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Gson append object to json file

I am currently working on a todo web application using Spring Boot. The problem I'm facing is that whenever I add an item it is not stored in my json file. This is my current json file:
[
{
"id": 0,
"task": "some task"
},
{
"id": 1,
"task": "some other task"
},
{
"id": 2,
"task": "some different task"
}
]
When I add my todo item with the application, it gets added to my dataprovider;
TodoDataProvider.java:
public class TodoDataProvider {
private static TodoDataProvider instance;
private List<TodoItem> todoItems = new ArrayList<>();
public static TodoDataProvider getInstance() {
if (instance == null) {
instance = new TodoDataProvider();
}
//read data from json
Gson gson = new Gson();
try {
FileReader reader = new FileReader("src/main/java/com/example/servingwebcontent/todos.json");
TodoItem[] tempTodos = gson.fromJson(reader, TodoItem[].class);
for (TodoItem tempTodo : tempTodos) {
instance.addTodo(tempTodo);
}
} catch (FileNotFoundException e) {
e.printStackTrace();
}
return instance;
}
... getters and setters
After I try to add this todo my json file becomes empty. What is the best way to add this new json object to the existing json file using gson?
This is the code where I add this item:
TodoController.java
#Controller
public class TodoController {
TodoDataProvider dataProvider = TodoDataProvider.getInstance();
#GetMapping("/")
public String greeting(Model model) {
model.addAttribute("todos", dataProvider.getTodoItems());
return "todo";
}
#PostMapping("/")
public String addTodo(Model model, #RequestParam("todoTask") String todoTask) {
TodoItem todoItem = new TodoItem(4, todoTask);
dataProvider.addTodo(todoItem);
Gson gson = new Gson();
try {
// TODO: 14-7-2022 add to json instead of replace
gson.toJson(dataProvider.getTodoItems(), new FileWriter("src/main/java/com/example/servingwebcontent/todos.json"));
} catch (IOException e) {
System.out.println(e.getMessage());
}
model.addAttribute("todos", dataProvider.getTodoItems());
return "redirect:/";
}
}

best approach to have multiple object as a service result

I have a java service that sends a JSON as a result.
my service depends on some condition creat a conversationVO object and converts it to JSON and in the other condition create a participantVO object and convert it to JSON and finally, it sends/returns the JSON object as a result.
the question is that is it rational that my service sends different objects as a result? is this approach correct? or it's better that I create another VO, for example, resultVO object that has 2 fields of conversationVO and participantVO and it the conditions I fill one of 2 fields and finally send the JSON of new resultVO?
you can find codes below.
public static void getThreadInfo(User currentUser, String uniqueName, long senderPeerId, String contactType, String uniqueId) {
try (HibernateSession session = SessionManager.openSession()) {
if (uniqueName == null || uniqueName.isEmpty()) {
throw new ChatException(ChatException.UNIQUE_NAME_IS_NEEDED, "UNIQUE_NAME_IS_NEEDED. " + uniqueId);
}
User participant = UserCRUD.getUserByUsername(uniqueName, session);
if (participant != null && participant.isActive()) {
sendParticipantThreadInfo(currentUser, participant, senderPeerId, uniqueId);
} else {
sendThreadInfo(currentUser,uniqueName, contactType, senderPeerId, uniqueId, session);
}
} catch (Throwable e) {
logger.warn("An exception occured", e);
ControllerHelper.sendError(
uniqueId,
new Long[]{senderPeerId},
e);
}
}
private static void sendParticipantThreadInfo(User currentUser, User participant, long senderPeerId, String uniqueId) {
ParticipantVO participantVO = ServiceHelper.fillParticipantSummary(participant);
ChatMessage chatMessage = new ChatMessage();
chatMessage.setType(ChatMessage.GET_THEAD_INFO);
chatMessage.setContent(JsonUtil.getJson(participantVO));
chatMessage.setUniqueId(uniqueId);
chatMessage.setSubjectId(participant.getId());
ControllerHelper.sendMessage(
new ResultVO(
chatMessage,
Collections.singletonList(senderPeerId),
null,
true,
ChatSettings.MESSAGE_NORMAL_PRIORITY,
currentUser
)
);
}
private static void sendThreadInfo(User currentUser,
String uniqueName,
String contactType,
long senderPeerId,
String uniqueId,
HibernateSession session) {
try {
MessageThread messageThread = getThreadByUniqueName(uniqueName, contactType, session);
if (messageThread == null) {
throw new ChatException(ChatException.USER_NOT_FOUND, "User or thread not found! " + uniqueId);
}
ConversationVO conversationVO = ServiceHelper.fillConversation(messageThread,
null,
true,
false,
contactType, session);
ChatMessage chatMessage = new ChatMessage();
chatMessage.setSubjectId(messageThread.getId());
chatMessage.setContent(JsonUtil.getJson(conversationVO));
chatMessage.setType(ChatMessage.GET_THEAD_INFO);
chatMessage.setUniqueId(uniqueId);
ControllerHelper.sendMessage(
new ResultVO(
chatMessage,
Collections.singletonList(senderPeerId),
null,
true,
ChatSettings.MESSAGE_NORMAL_PRIORITY,
currentUser
)
);
} catch (Throwable e) {
logger.warn("An exception occured", e);
ControllerHelper.sendError(
uniqueId,
new Long[]{senderPeerId},
e);
}
}

NewResultVO<T>{
private String code;
private T data; // T is your ResultVO or other VO.
}
Client get code "0000000" to do ...
Client get code "9999999" to do ...

Better Approach is send data in a single result like below format.
JSON Result
{
"ChatId": 1,
"Description": "test",
"conversation": [
{
"conversationId":1,
"conversationType":"content"
},
{
"conversationId":2,
"conversationType":"content"
}
],
"participant": [
{
"participantId":1,
"participantName":"Name1"
},
{
"conversationId":2,
"participantName":"Name2"
}
]
}

Converting JSON response to Java object

I need to convert a JSON response into Java Object, but I am gettin nullpointerException.
Here is my model class:
public class Cheque {
private String payeeName;
private String accountNumber;
private String ifsCode;
private String micr;
private String bankName;
public Cheque() {
super();
// TODO Auto-generated constructor stub
}
public Cheque(String payeeName, String accountNumber, String ifsCode, String micr, String bankName) {
super();
this.payeeName = payeeName;
this.accountNumber = accountNumber;
this.ifsCode = ifsCode;
this.micr = micr;
this.bankName = bankName;
}
public String getPayeeName() {
return payeeName;
}
public void setPayeeName(String payeeName) {
this.payeeName = payeeName;
}
public String getAccountNumber() {
return accountNumber;
}
public void setAccountNumber(String accountNumber) {
this.accountNumber = accountNumber;
}
public String getIfsCode() {
return ifsCode;
}
public void setIfscCode(String ifsCode) {
this.ifsCode = ifsCode;
}
public String getMicr() {
return micr;
}
public void setMicr(String micr) {
this.micr = micr;
}
public String getBankName() {
return bankName;
}
public void setBankName(String bankName) {
this.bankName = bankName;
}
Below I am posting the method in which I am invoking a Python program and getting a Json response from it:
public class RunPython {
public static void main(String[] args) throws IOException,ScriptException,NullPointerException{
// TODO Auto-generated method stub
Process p = Runtime.getRuntime().exec("python <path to file>/reg.py");
BufferedReader in = new BufferedReader(new InputStreamReader(p.getInputStream()));
String line ;
try {
while((line =in.readLine()) != null) {
System.out.println(line);
}
} finally {
in.close();
}
ObjectMapper mapper = new ObjectMapper();
try {
Cheque cheque = mapper.readValue(line, Cheque.class);
System.out.println("Java object :");
System.out.println(cheque);
}
catch (JsonGenerationException ex) {
ex.printStackTrace();
}
catch (JsonMappingException ex) {
ex.printStackTrace();
}
catch (IOException ex) {
ex.printStackTrace();
}}}
The JSON response which I am getting is:
{
"bankName": [
"Bank"
],
"accountNumber": [
"989898989898"
],
"ifsCode": [
"0000000"
],
"micr": [
"0000000"
],
"payeeName": [
"name"
]
}
After running the program I am getting the JSON response as expected, but while converting it to a Java object it is showing nullPointerException in the main thread. Help me out to find where I am making the mistake.

You consume/exhaust all your Process Inputstream here when printing it :
try {
while((line =in.readLine()) != null) {
System.out.println(line);
}
And later on when you call the below it's already null :
Cheque cheque = mapper.readValue(line, Cheque.class);
You'd have to process it in the above while loop, instead of only printing it out

When you reach the statement:
Cheque cheque = mapper.readValue(line, Cheque.class);
variable line is null.
So you can either rebuild the JSON string (with a StringBuilder), or remove the code that prints the response, and parse the JSON directly from p.getInputStream().

Your Java object has String value, but JSON has array type ([ ])

Read JSON Nested element using reflection without using JsonFactory or ObjectMapper

[Unable to access property of another object stored in Arraylist]
I am creating an function to get JSON input in object from RESTful Web service input and format it again in JSON format to call other web service.
I have limitation that I can not use any JSON API for object mapping hence using Java reflection core API.
I am able to create JSON format from Input for simple elements but unable to access nested elements (another user defined POJO class ). I am using arraylist.
Input
{
"GenesisIncidents": {
"service": "Transmission",
"affectedCI": "22BT_ORNC03",
"opt_additionalAffectedItems": [
{
"itemType": "NODE-ID",
"ItemName": "22BT_ORNC03"
},
{
"ItemType": "CCT",
"ItemName": "A_circuit_id"
}]
}
}
GenesisIncidents.class
import java.util.ArrayList;
import java.util.Date;
public class GenesisIncidents {
private String service;
private String affectedCI;
private ArrayList<AdditionalAffectedItems> opt_additionalAffectedItems;
public GenesisIncidents(){}
public String getService() {
return service;
}
public void setService(String service) {
this.service = service;
}
public String getAffectedCI() {
return affectedCI;
}
public void setAffectedCI(String affectedCI) {
this.affectedCI = affectedCI;
}
public ArrayList<AdditionalAffectedItems> getOpt_additionalAffectedItems() {
return opt_additionalAffectedItems;
}
public void setOpt_additionalAffectedItems(ArrayList<AdditionalAffectedItems> opt_additionalAffectedItems) {
this.opt_additionalAffectedItems = opt_additionalAffectedItems;
}
}
AdditionalAffectedItems.class
public class AdditionalAffectedItems {
private String itemType;
private String itemName;
public AdditionalAffectedItems(){
super();
}
public String getItemType() {
return itemType;
}
public void setItemType(String itemType) {
this.itemType = itemType;
}
public String getItemName() {
return itemName;
}
public void setItemName(String itemName) {
this.itemName = itemName;
}
}
Implemetation
public void updateTicketExt(GenesisIncidents genesisIncidents) {
try{
Field allFields[]=genesisIncidents.getClass().getDeclaredFields();
Method allMethods[] = genesisIncidents.getClass().getDeclaredMethods();
String jsonString ="{\r\n \""+genesisIncidents.getClass().getName().toString().substring(48)+"\": {";
final String preStr="\r\n \""; //To create a JSON object format.
final String postStr="\": "; //To create a JSON object format.
int totalNoOfFields=allFields.length;
for (Field field : allFields) {
System.out.println(field.getType());
String getter="get"+StringUtils.capitalize(field.getName());
Method method= genesisIncidents.getClass().getMethod(getter, null);
try{
if(field.getType().toString().contains("Integer"))
jsonString=jsonString + preStr + field.getName() + postStr +method.invoke(genesisIncidents).toString()+",";
else
jsonString=jsonString + preStr + field.getName() + postStr +"\""+method.invoke(genesisIncidents).toString()+"\",";
if(field.getType().toString().contains("ArrayList")){
System.out.println("ArrayListElement found");
genesisIncidents.getOpt_additionalAffectedItems().forEach(obj->{System.out.println(obj.getItemName());});
//convertArrayToJSON(field, genesisIncidents);
}
}catch(NullPointerException npe)
{
System.out.println("Null value in field.");
continue;
}
}
jsonString=jsonString.substring(0,jsonString.length()-1);
jsonString=jsonString+"\r\n }\r\n }";
System.out.println("\n"+jsonString);
}catch(Exception jex){
jex.printStackTrace();
}
}
My below code line is unable to access object stored under array list.
genesisIncidents.getOpt_additionalAffectedItems().forEach(obj->{System.out.println(obj.getItemName());});
OUTPUT
karaf#root>class java.lang.String
class java.lang.String
class java.lang.String
class java.util.ArrayList
ArrayListElement found
null
null
{
"GenesisIncidents": {
"service": "Transmission",
"affectedCI": "22BT_ORNC03",
"opt_additionalAffectedItems": " [org.apache.servicemix.examples.camel.rest.model.AdditionalAffectedItems#5881a 895, org.apache.servicemix.examples.camel.rest.model.AdditionalAffectedItems#399b4e eb]"
}
}

I have fiddled around with your example I have managed to get it working. This will produce the correct JSON string by passing in an instance of a GenesisIncident object. I guess that there is much room for improvement here but this can serve as an example.
public static String genesisToJson(GenesisIncidents incidents) {
try{
StringBuilder jsonBuilder = new StringBuilder();
jsonBuilder.append("{\r\n \"")
.append(incidents.getClass().getSimpleName())
.append("\": {");
Field allFields[] = incidents.getClass().getDeclaredFields();
for (Field field : allFields) {
String getter = getGetterMethod(field);
Method method = incidents.getClass().getMethod(getter, null);
try{
if(field.getType().isAssignableFrom(Integer.class)) {
jsonBuilder.append(preStr).append(field.getName()).append(postStr)
.append(method.invoke(incidents).toString()).append(",");
} else if (field.getType().isAssignableFrom(String.class)) {
jsonBuilder.append(preStr).append(field.getName()).append(postStr).append("\"")
.append(method.invoke(incidents).toString()).append("\",");
} else if (field.getType().isAssignableFrom(List.class)) {
System.out.println("ArrayListElement found");
getInnerObjectToJson(field, incidents.getOptItems(), jsonBuilder);
}
} catch(NullPointerException npe) {
System.out.println("Null value in field.");
continue;
}
}
jsonBuilder.append("\r\n } \r\n }");
return jsonBuilder.toString();
}catch(Exception jex){
jex.printStackTrace();
}
return null;
}
private static void getInnerObjectToJson(Field field, List<AdditionalAffectedItems> items, StringBuilder builder)
throws NoSuchMethodException, InvocationTargetException, IllegalAccessException {
builder.append(preStr).append(field.getName()).append(postStr).append("[");
for (var item : items) {
var fields = List.of(item.getClass().getDeclaredFields());
builder.append("{");
for (var f : fields) {
String getter = getGetterMethod(f);
Method method = item.getClass().getMethod(getter, null);
builder.append(preStr).append(f.getName()).append(postStr).append("\"")
.append(method.invoke(item).toString()).append("\"");
if (!(fields.indexOf(f) == (fields.size() - 1))) {
builder.append(",");
}
}
if (items.indexOf(item) == (items.size() - 1)) {
builder.append("}\r\n");
} else {
builder.append("},\r\n");
}
}
builder.append("]");
}
private static String getGetterMethod(Field field) {
return "get" + StringUtils.capitalize(field.getName());
}

Get User online/offline state

I have chat app where i need to develop user online state and for this i am calling API in every 15 seconds on server, which will return all logged in users and their online state in 0(Offline) and 1(Online).
I need to show whether user is online or offline (leaves app not logged out) while chatting.I have 1 array list which shows when app is launch with all logged in user id and their details including their online state and i created second API which return users online state in JSON.I have following option to achieve users online state
Replace existing array list item:-I am getting User ID and their online state in JSON but i need to run loop in every 15 second which replace values in existing arraylist
Store JSON in someway where i can easily find user id and its state:- If i store JSON in array list i need to run loop to find ID and its state which i dont want to,So which is best way to store JSON so i can easily get user state by its User ID.
Here is how i am getting JSON
protected List<WrapperClass> doInBackground(Void... params) {
userSession=new UserSession(context,"Elaxer");
UserState_Update=new ArrayList<>();
String data = null;
try {
String ID=userSession.getUserID(); //Getting Value from shared pref
data = URLEncoder.encode("User_ID", "UTF-8") + "=" + URLEncoder.encode(ID, "UTF-8");
Log.d(TAG,"Login ID "+ ID);
Log.d(TAG,"DO IN BACKGROUND START ");
URL url=new URL(URL_Path_NearBy);
connection= (HttpURLConnection) url.openConnection();
Log.d(TAG,connection.toString());
connection.setRequestMethod("POST");
connection.setDoInput(true);
connection.setDoOutput(true);
//For POST Only - Begin
OutputStream os = connection.getOutputStream();
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(os, "UTF-8"));
writer.write(data);
writer.flush();
writer.close();
os.close();
connection.connect();
InputStream inputStream=connection.getInputStream();
reader=new BufferedReader(new InputStreamReader(inputStream));
Log.d(TAG,"GET INPUT STREAM AND PUUTING INTO READER");
String line;
StringBuffer stringBuffer=new StringBuffer();
while ((line=reader.readLine())!=null){
stringBuffer.append(line);
}
String completeJSON=stringBuffer.toString();
Log.d(TAG,"JSON ARRAY START");
JSONObject parentArray=new JSONObject(completeJSON);
JSONArray jsonArray=parentArray.getJSONArray("uData");
String LastSeen;
int LoginStatus,User_ID;
int Rec_Online_Status;
for (int i = 0; i <jsonArray.length() ; i++) {
JSONObject childObject=jsonArray.getJSONObject(i);
LastSeen=childObject.getString("lastseen") ;
LoginStatus=childObject.getInt("login_status") ;
User_ID=childObject.getInt("User_ID");
String UseID= String.valueOf(User_ID);
Log.d(TAG,"JSON Values "+LastSeen+" "+LoginStatus+" "+User_ID);
WrapperClass wrapperClass=new WrapperClass(UseID,LoginStatus);
UserState_Update.add(wrapperClass);
}
return UserState_Update; //List<WrapperClass> UserState_Update
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
} catch (ProtocolException e) {
e.printStackTrace();
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (JSONException e) {
e.printStackTrace();
}
return null;
}
This is the response as JSON
{
"status": "SUCCESS",
"uData": [
{
"User_ID": "4",
"login_status": "1",
"lastseen": "0000-00-00 00:00:00"
},
{
"User_ID": "1",
"login_status": "0",
"lastseen": "0000-00-00 00:00:00"
},
{
"User_ID": "12",
"login_status": "1",
"lastseen": "0000-00-00 00:00:00"
},
{
"User_ID": "33",
"login_status": "0",
"lastseen": "0000-00-00 00:00:00"
}
]
}
Is this right way to get user online state? (I know FCM is right way but FCM currently not ready on app server side)
UPDATE 2:-As code recommended by #XngPro i implement on doinBackground
Map<String,Online_Status_Wrapper.User> map=new HashMap<>();
Online_Status_Wrapper wrapper=gson.fromJson(completeJSON,Online_Status_Wrapper.class);
Log.d(TAG,"Wrapper Get Data value "+wrapper.getuData());
Log.d(TAG,"Wrapper Get Status value "+wrapper.getStatus());
for (Online_Status_Wrapper.User u: wrapper.getuData()){
map.put(u.getUser_ID(),u);
}
Log.d(TAG,"State of Other User users "+map.get(12).getLogin_status());//HERE I AM GETTING NullPointerException
return map;

I think you should use a Java serialization/deserialization library like Gson.
Hope to help you~
Example
private static void bar() {
String jsonStr = "{\"status\":\"SUCCESS\",\"uData\":[{\"User_ID\":\"4\",\"login_status\":\"1\",\"lastseen\":\"0000-00-00 00:00:00\"},{\"User_ID\":\"1\",\"login_status\":\"0\",\"lastseen\":\"0000-00-00 00:00:00\"},{\"User_ID\":\"12\",\"login_status\":\"1\",\"lastseen\":\"0000-00-00 00:00:00\"},{\"User_ID\":\"33\",\"login_status\":\"0\",\"lastseen\":\"0000-00-00 00:00:00\"}]}";
Gson gson = new Gson();
UserFoo userFoo = gson.fromJson(jsonStr, UserFoo.class);
Map<String, UserFoo.User> map = new HashMap<>();
for (UserFoo.User u : userFoo.getUData()) {
map.put(u.getUser_ID(), u);
}
System.out.println("userId: 12, loginState: " + map.get("12").getLogin_status());
}
public static class UserFoo {
/**
* status : SUCCESS
* uData : [{"User_ID":"4","login_status":"1","lastseen":"0000-00-00 00:00:00"},{"User_ID":"1","login_status":"0","lastseen":"0000-00-00 00:00:00"},{"User_ID":"12","login_status":"1","lastseen":"0000-00-00 00:00:00"},{"User_ID":"33","login_status":"0","lastseen":"0000-00-00 00:00:00"}]
*/
private String status;
private List<User> uData;
public String getStatus() {
return status;
}
public void setStatus(String status) {
this.status = status;
}
public List<User> getUData() {
return uData;
}
public void setUData(List<User> uData) {
this.uData = uData;
}
public static class User {
/**
* User_ID : 4
* login_status : 1
* lastseen : 0000-00-00 00:00:00
*/
private String User_ID;
private String login_status;
private String lastseen;
public String getUser_ID() {
return User_ID;
}
public void setUser_ID(String User_ID) {
this.User_ID = User_ID;
}
public String getLogin_status() {
return login_status;
}
public void setLogin_status(String login_status) {
this.login_status = login_status;
}
public String getLastseen() {
return lastseen;
}
public void setLastseen(String lastseen) {
this.lastseen = lastseen;
}
}
}

public class Test {
public static void main(String[] args) {
bar();
}
private static void bar() {
String jsonStr = "{\"status\":\"SUCCESS\",\"uData\":[{\"User_ID\":\"4\",\"login_status\":\"1\",\"lastseen\":\"0000-00-00 00:00:00\"},{\"User_ID\":\"1\",\"login_status\":\"0\",\"lastseen\":\"0000-00-00 00:00:00\"},{\"User_ID\":\"12\",\"login_status\":\"1\",\"lastseen\":\"0000-00-00 00:00:00\"},{\"User_ID\":\"33\",\"login_status\":\"0\",\"lastseen\":\"0000-00-00 00:00:00\"}]}";
Gson gson = new Gson();
UserFoo userFoo = gson.fromJson(jsonStr, UserFoo.class);
Map<String, UserFoo.User> map = new HashMap<>();
for (UserFoo.User u : userFoo.getUData()) {
map.put(u.getUser_ID(), u);
}
System.out.println("userId: " + "12, loginState: " + map.get("12").getLogin_status());
}
public static class UserFoo {
/**
* status : SUCCESS
* uData : [{"User_ID":"4","login_status":"1","lastseen":"0000-00-00 00:00:00"},{"User_ID":"1","login_status":"0","lastseen":"0000-00-00 00:00:00"},{"User_ID":"12","login_status":"1","lastseen":"0000-00-00 00:00:00"},{"User_ID":"33","login_status":"0","lastseen":"0000-00-00 00:00:00"}]
*/
private String status;
private List<User> uData;
public String getStatus() {
return status;
}
public void setStatus(String status) {
this.status = status;
}
public List<User> getUData() {
return uData;
}
public void setUData(List<User> uData) {
this.uData = uData;
}
public static class User {
/**
* User_ID : 4
* login_status : 1
* lastseen : 0000-00-00 00:00:00
*/
private String User_ID;
private String login_status;
private String lastseen;
public String getUser_ID() {
return User_ID;
}
public void setUser_ID(String User_ID) {
this.User_ID = User_ID;
}
public String getLogin_status() {
return login_status;
}
public void setLogin_status(String login_status) {
this.login_status = login_status;
}
public String getLastseen() {
return lastseen;
}
public void setLastseen(String lastseen) {
this.lastseen = lastseen;
}
}
}
}
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