How to reuse Jersey's JSON/JAXB for serialization? - json

Question

I have a JAX-RS REST service implemented using Jersey. One of the cool features of JAX-RS/Jersey is how easily a POJO can be turned into a REST service, simply by sprinkling a few Java annotations... including a trivially easy mechanism for translating POJOs to JSON - using JAXB annotations.
Now, I'd like to be able to take advantage of this cool JSON-ifying functionality for non-REST purposes - I'd love to be able to just serialize some of these objects to disk, as JSON text. Here's an example JAXB object that I'd want to serialize:
#XmlRootElement(name = "user")
public class UserInfoImpl implements UserInfo {
public UserInfoImpl() {}
public UserInfoImpl(String user, String details) {
this.user = user;
this.details = details;
}
public String getUser() { return user; }
public void setUser(String user) { this.user = user; }
public String getDetails() { return details; }
public void setDetails(String details) { this.details = details; }
private String user;
private String details;
}
Jersey can turn one of these into json with no additional info. I'm wondering if Jersey has exposed this functionality in the API for needs like mine? I've had no luck finding it so far...
Thanks!
UPDATE 2009-07-09: I have learned that I can use the Providers object to almost do exactly what I want:
#Context Providers ps;
MessageBodyWriter uw = ps.getMessageBodyWriter(UserInfoImpl.class, UserInfoImpl.class, new Annotation[0], MediaType.APPLICATION_JSON_TYPE);
uw.writeTo(....)
... This writes the object as json to any outputstream, which would be perfect for me, but I can only get at the Providers object using #Context from a #Component object. Does anyone know how to access it from a regular, un-annotated POJO? Thanks!

Answer 1

Jersey uses a couple different frameworks depending on whether you use mapped(), badgerfish(), or natural() notation. Natural is usually the one people want. And that's implemented using the very good (and very fast) standalone Jackson JSON processor, I believe, which goes from Object->JAXB->JSON. However Jackson also provides it's own JAX-RS provider to go direct Object->JSON.
In fact, they even added support for JAXB annotations. Have a look at
http://wiki.fasterxml.com/JacksonJAXBAnnotations
I think that's ultimately what you are looking for. Jackson does Object<->JSON processing...Jersey just makes the calls for you

Answer 2

Here's a simple brief example of using JAXB to map objects to JSON (using Jackson):
http://ondra.zizka.cz/stranky/programovani/java/jaxb-json-jackson-howto.texy

Answer 3

ObjectMapper mapper = new ObjectMapper();
String str = mapper.writeValueAsString(pojoObject);

Answer 4

JAXB annotations work fine when serializing to XML.
The main problem is that JAXB does not support empty arrays. So when serializing something like this...
List myArray = new ArrayList();
...to json via jaxb anottations all your empty arrays become null instead of [].
To solve this you can just serialize your pojos directly to json via jackson.
Take a look at this from Jersey's user guide:
http://jersey.java.net/nonav/documentation/latest/user-guide.html#d0e1959
This is the best way to use Jackson provider without JAXB. Moreover, you can always use the latest version of jackson by downlaoding jackson-all-x.y.z-jar from its web.
This method will not interfere with your jaxb annotations so I would suggest to have a try!

Answer 5

Since Jersey is a reference implementation of JAX-RS and JAX-RS is focused completely on providing a standard way of implementing the end-point for the REST service the issues of serializing the payload is left to other standards.
I think that if they included object serialization in the JAX-RS standard it would quickly become a large multi-headed beast that would be difficult to implement and loose some of it's focus.
I appreciate how focused Jersey is on delivering clean and simple to use REST endpoints. In my case I've just subclassed a parent that has all the JAXB plumbing in it so marshalling objects between binary and XML is very clean.

Answer 6

With a little Jersey specific bootstrapping, you can use it to create the necessary JSON objects for you. You need to include the following dependencies (you can use bundle, but it will cause problems if you are using Weld for testing):
<dependency>
<groupId>com.sun.jersey</groupId>
<artifactId>jersey-json</artifactId>
<version>1.12</version>
</dependency>
<dependency>
<groupId>com.sun.jersey</groupId>
<artifactId>jersey-client</artifactId>
<version>1.12</version>
</dependency>
From there you can create a JAXB annotated class. The following is an example:
#XmlRootElement
public class TextMessage {
private String text;
public String getText() { return text; }
public void setText(String s) { this.text = text; }
}
Then you can create the following unit test:
TextMessage textMessage = new TextMessage();
textMessage.setText("hello");
textMessage.setUuid(UUID.randomUUID());
// Jersey specific start
final Providers ps = new Client().getProviders();
// Jersey specific end
final MultivaluedMap<String, Object> responseHeaders = new MultivaluedMap<String, Object>() {
#Override
public void add(final String key, final Object value) {
}
#Override
public void clear() {
}
#Override
public boolean containsKey(final Object key) {
return false;
}
#Override
public boolean containsValue(final Object value) {
return false;
}
#Override
public Set<java.util.Map.Entry<String, List<Object>>> entrySet() {
return null;
}
#Override
public List<Object> get(final Object key) {
return null;
}
#Override
public Object getFirst(final String key) {
return null;
}
#Override
public boolean isEmpty() {
return false;
}
#Override
public Set<String> keySet() {
return null;
}
#Override
public List<Object> put(final String key, final List<Object> value) {
return null;
}
#Override
public void putAll(
final Map<? extends String, ? extends List<Object>> m) {
}
#Override
public void putSingle(final String key, final Object value) {
}
#Override
public List<Object> remove(final Object key) {
return null;
}
#Override
public int size() {
return 0;
}
#Override
public Collection<List<Object>> values() {
return null;
}
};
final MessageBodyWriter<TextMessage> messageBodyWriter = ps
.getMessageBodyWriter(TextMessage.class, TextMessage.class,
new Annotation[0], MediaType.APPLICATION_JSON_TYPE);
final ByteArrayOutputStream baos = new ByteArrayOutputStream();
Assert.assertNotNull(messageBodyWriter);
messageBodyWriter.writeTo(textMessage, TextMessage.class,
TextMessage.class, new Annotation[0],
MediaType.APPLICATION_JSON_TYPE, responseHeaders, baos);
final String jsonString = new String(baos.toByteArray());
Assert.assertTrue(jsonString.contains("\"text\":\"hello\""));
The advantage to this approach is it keeps everything within the JEE6 API, no external libraries are explicitly needed except for testing and getting the providers. However, you need to create an implementation of MultivaluedMap since there is nothing provided in the standard and we don't actually use it. It may also be slower than GSON, and a lot more complicated than necessary.

Answer 7

I understand XML views but it would have shown some foresight to require JSON support for POJOs as standard equipment. Having to doctor up JSON identifiers with special characters makes no sense if your implementation is JSON and your client is a JavaScript RIA.
Also, not that Java Beans are NOT POJOs. I would like to use something like this on the outer surface of my web tier:
public class Model
{
#Property height;
#Property weight;
#Property age;
}
No default constructor, no getter/setter noise, just a POJO with my own annotations.