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Trouble deserializing JSON into Java objects with GSON

I have been trying to learn GSON, but I am struggling with it. I am trying to deserialize a JSON file into Java objects, using GSON. I have read a million other questions on here, and for the life of me, I can't understand what I'm doing wrong.
Here is my JSON text:
{
"movies": [
{
"name": "The Shawshank Redemption",
"url": "https://parsehub.com/sandbox/moviedetails?movie=The%20Shawshank%20Redemption",
"IMAX": "06:00 PM",
"rating": "9 . 2",
"cast": [
{
"character": "Andy Dufresne",
"actor": "Tim Robbins"
},
{
"character": "Ellis Boyd 'Red' Redding",
"actor": "Morgan Freeman"
},
{
"character": "Warden Norton",
"actor": "Bob Gunton"
},
{
"character": "Heywood",
"actor": "William Sadler"
}
]
},
{
"name": "Schindler's List",
"url": "https://parsehub.com/sandbox/moviedetails?movie=Schindler%27s%20List",
"IMAX": "06:15 PM",
"rating": "8 . 9",
"cast": [
{
"character": "Oskar Schindler",
"actor": "Liam Neeson"
},
{
"character": "Itzhak Stern",
"actor": "Ben Kingsley"
},
{
"character": "Amon Goeth",
"actor": "Ralph Fiennes"
}
]
}
]
}
And here is my Java code:
import com.google.gson.Gson;
import java.io.*;
import java.lang.reflect.Type;
import java.util.ArrayList;
public class Main {
public static void main(String[] args) throws FileNotFoundException {
Gson gson = new Gson();
Movies[] movies = gson.fromJson(new FileReader("src/main/input.json"), (Type) Movies.class);
System.out.println(movies[0]);
}
class Movies {
String name;
String url;
String IMAX;
String rating;
ArrayList<Cast> cast;
}
class Cast {
ArrayList<CastMember> castMembers;
}
class CastMember{
String character;
String actor;
}
}
When I run this, I get the following error:
Exception in thread "main" java.lang.ClassCastException: class com.Main$Movies cannot be cast to class [Lcom.Main$Movies; (com.Main$Movies and [Lcom.Main$Movies; are in unnamed module of loader 'app')
at com.Main.main(Main.java:13)

The JSON you are deserializing represents an object with a list of objects on it. The Java object you are trying to deserialize to needs to match that.
First, create a new class MovieList.
class MovieList {
List<Movie> movies;
}
Update your Movies class to be called Movie, since it represents a single movie.
class Movie {
String name;
String url;
String IMAX;
String rating;
List<Cast> cast;
}
Now try calling gson.fromJson(...) with the following
MovieList movieList = gson.fromJson(new FileReader("src/main/input.json"), MovieList.class);
System.out.println(movieList.getMovies().get(0));

Try using this
Gson gson = new Gson();
Movies[] movies = gson.fromJson(new JsonReader(new FileReader("src/main/input.json"))),Movies[].class);
or something like
Type listType = new TypeToken<List<Movies>>() {}.getType();
List<Movies> movies = new Gson().fromJson(new JsonReader(new FileReader("src/main/input.json"))), listType);

How to modify the value of a JsonNode recursively using Jackson

Requirements:
I want to apply some functions on the inner values of the JsonNode. The functions can be different eg:- lowercasing some values or appending something to the values or replace the values with something. How can I achieve that using Jackson library? Note that the structure of the JSON data can be different which means I want to build a generic system which will accept some path expression which will basically decide where to change. I want to use functional programming style, so that I can pass these functions as arguments.
eg:
input:
{
"name": "xyz",
"values": [
{
"id": "xyz1",
"sal": "1234",
"addresses": [
{
"id": "add1",
"name": "ABCD",
"dist": "123"
},
{
"id": "add2",
"name": "abcd3",
"dist": "345"
}
]
},
{
"id": "xyz2",
"sal": "3456",
"addresses": [
{
"id": "add1",
"name": "abcd",
"dist": "123"
},
{
"id": "add2",
"name": "XXXXX",
"dist": "345"
}
]
}
]
}
In this case I have to two functions basically, lowercase() and convert_to_number(). I want to apply lowercase() function on all the "name" attribute inside all the "addresses" of each "value".
same goes for convert_to_number() , but for all the "dist" attribute.
So, basically the JSON expressions will be something like below for the functions:
lowercase() : /values/*/addresses/*/name
convert_to_number() : /values/*/addresses/*/dist
output:
{
"name": "xyz",
"values": [
{
"id": "xyz1",
"sal": "1234",
"addresses": [
{
"id": "add1",
"name": "abcd",
"dist": 123
},
{
"id": "add2",
"name": "abcd3",
"dist": 345
}
]
},
{
"id": "xyz2",
"sal": "3456",
"addresses": [
{
"id": "add1",
"name": "abcd",
"dist": 123
},
{
"id": "add2",
"name": "xxxx",
"dist": 345
}
]
}
]
}
Client code:
JsonNode jsonNode = ...
applyFunctionsRecursivelyBasedOnExpr(JsonNode jsonNode, String expr, Function )

As #MichalZiober in his answer already pointed out,
JsonPath offers a much more powerful API than Jackson,
when you need to do JSON-path-based operations.
Using methods JsonPath.parse and WriteContext.map
you can solve your problem in just a few lines:
import java.io.File;
import com.jayway.jsonpath.Configuration;
import com.jayway.jsonpath.JsonPath;
public class Main {
public static void main(String[] args) throws Exception {
File file = new File("input.json");
String json = JsonPath.parse(file)
.map("$.values[*].addresses[*].name", Main::lowerCase)
.map("$.values[*].addresses[*].dist", Main::convertToNumber)
.jsonString();
System.out.println(json);
}
private static Object lowerCase(Object currentValue, Configuration configuration) {
if (currentValue instanceof String)
return ((String)currentValue).toLowerCase();
return currentValue;
}
private static Object convertToNumber(Object currentValue, Configuration configuration) {
if (currentValue instanceof String)
return Integer.valueOf((String)currentValue);
return currentValue;
}
}

JsonPath
You could use JsonPath library which has a better JSON Path handling. When Jackson supports only JSON Pointer draft-ietf-appsawg-json-pointer-03. Take a look on JsonPointer documentation. With JsonPath library you could do that in this way:
import com.jayway.jsonpath.DocumentContext;
import com.jayway.jsonpath.JsonPath;
import net.minidev.json.JSONArray;
import java.io.File;
import java.io.IOException;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.Arrays;
import java.util.List;
import java.util.Map;
import java.util.function.Function;
public class JsonPathApp {
public static void main(String[] args) throws Exception {
File jsonFile = new File("./resource/test.json").getAbsoluteFile();
JsonModifier jsonModifier = new JsonModifier(jsonFile);
Function<Map<String, Object>, Void> lowerCaseName = map -> {
final String key = "name";
map.put(key, map.get(key).toString().toLowerCase());
return null;
};
Function<Map<String, Object>, Void> changeDistToNumber = map -> {
final String key = "dist";
map.put(key, Integer.parseInt(map.get(key).toString()));
return null;
};
jsonModifier.update("$.values[*].addresses[*]", Arrays.asList(lowerCaseName, changeDistToNumber));
jsonModifier.print();
}
}
class JsonModifier {
private final DocumentContext document;
public JsonModifier(File json) throws IOException {
this.document = JsonPath.parse(json);
}
public void update(String path, List<Function<Map<String, Object>, Void>> transformers) {
JSONArray array = document.read(path);
for (int i = 0; i < array.size(); i++) {
Object o = array.get(i);
transformers.forEach(t -> {
t.apply((Map<String, Object>) o);
});
}
}
public void print() {
System.out.println(document.jsonString());
}
}
Your path, should work on JSON object-s which are represented by Map<String, Object>. You can replace keys in given object, add them, remove them just like replacing, adding and removing keys in Map.
Jackson
You can of course mock JsonPath feature by iterating over Json Pointer. For each * we need to create loop and iterate over it using counter and until node is not missing. Below you can see simple implementation:
import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.node.ObjectNode;
import java.io.File;
import java.util.Arrays;
import java.util.List;
import java.util.function.Function;
public class JsonApp {
public static void main(String[] args) throws Exception {
File jsonFile = new File("./resource/test.json").getAbsoluteFile();
ObjectMapper mapper = new ObjectMapper();
mapper.enable(SerializationFeature.INDENT_OUTPUT);
JsonNode root = mapper.readTree(jsonFile);
Function<ObjectNode, Void> lowerCaseName = node -> {
final String key = "name";
node.put(key, node.get(key).asText().toLowerCase());
return null;
};
Function<ObjectNode, Void> changeDistToNumber = node -> {
final String key = "dist";
node.put(key, Integer.parseInt(node.get(key).asText()));
return null;
};
JsonModifier jsonModifier = new JsonModifier(root);
jsonModifier.updateAddresses(Arrays.asList(lowerCaseName, changeDistToNumber));
System.out.println(mapper.writeValueAsString(root));
}
}
class JsonModifier {
private final JsonNode root;
public JsonModifier(JsonNode root) {
this.root = root;
}
public void updateAddresses(List<Function<ObjectNode, Void>> transformers) {
String path = "/values/%d/addresses/%d";
for (int v = 0; v < 100; v++) {
int a = 0;
do {
JsonNode address = root.at(String.format(path, v, a++));
if (address.isMissingNode()) {
break;
}
if (address.isObject()) {
transformers.forEach(t -> t.apply((ObjectNode) address));
}
} while (true);
if (a == 0) {
break;
}
}
}
}
This solution is slower than with JsonPath because we need to travers whole JSON tree n times where n number of matching nodes. Of course, our implementation could be a much faster using Streaming API.

Problem in parsing multiple json objects each having multiple Arrays from a validated JSON format-JavaString

Want to Process following JSON string (Validated with jsonlint.com)
[{
"label": "Hospital",
"domain": "Health_Care",
"synonymlabels": [{
"label": "SHCO"
}, {
"label": "HCO"
}],
"childrenlabels": [{
"label": "Childern_Hospital"
}, {
"label": "Mental_Hospital"
}, {
"label": "Heart_Hospital"
}, {
"label": "Orthopadic_Hospital"
}, {
"label": "General_Hospital"
}, {
"label": "Gynac_Hospital"
}, {
"label": "Cancer_Hospital"
}, {
"label": "Burn_Hospital"
}, {
"label": "Trauma_Care_Hospital"
}]
},
{
"label": "Doctor",
"domain": "Health_Care",
"synonymlabels": [{
"label": "Clinician"
}, {
"label": "Physician"
}, {
"label": "Medical_Practitioner"
}],
"childrenlabels": [{
"label": "Cardiaologist"
}, {
"label": "Allergist"
}, {
"label": "Nurologist"
}, {
"label": "Gynacologist"
}, {
"label": "General_Physician"
}, {
"label": "Anesthetist"
}, {
"label": "Physiotherapist"
}, {
"label": "Urologist"
}, {
"label": "Oncologist"
}, {
"label": "Homeopath"
}, {
"label": "Dentist"
}]
}
]
Sample Code
I am able to run the following sample code and able to get the desired output. If I change JSON string i.e. object "{}" to JSON ARRAY "[{},{},{}]" to parse and necessary change in the code (no idea that how to deal with the Array) then I'm getting no results in the console. Feeling paralytic in finding my error. Please help. Struggled for almost a day in tweaking the code.
import java.io.IOException;
import java.io.StringReader;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonToken;
public class gsontester {
public static void main(String args[]) {
String jsonString =
"{ \"name\":\"Mahesh Kumar\", \"age\":21,\"verified\":false,\"marks\": [100,90,85,100,14,95]}";
JsonReader reader = new JsonReader(new StringReader(jsonString));
try {
handleJsonObject(reader);
}
catch (IOException e) {
e.printStackTrace();
}
}
private static void handleJsonObject(JsonReader reader) throws IOException {
reader.beginObject();
String fieldname = null;
while (reader.hasNext()) {
JsonToken token = reader.peek();
if (token.equals(JsonToken.BEGIN_ARRAY)) {
System.out.print("Marks [ ");
handleJsonArray(reader);
System.out.print("]");
} else if (token.equals(JsonToken.END_OBJECT)) {
reader.endObject();
return;
} else {
if (token.equals(JsonToken.NAME)) {
//get the current token
fieldname = reader.nextName();
}
if ("name".equals(fieldname)) {
//move to next token
token = reader.peek();
System.out.println("Name: "+reader.nextString() );
}
if("age".equals(fieldname)) {
//move to next token
token = reader.peek();
System.out.println("Age:" + reader.nextInt());
}
if("verified".equals(fieldname)) {
//move to next token
token = reader.peek();
System.out.println("Verified:" + reader.nextBoolean());
}
}
}
}
Output
Name: Mahesh Kumar
Age:21
Verified:false
Marks [ 100 90 85 100 14 95 ]

Your JSON has one tricky element - label arrays contain one-element JSON object. We can unwrap it using custom deserialiser. To do that let's create simple POJO structure which fit's JSON payload. JSON starts from [ so it means we need to parse it as an array. All elements have the same structure. We can define it like below:
class Phrase {
private String label;
private String domain;
#JsonAdapter(StringWrapperJsonDeserializer.class)
#SerializedName("synonymlabels")
private List<String> synonymLabels;
#JsonAdapter(StringWrapperJsonDeserializer.class)
#SerializedName("childrenlabels")
private List<String> childrenLabels;
public String getLabel() {
return label;
}
public void setLabel(String label) {
this.label = label;
}
public String getDomain() {
return domain;
}
public void setDomain(String domain) {
this.domain = domain;
}
public List<String> getSynonymLabels() {
return synonymLabels;
}
public void setSynonymLabels(List<String> synonymLabels) {
this.synonymLabels = synonymLabels;
}
public List<String> getChildrenLabels() {
return childrenLabels;
}
public void setChildrenLabels(List<String> childrenLabels) {
this.childrenLabels = childrenLabels;
}
#Override
public String toString() {
return "Phrase{" +
"label='" + label + '\'' +
", domain='" + domain + '\'' +
", synonymLabels=" + synonymLabels +
", childrenLabels=" + childrenLabels +
'}';
}
}
When we want to use another name for property in Java comparing to what we have in JSON we use SerializedName annotation. To inform Gson library that we would like to handle given element in a specific way we use JsonAdapter annotation. In case we do not know how to write custom deserialiser it is always safe to use Map<String, Object> type for unknown or random JSON object. In case we have list of objects we can use List<Map<String, Object>>. Let's write simple deserialiser for labels arrays:
class StringWrapperJsonDeserializer implements JsonDeserializer<List<String>> {
#Override
public List<String> deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context) throws JsonParseException {
if (json.isJsonArray()) {
final JsonArray array = (JsonArray) json;
final int size = array.size();
if (size == 0) {
return Collections.emptyList();
}
List<String> labels = new ArrayList<>(size);
for (int i = 0; i < size; i++) {
JsonObject jsonElement = (JsonObject) array.get(i);
Set<String> keys = jsonElement.keySet();
for (String key : keys) {
labels.add(jsonElement.getAsJsonPrimitive(key).getAsString());
}
}
return labels;
}
return Collections.emptyList();
}
}
Algorithm is quite simple: if given element is an array, iterate over it and take each object one-by-one. For each object get all keys and add corresponding values to labels list which is our result of deserialisation process. Example usage, could look like this:
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.JsonObject;
import com.google.gson.JsonParseException;
import com.google.gson.annotations.JsonAdapter;
import com.google.gson.annotations.SerializedName;
import java.io.File;
import java.io.FileReader;
import java.lang.reflect.Type;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.Set;
import java.util.stream.Stream;
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();
Phrase[] phrases = gson.fromJson(new FileReader(jsonFile), Phrase[].class);
Stream.of(phrases).forEach(System.out::println);
}
}
Above code prints:
Phrase{label='Hospital', domain='Health_Care', synonymLabels=[SHCO, HCO], childrenLabels=[Childern_Hospital, Mental_Hospital, Heart_Hospital, Orthopadic_Hospital, General_Hospital, Gynac_Hospital, Cancer_Hospital, Burn_Hospital, Trauma_Care_Hospital]}
Phrase{label='Doctor', domain='Health_Care', synonymLabels=[Clinician, Physician, Medical_Practitioner], childrenLabels=[Cardiaologist, Allergist, Nurologist, Gynacologist, General_Physician, Anesthetist, Physiotherapist, Urologist, Oncologist, Homeopath, Dentist]}
Read also:
JsonAdapter
Serializing and Deserializing a List with Gson

Parse JSON object recursively - Java

I have a JSON with list of Objects and any of the item in the list can have null or the same object as a value for a key. I am looking for a faster way to parse the json to arrive at my final result.
The data structure looks like -
[
{
"firstName": "Bruce",
"familyMembers": null
},
{
"firstName": "Gates Family",
"familyMembers": [
{
"firstName": "Bill",
"familyMembers": null
},
{
"firstName": "Steve",
"familyMembers": null
}
]
},
{
"firstName": "Lee",
"familyMembers": null
},
{
"firstName": "Chan",
"familyMembers": null
}
]
The output should be a set = ("Bruce", "Bill", "Steve", "Lee", "Chan").
I am looking for a best possible way to do this in Java, such that i dont increase my response time by getting caught in this parsing hell.
Appreciate your time on this.

Try my recursive implementation.
public static void jsonArrayToSet(JSONArray jAry, Set<String> result, String targetKey, String subArrayKey, boolean includeNode){
try {
for (int i = 0; i < jAry.length(); i++) {
JSONObject jObj = jAry.getJSONObject(i);
boolean hasSubArray = false;
JSONArray subArray = null;
if(jObj.has(subArrayKey)){
Object possibleSubArray = jObj.get(subArrayKey);
if(possibleSubArray instanceof JSONArray){
hasSubArray = true;
subArray = (JSONArray) possibleSubArray;
}
}
if(hasSubArray){
if(includeNode){
result.add(jObj.getString(targetKey));
}
jsonArrayToSet(subArray, result, targetKey, subArrayKey, includeNode);
} else {
result.add(jObj.getString(targetKey));
}
}
} catch (JSONException e){
e.printStackTrace();
}
}
jAry: The source JSONArray.
result: The Set you want to write in.
targetKey: The key that maps to an entry which you want to add to result.
subArrayKey: The key that map to a sub-JSONArray.
includeNode: When current JSONOnject is a node containing sub-array, add it to result or not.
In your case, you can call:
jsonArrayToSet(yourJsonArray, yourSet, "firstName", "familyMembers", false);

As mentioned in my comment.
Your first issue would be the content in your JSON file. Based on the standard, it should be wrapped around with a set of { }.
Example
{
"members": [
{
"firstName": "Bruce",
"familyMembers": null
},
{
"firstName": "Gates Family",
"familyMembers": [
{
"firstName": "Bill",
"familyMembers": null
},
{
"firstName": "Steve",
"familyMembers": null
}
]
},
{
"firstName": "Lee",
"familyMembers": null
},
{
"firstName": "Chan",
"familyMembers": null
}
]
}
Also, I think the value "Gates Family" should be part of the output? Since it is under the "FirstName" attribute.
Anyway, here is my solution that is based on the org.json library. It also uses Goggle's GSon library where I use it for reading the JSON file.
import org.json.*;
import java.io.File;
import java.util.HashMap;
import java.util.Map;
import com.google.common.base.Charsets;
import com.google.common.io.Files;
public class solution {
public static final String JSON_DATA_FILE_PATH = "./data/source_37848106.json";
private static boolean hasMoreFamilyName(JSONObject json) {
return json.has("familyMembers") && json.get("familyMembers") != JSONObject.NULL;
}
private static void trackFirstName(Map<String, Integer> nameTracker, JSONObject json) {
if (!nameTracker.containsKey(json.getString("firstName"))) {
nameTracker.put(json.getString("firstName"), /*DUMMY VALUE =*/1);
}
}
private static void getNames(Map<String,Integer> nameTracker, JSONArray jsonArr) {
for (int i = 0; i< jsonArr.length(); i++) {
JSONObject item = jsonArr.getJSONObject(i);
if (hasMoreFamilyName(item)) {
getNames(nameTracker, item.getJSONArray("familyMembers"));
}
trackFirstName(nameTracker, item);
}
}
public static void main(String[] args) {
Map<String, Integer> nameTracker = new HashMap<>();
try {
String text = Files.toString(new File(JSON_DATA_FILE_PATH), Charsets.UTF_8);
JSONObject json = new JSONObject(text);
getNames(nameTracker, json.getJSONArray("members"));
}
catch (Exception ex) {
System.out.println("Something is wrong.");
}
for (Map.Entry<String,Integer> entry : nameTracker.entrySet()) {
System.out.println(entry.getKey());
}
}

You can use ObjectMapper defined in jackson-databind-2.6.5.jar
Define a java class with fields similar to json pattern and then just bind them like:
import com.fasterxml.jackson.databind.DeserializationFeature;
import com.fasterxml.jackson.databind.ObjectMapper;
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
objectMapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
Family family=objectMapper.readValue(jsonString, Family.class);
Now you will have a java object similar to your json string pattern and you can print it the way you like.

Add a Key value to Json String generated from GSON.toJSON

String jsonResponse=Utils.getGsonInstance().toJson(Object);
jsonResponse returns :
[
{
"Key":"1",
"Code": "11",
},
{
"key":"2",
"code": "22",
}
]
End result I am looking for is to wrap this JSON-String in another Key E.g.
{
"MainObj":
[
{
"Key":"1",
"Code": "11",
},
{
"key":"2",
"code": "22",
}
]
}
Is there a way I can achieve this using GSON Api ?
I tried ::
JSONObject jsonObject = new JSONObject();
jsonObject.put("MainObj",jsonResponse);
Output I am getting is :
{"MainObj": "[{\"Key\":\"1",\"Code\":\"11\"}, {\"Key\":\"2",\"Code\":\"22\"}]"}

Continue with GSON like :
public class MainObj {
#SerializedName("MainObj")
public List<Key> Main;
public class Key {
#SerializedName("Key")
public String Key;
#SerializedName("code")
public String Code;
}
}
And change
JSONObject jsonObject = new JSONObject();
jsonObject.put("MainObj",jsonResponse);
by
String tmp = new Gson().toJson(new MainObj());