Jersey Jackson unmarshall JSON

Question

I am working on an embedded jersey instance which will run a JAXB RESTful service. I have configured Jackson with two steps:

Adding this to my POM

<dependency>
    <groupId>org.glassfish.jersey.media</groupId>
    <artifactId>jersey-media-json-jackson</artifactId>
    <version>2.23.2</version>
</dependency>

Registering it in my application

public HandheldApplication() {
scripts.add(HandheldServer.class);
scripts.add(BasicScript.class);
// Add JacksonFeature.
scripts.add(JacksonFeature.class);
scripts.add(LoggingFilter.class);

}

I have a complex object being passed back and forth as shown below:

package com.ziath.handheldserver.valueobjects;

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

import javax.xml.bind.annotation.*;

@SuppressWarnings("restriction")
@XmlRootElement
public class Widget {

private String key;
private String name;
private List<String> options = new ArrayList<String>();
private String value;
private String type;

public Widget(){
    super();
}

public Widget(String key, String name, List<String> options, String value,
        String type) {
    super();
    this.key = key;
    this.name = name;
    this.options = options;
    this.value = value;
    this.type = type;
}

public String getKey() {
    return key;
}
public void setKey(String key) {
    this.key = key;
}
public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public List<String> getOptions() {
    return options;
}
public void setOptions(List<String> options) {
    this.options = options;
}
public String getValue() {
    return value;
}
public void setValue(String value) {
    this.value = value;
}
public String getType() {
    return type;
}
public void setType(String type) {
    this.type = type;
}

}

When I execute this in a GET method as shown below:

@Override
@GET
@Path("getKeys")
@Produces(MediaType.APPLICATION_JSON)
public List<Widget> getKeys(@QueryParam(value = "page") int page)

This works fine and I get JSON back; however when I execute it is a PUT as shown below:

@Override
@PUT
@Path("validateKeys")
@Produces({MediaType.APPLICATION_JSON})
@Consumes(MediaType.APPLICATION_JSON)
public boolean validateKeys(@QueryParam(value = "page")int page, @QueryParam(value = "widgets")List<Widget> widgets)

When I execute a PUT to access this method I get a stack trace as follows:

Caused by: org.glassfish.jersey.internal.inject.ExtractorException: Error un-marshalling JAXB object of type: class com.ziath.handheldserver.valueobjects.Widget.
    at org.glassfish.jersey.jaxb.internal.JaxbStringReaderProvider$RootElementProvider$1.fromString(JaxbStringReaderProvider.java:195)
    at org.glassfish.jersey.server.internal.inject.AbstractParamValueExtractor.convert(AbstractParamValueExtractor.java:139)
    at org.glassfish.jersey.server.internal.inject.AbstractParamValueExtractor.fromString(AbstractParamValueExtractor.java:130)
    at org.glassfish.jersey.server.internal.inject.CollectionExtractor.extract(CollectionExtractor.java:88)
    at org.glassfish.jersey.server.internal.inject.CollectionExtractor$ListValueOf.extract(CollectionExtractor.java:107)
    at org.glassfish.jersey.server.internal.inject.QueryParamValueFactoryProvider$QueryParamValueFactory.provide(QueryParamValueFactoryProvider.java:89)
    ... 38 more
Caused by: javax.xml.bind.UnmarshalException
 - with linked exception:
[org.xml.sax.SAXParseException; lineNumber: 1; columnNumber: 1; Content is not allowed in prolog.]
    at javax.xml.bind.helpers.AbstractUnmarshallerImpl.createUnmarshalException(AbstractUnmarshallerImpl.java:335)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.createUnmarshalException(UnmarshallerImpl.java:563)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:249)
    at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal(UnmarshallerImpl.java:214)
    at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:140)
    at javax.xml.bind.helpers.AbstractUnmarshallerImpl.unmarshal(AbstractUnmarshallerImpl.java:123)
    at org.glassfish.jersey.jaxb.internal.JaxbStringReaderProvider$RootElementProvider$1.fromString(JaxbStringReaderProvider.java:190)
    ... 43 more

So it seems to me that Jackson is correctly marshalling my POJO into JSON but trying to unmarshall it as XML. Note that I switched to Jackson away from MOXy because I needed to be able to handle collections coming back and forth and apparently MOXy cannot do that.

Is there a setting I've missed to tell Jackson/Jersey to go both ways for JSON?

How are you executing the PUT? From javascript, a command line, a JAX-RS client, or something else? — Jason Hoetger, Sep 07 '16 at 02:25
Sorry; I found the answer the problem was that Jackson/Jersey automatically marshalls a POJO into JSON but doesn't unmarshall it so I needed to remove the XML related annotation at the top and manually put in a fromString method to unmarhsall JSON into the object. — Neil Benn, Sep 07 '16 at 09:55
Jersey definitely can unmarshall entities from the body as well. I suspect you were not supplying the proper Content-Type header (application/json) when making the request, which is why I was wondering how you were invoking it (when I `curl` from the command line, I often forget the `-H 'Content-Type: application/json`). — Jason Hoetger, Sep 09 '16 at 04:31
Hello - no the problem was that I needed to provide a unmarshall method for the object - see my own answer. The correct content type was provided but it could not automatically unmarshall the JSON without a helper method - the helper method fromString is sought by Jersey/Jackson and it all works just fine. Now I have provided the unmarshall method it works just fine. Thanks for taking the time to comment however. — Neil Benn, Sep 09 '16 at 12:23
I suspect you could've done it without a static fromString method and without a @FormParam. But it sounds like your solution works for you! — Jason Hoetger, Sep 09 '16 at 18:31

score 0 · Answer 1 · edited May 23 '17 at 10:33

Try removing @QueryParam(value = "widgets") because you should pass it as entity body - not query param.

@PUT
@Path("validateKeys")
@Produces({MediaType.APPLICATION_JSON})
@Consumes(MediaType.APPLICATION_JSON)
public boolean validateKeys(@QueryParam(value = "page")int page, List<Widget> widgets)

Also you can make wrapper class:

@XmlRootElement
public class Widgets {

    private List<Widget> widgets; 

    // other fields, setters and getters
}

And then:

@PUT
@Path("validateKeys")
@Produces({MediaType.APPLICATION_JSON})
@Consumes(MediaType.APPLICATION_JSON)
public boolean validateKeys(@QueryParam(value = "page")int page, Widgets widgets)

I would suggest to read some discussions about REST design because you're using verbs in your paths:

Thanks I posted the answer above and thanks for the REST grammar; I autogenerated the code stubs at first which I'll clean up later when going into deployment. — Neil Benn, Aug 31 '16 at 13:21

score 0 · Accepted Answer · answered Aug 31 '16 at 13:20

I was switching between QueryParam and FormParam to try and get one of them to work. If I use FormParam I also need to change the consumes to APPLICATION_FORM_URLENCODED.

The actual issue was that the default unmarshalling with Jackson was using XML because it was tagged as an XML resource - take that out! I finally managed to work out how to unmarshall from JSON by using a static fromString method. Then to handle the list; I cannot use a wrapper class because this needs to be highly cross language and exposing a wrapper with a list would have complicated the implementation from Python, C#, etc. The way to get it to accept a list with a wrapper is to post the name of the param (in this case widgets) multiple time. Then each JSON passed in will be called against the fromString method.

Jersey Jackson unmarshall JSON

2 Answers2