Jersey: Json array with 1 element is serialized as object

Question

I'm creating a REST server with Jersey/Java and I found a strange behavior.

I have a method on the server that returns an array of objects as Json

@GET
@Path("/files")
@Produces(MediaType.APPLICATION_JSON)
public Object getFiles() throws Exception{
    DatabaseManager db = new DatabaseManager();
    FileInfo[] result = db.getFiles();
    return result;
}

The code is executed correctly and data is returned to the client (a jQuery ajax call). The problem is that the format of the returned data changes if the "result" array has one element or more than one.

Response with one element:

{"fileInfo":{"fileName":"weather.arff","id":"10"}}

Response with two elements:

{"fileInfo":[{"fileName":"weather.arff","id":"10"},{"fileName":"supermarket.arff","id":"11"}]}

As you can see, in the first scenario the value of the "fileInfo" property of the returned object is an object, and in the second case the value is an array. What am I doing wrong? Shouldn't the first case return something like this:

{"fileInfo":[{"fileName":"weather.arff","id":"10"}]}

i.e. an array with a single object inside?

I know that I can detect this on the client side, but it seems like a very ugly hack.

Thanks for your time.

score 12 · Answer 1 · answered Nov 27 '12 at 23:05

I ended up using Jackson, also described in the official Jersey documentation (http://jersey.java.net/nonav/documentation/latest/user-guide.html#json.pojo.approach.section).

I had tried that before but it wasn't working because I didn't have the jackson jar in the buildpath of my project (Based on the documentation I thought it was built into jersey's core library).

I just added the jackson-all.jar file (http://wiki.fasterxml.com/JacksonDownload) and enabled the POJO support in the configuration

    <init-param>
          <param-name>com.sun.jersey.api.json.POJOMappingFeature</param-name>
          <param-value>true</param-value>
    </init-param>

And voilá!

This answered my question without modifying anything in my code. Thanks. — Solostaran14, Oct 06 '14 at 15:07

score 5 · Answer 2 · answered Nov 27 '12 at 04:42

If you were using JAXB to build JSON result, you can configure Jersey JSON procesor to get more important JSON format.

jersey official document has detailed config:

To achieve more important JSON format changes, you will need to configure Jersey JSON procesor itself. Various configuration options could be set on an JSONConfiguration instance. The instance could be then further used to create a JSONConfigurated JSONJAXBContext, which serves as a main configuration point in this area. To pass your specialized JSONJAXBContext to Jersey, you will finally need to implement a JAXBContext ContextResolver:

    @Provider
public class JAXBContextResolver implements ContextResolver<JAXBContext> {
    private final JAXBContext context;
    private final Set<Class> types;
    private Class[] ctypes = { FileInfo.class}; //your pojo class
    public JAXBContextResolver() throws Exception {
        this.types = new HashSet(Arrays.asList(ctypes));
        this.context = new JSONJAXBContext(JSONConfiguration.natural().build(),
                ctypes); //json configuration
    }

    @Override
    public JAXBContext getContext(Class<?> objectType) {
        return (types.contains(objectType)) ? context : null;
    }
}

How will the response get formatted after adding this Resolver, so I need to tell code somewhere to use this Resolver? — Sankalp, Mar 12 '18 at 13:20

score 1 · Answer 3 · edited May 23 '17 at 10:30

1

Also have a look at the following answer, that solved it for me:

How can I customize serialization of a list of JAXB objects to JSON?

edited May 23 '17 at 10:30

Community

1
1

answered May 11 '13 at 15:25

chrset

581
6
11

score 1 · Answer 4 · edited Sep 02 '19 at 14:29

I'm using cxf, here is my applicationContext.xml to force array in JSON:

<jaxrs:server id="myService" serviceName="MyService"
address="/mysvc">
<jaxrs:serviceBeans>
    <ref bean="myServiceImpl"/>
</jaxrs:serviceBeans>
<jaxrs:providers>
    <bean class="org.apache.cxf.jaxrs.provider.json.JSONProvider">
   <property name="dropRootElement" value="true" />
   <property name="supportUnwrapped" value="true" />
   <property name="namespaceMap">
      <map>
        <entry key="http://example.com/myservice" value=""/>
      </map>
   </property>
   <property name="arrayKeys">
      <list>
    <value>fileInfo</value>
      </list>
   </property>                          
    </bean>
</jaxrs:providers>
</jaxrs:server>

This is apparently the only way to do it using the spring configuration. It's not optimal but for my case this is the only way to go. — рüффп, Sep 02 '19 at 14:28

Fabian Lange · Answer 5 · 2012-11-27T00:34:06.083

0

You could use Jettison (coming with Jersey) and prepare the structure you would like to have yourself using JSONObject and JSONArray as return values. They are in the package org.codehaus.jettison.json of jettison-1.3.2.jar which is a transitive dependency of jerysey-json

edited Nov 27 '12 at 00:34

answered Nov 27 '12 at 00:24

Fabian Lange

1,786
1
14
18

Where is the JSONArray class defined? Do I have to add the json library from json.org? – willvv Nov 27 '12 at 00:32

score 0 · Answer 6 · answered Nov 27 '12 at 09:01

You could also try Genson library http://code.google.com/p/genson/. It integrates well with jersey, just drop the jar in your classpath and everything will work. It doesnt require you to write additional code, it should work like what you have now but without any weird result.

score 0 · Answer 7 · answered Jun 11 '16 at 06:15

I've struggled quite a bit and found this simple solution

In your pom.xml:

<dependency>
    <groupId>org.codehaus.jackson</groupId>
    <artifactId>jackson-jaxrs</artifactId>
    <version>1.9.13</version>
</dependency>
<dependency>
    <groupId>org.codehaus.jackson</groupId>
    <artifactId>jackson-xc</artifactId>
    <version>1.9.13</version>
</dependency>

In your web.xml:

<servlet-class>com.sun.jersey.spi.container.servlet.ServletContainer</servlet-class>
<init-param>
    <param-name>com.sun.jersey.config.property.packages</param-name>
    <param-value>com.other-packages;org.codehaus.jackson.jaxrs</param-value>
</init-param>

score 0 · Answer 8 · answered May 08 '19 at 11:19

Converting the Array into ArrayList would suffice the requirement here. Similar kind of contradictory issue I have faced, where I had to return the Json Array Object instead of list in case of single element.

There i took the help of below annotation to get my job done-

@JsonFormat(with = JsonFormat.Feature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED). Below is the example of a JSON Pojo class:

import java.util.List;

import com.fasterxml.jackson.annotation.JsonFormat;
import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
import com.fasterxml.jackson.annotation.JsonProperty;

@JsonIgnoreProperties(ignoreUnknown = true)
public class TAResponseMapper {

    @JsonProperty("Response")
    @JsonFormat(with = JsonFormat.Feature.WRITE_SINGLE_ELEM_ARRAYS_UNWRAPPED) 
    private List<TAResponse> responses;

    public List<TAResponse> getResponses() {
        return responses;
    }

    public void setResponses(List<TAResponse> responses) {
        this.responses = responses;
    }

}

Jersey: Json array with 1 element is serialized as object

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