How to Deserialize JSON Array?

Question

I'm using Jackson within CXF to serialize/deserialize data. Unfortunately, I am having difficulty configuring CXF/Jackson to deserialize a JSON array. I'd appreciate help in resolving the issue.

Up to this point most of the json data has been in object format, ie

{ "objectCollection": [ {...}, {...}, {...}... ] }

However, the json data in question is of the form:

[ {...}, {...}, {...} ]

The web service endpoint expects a "GroupsDto" object (see following) that has a single property -- a collection of groups, which is transmitted via the JSON array.

@PATH(...)
public Response createGroups(GroupsDto groups) {
...
}

I added @JsonDeserialize as follows to the GroupsDto collection property, but it does NOT work. I continue to get: "Can not deserialize instance of GroupsDto out of START_ARRAY token"

public class GroupsDto {

       private Collection<GroupDto> groups;

       /**
        * @return the groups
        */
       @XmlElement(name="group")
       @JsonDeserialize(contentAs=GroupDto.class)
       public Collection<GroupDto> getGroups() {
               return groups;
       }
...
}

Answer 1

If json data is of the form:

[ {...}, {...}, {...} ]

You got to use add another class say 'wrapper':

@JsonIgnoreProperties(ignoreUnknown = true)
public class ListDto extends ArrayList<GroupDto> {

    public ListDto() {
    }
}

And use this class while deserailizing. This approach worked for me.

Answer 2

You just need to specify the @JsonDeserialize(contentAs=GroupDto.class) in your setter . Serialization is always on get desserialization is always on set , or if you prefer you can specify both on the field.

Documentation for Serialize and Deserialize

Code sample:

import java.io.IOException;
import java.util.List;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import org.codehaus.jackson.map.annotate.JsonDeserialize;

public class JacksonDeserialize {

    public static class ModelClass {

        private String name;

        public ModelClass() {
        }

        public String getName() {
            return name;
        }

        public void setName(final String name) {
            this.name = name;
        }

        public ModelClass(final String name) {
            super();
            this.name = name;
        }

        @Override
        public String toString() {
            return "ModelClass [name=" + name + "]";
        }

    }

    public static class ListModelClass {

        private List<ModelClass> list;

        @JsonDeserialize(contentAs = ModelClass.class)
        public void setList(final List<ModelClass> list) {
            this.list = list;
        }

        @Override
        public String toString() {
            return "ListModelClass [list=" + list + "]";
        }

    }

    public static void main(final String[] args) throws JsonGenerationException, JsonMappingException, IOException {
        ObjectMapper objectMapper = new ObjectMapper();
        System.out.println(objectMapper.readValue("{\"list\":[{\"name\":\"name1\"},{\"name\":\"name2\"}]}",
                ListModelClass.class));
    }

}