Develop Reference

GSON: JSON deserialization to variable type (List/String)

At this point it's already an old question and I've probably read every related topic on SO.
But to the point. I need some advice or correction maybe?
For some reason we have generatable Jsons of 2 types:
{"data": {"id": "value"}} and {"data":[{"id": "value"}]}
Object and Array. There are also other params but they doesn't matter here. "id" is differ for every request. Sometimes it's userId, portfolioId etc. So I get "id" and pass it to related var.
For a long time I was working with the first case. And created POJO like this:
Data.class
public class Data {
#SerializedName("id")
#Expose
private String id;
public Data() {
}
public Data(String id) {
super();
this.id = id;
}
protected String getId() {
return id;
}
And I adress "data" paramets via User.class.
#JsonAdapter(UserDeserializer.class)
public Data data;
public Data getData() {
return data;
}
public void setData(Data data) {
this.data = data;
}
public User() {
}
public User(Data data) {
super();
this.data = data;
}
Gson gson = new Gson();
public String getPortfolioList(String tokenId, String userId) {
Call<User> call = apiRequest.getPortfolioList(userId, tokenId);
try {
User newResult = gson.fromJson(String.valueOf(call.execute().body()), User.class);
System.out.println(newResult.getData().getId());
} catch (IOException e) {
e.printStackTrace();
}
return getPortfolioId();
}
Deserializer.class
public class UserDeserializer implements JsonDeserializer<User> {
private Type listType = new TypeToken<List<Data>>(){}.getType();
#Override
public User deserialize(JsonElement json, Type type, JsonDeserializationContext context) throws JsonParseException {
User user = new User();
JsonElement jsonElement;
if (json.isJsonArray()) {
jsonElement = json.getAsJsonArray();
user.data = context.deserialize(jsonElement,listType);
// user.data = new Gson().fromJson(jsonElement, new TypeToken<List<Data>>() {}.getType());
} else {
jsonElement = json.getAsJsonObject();
user.data = context.deserialize(jsonElement, Data.class);
// user.setData(new Gson().fromJson(jsonElement, new TypeToken<Data>() {}.getType()));
}
return user;
}
}
Gson builder in BaseApi class just in case:
Gson gson = new GsonBuilder().registerTypeAdapter(UserDeserializer.class, new UserDeserializer()).setLenient().create();
Without custom deserialization and Array JSON issue this would work perfectly. But now I have to determine "data" 's exact type I get.
In above case I get java.lang.ClassCastException: java.util.ArrayList cannot be cast to auto.Rest.Data
I assume I have to create another Data class (for example there will be DataObject & DataArray) and describe every parameter as I did before in Data.class to get this work? I think I do something wrong during deserialization but I'm not sure where tbh.
Or am I wrong and it is possible to invoke Data as List and Data as an Object of the same class?
I'm working on this for several days already(?) and was thinking about use generics instead of Gson help, yeah, I'm desperate. So any help appreciated.

if there is always one object, just add
json.getAsJsonArray().get(0);
public class UserDeserializer implements JsonDeserializer<User> {
private Type listType = new TypeToken<List<Data>>(){}.getType();
#Override
public User deserialize(JsonElement json, Type type, JsonDeserializationContext context) throws JsonParseException {
User user = new User();
JsonElement jsonElement;
if (json.isJsonArray()) {
jsonElement = json.getAsJsonArray().get(0);
user.data = context.deserialize(jsonElement,listType);
// user.data = new Gson().fromJson(jsonElement, new TypeToken<List<Data>>() {}.getType());
} else {
jsonElement = json.getAsJsonObject();
user.data = context.deserialize(jsonElement, Data.class);
// user.setData(new Gson().fromJson(jsonElement, new TypeToken<Data>() {}.getType()));
}
return user;
}
}
if there are more objects, change field data to the list
public class UserDeserializer implements JsonDeserializer<User> {
private Type listType = new TypeToken<List<Data>>(){}.getType();
#Override
public User deserialize(JsonElement json, Type type, JsonDeserializationContext context) throws JsonParseException {
User user = new User();
JsonElement jsonElement;
if (json.isJsonArray()) {
jsonElement = json.getAsJsonArray();
user.data = context.deserialize(jsonElement,listType);
// user.data = new Gson().fromJson(jsonElement, new TypeToken<List<Data>>() {}.getType());
} else {
jsonElement = json.getAsJsonObject();
List<Data> data = new ArrayList<Data>();
data.add(context.deserialize(jsonElement, Data.class)) ;
user.data = data ;
// user.setData(new Gson().fromJson(jsonElement, new TypeToken<Data>() {}.getType()));
}
return user;
}
}
and change User.class field data to List
public List<Data> data;
this is a similar topic in kotlin language link

If you always have object or one-element array you can write custom deserialiser as below:
class OneOrElementJsonDeserializer<T> implements JsonDeserializer<T> {
#Override
public T deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
if (json instanceof JsonArray) {
final JsonArray array = (JsonArray) json;
final int size = array.size();
if (size == 0) {
return null;
}
return context.deserialize(array.get(0), typeOfT);
}
return context.deserialize(json, typeOfT);
}
}
Your example model after simplification looks like below:
class User {
#JsonAdapter(OneOrElementJsonDeserializer.class)
private Data data;
public User() {
}
public User(Data data) {
super();
this.data = data;
}
public Data getData() {
return data;
}
public void setData(Data data) {
this.data = data;
}
#Override
public String toString() {
return "User{" +
"data=" + data +
'}';
}
}
class Data {
private String id;
protected String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
#Override
public String toString() {
return "Data{" +
"id='" + id + '\'' +
'}';
}
}
Example usage:
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonArray;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
import com.google.gson.annotations.JsonAdapter;
import java.io.File;
import java.io.FileReader;
import java.lang.reflect.Type;
public class GsonApp {
public static void main(String[] args) throws Exception {
File jsonFile = new File("./resource/test.json").getAbsoluteFile();
Gson gson = new GsonBuilder()
.setPrettyPrinting()
.create();
User root = gson.fromJson(new FileReader(jsonFile), User.class);
System.out.println(root);
}
}
Above code for below JSON payload:
{
"data": [
{
"id": "c87ca3fe85781007869b83f"
}
]
}
prints:
User{data=Data{id='c87ca3fe85781007869b83f'}}
And for object case JSON payload:
{
"data": {
"id": "c87ca3fe85781007869b83f"
}
}
prints:
User{data=Data{id='c87ca3fe85781007869b83f'}}
In case your property could contain JSON object or multi-element array see my answer to this question Mapping Json Array to Java Models. There is implemented deserialiser which handle cases like this.

Gson- Parsing a JSON array of JSON objects to ArrayList<org.json.JSONObject>

I have a JSON string like this:
{
"r": [
{
"pic": "1.jpg",
"name": "Name1"
},
{
"pic": "2.jpg",
"name": "Name2"
},
{
"pic": "3.jpg",
"name": "Name3"
}
]
}
I want to parse to this POJO model:
public class Catalog {
#SerializedName("r")
#Expose
private List<JSONObject> r = new ArrayList<JSONObject>();
public List<JSONObject> getR() {
return r;
}
public void setR(List<JSONObject> r) {
this.r = r;
}
}
I am parsing this way:
Catalog cat = new Gson().fromJson(jsonString,Catalog.class);
But finally am getting this json
{
"r": [
{
"nameValuePairs": {}
},
{
"nameValuePairs": {}
},
{
"nameValuePairs": {}
}
]
}
Please note that I don't want to use com.google.gson.JsonObject.
I want to use org.json.JSONObject. How to achieve this because almost all of my code uses it?

As it was already mentioned in other answer and comments, you probably might not really want to use org.json.JSONObject for several reasons. But if it's a must for you, you just have to create your org.json.JSONObject-aware Gson instance.
final class JSONObjectJsonDeserializer
implements JsonDeserializer<JSONObject> {
// The implementation is fully thread-safe and can be instantiated once
private static final JsonDeserializer<JSONObject> jsonObjectJsonDeserializer = new JSONObjectJsonDeserializer();
// Type tokens are immutable values and therefore can be considered constants (and final) and thread-safe as well
private static final TypeToken<Map<String, Object>> mapStringToObjectTypeToken = new TypeToken<Map<String, Object>>() {
};
private JSONObjectJsonDeserializer() {
}
static JsonDeserializer<JSONObject> getJsonObjectJsonDeserializer() {
return jsonObjectJsonDeserializer;
}
#Override
public JSONObject deserialize(final JsonElement jsonElement, final Type type, final JsonDeserializationContext context) {
// Convert the input jsonElement as if it were a Map<String, Object> (a generic representation for JSON objectS)
final Map<String, Object> map = context.deserialize(jsonElement, mapStringToObjectTypeToken.getType());
// And forward the map to the JSONObject constructor - it seems to accept it nice
return new JSONObject(map);
}
}
Gson is designed thread-safe and does not need to be instantiated every time serialization or deserialization is necessary:
private static final Gson gson = new GsonBuilder()
.registerTypeAdapter(JSONObject.class, getJsonObjectJsonDeserializer())
.create();
And finally:
final Catalog catalog = gson.fromJson(jsonString, Catalog.class);
out.println(catalog.getR());
with the following result:
[{"name":"Name1","pic":"1.jpg"}, {"name":"Name2","pic":"2.jpg"}, {"name":"Name3","pic":"3.jpg"}]
Anyway, I would suggest you to redesign your mappings model.

I think you don't need JSONObject.
Try this
// is wrapped class for serialized json.
public class JsonExample
{
List<Catalog> r;
}
public class Catalog {
private String pic;
private String name;
public String getPic() {
return pic;
}
public String getName() {
return name;
}
}
JsonExample example = new Gson().fromJson(json, JsonExample.class);
Additional - using JSONObject
JSONObject obj = new JSONObject(json);
JSONArray arr = obj.getJSONArray("r");
List<Catalog> cataList = new ArrayList<>();
for(int i = 0 ; i < arr.length() ; ++i)
{
cataList.add(new Catalog(arr.getJSONObject(i)));
}
public class Catalog {
private String pic;
private String name;
public Catalog(JSONObject obj) throws JSONException
{
pic = obj.getString("pic");
name = obj.getString("name");
}
public String getPic() {
return pic;
}
public String getName() {
return name;
}
}

I think in your case, usage of gson library is not required at all.
Only org.json can solve the entire problem.
E.g.:
JSONObject json = new JSONObject(jsonString);
JSONArray jsonArray = json.getJSONArray("r");
List<JSONObject> jsonList = new ArrayList<>();
for (int i = 0; i < jsonArray.length(); i++) {
jsonList.add(jsonArray.getJSONObject(i));
}
Catalog catalog = new Catalog();
catalog.setR(jsonList);

Mask value of selected key in JSON

I have JSON request and response, I want to print the JSONs in the log, but there are some secured fields which I want to avoid to print in the log, I am trying to mask fields keys:
example:
before masking:
{"username":"user1","password":"123456","country":"US","creditCardNumber":"1283-1238-0458-3458"}
after masking
{"username":"user1","password":"XXXXXX","country":"US","creditCardNumber":"XXXXXX"}
I am using java Gson lib, please help me to do that
EDIT
I want to pass the keys dynamically, so in function a I want to mask these fields, but in function b different fields.

I think you should exclude that fields from log. Below is a simple example using Gson and #Expose annotation.
public static void main(String[] args) throws IOException {
String json = "{\"username\":\"user1\",\"password\":\"123456\",\"country\":\"US\",\"creditCardNumber\":\"1283-1238-0458-3458\"}";
Gson gson = new Gson();
User user = gson.fromJson(json, User.class);
System.out.println(gson.toJson(user));
Gson gsonExpose = new GsonBuilder().excludeFieldsWithoutExposeAnnotation().create();
System.out.println(gsonExpose.toJson(user));
}
public class User {
#Expose
private String username;
private String password;
#Expose
private String country;
private String creditCardNumber;
}
Output will be:
{"username":"user1","password":"123456","country":"US","creditCardNumber":"1283-1238-0458-3458"}
{"username":"user1","country":"US"}
Another solution using Reflection:
public static void main(String[] args) throws IOException {
String json = "{\"username\":\"user1\",\"password\":\"123456\",\"country\":\"US\",\"creditCardNumber\":\"1283-1238-0458-3458\"}";
Gson gson = new Gson();
User user = gson.fromJson(json, User.class);
List<String> fieldNames = Arrays.asList("password", "creditCardNumber");
System.out.println(mask(user, fieldNames, "XXXXXXX"));
}
public static String mask(Object object, List<String> fieldNames, String mask) {
Field[] fields = object.getClass().getDeclaredFields();
for (int i = 0; i < fields.length; i++) {
if (fieldNames.contains(fields[i].getName())) {
try {
fields[i].setAccessible(true);
if (fields[i].get(object) != null) {
fields[i].set(object, mask);
}
} catch (Exception ex) {
ex.printStackTrace();
}
}
}
Gson gson = new Gson();
return gson.toJson(object);
}

I like the above solution to mask using reflection but wanted to extend same for other field types and saving masked field to unmask again.
Create annotation #MaskedField on top of field.
#Retention(RetentionPolicy.RUNTIME)
#Target(ElementType.FIELD)
public #interface MaskedField {
}
public <T> Map<String,? super Object> maskObjectFields(T object){
Map<String,? super Object> values = new HashMap<>();
Arrays.stream(object.getClass().getDeclaredFields()).filter(field->null != field.getAnnotation(MaskedField.class)).
forEach(annotatedField->{
try {
if(annotatedField.getType().isAssignableFrom(String.class)) {
annotatedField.setAccessible(true);
values.put(annotatedField.getName(),annotatedField.get(object));
annotatedField.set(object, maskString((String) annotatedField.get(object)));
}
} catch (IllegalAccessException e) {
e.printStackTrace();
}
});
return values;
}
public <T> void unMaskObjectFields(T object,Map values){
Arrays.stream(object.getClass().getDeclaredFields()).filter(field->null != field.getAnnotation(MaskedField.class)).
forEach(annotatedField->{
try {
annotatedField.setAccessible(true);
annotatedField.set(object,values.get(annotatedField.getName()));
} catch (IllegalAccessException e) {
e.printStackTrace();
}
});
}
private String maskString(String value){
if(Objects.isNull(value)) return null;
return null; //TODO: your logic goes here for masking
}

Deserialize a JSON response with Gson containing a field of variable type

The responses of a REST API always return a JSON with the following structure:
{
"status": "<status_code>",
"data": <data_object>
}
My problem is that the value of data doesn't have an unique type, but it can be a String, a JSON Object or a JSON Array, depending on the called endpoint. I can't figure out how to deserialize it in the right way to create the different Java objects...
For example, I've already prepared some POJOs: the root element
public class ApiResult {
#SerializedName("status")
public String status;
#SerializedName("data")
public JsonElement data; // should I define it as a JsonElement??
}
and two objects that reflects two of the endpoints:
// "data" can be a list of NavItems
public class NavItem {
#SerializedName("id")
public String id;
#SerializedName("name")
public String name;
#SerializedName("icon")
public String icon;
#SuppressWarnings("serial")
public static class List extends ArrayList<NavItem> {}
}
and
// "data" can be a single object representing a Profile
public class Profile {
#SerializedName("id")
public String id;
#SerializedName("fullname")
public String fullname;
#SerializedName("avatar")
public String avatar;
}
Reading some StackOverflow questions, I've seen I should use the JsonDeserializer<T> interface. But how if the type of data in ApiResult is variable?

You should use a a custom JsonDeserializer and write all your logic there, like this
ApiResult.java
public class ApiResult {
#SerializedName("status")
public String status;
#SerializedName("data")
public Object data;
}
ApiResultDeserializer.java
import java.lang.reflect.Type;
import java.util.List;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.reflect.TypeToken;
public class ApiResultDeserializer implements JsonDeserializer<ApiResult> {
private Type listType = new TypeToken<List<NavItem>>(){}.getType();
#Override
public ApiResult deserialize(JsonElement value, Type type,
JsonDeserializationContext context) throws JsonParseException {
final JsonObject apiResultJson = value.getAsJsonObject();
final ApiResult result = new ApiResult();
result.status = apiResultJson.get("status").getAsString();
JsonElement dataJson = apiResultJson.get("data");
if(dataJson.isJsonObject()) {
result.data = context.deserialize(dataJson, NavItem.class);
} else if(dataJson.isJsonPrimitive()) {
result.data = context.deserialize(dataJson, String.class);
} else if(dataJson.isJsonArray()) {
result.data = context.deserialize(dataJson, listType);
}
return result;
}
}
and try to create different kinds of data (List, Object, or String) as you mentioned
Main.java
Gson gson = new GsonBuilder()
.registerTypeAdapter(ApiResult.class, new ApiResultDeserializer())
.create();
List<NavItem> navItems = new ArrayList<NavItem>();
for(int i = 1 ; i < 6 ; ++i) {
navItems.add(new NavItem(i+"", "Name-" + i, "Icon-" + i ));
}
ApiResult result = new ApiResult();
result.status = "OK";
result.data = navItems;
// Serialization
System.out.println(gson.toJson(result)); // {\"status\":\"OK\",\"data\":[{\"id\":\"1\",\"name\":\"Name-1\",\"icon\":\"Icon-1\"},{\"id\":\"2\",\"name\":\"Name-2\",\"icon\":\"Icon-2\"},{\"id\":\"3\",\"name\":\"Name-3\",\"icon\":\"Icon-3\"},{\"id\":\"4\",\"name\":\"Name-4\",\"icon\":\"Icon-4\"},{\"id\":\"5\",\"name\":\"Name-5\",\"icon\":\"Icon-5\"}]}
result.data = navItems.get(0);
System.out.println(gson.toJson(result)); // {\"status\":\"OK\",\"data\":{\"id\":\"1\",\"name\":\"Name-1\",\"icon\":\"Icon-1\"}}
result.data = "Test";
System.out.println(gson.toJson(result)); // {\"status\":\"OK\",\"data\":\"Test\"}
// Deserialization
String input = "{\"status\":\"OK\",\"data\":[{\"id\":\"1\",\"name\":\"Name-1\",\"icon\":\"Icon-1\"},{\"id\":\"2\",\"name\":\"Name-2\",\"icon\":\"Icon-2\"},{\"id\":\"3\",\"name\":\"Name-3\",\"icon\":\"Icon-3\"},{\"id\":\"4\",\"name\":\"Name-4\",\"icon\":\"Icon-4\"},{\"id\":\"5\",\"name\":\"Name-5\",\"icon\":\"Icon-5\"}]}";
ApiResult newResult = gson.fromJson(input, ApiResult.class);
System.out.println(newResult.data); // Array
input = "{\"status\":\"OK\",\"data\":{\"id\":\"1\",\"name\":\"Name-1\",\"icon\":\"Icon-1\"}}";
newResult = gson.fromJson(input, ApiResult.class);
System.out.println(newResult.data); // Object
input = "{\"status\":\"OK\",\"data\":\"Test\"}";
newResult = gson.fromJson(input, ApiResult.class);
System.out.println(newResult.data); // String

I managed to make it work as I wanted, and without using any custom deserializer!
For each endpoint, I wait for the response (btw I'm using Volley), then I first generate the "root" ApiResult object, check if the status is OK, then I proceed instantiating the data field as the requested type.
The POJOs are the same of the question. In ApiResult, "data" is a JsonElement.
// ... called the endpoint that returns a NavItem list
public void onResponse(String response) {
ApiResult rootResult = gson.fromJson(response.toString(), ApiResult.class);
if (rootResult.status.equals(STATUS_OK)) {
Log.d(LOG_TAG, response.toString());
NavItem.List resData = gson.fromJson(rootResult.data, NavItem.List.class); // <-- !!!!!
callback.onSuccess(resData);
}
else {
Log.e(LOG_TAG, response.toString());
callback.onError(-1, null);
}
}
Obviously the only thing to change for the "Profile" endpoint is the line with !!!!!

Converting Java objects to JSON with Jackson

I want my JSON to look like this:
{
"information": [{
"timestamp": "xxxx",
"feature": "xxxx",
"ean": 1234,
"data": "xxxx"
}, {
"timestamp": "yyy",
"feature": "yyy",
"ean": 12345,
"data": "yyy"
}]
}
Code so far:
import java.util.List;
public class ValueData {
private List<ValueItems> information;
public ValueData(){
}
public List<ValueItems> getInformation() {
return information;
}
public void setInformation(List<ValueItems> information) {
this.information = information;
}
#Override
public String toString() {
return String.format("{information:%s}", information);
}
}
and
public class ValueItems {
private String timestamp;
private String feature;
private int ean;
private String data;
public ValueItems(){
}
public ValueItems(String timestamp, String feature, int ean, String data){
this.timestamp = timestamp;
this.feature = feature;
this.ean = ean;
this.data = data;
}
public String getTimestamp() {
return timestamp;
}
public void setTimestamp(String timestamp) {
this.timestamp = timestamp;
}
public String getFeature() {
return feature;
}
public void setFeature(String feature) {
this.feature = feature;
}
public int getEan() {
return ean;
}
public void setEan(int ean) {
this.ean = ean;
}
public String getData() {
return data;
}
public void setData(String data) {
this.data = data;
}
#Override
public String toString() {
return String.format("{timestamp:%s,feature:%s,ean:%s,data:%s}", timestamp, feature, ean, data);
}
}
I just missing the part how I can convert the Java object to JSON with Jackson:
public static void main(String[] args) {
// CONVERT THE JAVA OBJECT TO JSON HERE
System.out.println(json);
}
My Question is: Are my classes correct? Which instance do I have to call and how that I can achieve this JSON output?

To convert your object in JSON with Jackson:
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.ObjectWriter;
ObjectWriter ow = new ObjectMapper().writer().withDefaultPrettyPrinter();
String json = ow.writeValueAsString(object);

I know this is old (and I am new to java), but I ran into the same problem. And the answers were not as clear to me as a newbie... so I thought I would add what I learned.
I used a third-party library to aid in the endeavor: org.codehaus.jackson
All of the downloads for this can be found here.
For base JSON functionality, you need to add the following jars to your project's libraries:
jackson-mapper-asl
and
jackson-core-asl
Choose the version your project needs. (Typically you can go with the latest stable build).
Once they are imported in to your project's libraries, add the following import lines to your code:
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.map.JsonMappingException;
import com.fasterxml.jackson.databind.ObjectMapper;
With the java object defined and assigned values that you wish to convert to JSON and return as part of a RESTful web service
User u = new User();
u.firstName = "Sample";
u.lastName = "User";
u.email = "sampleU#example.com";
ObjectMapper mapper = new ObjectMapper();
try {
// convert user object to json string and return it
return mapper.writeValueAsString(u);
}
catch (JsonGenerationException | JsonMappingException e) {
// catch various errors
e.printStackTrace();
}
The result should looks like this:
{"firstName":"Sample","lastName":"User","email":"sampleU#example.com"}

Just follow any of these:
For jackson it should work:
ObjectMapper mapper = new ObjectMapper();
return mapper.writeValueAsString(object);
//will return json in string
For gson it should work:
Gson gson = new Gson();
return Response.ok(gson.toJson(yourClass)).build();

You could do this:
String json = new ObjectMapper().writeValueAsString(yourObjectHere);

This might be useful:
objectMapper.writeValue(new File("c:\\employee.json"), employee);
// display to console
Object json = objectMapper.readValue(
objectMapper.writeValueAsString(employee), Object.class);
System.out.println(objectMapper.writerWithDefaultPrettyPrinter()
.writeValueAsString(json));

You can use Google Gson like this
UserEntity user = new UserEntity();
user.setUserName("UserName");
user.setUserAge(18);
Gson gson = new Gson();
String jsonStr = gson.toJson(user);

Well, even the accepted answer does not exactly output what op has asked for. It outputs the JSON string but with " characters escaped. So, although might be a little late, I am answering hopeing it will help people! Here is how I do it:
StringWriter writer = new StringWriter();
JsonGenerator jgen = new JsonFactory().createGenerator(writer);
jgen.setCodec(new ObjectMapper());
jgen.writeObject(object);
jgen.close();
System.out.println(writer.toString());

Note: To make the most voted solution work, attributes in the POJO have to be public or have a public getter/setter:
By default, Jackson 2 will only work with fields that are either
public, or have a public getter method – serializing an entity that
has all fields private or package private will fail.
Not tested yet, but I believe that this rule also applies for other JSON libs like google Gson.

public class JSONConvector {
public static String toJSON(Object object) throws JSONException, IllegalAccessException {
String str = "";
Class c = object.getClass();
JSONObject jsonObject = new JSONObject();
for (Field field : c.getDeclaredFields()) {
field.setAccessible(true);
String name = field.getName();
String value = String.valueOf(field.get(object));
jsonObject.put(name, value);
}
System.out.println(jsonObject.toString());
return jsonObject.toString();
}
public static String toJSON(List list ) throws JSONException, IllegalAccessException {
JSONArray jsonArray = new JSONArray();
for (Object i : list) {
String jstr = toJSON(i);
JSONObject jsonObject = new JSONObject(jstr);
jsonArray.put(jsonArray);
}
return jsonArray.toString();
}
}