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How get just one value from json as string

I have following json as string:
{
"field1":"value1",
"field2":"value2",
"field3":{
"field31":"value31",
"field32":"value32"
},
"field4":{
"field41":"value41"
}
}
What is the best and the most modern way to get from this json just value from field41, so I would return "value41". I know that I can use JSONObject but I'm wondering weather we have any other better option?

Try JSONPath
String json = "...";
String field41 = JsonPath.read(json, "$.field4.field41");
You can test it here - https://jsonpath.herokuapp.com/

If you want to generate a real object out of it you can use Gson. You need to describe the class first. There are online json to Java objects converters out there. And then you can just call:
YourObject obj = new Gson().fromJson(json,YourObject.class);
System.out.println(obj.getField4().getField41());
And there you have it!

How to map values of a json object without knowing about the format?

I have the following programming requirement:
problem:
Given two JSONs A and B, if the fields x,y,z in JSON A match the fields i,o,p in B return true else false.
approach:
I want to stay away from building a matching engine that depends on the json's format. I don't want to format the jsons by using pojos and then do object matching. My approach is to convert all the jsons into a hash map and then specify the location of the fields by using a string:
Example:
money -> a,b,c
{
a :
{
b : {
c: {
money : "100"
}
}
}
}
However this approach seems to be a bit tricky as we have to take into account collections. I have to cover all of the edge cases. Is there any spring library or java tool I can use to fulfill this purpose?.

There are many libraries being used for this purpose.The most popular one is com.google.gson
Usage:
JsonObject jo = (JsonObject)(jsonParser.parse("{somejsonstring}");<br>
jo.has("objectProperty") //Check if property exists
jo.get("objectProperty") // returns JsonElement,
jo.get("objectProperty").isJsonArray() // check if the property is the type that want
jo.getAsJsonArray("objectProperty") get the property

You may simplify this work by using im.wilk.vor:Voritem library gitHub or in Maven repository.
JsonElement je_one = jsonParser.parse("{some_json_string"})
JsonElement je_two = jsonParser.parse("{other_json_string"})
VorItem vi_one = vorItemFactory.from(je_one);
VorItem vi_two = vorItemFactory.from(je_two);
if (vi_one.get("a.b.c").optionalLong().isPresent() ) {
return vi_one.get("a.b.c").optionalLong().equals(vi_one.get("i.o.p").optionalLong())
}
return false;

Trying to see content of map

Hi I'm trying to see the content of a map with the following code:
#RequestMapping(value = "/demos", method = RequestMethod.GET,headers="Accept=application/json")
public #ResponseBody String getPartnerIdClusters(Model model) {
Map<Integer, List<Cluster>> partnerIdClusterMap = partnerService.getPartnerIdClusterMap(true, true);
return "partnerIdClusterMap: " + partnerIdClusterMap;
}
Which gave me the following output in the browser:
partnerIdClusterMap: {2=nl.irp.globetrotting.model.Cluster#7499394a}
After that I had tried this code:
String line = "test: /n";
for (Object entry : partnerIdClusterMap.values().toArray()) {
line += entry;
line += "/n";
}
return "partnerIdClusterMap " + line;
But that didn't work either because it has given me the following output:
partnerIdClusterMap test: /nnl.irp.globetrotting.model.Cluster#63769066/n
I already know that it is the Cluster with id 3. Here is a screenshot of it:
link: http://i.imgur.com/pKLu6gf.png
Here is how the getPartnerIdClusterMap() method looks like (in case you want to know):
#Override
public Map<Integer, List<Cluster>> getPartnerIdClusterMap(boolean minorOnly, boolean foreignCountriesOnly) {
BooleanBuilder predicate = new BooleanBuilder();
if (minorOnly) {
predicate.and(qCluster.type.eq(ClusterType.MINOR));
}
if (foreignCountriesOnly) {
predicate.and(qPartner.country.code2.ne("nl"));
}
return from(qCluster)
.innerJoin(qCluster.partner, qPartner)
.where(predicate)
.where(qPartner.country.code2.ne("nl"))
.map(qPartner.id, GroupBy.list(qCluster));
}
This is what I gladly want to know:
So I gladly want to see the Clusterwith all the values from the row.

Spring MVC should be able to convert maps to JSON, with the help of a message converter. Using Spring Boot, just returning maps works for me - a MappingJackson2HttpMessageConverter is automatically configured. As I recall, before using Spring Boot, I just had to configure an ObjectMapper, and then returning the map was working. So, in summary, I think that
Using Spring Boot, returning a map should work.
If not using Spring Boot, an ObjectMapper (and maybe also a MappingJackson2HttpMessageConverter) might be needed to be configured.
If the returned map holds things that the ObjectMapper can't convert by default, it might need you to supply some converting customization. Help material on Jackson (now called fasterxml) ObjectMapper will have more details.

partnerIdClusterMap: {2=nl.irp.globetrotting.model.Cluster#7499394a}
Basically you are printing the values that you specified. In this case you are printing the integer and then the list object populated with Cluster Objects.
Consider to iterate partnerIdClusterMap in your weblogic and extract the values with the methods provided by it or override the toString method of the object to get a full line with details.

How to convert arbitrary JSON into a usable structure in Java

I'm trying to use gson to convert this returned JSON into some kind of
data structure such that I can extract useful data.
For Example:
http://search.twitter.com/search.json?q=test&rpp=1
Returns:
{
"completed_in":0.028,
"max_id":196386333906837504,
"max_id_str":"196386333906837504",
"next_page":"?page=2&max_id=196386333906837504&q=test&rpp=1",
"page":1,
"query":"test",
"refresh_url":"?since_id=196386333906837504&q=test",
"results":[
{
"created_at":"Sat, 28 Apr 2012 23:52:05 +0000",
"from_user":"della_ky",
"from_user_id":525641596,
"from_user_id_str":"525641596",
"from_user_name":"kydella modeste",
"geo":null,
"id":196386333906837504,
"id_str":"196386333906837504",
"iso_language_code":"en",
"metadata":{
"result_type":"recent"
},
"profile_image_url":"http:\/\/a0.twimg.com\/profile_images\/2159990525\/webcam-toy-photo3_20_2__normal.jpg",
"profile_image_url_https":"https:\/\/si0.twimg.com\/profile_images\/2159990525\/webcam-toy-photo3_20_2__normal.jpg",
"source":"<a href="http:\/\/mobile.twitter.com" rel="nofollow">Mobile Web<\/a>",
"text":"RT #Y__U__NOOO: #SongsIKnowOffByHeart ALL SONGS I LISTEN TO. BRAIN, Y U NO REMEMBER TEST ANSWERS LIKE THAT?!?",
"to_user":null,
"to_user_id":null,
"to_user_id_str":null,
"to_user_name":null
}
],
"results_per_page":1,
"since_id":0,
"since_id_str":"0"
}
Ultimately, I would like to be able to output a list of tweets with the
name of the sender and the date/time of the tweet.
I have read through the gson documentation but it's going over my head
to be honest - lots of new concepts there for me.
Do I need to define a class which maps exactly to the structure of the
JSON in order to then populate an instance of that class? If so this
seems very inflexible/laborious. Ideally I'm looking for something
which will handle JSON in any form and give me a structure I can use
automatically...
Is anyone able to give me some pointers? Being new to this - the more
detailed and in words of the fewest syllables the better!
Update - Thanks to the responses I've already had on this I've had a go at putting a class together to capture the twitter JSON. However, since the JSON has an embedded ArrayList of Objects I'm struggling a bit... So far I have
public class tweetData {
private double completed_in;
private long max_id;
private long max_id_str;
private String next_page;
private int page;
private String query;
private String refresh_url;
private List<tweetDetails> tweets = new ArrayList<tweetDetails>();
}
and
public class tweetDetails {
private String created_at;
private String from_user;
private long from_user_id;
private long from_user_id_str;
private String from_user_name;
private String geo;
private long id;
private long id_str;
private String iso_language_code;
// "metadata":
// {
// "result_type":"recent"
// },
private String profile_image_url;
private String profile_image_url_https;
private String source;
private String text;
private String to_user;
private String to_user_id;
private String to_user_id_str;
private String to_user_name;
}
Which I'm instantiating with
URI uri = new URI("http", "search.twitter.com", "/search.json", "q="+ searchTerms + "&rrp=" + RRP, null);
URL twitterSearch = uri.toURL();
URLConnection yc = twitterSearch.openConnection();
JsonReader reader = new JsonReader(new InputStreamReader(yc.getInputStream()));
Gson gson = new Gson();
tweetData data = gson.fromJson(reader, tweetData.class);
System.out.println(data);
The basic name:values are being populated correctly but the ArrayList is not.
tweetData : 0.17196614959919140865196614959919140865?page=2&max_id=196614959919140865&q=test1test?since_id=196614959919140865&q=testSIZE 0[]
So, I'm still struggling a bit - any more tips hugely appreciated!
Tia,
Tom

Do I need to define a class which maps exactly to the structure of the JSON in order to then populate an instance of that class? If so this seems very inflexible/laborious.
Yes. GSON is a library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object. This is really powerful because you can automagically instantiate your Java objects from the JSON representation. Assuming your JSON doesn't change its structure, you only have to define the appropriate Java object representation once.
Ideally I'm looking for something which will handle JSON in any form and give me a structure I can use automatically...
However, if you don't want automagical serialisation/deserialisation, then try looking at a simpler library such as java.net/projects/jsonp.
You can extract stuff from it just by querying the keys:
final JSONObject json = new JSONObject(theJsonString);
final String id = json.getString("max_id");
final JSONArray results = json.getJSONArray("results");
final String user = results.getJSONObject(2).getString("from_user");

Gson actually does all the serialization for you. So yes, you would have to write the classes yourself. To you, this seams inflexible and laborious, but that's only because that library isn't made for what you're asking for (it doesn't parse 'arbitrary' JSON).
I would suggest at least considering writing the classes and using gson. The reason I say that is because either way your application's logic will have to expect a very specific format, and writing out that format in a Java class will make things tidier. Here's a nice guide that will help you get started that way.
If you want to simply decode the JSON without serializing it into a Java class (IMHO the only way to use 'arbitrary' JSON), you'll want to use another library. Try this one. It allows you to decode the JSON, and use it by getting values from it (as described in this question: Convert a JSON string to object in Java ME?).

There are some tools that do gson to schema mapping. You give some sample JSON responses, and the java classes to access them are created for you.
http://www.jsonschema2pojo.org/

Gson is a slick beast!
Or at least it became so over the years that have passed since the question had been asked.
You can pass it an Object.class as a second parameter to the fromJson() method and it will parse your Json into a reasonable structure of LinkedTreeMaps and ArrayLists.
Object result = (new Gson()).fromJson(jsonString, Object.class)
More than that, you can really do partial parsing and leave loose ends at any level of your object structure by defining a certain field as Object!
Gson will then parse Json into your structure and your field of type Object will contain the above mentioned structure of LinkedTreeMaps and ArrayLists.
E.g., you may define a class
Person {
String name;
Object details;
}
(Imagine, you care mostly about the person's name but may want the details also somewhere. To log them, for instance.)
Then you can pass the following Json to the fromJson(input, Person.class) method as a first parameter
{
"name": "Carlsson",
"details": {
"address": "Stockholm",
"phones": [
"work": "233-322-233-322",
"home": "none"
]
}
}
The result will have the name field filled with "Carlsson" string and details field will contain a LinkedTreeMap with keys "address" and "phones", etc.

Can't parse JSON property "null"

I faced with one trouble when tried to parse JSON "null" property, please help me to understand what's the real problem. I had a following JSON:
{
"properties" : {
"null" : {
"value" : false
}
}
}
I used http://jsonlint.com to validate that this JSON is valid. I tried to parse it from java:
import net.sf.json.JSONObject;
import java.io.IOException;
public class Test {
public static void main(String[] args) throws IOException {
String st = "{" +
" 'properties' : {" +
" 'null' : {" +
" 'value' : false" +
" }" +
" }" +
"}";
JSONObject.fromObject(st);
}
}
But got the exception:
Exception in thread "main" java.lang.ClassCastException: JSON keys must be strings.
at net.sf.json.JSONObject._fromJSONObject(JSONObject.java:927)
at net.sf.json.JSONObject.fromObject(JSONObject.java:155)
at net.sf.json.JSONSerializer.toJSON(JSONSerializer.java:108)
at net.sf.json.AbstractJSON._processValue(AbstractJSON.java:238)
at net.sf.json.JSONObject._processValue(JSONObject.java:2655)
at net.sf.json.JSONObject.processValue(JSONObject.java:2721)
at net.sf.json.JSONObject.element(JSONObject.java:1786)
at net.sf.json.JSONObject._fromJSONTokener(JSONObject.java:1036)
at net.sf.json.JSONObject._fromString(JSONObject.java:1201)
at net.sf.json.JSONObject.fromObject(JSONObject.java:165)
at net.sf.json.JSONObject.fromObject(JSONObject.java:134)
I used json-lib-2.4-jdk15.jar from http://json-lib.sourceforge.net to parse it. Could anybody please clarify this? Why this library throws exception, but online validator said that it's valid JSON? It is a bug in the library or I made something wrong?

JSON-lib initially parses and populates a Java Map with the input JSON. Unfortunately, JSON-lib then checks whether every JSON object element name is a JSON null. It's null check is performed in the JSONNull.equals(Object) method. This method returns true for a "null" JSON string, which of course is not actually a JSON null value.
I recommend filing a bug with the JSON-lib project for this issue. The implementation of JSONNull.equals(Object) is flawed.
Unfortunately, it's not possible to handle this with a custom PropertyNameProcessor.
Options available for a more immediate solution include altering the JSON-lib code yourself, or switching libraries.
If you can switch libraries, I highly recommend Jackson. Following is an example of using it to deserialize the example JSON in the original question.
/*
{
"properties" : {
"null" : {
"value" : false
}
}
}
*/
String json = "{\"properties\":{\"null\":{\"value\":false}}}";
ObjectMapper mapper = new ObjectMapper();
Map<String, Object> map = mapper.readValue(json, Map.class);
System.out.println(map);
// output: {properties={null={value=false}}}
Map<String, Object> propertiesMap = (Map) map.get("properties");
System.out.println(propertiesMap);
// output: {null={value=false}}
Map<String, Object> nullMap = (Map) propertiesMap.get("null");
System.out.println(nullMap);
// output: {value=false}

The first JSON posted is valid JSON: the JSON in the Java, however, is not valid -- only " is valid for the [required] key quote. From json.org:
A string is a sequence of zero or more Unicode characters, wrapped in double quotes, using backslash escapes....
However, that sounds like a bug, assuming it was not triggered by the invalid JSON fed to it (the library can do whatever it wants with invalid JSON)... one would have to look at the source (or bug reports / user experience) to say conclusively if this is indeed a "bug". I have added some suggestions of things to try below which may either show expected behavior or outline the cause/issue in further detail.
Consider this minimal test-case (with valid JSON):
String st = "{ \"null\": \"hello world!\" }";
This may also shed more light, depending on if the first item is "null" or null when extracted:
String st = "[ \"null\" ]";
Happy coding.

The gson library link is:
http://code.google.com/p/google-gson/
I normally usr gson to generate the josn string,so I found some example someone else posted in stackoverflow to parse json string with gson,see the link:
Converting JSON to Java

suggest you to use Gson,
and construct the json string using java Map and List,
then use Gson to output the Map or List object