Develop Reference

Create some hash type structure in Gson to access values in json instead of creating custom classes

If I have a json object inside a bigger json:
customer_data: {
details: {
personal_info: {
first: “George”
last: “Washington”
}
order_details: {
canceled: “true”
id:”1234”
}
}
If I want only specific values of customer_data besides traversing the structure using getAsJsonObject etc is there any other way to access them if I would like to avoid creating a CustomerData class since I won’t need to access all the data of customer_data?
Note: I am using Gson

Gson don't supports data access via XPath anology, and if you don't want to use Data Binding, you have only two ways: Tree Model or Streaming API.
The simpliest will be Tree Model:
JsonObject customerData = someBiggerJson.get("customer_data").getAsJsonObject();
String someInfo = customerData.get("some_field").gatAsString();
...
In case of using Streaming API you should iterate in your json to field you need by your hands.
reader = new JsonReader((<input_stream>)
reader.nextString()
reader.beginObject()
reader.endObject() etc..

Creating Map or Set using GSON

I am using Google's GSON library and want to create a JSON which looks something like this:
{
"teamSet":[],
"classificationMap" : {}
}
Notice that [] and {} are empty Set and Map respectively and are not surrounded with double quotes.
I want to create this Json using the GSON add/addproperty method and not by converting a Java class into JSON using the Gson.toJson() method. The business use-case restricts me from creating specific Java classes as the JSON structure may change overtime. So I cannot use a class like this as this would rigidly tie a JSON structure with a concrete class
class Linkage{
private Set<String> teamSet;
private Map<String, String> classificationMap;
// getter, setter methods follow
...
}
When I use the GSON.addproperty(genericObject, type), it is appending double quotes around [] and {}. I am using couchbase for my DB requirements and the double quotes around [] and {} makes couchbase treat them as string and not as Set/Map. This renders my Map-Reduce views useless and buggy :(
Please let me know if its possible to create such a JSON without having to tie it up with a concrete JAVA class. Thanks !
My current code looks like this:
// create types
Type setType = new TypeToken<Set<String>>() {}.getType();
Type mapType = new TypeToken<Map<String, String>>() {}.getType();
Gson GSON = new Gson();
Set<String> teams = new HashSet<String>();
Map<String, String> classificationMap = new HashMap<String, String>();
JsonObject linkageJson = new JsonObject();
linkageJson.addProperty("teamSet", GSON.toJson(teams, setType));
linkageJson.addProperty("classificationMap", GSON.toJson(classificationMap, mapType));

In the 2.x line of the couchbase java sdk, there is the JsonObject class that could have fit your need.
It is perfect to create Json "by hand" and still have a simple generic object representation, and is the official way of putting json into the database via the sdk.
It would go like this :
JsonObject obj = JsonObject.create();
obj.put("teamSet", JsonArray.from(new ArrayList(teams)))
.put("classificationMap", JsonObject.from(classificationMap));
Granted this is a little bit contrived because arrays and sub-objects can only be constructed from respectively List<?> and Map<String, ?> factory methods. Also the class support a limited set of value types (no custom classes, only String, Number, Boolean, etc...).

i feel somewhere it is storing it toString() representation.
Please refer below link, it might help you.
click here

Get JSONobjects from string

So i have a string which contains multiple JSONobjects and looks like this:
[{"one":"1","two":"2","three":"3"},
{"one":"4","two":"5","three":"6"},
{"one":"7","two":"8","three":"9"}]
How can i iterate through this string using java and get every object? Is it possible using JSON api, or i should make parser by myself?

GSON library is a good option to convert java object to json string and vise versa.
for converting json to java object use: fromJson(String, Class)
for converting java object to json string use: toJson(Object)
In your case it's a List of Object.
sample code:
class MyPOJO {
private String one;
private String two;
private String three;
// getter & setter
}
String jsonString = "[{\"one\":\"1\",\"two\":\"2\",\"three\":\"3\"}, {\"one\":\"4\",\"two\":\"5\",\"three\":\"6\"}, {\"one\":\"7\",\"two\":\"8\",\"three\":\"9\"}]";
Type type = new TypeToken<ArrayList<MyPOJO>>() {}.getType();
ArrayList<MyPOJO> obj = new Gson().fromJson(jsonString, type);
System.out.println(new GsonBuilder().setPrettyPrinting().create().toJson(obj));
Note: The name of variable in your java POJO class should be same as JSON string.
Find more examples...

You Should defiantly use the Json API, you can download the jar from here and simply use
JSONArray myArray = new JSONArray(yourString);
for (int i=0; i < myArray.length(); i++)
{
JSONObject currentOb = myArray.get(i);
doSomthing(currentOb);
}

It is obvious that you should use a JSON library. Existing libraries are tested and validated. In very rare conditions you may need to write your own parser, your own implementation. If that is the case, I think you should double check your design. Because you might be doing something wrong if the existing library is in conflict with your design.
Library selection depends on your environment and your performance requirements.
In my case, Spring3 is the environment and the JSON objects are huge (10-20MB), and inserts occur on existing JSON objects. We prefer Jackson. Jackson's performance is outstanding. An independent performance comparison is in here. You will see that the Jackson outperforms GSon in here.

Creating json object from POJO object in restful web service

I am developing a web application. I have database used by the web service. I want to send the same data to the web pages which are calling web service.
I get the data i.e. single row from the database by using hibernate and POJO classes(getColumn). Now I have object(POJO class) of the Table which represent single row of the database. For sending it back to the web pages (html, jsp), I need to convert it to the json object as my web service returns the json object.
How can I make Json object from POJO classes. There are many other ways to generate Json String but i want json object.
How can do this?
Thank you

You can use GSon to convert json object to java object
Link
to refer example.
Gson gson = new Gson();
//to get json object use toJson
String json = gson.toJson(obj);
//to get java object use fromJson
MyClass obj = gson.fromJson(jsonObj, MyClass.class);
or
jackson is also pretty fast and easy to use

private ObjectMapper mapper = new ObjectMapper();
JsonNode node = mapper.convertValue(YOUR POJO CLASS, JsonNode.class);
You can use Jackson and achieve this as above. GSON also does the job.

The way I use is with Google's Gson library. Very simple and powerful

Spring and Jackson as it is so simple. You can find a very basic example below Jackson/spring JSON example

What is the most suitable Java data structure for representing JSON?

I' m developing a RESTful Android mobile client. Information exchange between my app and server is in JSON. So I' m now a little bit confused what data structure choose for represent JSON responses and data because there a lot of them. I've just stopped with LinkedHashMap<> but as far as i know JSON is unordered. Across the Internet I saw people use Map<> or HashMap<> for this. So the question - what is the best data structure for this purpose? Or if there is no a univocal answer - pros and cons of using data structures I' ve mentioned.

I would disagree with the first answer. The REST paradigm was developed so that you would operate with objects, rather than operations.
For me the most sensible approach will be if you declare beans on the client side and parse the json responses and request through them. I would recommend using the GSON library for the serialization/ deserialization. JsonObject/ JsonArray is almost never the best choice.
Maybe if you give examples of the operations you are about to use we might be able to help more precisely.
EDIT: Let me also give a few GSON Examples. Let's use this thread to compare the different libraries.
In the most cases REST services communicate objects. Let's assume you make a post of product, which has reference to shop.
{ "name": "Bread",
"price": 0.78,
"produced": "08-12-2012 14:34",
"shop": {
"name": "neighbourhood bakery"
}
}
Then if you declare the following beans:
public class Product {
private String name;
private double price;
private Date produced;
private Shop shop;
// Optional Getters and setters. GSON uses reflection, it doesn't need them
// However, better declare them so that you can access the fields
}
public class Shop {
private String name;
// Optional Getters and setters. GSON uses reflection, it doesn't need them
// However, better declare them so that you can access the fields
}
You can deserialize the json using:
String jsonString; // initialized as you can
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.setDateFormat("MM-dd-yyyy HH:mm"); // setting custom date format
Gson gson = gsonBuilder.create();
Product product = gson.fromJson(jsonString, Product.class);
// Do whatever you want with the object it has its fields loaded from the json
On the other hand you can serialize to json even more easily:
GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.setDateFormat("MM-dd-yyyy HH:mm"); // setting custom date format
Gson gson = gsonBuilder.create();
String jsonString = gson.toJson(product);

Are you talking about receiving and parsing the JSON string from a server request?
For that you can use:
import org.json.JSONArray;
import org.json.JSONObject;
Using these, I read through my JSON array from my POST request and store the resulting information in Class objects in my project.
For each item in JSONArray, you can extract the JSONObject and attributes like this:
for (int i = 0; i < jsonArray.length(); i++) {
JSONObject jsonObject = jsonArray.getJSONObject(i);
jsonObject.getString("text");
}
As far as actually storing the data, like mentioned above, JSON data can come in a wide array of formats depending on the source, and as such, it is usually parsed on the client end and saved in your application Class objects for use. Or more generically, you could store the data using Map<String, Object>

This is easily the best answer I've seen:
https://dzone.com/articles/which-is-the-right-java-abstraction-for-json
Summary: there are three abstrations: pojos, maps and lists, and custom classes to represent objects, arrays, and primitives. There are advantages and disadvantages to each, with no clear winner.
Pojos have the biggest advantages, but you can't always use them. Use them if you can, and use the others if you must.

If you are doing anything other than the most simple mapping then you should use a full class structure. Create your class hierarchy as a mirror of the data structure in JSON and use Jackson to map the JSON directly to the class hierarchy using the ObjectMapper.
With this solution you don't have any casting of Object to Map or messing around with JSONObject or JSONArray and you don't have any multi-level map traversal in your code. You simply take the JSON string, feed it to the ObjectMapper, and get a your Object, which contains child objects (even collections) automatically mapped by the ObjectMapper.

I've used xstream to serialize JSON, in the following way:
XStream xstream = new XStream(new JsonHierarchicalStreamDriver());
xstream.setMode(XStream.NO_REFERENCES);
xstream.alias("myAlias", MyClass.class); // requires a no argument constructor
System.out.println(xstream.toXML(product));
Ok, the gentleman in the comments wants a deserialization example, here you are:
XStream xstream = new XStream(new JsonHierarchicalStreamDriver());
xstream.alias("myAlias", MyClass.class);
Product product = (Product)xstream.fromXML(json);
System.out.println(product.getName());
Let me know if you need further assistance...