserialize(
                final JsonParser p,
                final DeserializationContext ctxt) throws IOException {
            final Map<String, String> params = p.readValueAs(
                    new TypeReference<Map<String, String>>() {});
            if (params == null) {
                return null;
            }
            final String uri = params.remove(HREF);
            if (uri == null) {
                return null;
            }
            final Builder builder = Link.fromUri(uri);
            params.forEach(builder::param);
            return builder.build();
        }
    }
}

Example:

final ObjectMapper mapper = new ObjectMapper();
mapper.addMixIn(Link.class, LinkMixin.class);
final Link link = Link.fromUri("http://example.com")
        .rel("self")
        .title("xxx")
        .param("custom", "my")
        .build();
final String json = mapper
        .writerWithDefaultPrettyPrinter()
        .writeValueAsString(Collections.singleton(link));
System.out.println(json);
final List<Link> o = mapper.readValue(json, new TypeReference<List<Link>>() {});
System.out.println(o);

Output:

[ {
  "href" : "http://example.com",
  "rel" : "self",
  "title" : "xxx",
  "custom" : "my"
} ]
[<http://example.com>; rel="self"; title="xxx"; custom="my"]

Answer 3

Using the suggested update solution I was still getting the rel part inside the params map.

I have made some changes in the Link adapter class

public class LinkAdapter
    extends XmlAdapter<LinkJaxb, Link> {

    public LinkAdapter() {
    }

    public Link unmarshal(LinkJaxb p1) {

        Link.Builder builder = Link.fromUri(p1.getUri());

        return builder.build();
    }

    public LinkJaxb marshal(Link p1) {

        return new LinkJaxb(p1.getUri(), p1.getRel());
    }
}

class LinkJaxb  {

    private URI uri;
    private String rel;


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

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

    public LinkJaxb(URI uri,String rel) {

        this.uri = uri;
        this.rel = rel;

    }

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

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


}

It now holds only the two params which are needed (rel,href) I did not really understand when do I need to unmarshal a Jaxb link to a Link. What mattered to me was the Link to Jaxb link marshaling.

Answer 4

Thank You, @peeskillet and @Nir Sivan, for your answers. But I was able to make it work without using the LinkAdapter or ContextResolver<ObjectMapper> .

I just added a instance variable of the custom Link type (here ResourceLink which is analogous to your LinkJaxb ) to my entity class as a @Transient property and after that Jackson configuration automatically included that attribute in the Response JSON

Resource Link - Class

import javax.xml.bind.annotation.XmlAttribute;

import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;

@JsonInclude(Include.NON_EMPTY)
public class ResourceLink  {

private String uri;
private String rel;


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

public ResourceLink(String uri) {
    this(uri, null);
}

public ResourceLink(String uri,String rel) {

    this.uri = uri;
    this.rel = rel;

}

@XmlAttribute(name = "href")
public String getUri() {
    return uri;
}
@XmlAttribute(name = "rel")
public String getRel(){return rel;}

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


}

Entity Class

package com.bts.entities;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.persistence.Transient;

import com.bts.rs.root.util.ResourceLink;
import com.fasterxml.jackson.annotation.JsonInclude;
import com.fasterxml.jackson.annotation.JsonInclude.Include;

@Entity
@Table(name="cities")
@JsonInclude(Include.NON_EMPTY)
public class City {
    @Id
    @Column(name="city_id")
    private Integer cityId;

    @Column(name="name")
    private String name;

    @Column(name="status")
    private int status;

    @Column(name="del_status")
    private int delStatus;

    @Transient
    List<ResourceLink> links = new ArrayList<ResourceLink>();

    // private 
    public City () {

    }

    public City (Integer id, String name) {
        this.cityId = id;
        this.name = name;
        this.status = 0;
        this.delStatus = 0;
    }

    // getters and setters for Non-transient properties

    // Below is the getter for lInks transient attribute
    public List<ResourceLink> getLinks(){
        return this.links;
    }

    // a method to add links - need not be a setter necessarily
    public void addResourceLink (String uri,String rel) {
        this.links.add(new ResourceLink(uri, rel));
    }   
}

Jersy Resource Provider

@GET
@Path("/karanchadha")
@Produces({MediaType.APPLICATION_JSON})
@Transactional
public Response savePayment() {
    City c1 = new City();
    c1.setCityId(11);
    c1.setName("Jamshedpur");
    c1.addResourceLink("http://www.test.com/home", "self");
    c1.addResourceLink("http://www.test.com/2", "parent");

    return Response.status(201).entity(c1).build();
}

Answer 5

First dependencies :

<dependency> 
    <groupId>org.glassfish.jersey.media</groupId>
    <artifactId>jersey-media-json-jackson</artifactId> 
    <version>2.34</version>
</dependency> 
<dependency>
    <groupId>org.glassfish.jersey.ext</groupId>
    <artifactId>jersey-declarative-linking</artifactId>
    <version>2.34</version>
</dependency>

Second - config class:

package app.rest.config;

import javax.ws.rs.ApplicationPath;
import org.glassfish.jersey.jackson.internal.jackson.jaxrs.json.JacksonJaxbJsonProvider;
import org.glassfish.jersey.linking.DeclarativeLinkingFeature;
import org.glassfish.jersey.server.ResourceConfig;


@ApplicationPath(value = "rest")
public class RestApplicationConfig extends ResourceConfig{

    public RestApplicationConfig() {
        register(JacksonJaxbJsonProvider.class);
        register(DeclarativeLinkingFeature.class);
        packages("app.rest.controllers");
    }


}

Third - create adapter class

package app.rest.config;

import javax.json.Json;
import javax.json.JsonObject;
import javax.json.bind.adapter.JsonbAdapter;
import javax.ws.rs.core.Link;
import javax.ws.rs.core.UriBuilder;


public class CustomJsonAdapter implements JsonbAdapter<Link, JsonObject> {

    @Override
    public JsonObject adaptToJson(Link link) throws Exception {
        StringBuilder builder = new StringBuilder();
        builder.append("http://");
        builder.append(link.getUri().getHost());
        builder.append(":");
        builder.append(link.getUri().getPort());
        builder.append(link.getUri().getRawPath());
        return Json.createObjectBuilder().add("href", builder.toString()).add("rel", link.getRel()).build();
    }

    @Override
    public Link adaptFromJson(JsonObject json) throws Exception {
        Link link = Link.fromUriBuilder(UriBuilder.fromUri(json.getString("href"))).rel(json.getString("rel")).build();
        return link;
    }

}

Forth - register JsonbConfig with custom adapter class from Link class to Json

package app.rest.config;

import javax.json.bind.Jsonb;
import javax.json.bind.JsonbBuilder;
import javax.json.bind.JsonbConfig;
import javax.ws.rs.ext.ContextResolver;
import javax.ws.rs.ext.Provider;

@Provider
public class AppJsonConfig implements ContextResolver<Jsonb>  {

    @Override
    public Jsonb getContext(Class<?> type) {
        JsonbConfig jsonbConfig = new JsonbConfig();
        jsonbConfig.withAdapters(new CustomJsonAdapter());
        return JsonbBuilder.create(jsonbConfig);
    }

}

Fifth : create the model with Link and @InjectLink annotation

package app.rest.model;

import java.io.Serializable;
import java.util.Objects;
import javax.json.bind.annotation.JsonbTypeAdapter;
import javax.ws.rs.core.Link;
import javax.xml.bind.annotation.XmlRootElement;
import org.glassfish.jersey.linking.Binding;
import org.glassfish.jersey.linking.InjectLink;
import org.glassfish.jersey.linking.InjectLink.Style;
import app.rest.config.CustomJsonAdapter;
import app.rest.controllers.RestController;


@XmlRootElement
public class StudentResource implements Serializable{

    private static final long serialVersionUID = 1L;

    private Long id;

    private String name;

    private String surname;

    @InjectLink(resource = RestController.class, 
                style = Style.ABSOLUTE,
                rel = "self", 
                method = "getStudentById",
                bindings = @Binding(name = "id", value = "${instance.id}"))
    @JsonbTypeAdapter(value = CustomJsonAdapter.class)
    private Link link;

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getSurname() {
        return surname;
    }
    public void setSurname(String surname) {
        this.surname = surname;
    }

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

    public Link getLink() {
        return link;
    } 
    public void setLink(Link link) {
        this.link = link;
    }
    @Override
    public int hashCode() {
        return Objects.hash(name, surname);
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        StudentResource other = (StudentResource) obj;
        return Objects.equals(name, other.name) && Objects.equals(surname, other.surname);
    }
}

And at the end rest controller

package app.rest.controllers;

import java.net.URISyntaxException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.PathParam;
import javax.ws.rs.Produces;
import javax.ws.rs.core.Context;
import javax.ws.rs.core.GenericEntity;
import javax.ws.rs.core.Link;
import javax.ws.rs.core.MediaType;
import javax.ws.rs.core.Response;
import javax.ws.rs.core.UriInfo;
import app.rest.model.*;

@Path(value = "student")
public class RestController {

    @Context
    private UriInfo uriInfo;

    private final ArrayList<StudentResource> students = new ArrayList<>();

    public RestController() {
        StudentResource s1 = new StudentResource();
        s1.setId(1L);
        s1.setName("test1");
        s1.setSurname("surTest1");
        students.add(s1);
        StudentResource s2 = new StudentResource();
        s2.setId(2L);
        s2.setName("new_St");
        s2.setSurname("surNew");
        students.add(s2);
    
    }

    @GET
    @Produces(MediaType.APPLICATION_JSON)
    public Response  getAllStudents() throws URISyntaxException{
        Link link = Link.fromUriBuilder(uriInfo.getAbsolutePathBuilder()).rel("self").type("GET").build();
        GenericEntity<List<StudentResource>> studentsEntities = new GenericEntity<List<StudentResource>>(students) {};
        return Response.status(Response.Status.OK).entity(studentsEntities).links(link).build();
    }

    @GET
    @Path(value = "/{id}")
    @Produces(MediaType.APPLICATION_JSON)
    public Response getStudentById(@PathParam(value = "id") Long id ) {
        Iterator<StudentResource> iterator = students.iterator();
        StudentResource studentById= null;
        while (iterator.hasNext()) {
            StudentResource next = iterator.next();
            if(next.getId().equals(id)) {
                studentById = next;
                break;
            }
        }
        if(null!=studentById) return Response.status(Response.Status.OK).entity(studentById).build();
    
        else return Response.status(Response.Status.NO_CONTENT).build();
    }   

}

Deployed and tested on Payara 5.201

Produces : get http://localhost:8080/sampleappee/rest/student/1

{"id":1,"link":{"href":"http://localhost:8080/sampleappee/rest/student/1","rel":"self"},"name":"test1","surname":"surTest1"}