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using JsonObject as entity for Jersey 2 response

I have this simple code:
package com.example
import javax.json.Json;
import javax.json.JsonObject;
...
#Path("/")
public class Resource {
#GET
#Produces(MediaType.APPLICATION_JSON)
public Response defaultEntry() {
JsonObject result = (Json.createObjectBuilder()
.add("hello", "world")
.build());
return Response.status(200).entity(result.toString()).build();
}
}
I am new to Java, could someone please explain why, if I omit the call to result.toString() and simply pass result to .entity (like so: return Response.status(200).entity(result).build()), I get JSON on the client that includes type information etc, but not what I expected:
{"hello":{"chars":"world","string":"world","valueType":"STRING"}}
what is the intention of this? How is passing JsonObject to it different from passing a string?
Also, I did not find Response.entity method in the documentation (https://jersey.java.net/apidocs/2.11/jersey/javax/ws/rs/core/Response.html). I copied this code from a tutorial, that did not explain properly what is going on...

I wish I had a better answer for you, this more of a hint until a better answer arrives. There are a few moving parts here. JsonObject is an interface. Its implementation is not described. Furthermore there is a Json serializer that is turning your returned objects into Json text. It is both these things together that is leading to this Json schema output. When you did the .toString() variation, the serializer just returned the String as is. But when you return the JsonObject now you have these two dynamics at play, the implementation of the JsonObject and the implementation of the serializer. Since you are using Jersey 2.0 you could be using Jackson, Moxy, or Jettison serializers. These all might have different output when serializing the JsonObject, but we would have to test to be sure. Furthermore, the JsonObject implementation might be configured in a way that when serialized by your chosen serializer leads to its output being a Json schema, versus just regular Json. This can be done using annotations that are specific to the chosen Json serializer.
In my career I have used multiple Json serializers. Jackson is probably the most popular one out there. But I have also used Gson extensively. In one project we configured Gson in some way where its serialized Json output came out as a Json schema when serializing POJO's. So its not far fetched to have a Json serializer output Json schema under certain conditions.
When serializing POJO's (aka Java Beans), you expect a regular Json output when using default settings of your serializer on a Java Bean. But when sending back objects that could have complex interworkings with specific Json serializers you may get varying Json output.
In this situation you would have to run tests to dive deeper into what is going on. For example, I would first test out the serializer against a POJO that matches the JsonObject you created. Then I would also test out other Json serializers on the same JsonObject. See if you can pick up on a pattern.

The datatype module jackson-datatype-jsr353 provides support for javax.json types. The page includes instructions how to add the dependency and register the module with Jackson's ObjectMapper.
Starting with Jackson 2.11.0, the module moved under the umbrella of jackson-datatypes-misc.

Convert Json to a java object

What is the way to generate a Java object with get and set methods?

You should write a java bean with properties maching the JSON key's, from that point since you already have a reader its a simple as
YourObject obj = gson.fromJson(br, YourObject.class);
UPDATE
With respect to your comment, when you don't want or can't create a bean it usually boils down to parsing JSON to map. GSON (afaik) doesn't have a built-in for this, but its not hard to build a method that will traverse GSON's objects. You have an example in this blog
http://itsmyviewofthings.blogspot.it/2013/04/jsonconverter-code-that-converts-json.html
As you seem to be open to alternatives, take a look at Jackson as well (the two libs are the de-facto standard in JAVA).
With jackson you don't have to create a bean to support deserialization, e.g.
String json = "{\"id\":\"masterslave\"}";
Map<String,String> map = new HashMap<String,String>();
ObjectMapper mapper = new ObjectMapper();
//convert JSON string to Map
map = mapper.readValue(json,
new TypeReference<HashMap<String,String>>(){});

http://www.jsonschema2pojo.org/
That link helps generate the Java object format based on the GSON you feed in. Just make sure you set the settings exactly as you need it. As always, it's not a good idea to just copy-paste generated code, but it might be of help.

how can an object be not json serializable?

private FbDataToServer generateFbDataToServer(
GraphObject graphObject) {
Gson gson = new Gson();
FbDataToServer fbDataToServer = new FbDataToServer();
ֿ fbDataToServer.fbJson = gson.toJson(graphObject
.getInnerJSONObject());
Everytime I try to parse FB graphObject object to Json -
an error is thrown.
How can it be? how can an object be not json serializable?
The error said the object is not serializable.
I'll try to get reproduce the error again.

Actually, i have never use the facebook API. But when it comes to serialize the object to json string, i recommend you that fastjson is much better.
https://github.com/alibaba/fastjson
Fastjson is the fastest processing library in java (even than jackson, let alone gson) and it has no special requirements to the object ,just be the pain java bean is enough.
By the way, fastjson is developed by Chinese, so its documentation are mainly in Chinese. If you have trouble with its api, you can contact with me.

Serialize an object to json in Java?

I am trying to serialize an instance of Campaign in Adwords API with gson at first with the code below:
Campaign c = new Campaign();
c.setName("beijing");
c.setId(23423L);
Gson gson = new Gson();
String json = gson.toJson(c);
and I get the exception that class Money declares multiple JSON fields named __equalsCalc. When I try to serialize the instance with json plugin of struts2 with the code below
String str = org.apache.struts2.json.JSONUtil.serialize(c);
System.out.println(str);
It works and output the correct result
{"adServingOptimizationStatus":null,"biddingStrategy":null,"budget":null,"campaignStats":null,"conversionOptimizerEligibility":null,"endDate":null,"frequencyCap":null,"id":23423,"name":"beijing","networkSetting":null,"servingStatus":null,"settings":null,"startDate":null,"status":null}
Then my question is that why can the json plugin of struts2 can serialize the instance correctly while gson cannot? Can I use the json plugin of struts2 to serialize objects to json since it is design to produce json result in struts2 not for this situation.

You can use the json plugin in struts2 to serialize your object manually to json string. You can do that by calling the serialize static method.
String jsonString = JSONUtil.serialize(your_object);
Don't forget to include xwork-core jar in your classpath because it depends on it.

Sounds like either a bug in Gson or it is more particular/less robust. Without looking at the code for either it would be hard to know more.
Personally I use Jackson for JSON to POJO transformations.
Ultimately as long as the Structs2 plugin is available on your classpath I don't see why you couldn't leverage it's classes to handle JSON transformations. Ultimately JSON is a format therefore all JSON libraries need to produce commonly understandable data.

I had a similar problem and solved it by moving my use of SimpleDateFormat from the class level to inside a method. GSON doesn't have to serialize SimpleDateFormat this way.
Hope this helps someone - 45 minutes of head banging for me! :-)

Parsing Json Feeds with google Gson

I would like to know how to parse a JSON feed by items (eg. url / title / description for each item). I have had a look to the doc / api but, it didn't help me.
This is what I got so far
import com.google.gson.Gson;
import com.google.gson.JsonObject;
public class ImportSources extends Job {
public void doJob() throws IOException {
String json = stringOfUrl("http://feed.test/all.json");
JsonObject jobj = new Gson().fromJson(json, JsonObject.class);
Logger.info(jobj.get("responseData").toString());
}
public static String stringOfUrl(String addr) throws IOException {
ByteArrayOutputStream output = new ByteArrayOutputStream();
URL url = new URL(addr);
IOUtils.copy(url.openStream(), output);
return output.toString();
}
}

Depends on the actual JSON format. You can in fact just create a custom Javabean class which matches the JSON format. Any fields in JSON can be mapped as String, Integer, Boolean, etc Javabean properties. Any arrays can be mapped as List properties. Any objects can be mapped as another nested Javabean property. It greatly eases further processing in Java.
Without a JSON string example from your side, it's only guessing how it would look like, so I can't give a basic example here. But I've posted similar answers before here, you may find it useful:
Converting JSON to Java
Generate Java class from JSON?
Gson has also an User Guide, you may find it useful as well.

Gson 1.4 has a new API JsonStreamParser that lets you parse multiple JSON objects one by one from a stream.

You can create corresponding java classes for the json objects. The integer, string values can be mapped as is. Json can be parsed like this-
Gson gson = new GsonBuilder().create();
Response r = gson.fromJson(jsonString, Response.class);
Here is an example- http://rowsandcolumns.blogspot.com/2013/02/url-encode-http-get-solr-request-and.html

I don't know if GSON can do streaming/incremental binding (I thought it did not).
But is there specific reason to only consider that particular library? Other Java JSON processing libraries do allow such processing (you can check links the other answer has for some ideas), since it is quite important feature when processing large feeds.