Develop Reference

Java reading JSON with multiple objects

The json looks like:
{
"id": "SMAAZGD20R",
"data": [
{
"blukiiId": "CC78AB5E73C8",
"macAddress": "CC78AB5E73C8",
"type": "SENSOR_BEACON",
"battery": 97,
"advInterval": 1000,
"firmware": "003.007",
"rssi": [
{
"rssi": -96,
"timestamp": 1594642177138
}
],
"beaconSensorData": {
"environment": [
{
"airPressure": 994.4,
"light": 5,
"humidity": 26,
"temperature": 28.4,
"timestamp": 1594642177138
}
]
}
}
]
}
The code looks like:
public class getJSON
{
public static void main(String[] args) throws Exception{
JSONParser parser = new JSONParser();
try{
Object obj = parser.parse(new FileReader("C:\\test.json"));
JSONObject jsonObj = (JSONObject)obj;
JSONArray jsonArr = (JSONArray)jsonObj.get("data");
Iterator itr = jsonArr.iterator();
while(itr.hasNext()){
System.out.println(itr.next());
}
}catch(Exception e){
e.printStackTrace();
}
}
}
Output:
{"macAddress":"CC78AB5E73C8","rssi":[{"rssi":-96,"timestamp":1594642177138}],"advInterval":1000,"blukiiId":"CC78AB5E73C8","type":"SENSOR_BEACON","battery":97,"firmware":"003.007","beaconSensorData":{"environment":[{"light":5,"airPressure":994.4,"temperature":28.4,"humidity":26,"timestamp":1594642177138}]}}
I get an Array with the object "data", but the array includes only one value with all objects from "data".
How can i address the array "environment" and get the values tempreature, light,...

Have you should try to use a framework like jackson
Who will let unmarshall your json to real java object of your choice
for example :
public class Data
{
private String blukiiId;
private String macAddress;
private String type;
...
private List<RSSI> rssi;
private BeaconSensorData beaconSensorData;
}
With Rssi,BeaconSensorData another class like that etc...
Now your code will get Converted as below
public class getJSON
{
public static void main(String[] args) throws Exception{
ObjectMapper mapper = new ObjectMapper();
// Set any extra configs like ignore fields etc here
try{
Data data = mapper.convertValue(Files.readAllBytes(Paths.get("test.json"), Data.class);
//Now you can access the value as below
data.getBeaconSensorData().getEnvironment().getTemperature();
}catch(Exception e){
e.printStackTrace();
}
}
}

If you can only use org.json.simple.parser.JSONParser, please refer to the following java code by recursion.
import java.io.FileReader;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import org.json.simple.JSONArray;
import org.json.simple.JSONObject;
import org.json.simple.parser.JSONParser;
public class getJSON
{
private static void visitElement(Object obj, Map<Object, Object> map) throws Exception {
if(obj instanceof JSONArray) {
JSONArray jsonArr = (JSONArray)obj;
Iterator<?> itr = jsonArr.iterator();
while(itr.hasNext())
visitElement(itr.next(), map);
}
else if(obj instanceof JSONObject) {
JSONObject jsonObj = (JSONObject)obj;
Iterator<?> itr = jsonObj.keySet().iterator();
while(itr.hasNext()) {
Object key = itr.next();
Object value = jsonObj.get(key);
map.put(key, value);
visitElement(value, map);
}
}
}
public static void main(String[] args) throws Exception{
JSONParser parser = new JSONParser();
try{
Object obj = parser.parse(new FileReader("C:/test.json"));
Map<Object, Object> map = new HashMap<>();
visitElement(obj, map);
for(Object key : map.keySet())
System.out.println(key + ": " + map.get(key));
}catch(Exception e){
e.printStackTrace();
}
}
}

Look at my code below.
[
{
"title": "Dywizjon 303",
"year": 2019,
"type": "Dokumentalny",
"director": "Zbigniew Zbigniew",
"actors": ["Maria Joao","Jose Raposo"]
},
{
"title": "Szeregowiec Ryan",
"year": 2006,
"type": "Historyczny",
"director": "Stanislaw Stanislaw",
"actors": ["Rosa Canto","Amalia Reis","Maria Garcia"]
},
{
"title": "Duzy",
"year": 1988,
"type": "Dramat",
"director": "Penny Marshall",
"actors": ["Rosa Canto"]
},
{
"title": "Syberiada Polska",
"year": 2013,
"type": "Wojenny",
"director": "Janusz Zaorski",
"actors": ["Harvey Glazer"]
}
]
This is my Entity named Movie
package objectMapper.app;
import java.util.Arrays;
import java.util.List;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlProperty;
import com.fasterxml.jackson.dataformat.xml.annotation.JacksonXmlRootElement;
#JacksonXmlRootElement(localName="Class")
public class Movie {
#JacksonXmlProperty(localName="title")
private String title;
#JacksonXmlProperty(localName="year")
private int year;
#JacksonXmlProperty(localName="type")
private String type;
#JacksonXmlProperty(localName="director")
private String director;
#JacksonXmlProperty(localName="actors")
private String[] actors;
public Movie()
{
}
public Movie(String title, int year, String type, String director, String[] actors) {
this.title = title;
this.year = year;
this.type = type;
this.director = director;
this.actors = actors;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public int getYear() {
return year;
}
public void setYear(int year) {
this.year = year;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
public String getDirector() {
return director;
}
public void setDirector(String director) {
this.director = director;
}
public String[] getActors() {
return actors;
}
public void setActors(String[] actors) {
this.actors = actors;
}
#Override
public String toString() {
return "Movie [title=" + title + ", year=" + year + ", type=" + type + ", director=" + director + ", actors="
+ Arrays.toString(actors) + "]";
}
}
Function to read movies.json
public Movie[] readJSONFile() throws JsonParseException, JsonMappingException, IOException
{
ObjectMapper mapper= new ObjectMapper();
Movie[] jsonObj=mapper.readValue(new File("movies.json"),Movie[].class);
return jsonObj;
}
Lets do something with our POJO class
List<String> titles=new ArrayList();
for(Movie itr: tempMovies)
{
titles.add(itr.getTitle().toLowerCase());
}
if(titles.contains(tempTitle.toLowerCase()))
{
for(Movie itr2 : tempMovies)
{
if(tempTitle.toLowerCase().equals(itr2.getTitle().toLowerCase()))
{
System.out.println("Title: "+itr2.getTitle());
System.out.println("Year: "+itr2.getYear());
System.out.println("Type: "+itr2.getType());
System.out.println("Director: "+itr2.getDirector());
String[] tempActorsStrings=itr2.getActors();
int size=tempActorsStrings.length;
for(int i=0;i<size;i++)
{
System.out.println("Actor: "+tempActorsStrings[i]);
}
status=false;
}
}
}
else
{
System.out.println("Movie title does not exist!");
}

how to convert a json file to some object calsses

I have a json file like this:
{
"Student" : [
{
"name": "john",
"age": 12
}, {
"name": "jack",
"age": 20
}
]
}
and my Student class is:
public class Student {
private String name;
private int age;
public Student(String name, int age) {
this.name = name;
this.age = age;
}
public String getName() {
return name;
}
public int getAge() {
return age;
}
}
I want to make a Student Instance with name "jack" by using json
how can I do it?

Make Another Class Students which contain List<Student>.
public class Students {
List<Student> Student;
public List<Student> getStudents() {
return Student;
}
public void setStudent(List<Student> students) {
this.Student=students;
}
}
Gson gson = new Gson();
String jsonString = "Your Json String";
Students student = gson.fromJson(jsonString, Students.class);

I use org.json.simple library when I parse JSON
Excample:
excample App.java,
excample Information.java
List<Information> parseInformationObject(JSONArray infoList) {
List<Information> in = new ArrayList<>();
infoList.forEach(emp -> {
JSONObject info = (JSONObject) emp;
String id = info.get("id").toString();
String state = info.get("state").toString();
String type = null;
if (info.get("type") != null) {
type = info.get("type").toString();
}
String host = null;
if (info.get("host") != null) {
host = info.get("host").toString();
}
long timestamp = (long) info.get("timestamp");
in.add(new Information(id, state, type, host, timestamp));
});
return in;
}

exception while convert string to jsonobject

I have a String in my servlet which is of the following format.
{
"name": "Jam",
"noOfBooksRequired": "2",
"type": "Type 1",
"bookName": [
"The Magic",
"The Power"
]
}
where the bookName is an array. I want to access the values in the array and populate in the bean. But, when I try to convert the string to jsonobject, I am getting the following exception because bookName is an array com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected a string but was BEGIN_ARRAY This is how I am trying to do it
JSONObject js= new JSONObject();
String inputData= request.getParameter("inputData");
HashMap<String, String> hmap= new HashMap<String, String>();
Type type = new TypeToken<HashMap<String, String>>(){}.getType();
hmap = gson.fromJson(inputData, type);
js.putAll(hmap);
What I am doing is, I convert the string to a map and then add it to the JSONObject.
Since there are many json serializers and not sure which is the best. Right now, I have net.sf.json.JSONObject and com.google.gson.JsonObject
Can someone help me to get this solved.
Thanks in advance

You can map your JSON to a POJO.
If the book will have more attributes besides the name, you'll need two POJOs, as you can see below.
A POJO for the book:
class Book {
private String name;
private String author;
public Book() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAuthor() {
return author;
}
public void setAuthor(String author) {
this.author = author;
}
}
And a POJO for the shelf, which have a list of books:
class Shelf {
private String name;
private Integer noOfBooksRequired;
private String type;
private List<Book> books;
public Shelf() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getNoOfBooksRequired() {
return noOfBooksRequired;
}
public void setNoOfBooksRequired(Integer noOfBooksRequired) {
this.noOfBooksRequired = noOfBooksRequired;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
public List<Book> getBooks() {
return books;
}
public void setBooks(List<Book> books) {
this.books = books;
}
}
Your JSON will look like this:
{
"name": "Jam",
"noOfBooksRequired": "2",
"type": "Type 1",
"books": [
{"name": "The Magic", "author": "John Doe"},
{"name": "The Power", "author": "Jane Doe"}
]
}
And then you can use Gson to parse your JSON:
Gson gson = new Gson();
Shelf shelf = gson.fromJson(inputData, Shelf.class);
Update
Considering your JSON looks like this (the book can be represented as a String):
{
"name": "Jam",
"noOfBooksRequired": "2",
"type": "Type 1",
"books": [
"The Magic",
"The Power"
]
}
Only one POJO with a list of String is enough:
class Shelf {
private String name;
private Integer noOfBooksRequired;
private String type;
private List<String> books;
public Shelf() {
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getNoOfBooksRequired() {
return noOfBooksRequired;
}
public void setNoOfBooksRequired(Integer noOfBooksRequired) {
this.noOfBooksRequired = noOfBooksRequired;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
public List<String> getBooks() {
return books;
}
public void setBooks(List<String> books) {
this.books = books;
}
}

Binding json, that has a list, with an object using Jackson

Given I have the following json:
{
"Company": {
"name": "cookieltd",
"type": "food",
"franchise_location": [
{
"location_type": "town",
"address_1": "5street"
},
{
"location_type": "village",
"address_1": "2road"
}
]
}
}
How can it be binded to the following object classes using Jackson?:
1) Company class
public class Company
{
String name, type;
List<Location> franchise_location = new ArrayList<Location>();
[getters and setters]
}
2) Location class
public class Location
{
String location_type, address_1;
[getters and setters]
}
I have done:
String content = [json above];
ObjectReader reader = mapper.reader(Company.class).withRootName("Company"); //read after the root name
Company company = reader.readValue(content);
but I am getting:
com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "franchise_location"

As far as I can tell, you are simply missing an appropriately named getter for the field franchise_location. It should be
public List<Location> getFranchise_location() {
return franchise_location;
}
(and the setter)
public void setFranchise_location(List<Location> franchise_location) {
this.franchise_location = franchise_location;
}
Alternatively, you can annotate your current getter or field with
#JsonProperty("franchise_location")
private List<Location> franchiseLocation = ...;
which helps to map JSON element names that don't really work with Java field name conventions.
The following works for me
public static void main(String[] args) throws Exception {
String json = "{ \"Company\": { \"name\": \"cookieltd\", \"type\": \"food\", \"franchise_location\": [ { \"location_type\": \"town\", \"address_1\": \"5street\" }, { \"location_type\": \"village\", \"address_1\": \"2road\" } ] } }";
ObjectMapper mapper = new ObjectMapper();
ObjectReader reader = mapper.reader(Company.class).withRootName(
"Company"); // read after the root name
Company company = reader.readValue(json);
System.out.println(company.getFranchise_location().get(0).getAddress_1());
}
public static class Company {
private String name;
private String type;
private List<Location> franchise_location = new ArrayList<Location>();
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getType() {
return type;
}
public void setType(String type) {
this.type = type;
}
public List<Location> getFranchise_location() {
return franchise_location;
}
public void setFranchise_location(List<Location> franchise_location) {
this.franchise_location = franchise_location;
}
}
public static class Location {
private String location_type;
private String address_1;
public String getLocation_type() {
return location_type;
}
public void setLocation_type(String location_type) {
this.location_type = location_type;
}
public String getAddress_1() {
return address_1;
}
public void setAddress_1(String address_1) {
this.address_1 = address_1;
}
}
and prints
5street

my solution for JSON is always GSON, you can do some research on that, as long as you have the correct structure of class according to the JSON, it can automatically transfer from JSON to object:
Company company = gson.fromJson(json, Company.class);
GSON is so smart to do the convertion thing!
enjoy GSON !

Jackson Custom Deserialize

I would like deserialize my custom serialized objects. My objects are basically consisting a simple Pair implementation.
class School{
Integer id;
String schoolName;
}
class Student{
Integer id;
Integer schoolId;
String studentName;
}
#JsonSerialize(using=PairSerializer.class)
public class Pair<V,K>{
V v;
K k;
}
Here is the result
[
{
"v":{
"id":1,
"schoolId":3,
"studentName":"O. Bas"
},
"k":{
"id":3,
"schoolName":"School 3"
}
},
{
"v":{
"id":2,
"schoolId":3,
"studentName":"C. Koc"
},
"k":{
"id":3,
"schoolName":"School 3"
}
}
]
v and k as field name in json is pretty ugly. That is why I have written a custom serializer as this:
#Override
public void serialize(Pair pair, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {
jsonGenerator.writeStartObject();
jsonGenerator.writeObjectField(CaseFormat.UPPER_CAMEL.to(CaseFormat.LOWER_CAMEL,pair.getK().getClass().getSimpleName() ), pair.getK());
jsonGenerator.writeObjectField(CaseFormat.UPPER_CAMEL.to(CaseFormat.LOWER_CAMEL,pair.getV().getClass().getSimpleName() ), pair.getV());
jsonGenerator.writeEndObject();
}
The result is exactly what I want. v and k field names are replaced by their class names.
[
{
"school":{
"id":3,
"schoolName":"School 3"
},
"student":{
"id":1,
"schoolId":3,
"studentName":"O. Bas"
}
},
{
"school":{
"id":3,
"schoolName":"School 3"
},
"student":{
"id":2,
"schoolId":3,
"studentName":"C. Koc"
}
}
]
Here is the my question. How can I deserialize my json string to List<Pair<V, K> ? The real problem is that V and K are depends on the deserialized context it might vary as Student, School or another pair implementation.
public class PairDeserializer extends JsonDeserializer<Pair> {
public PairDeserializer() {
}
#Override
public Pair deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
// I need to Deserialized generic type information of Pair
}
}

I think, you should create your own PropertyNamingStrategy. For example see my simple implementation:
class MapTransformNamingStrategy extends LowerCaseWithUnderscoresStrategy {
private static final long serialVersionUID = 1L;
private Map<String, String> mapping;
public MapTransformNamingStrategy(Map<String, String> mapping) {
this.mapping = mapping;
}
#Override
public String translate(String property) {
if (mapping.containsKey(property)) {
return mapping.get(property);
}
return property;
}
}
Now you can use it in this way:
Map<String, String> mapping = new HashMap<String, String>();
mapping.put("k", "student");
mapping.put("v", "school");
ObjectMapper objectMapper = new ObjectMapper();
objectMapper.setPropertyNamingStrategy(new MapTransformNamingStrategy(mapping));
//etc
Example JSON output:
{ "school" : { "id" : 1,
"schoolName" : "The Best School in the world"
},
"student" : { "id" : 1,
"schoolId" : 1,
"studentName" : "Arnold Shwarz"
}
}
EDIT
Because my answer is not clear for everyone I present full example source code which serialize Java POJO objects into JSON and "vice versa".
import java.io.StringWriter;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import com.fasterxml.jackson.core.JsonFactory;
import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.PropertyNamingStrategy.LowerCaseWithUnderscoresStrategy;
public class JacksonProgram {
#SuppressWarnings("unchecked")
public static void main(String[] args) throws Exception {
List<Pair<Student, School>> pairs = createDataForSerialization();
Map<String, String> mapping = createSchoolStudentMapping();
JsonConverter jsonConverter = new JsonConverter(mapping);
String json = jsonConverter.toJson(pairs);
System.out.println("JSON which represents list of pairs:");
System.out.println(json);
List<Pair<Student, School>> value = jsonConverter.fromJson(json, List.class);
System.out.println("----");
System.out.println("Deserialized version:");
System.out.println(value);
}
private static Map<String, String> createSchoolStudentMapping() {
Map<String, String> mapping = new HashMap<String, String>();
mapping.put("k", "student");
mapping.put("v", "school");
return mapping;
}
private static List<Pair<Student, School>> createDataForSerialization() {
List<Pair<Student, School>> pairs = new ArrayList<Pair<Student, School>>();
pairs.add(new Pair<Student, School>(new Student(1, 3, "O. Bas"), new School(3, "School 3")));
pairs.add(new Pair<Student, School>(new Student(2, 4, "C. Koc"), new School(4, "School 4")));
return pairs;
}
}
class JsonConverter {
private Map<String, String> mapping;
private ObjectMapper objectMapper;
private JsonFactory jsonFactory;
public JsonConverter(Map<String, String> mapping) {
this.mapping = mapping;
initJsonObjects();
}
private void initJsonObjects() {
objectMapper = new ObjectMapper();
objectMapper.setPropertyNamingStrategy(new MapTransformNamingStrategy(mapping));
jsonFactory = new JsonFactory();
}
public String toJson(Object object) throws Exception {
StringWriter stringWriter = new StringWriter();
JsonGenerator jsonGenerator = jsonFactory.createGenerator(stringWriter);
objectMapper.writeValue(jsonGenerator, object);
return stringWriter.toString();
}
public <T> T fromJson(String json, Class<T> expectedType) throws Exception {
JsonParser jsonParser = jsonFactory.createJsonParser(json);
return objectMapper.readValue(jsonParser, expectedType);
}
}
class MapTransformNamingStrategy extends LowerCaseWithUnderscoresStrategy {
private static final long serialVersionUID = 1L;
private Map<String, String> mapping;
public MapTransformNamingStrategy(Map<String, String> mapping) {
this.mapping = mapping;
}
#Override
public String translate(String property) {
if (mapping.containsKey(property)) {
return mapping.get(property);
}
return property;
}
}
class School {
private Integer id;
private String schoolName;
public School() {
}
public School(Integer id, String schoolName) {
this.id = id;
this.schoolName = schoolName;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getSchoolName() {
return schoolName;
}
public void setSchoolName(String schoolName) {
this.schoolName = schoolName;
}
#Override
public String toString() {
return "School [id=" + id + ", schoolName=" + schoolName + "]";
}
}
class Student {
private Integer id;
private Integer schoolId;
private String studentName;
public Student() {
}
public Student(Integer id, Integer schoolId, String studentName) {
this.id = id;
this.schoolId = schoolId;
this.studentName = studentName;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getSchoolId() {
return schoolId;
}
public void setSchoolId(Integer schoolId) {
this.schoolId = schoolId;
}
public String getStudentName() {
return studentName;
}
public void setStudentName(String studentName) {
this.studentName = studentName;
}
#Override
public String toString() {
return "Student [id=" + id + ", schoolId=" + schoolId + ", studentName=" + studentName
+ "]";
}
}
class Pair<V, K> {
private V v;
private K k;
public Pair() {
}
public Pair(V v, K k) {
this.v = v;
this.k = k;
}
public V getV() {
return v;
}
public void setV(V v) {
this.v = v;
}
public K getK() {
return k;
}
public void setK(K k) {
this.k = k;
}
#Override
public String toString() {
return "Pair [v=" + v + ", k=" + k + "]";
}
}
The full output log:
JSON which represents list of pairs:
[{"school":{"id":1,"schoolId":3,"studentName":"O. Bas"},"student":{"id":3,"schoolName":"School 3"}},{"school":{"id":2,"schoolId":4,"studentName":"C. Koc"},"student":{"id":4,"schoolName":"School 4"}}]
----
Deserialized version:
[{school={id=1, schoolId=3, studentName=O. Bas}, student={id=3, schoolName=School 3}}, {school={id=2, schoolId=4, studentName=C. Koc}, student={id=4, schoolName=School 4}}]
Because the output JSON is not formatted I present it in more understandable version:
[
{
"school":{
"id":1,
"schoolId":3,
"studentName":"O. Bas"
},
"student":{
"id":3,
"schoolName":"School 3"
}
},
{
"school":{
"id":2,
"schoolId":4,
"studentName":"C. Koc"
},
"student":{
"id":4,
"schoolName":"School 4"
}
}
]
As you can see, we create new JsonConverter object with definition of mapping between Pair property names and which names we want to see in JSON string representation. Now if you have for example Pair<School, Room> you can create mapping Map in this way:
private static Map<String, String> createSchoolRoomMapping() {
Map<String, String> mapping = new HashMap<String, String>();
mapping.put("k", "school");
mapping.put("v", "room");
return mapping;
}

I was going for an answer with some annotation (JsonTypeInfo and JsonUnwrapped), but those two don't work well together apparently (see this issue). That would of handled both the serialization and deserialization part of your problem, without relying on custom de/serializer. Instead, you'll need a custom deserializer, which does something along those line:
class PairDeserializer extends JsonDeserializer<Pair>{
static Map<String, Class> MAPPINGS = new HashMap<String, Class>();
#Override
public Pair deserialize(JsonParser jp, DeserializationContext ctxt) throws IOException, JsonProcessingException {
Object key = deserializeField(jp);
Object value = deserializeField(jp);
Pair pair = new Pair();
pair.k = key;
pair.v = value;
jp.nextToken();
return pair;
}
private Object deserializeField(JsonParser jp) throws IOException, JsonParseException, JsonProcessingException {
jp.nextValue();
String className = jp.getCurrentName();
return jp.readValueAs(MAPPINGS.get(className));
}
}
Then you only need to register the mappings you need