How to return a List>> as an output of a rest service class in json?

Question

I wrote a java application (commandline app) initially and now I am trying to convert it to a web application using jersey and jetty embeded. Now as the output of a service rest class I need to output a List<List<Map<String,String>>> in json format. Is there a way to accomplish this? I am new to the world of servlets and stuff.. i tried like this : @GET @Path("test") @Produces(MediaType.APPLICATION_JSON) public List>> test( ) throws FileNotFoundException, IOException, GateException, URISyntaxException {

    System.out.println("Starting the App");
    System.out.println("Locations added successfully");
    DataFile file = new CSVDataFile();
    CommandsExecutor.outputsList.add(0, file.performReadOperation(inputFilePath));

    System.out.println("Begining execution");
    List<List<Map<String, String>>> finalOutput = CommandsExecutor.executeSequentialCommands(file);
/*  for (List<Map<String, String>> tmpList : finalOutput) { 
        for (int i = 0; i < tmpList.size(); i++) {
            Map<String, String> insideMap = tmpList.get(i);
            // for (Map.Entry<String, String> entry : insideMap.entrySet())
            // {
            // System.out.println(entry.getKey() + "/" + entry.getValue());
            // }
            JSONObject obj = new JSONObject(insideMap);
            System.out.println(obj);
        }
        System.out.println("\n");
    } */
    return finalOutput;
}  

Now I get an error like this : Could not send response error 500: javax.servlet.ServletException: org.glassfish.jersey.message.internal.MessageBodyProviderNotFoundException: MessageBodyWriter not found for media type=application/json, type=class java.util.ArrayList, genericType=java.util.List>>.

What could be the work around ? Thanks in advance!!

Answer 1

The root cause of the problem is missing jars. Jersey makes use of Jackson, so you're going to need to download the Jackson jars.

Here's a list of all the Jackson jars I needed for a sample app I put together:

• jackson-annotations-2.7.0.jar
• jackson-core-2.7.0.jar
• jackson-databind-2.7.0.jar
• jackson-jaxrs-1.9.13.jar
• jackson-jaxrs-base-2.2.0.jar
• jackson-jaxrs-json-provider-2.2.0.jar
• jackson-xc.jar

In my example I created the following class:

package foo;

import java.util.*;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class ListWrapper 
{
    private List<List<Map<String,String>>> list;

    public List<List<Map<String,String>>> getList()
    {
        return list;
    }

    public void setList(List<List<Map<String,String>>> list)
    {
        this.list = list;
    }
}

And this servlet class:

    package foo;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.ws.rs.GET;
import javax.ws.rs.Path;
import javax.ws.rs.Produces;
import javax.ws.rs.core.MediaType;


@Path("/foo")
public class Foo  {

    @GET
    @Produces({ MediaType.APPLICATION_XML, MediaType.APPLICATION_JSON })
    public ListWrapper getList()
    {       
        ListWrapper lw = new ListWrapper();

        HashMap<String,String> map = new HashMap<String,String>();
        map.put("foo", "bar");
        map.put("hello", "world");

        ArrayList<Map<String,String>> innerList = new ArrayList<Map<String,String>>();
        innerList.add(map);

        ArrayList<List<Map<String,String>>> outerList = new ArrayList<List<Map<String,String>>>();
        outerList.add(innerList);

        lw.setList(outerList);
        return lw;
    }
}

The following XML was added to the web.xml config in my app:

<servlet>
    <servlet-name>Jersey REST Service</servlet-name>
    <servlet-class>org.glassfish.jersey.servlet.ServletContainer</servlet-class>
    <init-param>
        <param-name>jersey.config.server.provider.packages</param-name>
        <param-value>foo,com.fasterxml.jackson.jaxrs.json</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>Jersey REST Service</servlet-name>
    <url-pattern>/rest/*</url-pattern>
  </servlet-mapping>

Requesting the servlet in a browser returned the following JSON:

{
   "list": [
      [
         {
            "foo": "bar",
            "hello": "world"
         }
      ]
   ]
}

Once you get over the struggle of identifying all of the dependencies of Jersey and Jackson, they're really quite powerful libraries.

Answer 2

Your result JSON could be something like:

{
    "result": 
    [
        [ 
            {
                "key1":"value1",
                "key2":"value2",
                ...
            },
            {
                "key3":"value3", 
                "key4":"value4"
                ...
            }, 
            ...
        ],
        ...
     ]
}

In your commented code, you actually already created the inner JSONObject for the maps, you just need to append them to a JSONArray, and then put these JSONArrays into another JSONArray, then put this final JSONArray in a result JSONObject and return it.

JSONObject result = new JSONObject();
JSONArray array1 = new JSONArray();
for (List<Map<String, String>> tmpList : finalOutput) { 
    JSONArray array2 = new JSONArray();
    for (int i = 0; i < tmpList.size(); i++) {
        Map<String, String> insideMap = tmpList.get(i);
        JSONObject obj = new JSONObject(insideMap);
        array2.add(obj);
    }
    array1.add(array2);
}
result.append("result", array1);

return result.toString();