Append text field data to an existing JSON file in java

Question

I have a text field where user can enter data, once data is received i want to append it to existing JSON file.

I am able to read the existing data and getting the text field value, but while appending the new data with existing data I'm facing problem.

Error:

com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 18 path $

Below code :

JSON File :

{"vins":[{"vin":"544554"},{"vin":"54554"}]}

Text field value : String test ="3689"; so it should be appended as :

{"vins":[{"vin":"544554"},{"vin":"54554"},{"vin":"3689"}]}

Filereadwrite class :

public class JSONFIlewrite {
    public static String Vinno_Read;
    public static List<String> linststring;
    public static void main(String[] args) {
        try {

            JSONParser parser = new JSONParser();
            Object obj = parser.parse(new 
            FileReader("C:\\Amaresh\\Test\\sample_json.json"));

            JSONObject jsonObject = (JSONObject) obj;
            System.out.println(jsonObject);
            linststring= new ArrayList();
            // loop array
            JSONArray msg = (JSONArray) jsonObject.get("vins");
            Iterator iterator = msg.iterator();

            while (iterator.hasNext()) {
                Vinno_Read = iterator.next().toString();

                linststring.add(Vinno_Read);


            }

            String list_string = "";
            System.out.println(linststring);

            for(String temp:linststring){
                list_string += temp;
                System.out.println("amar1"+list_string);
            }

            System.out.println("amar"+list_string);
            Vin vin4 = new Vin();
            vin4.setVin("76354273462");
            Vins vins = new Vins();
            vins.addVins(vin4);

            Gson gson = new Gson();
            //String jsonValue=list_string;

            String jsonValue = gson.toJson(list_string).toString();
            System.out.println("json--"+jsonValue);
            Vins vins1 = gson.fromJson(jsonValue, Vins.class);
            System.out.println("ddd"+vins1);
            Vin vin = new Vin();
            vin.setVin("544554");

            vins1.addVins(vin);

            jsonValue = gson.toJson(vins1).toString();

            BufferedWriter writer = null;
            try {
                writer = new BufferedWriter(new FileWriter("C:\\Amaresh\\Test\\sample_json.json"));
                writer.write(jsonValue);
                System.out.println("Test"+jsonValue);

            } catch (IOException e) {
            } finally {
              try {
                if (writer != null)
                    writer.close();
              } catch (IOException e) {
            }
        }

        } catch (Exception e) {
            e.printStackTrace();
        }

    }

}

Setter Getter classes :

public class Vin {
    @Expose
    private String vin;

    public String getVin() {
        return vin;
    }

public void setVin(String vin) {
    this.vin = vin;
}

}


public class Vins {
    @Expose
    List<Vin> vins = new ArrayList<>();

    public List<Vin> getVins() {
        return vins;
    }

    public void addVins(Vin vin) {
        this.vins.add(vin);
    }
}

if i comment line //String jsonValue = gson.toJson(list_string).toString(); and pass this value String jsonValue=list_string; which i am reading from file error i am getting below error {"vins":[{"vin":"544554"},{"vin":"54554"}]} [{"vin":"544554"}, {"vin":"54554"}] amar1{"vin":"544554"} amar1{"vin":"544554"}{"vin":"54554"} amar{"vin":"544554"}{"vin":"54554"} json--{"vin":"544554"}{"vin":"54554"} com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Use JsonReader.setLenient(true) to accept malformed JSON at line 1 column 18 path $ — user8113638
– user8113638, Commented Jun 9, 2017 at 18:42
i am not getting how the new text field value i will append to existing data than write. Please help. — user8113638
– user8113638, Commented Jun 9, 2017 at 18:46
what library are you using ? since JSONParser is actually defined as JsonParser in GSON library google.github.io/gson/apidocs/com/google/gson/JsonParser.html — emecas
– emecas, Commented Jun 9, 2017 at 19:11

emecas · Accepted Answer · 2017-06-09 21:56:30Z

1

Main Logic:

you can see 4 blocks in the code

Reading the json file and parsing to a java Object
Casting de java Object to a JSonObject, parsing to a JsonArray and iterating the array printing the JsonElements
Creating a new Vin Object and converting it to a JSON String using Gson.toJson method (2nd part is not required only illustrative purposes)
Creating a JsonWriter, creating a Vins Object and loading it with the original JsonArray and then adding a new element (that correspondents to the new Vin Object created in step #3, finally writing the Vins Object to the [new] file.

Input:

{"vins":[{"vin":"544554"},{"vin":"54554"}]}

Output:

{"vins":[{"vin":"544554"},{"vin":"54554"},{"vin":"3689"}]}

Code

import java.io.FileReader;
import java.io.FileWriter;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import com.google.gson.Gson;
import com.google.gson.JsonArray;
import com.google.gson.JsonElement;
import com.google.gson.JsonObject;
import com.google.gson.JsonParser;
import com.google.gson.stream.JsonWriter;

public class JSONFIlewrite {
    public static String Vinno_Read;
    public static List<String> linststring;

    public static void main(String[] args) {
        try {

            JsonParser parser = new JsonParser();
            Object obj = parser.parse(new FileReader("C:\\Amaresh\\Test\\sample_json.json"));



            JsonObject jsonObject = (JsonObject) obj;
            System.out.println(jsonObject);
            linststring = new ArrayList<String>();
            // loop array
            JsonArray msg = (JsonArray) jsonObject.get("vins");
            Iterator<JsonElement> iterator = msg.iterator();
            while (iterator.hasNext()) {
                Vinno_Read = iterator.next().toString();
                System.out.println("Vinno_Read---->" + Vinno_Read);
            }



            Vin newVin = new Vin();
            newVin.setVin("3689");
            Gson gson = new Gson();
            String json = gson.toJson(newVin);
            System.out.println("json---->" + json);



            FileWriter file = new FileWriter("C:\\Amaresh\\Test\\sample_json2.json", false);
            JsonWriter jw = new JsonWriter(file);
            iterator = msg.iterator();
            Vins vins = new Vins();
            while (iterator.hasNext()) {
                vins.addVin(gson.fromJson(iterator.next().toString(), Vin.class));
            }
            vins.addVin(newVin);
            gson.toJson(vins, Vins.class, jw);
            file.close();

        } catch (Exception e) {
            e.printStackTrace();
        }

    }
}

Notes:

Since I don't know what library you are using, I have updated the class names to be compatible to GSON.

I have also changed the method: public void addVins(Vin vin) in Vins Class to public void addVin(Vin vin)

answered Jun 9, 2017 at 21:56

emecas

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3 Comments

user8113638 Over a year ago

Thanks a lot, Its working. I have one question if user enters a text data which is already present say Vin : 3689 already exist than i don't want to write duplicate entries to file how to achieve it.

emecas Over a year ago

For that, in the 4 step, you need to ask IF the array CONTAINS already the new Vin , IF it does then skipping it ELSE then go ahead and adding the new Vin. So, now is your turn for coding it :)

emecas Over a year ago

I meant the ArrayList inside the Vins object.

Eric Aya · Accepted Answer · 2018-09-04 08:27:03Z

1

To keep the existing content and append the new content to the end of JSON file:

Example1:

new FileWriter(file,true);

or you can try example02:

FileWriter file= new FileWriter(JSONLPATH,true)

edited Sep 4, 2018 at 8:27

Eric Aya

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answered Sep 4, 2018 at 3:50

RANA DINESH

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