Gson – Read and write JSON as a stream

By mkyong | Updated: April 23, 2019

Viewed: 6,367 (+86 pv/w)

Tags: gson json streaming

Since Gson version 1.6, two new classes JsonReader and JsonWriter are introduced to provide streaming processing on JSON data. Read this Gson streaming documentation to understand what are the benefits of using it.

• JsonWriter – Write JSON as a stream.
• JsonReader – Read JSON as a stream.

1. JsonWriter

package com.mkyong;

import com.google.gson.stream.JsonWriter;

import java.io.FileWriter;
import java.io.IOException;

public class GsonExample1 {

    public static void main(String[] args) {

        try (JsonWriter writer = new JsonWriter(new FileWriter("c:\\projects\\user.json"))) {

            writer.beginObject();                   // {
            writer.name("name").value("mkyong");    // "name" : "mkyong"
            writer.name("age").value(29);           // "age" : 29

            writer.name("messages");                // "messages" :
            writer.beginArray();                    // [
            writer.value("msg 1");                  // "msg 1"
            writer.value("msg 2");                  // "msg 2"
            writer.value("msg 3");                  // "msg 3"
            writer.endArray();                      // ]

            writer.endObject();                     // }

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

    }

}

Output

{"name":"mkyong","age":29,"messages":["msg 1","msg 2","msg 3"]}

2. JsonReader

Token
In streaming mode, every JSON data consider as an individual token. When you use JsonReader to process it, each token will be processed sequentially. For example,

{
	"url":"www.mkyong.com"
}

• Token 1 = {
• Token 2 = url
• Token 3 = www.mkyong.com
• Token 4 = }

package com.mkyong;

import com.google.gson.stream.JsonReader;

import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;

public class GsonExample2 {

    public static void main(String[] args) {

        try (JsonReader reader = new JsonReader(new FileReader("c:\\projects\\user.json"))) {

            reader.beginObject();

            while (reader.hasNext()) {

                String name = reader.nextName();

                if (name.equals("name")) {

                    System.out.println(reader.nextString());

                } else if (name.equals("age")) {

                    System.out.println(reader.nextInt());

                } else if (name.equals("messages")) {

                    // read array
                    reader.beginArray();

                    while (reader.hasNext()) {
                        System.out.println(reader.nextString());
                    }

                    reader.endArray();

                } else {
                    reader.skipValue(); //avoid some unhandle events
                }
            }

            reader.endObject();

        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }
}

Output

mkyong
29
msg 1
msg 2
msg 3

References

• Gson Streaming examples
• Jackson Streaming examples
• Gson – How to parse JSON