Similar to Jackson Streaming API, Google GSON too provides streaming API for the situation where performance is utmost important. Gson Streaming API is based on Sequential read/write and is very useful in situations where it is not desirable to load complete object model in memory(e.g complete data is not available). This is commonly known as token by token approach. You write as you read and your memory footprints remains low.

JsonWriter & JsonReader are core classes used for Streaming Write and Read in Streaming API.

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Step 1: Include GSON dependency in pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
 <modelVersion>4.0.0</modelVersion>

 <groupId>com.websystique.json</groupId>
 <artifactId>GsonStreamingExample</artifactId>
 <version>1.0.0</version>
 <packaging>jar</packaging>

 <name>GsonStreamingExample</name>

 <dependencies>
  <dependency>
   <groupId>com.google.code.gson</groupId>
   <artifactId>gson</artifactId>
   <version>2.3.1</version>
  </dependency>
 </dependencies>
</project>

Step 3: Write JSON using JsonWriter

Write the JSON content in a file. Basically in this approach, you build the JSON string structure from java object by yourself. You create the required opening/closing boundaries/brackets {}[] for elements(objects/arrays) using JsonWriter beginObject/endObject & beginArray/endArray methods. And you set the actual elements using name , their values using value methods.

package com.websystique.json.gson;

import java.io.FileReader;
import java.io.FileWriter;

import com.google.gson.stream.JsonWriter;

public class GsonStreamingDemo {

 public static void main(String args[]) {

  JsonWriter writer;
  try {
   writer = new JsonWriter(new FileWriter("result.json"));

   writer.beginObject();       // {
   writer.name("name").value("AUDI");    //  "name" : "AUDI"
   writer.name("model").value(2014);    //   "model" : 2014
   writer.name("price").value(30000);    //   "price" : 30000

   writer.name("colors");       //   "colors" :
   writer.beginArray();       //   [
   writer.value("GRAY");       //    "GRAY"
   writer.value("BLACK");       //    "BLACK"
   writer.value("WHITE");       //    "WHITE"
   writer.endArray();        //   ]

   writer.endObject();       // }
   writer.close();

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

Following is the content of result.json

{"name":"AUDI","model":2014,"price":30000,"colors":["GRAY","BLACK","WHITE"]}

Step 4: Read JSON using JsonReader

Read the JSON content from a file. Basically you iterate over the input json string and read each token subsequently using JsonReader hasNext & nextName methods. Depending on contract/element type, you get the actual element’s value using nextXXX().
As for writing, using JsonReader beginObject/endObject & beginArray/endArray methods to read opening/closing brackets.
We will use the output file generated from above.

package com.websystique.json.gson;

import java.io.FileReader;
import java.io.FileWriter;

import com.google.gson.stream.JsonReader;

public class GsonStreamingDemo {

 public static void main(String args[]) {

  JsonReader reader;
  try {
   reader = new JsonReader(new FileReader("result.json"));
   reader.beginObject();

   while (reader.hasNext()) {
    String name = reader.nextName();

    if (name.equals("name")) {
     System.out.println(reader.nextString());
    } else if (name.equals("model")) {
     System.out.println(reader.nextInt());
    } else if (name.equals("price")) {
     System.out.println(reader.nextDouble());
    } else if (name.equals("colors")) {

     // it's an array.
     reader.beginArray();
     while (reader.hasNext()) {
      System.out.println(reader.nextString());
     }
     reader.endArray();
    } else {// unexpected value, skip it or generate error
     reader.skipValue();
    }
   }

   reader.endObject();
   reader.close();
  } catch (Exception e) {
   e.printStackTrace();
  }
 }
}

Following is the output from above main.

AUDI
2014
30000.0
GRAY
BLACK
WHITE

That’s it. In the next post we will learn about custom serialization using Google GSON serialization API.

References

• Google Gson official Reference
• Json Official Reference