So, I've got a String that is the result of a toJson method I've implemented on a class, and have confirmed in my test code that it is the correct Json representation of my class. My goal is to turn this String into a JsonObject and pass it to a constructor, using Gson. However, I'm running into an odd problem.
This is the code I'm calling:
Gson gson = new Gson();
JsonObject jObj = gson.fromJson(jsonString, JsonObject.class);
I have used literally this exact same snippet of code before in many places in my project, for other classes, and it has worked fine. I even copied one of those functional snippets of code into this test class and tried it. However, every version I try results in the same thing--jObj is an empty set of brackets.
I don't understand how it's happening. I've confirmed that jsonString has all the fields it should need. Why is Gson returning an empty JsonObject? No exceptions are being thrown.
Ok so i know this is a little old but I had the same exact issue. The resolution was changing the jar file. I had a similar code sample working in another project but then I was experiencing the exact same problem in another. Well the problem was an older gson-2.1.jar. Updated the problem application to the matching gson-2.3.1.jar and all was working. Hope this helps.
From your comment that says the string is {"varName":1, "otherVarName":2, "thirdVarName":3.4}, looks like the serialized object was a Map. You may need to specify the type token:
gson.fromJson(jsonString, new TypeToken<Map<K,V>>() {}.getType());
where K and V are the key and value types of the map. If it is not a Map, specify whatever class the jsonString was obtained from.
For my case, I was accidentally using the following initializer in my dagger module.
Gson gson = new GsonBuilder()
.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
.create()
I removed the setFieldNamingPolicy and it worked.
I had this issue as well.
I was using
return gson.fromJson(response, JsonObject::class.java)
And I was receiving an object with only the default values populated.
I had to explicitly define the serialized names for each property in my JsonObject class that was different from how it was named in the json response.
For example, if in the json I was parsing the field name was "total_score', but my JsonObject had a field named "totalProperty", I had to use the #SerialedName annotation to define the relationship.
#SerializedName("total_score")
val TotalScore : Int = 0
Related
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.
I am trying call a REST service and using gson I am getting the following json for the following java pojo.
pojo
public class AlphaParameters {
private int one;
private int two;
private int three;
//getter setters
//constructors
}
Json
{"one":4,
"two":5,
"three":10
}
I am using the following code
Gson gson = new Gson()
AlphaParameters alphaParameters = new AlphaParameters(one,two,three);
gson.toJson(alphaParameters );
Earlier this code used to work, but now seems the server side which is on .net changed their implementation and now they are expecting the json in the following format. Everything is same but seems now they want the toplevel variable name in the json.
{"alphaParameters":
{"one":4,
"two":5,
"three":10
}
}
Question : Is there a specific api of Gson which I can use to generate the above json without refactoring my code ?
Or writing a wrapper class to include alphaParameters will be a better approach .
( I will have to write a lot of boilerplate code for latter ).
Thanks for your help.
I don't think Gson itself allows this kind of serialization but there is a number of ways you could tackle this problem without creating wrapper classes.
In my comment, I suggested putting the object in a map but that's a bit strange and you can do it so it looks more obvious in the code and probably performs better.
public Gson wrapJson(Object objectToSerialize) {
Gson gson = new Gson();
JsonObject result = new JsonObject();
//Obtain a serialized version of your object
JsonElement jsonElement = gson.toJsonTree(objectToSerialize);
result.add(objectToSerialize.getClass().getSimpleName(), jsonElement);
return result;
}
Then you can use it like this:
AlphaParameters alphaParameters = new AlphaParameters(one,two,three);
wrapJson(alphaParameters);
This allows you to use one pretty universal method in every case like this without writing boilerplate classes.
I used the class name to generate the key but feel free to modify this as it suits you. You could pass the key name as a parameter to make this wrapper utility more flexible.
I am parsing JSON data, and storing the results in a Java object using GSON. My question is, should the fields in the JSON String match the instance variables in the class? Including the class names? For eg,
If this is my JSON string -
"telephone":
{
"landline":"1-818-502 8310"
}
Should I have a class as below?
public class Telephone
{
private String landline;
}
The reason why I am asking this is, when I use gson's fromJson(obj), the object doesn't contain any values. It shows all records as null. I am wondering if it is throwing the error due to this.
Please note - This is not the entire code. My JSON data is quite huge, so I can't paste it here. The above telephone string is just one of the many embedded strings within my json string.
This is wrong JSON:
"telephone":{"landline":"1-818-502 8310"}
The JSON objects start with a { and end with a }. SO, it should be something like
{"name": "somename", "telephone":{"landline":"1-818-502 8310"}, ...}
Yes. Attributes in class should have exact same case and character as in the JSON String in case you are using default Gson instance as correctly mentioned by Eliran. Please note that you must have attributes just having getter/setter and not attribute wouldn't work.
You mentioned you are using inner class. It may not work with default Gson instance. You may need to use registerTypeAdapter like this:
gson.registerTypeAdapter(MyType.class, new MyInstanceCreator());
refer: https://sites.google.com/site/gson/gson-user-guide#TOC-Custom-Serialization-and-Deserialization
I have the following type of JSON I want to send to Java (I'm using Jersey and the default JSON Parser it comes with)
{ "something" : "1", "someOtherThing" : "2" , ... }
But instead of creating an Object with all these properties in Java, I would like to have a Single HashMap (or whatever) that will allow me to still have access to the Key and the Value
Is such a thing possible?
I don't really have any code that does the transformation, I use Jersey like this
#POST
#Path("/purchase")
#Produces(MediaType.APPLICATION_JSON)
#Consumes(MediaType.APPLICATION_JSON)
public StatusResult purchase(UserPurchaseRequest upr) {
}
If i put properties something and someOtherThing as Strings in my UserPurchaseRequest object, everything will come in fine, but I want to have everything in one structure (because I don't know how many values I will get, and I need their names as well)
Yes, it is possible. But still, it depends on what JSON java API you are using. For example using Jackson JSON you can create HashMap json string like this
ObjectMapper obj = new ObjectMapper();
String json = pbj.writeValue(<HashMap object>);
or vice-versa
HashMap obj = obj.readValue(json, HashMap.class);
Note - org.codehaus.jackson.map.ObjectMapper
You just need to add a Property to your Object like this
private HashMap<String,String> purchaseValues;
Jersey takes care of the rest, for some reason while you are debugging, most of the entries appear as null in the HashMap
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! :-)