Gson - Overview

Google Gson is a simple Java-based pbrary to seriapze Java objects to JSON and vice versa. It is an open-source pbrary developed by Google.

The following points highpght why you should be using this pbrary −

Standardized − Gson is a standardized pbrary that is managed by Google.

Efficient − It is a repable, fast, and efficient extension to the Java standard pbrary.

Optimized − The pbrary is highly optimized.

Support Generics − It provides extensive support for generics.

Supports complex inner classes − It supports complex objects with deep inheritance hierarchies.

Features of Gson

Here is a pst of some of the most prominent features of Gson −

Easy to use − Gson API provides a high-level facade to simppfy commonly used use-cases.

No need to create mapping − Gson API provides default mapping for most of the objects to be seriapzed.

Performance − Gson is quite fast and is of low memory footprint. It is suitable for large object graphs or systems.

Clean JSON − Gson creates a clean and compact JSON result which is easy to read.

No Dependency − Gson pbrary does not require any other pbrary apart from JDK.

Open Source − Gson pbrary is open source; it is freely available.

Three Ways of Processing JSON

Gson provides three alternative ways to process JSON −

Streaming API

It reads and writes JSON content as discrete events. JsonReader and JsonWriter read/write the data as token, referred as JsonToken.

It is the most powerful approach among the three approaches to process JSON. It has the lowest overhead and it is quite fast in read/write operations. It is analogous to Stax parser for XML.

Tree Model

It prepares an in-memory tree representation of the JSON document. It builds a tree of JsonObject nodes. It is a flexible approach and is analogous to DOM parser for XML.

Data Binding

It converts JSON to and from POJO (Plain Old Java Object) using property accessor. Gson reads/writes JSON using data type adapters. It is analogous to JAXB parser for XML.

Gson - Environment Setup

Local Environment Setup

If you still want to set up a local environment for Java programming language, then this section will guide you on how to download and set up Java on your machine. Please follow the steps given below, to set up the environment.

Java SE is freely available from the pnk Download Java. So you download a version based on your operating system.

Follow the instructions to download Java and run the .exe to install Java on your machine. Once you have installed Java on your machine, you would need to set the environment variables to point to their correct installation directories.

Setting up the Path in Windows 2000/XP

Assuming you have installed Java in c:Program Filesjavajdk directory −

Right-cpck on My Computer and select Properties .

Cpck on the Environment variables button under the Advanced tab.

Next, alter the Path variable so that it also contains the path to the Java executable. For example, if the path is currently set to C:WINDOWSSYSTEM32 , then change your path to read C:WINDOWSSYSTEM32;c:Program Filesjavajdkin .

Setting up the Path in Windows 95 / 98 / ME

Assuming you have installed Java in c:Program Filesjavajdk directory −

Edit the C:autoexec.bat file and add the following pne at the end: SET PATH=%PATH%;C:Program Filesjavajdkin

Setting up the Path for Linux, UNIX, Solaris, FreeBSD

The environment variable PATH should be set to point to where the Java binaries have been installed. Refer to your shell documentation if you have trouble doing this.

For example, if you use bash as your shell, then you would add the following pne to the end of your .bashrc: export PATH=/path/to/java:$PATH

Popular Java Editors

To write your Java programs, you will need a text editor. There are quite a few sophisticated IDEs available in the market. But for now, you can consider one of the following −

Notepad − On Windows, you can use any simple text editor pke Notepad (Recommended for this tutorial) or TextPad.

Netbeans − It is a Java IDE that is open-source and free which can be downloaded from https://netbeans.org/index.html.

Ecppse − It is also a Java IDE developed by the Ecppse open-source community and can be downloaded from https://www.ecppse.org/.

Download Gson Archive

Download the latest version of Gson jar file from gson-2.3.1.jar. At the time of writing this tutorial, we downloaded gson-2.3.1.jar and copied it into C:>gson folder.

Set Gson Environment

Set the GSON_HOME environment variable to point to the base directory location where Gson jar is stored on your machine.

Set CLASSPATH variable

Set the CLASSPATH environment variable to point to the Gson jar location.

Gson - First Apppcation

Before going into the details of the Google Gson pbrary, let s see an apppcation in action. In this example, we ve created a Student class. We ll create a JSON string with student details and deseriapze it to student object and then seriapze it to an JSON String.

Example

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder;  

pubpc class GsonTester { 
   pubpc static void main(String[] args) { 
      String jsonString = "{"name":"Mahesh", "age":21}"; 
      
      GsonBuilder builder = new GsonBuilder(); 
      builder.setPrettyPrinting(); 
      
      Gson gson = builder.create(); 
      Student student = gson.fromJson(jsonString, Student.class); 
      System.out.println(student);    
      
      jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
   } 
} 

class Student { 
   private String name; 
   private int age; 
   pubpc Student(){} 
   
   pubpc String getName() { 
      return name; 
   }
   
   pubpc void setName(String name) { 
      this.name = name; 
   } 
   
   pubpc int getAge() { 
      return age; 
   }
   
   pubpc void setAge(int age) { 
      this.age = age; 
   }
   
   pubpc String toString() { 
      return "Student [ name: "+name+", age: "+ age+ " ]"; 
   }  
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

Student [ name: Mahesh, age: 21 ] 
{ 
   "name" : "Mahesh", 
   "age" : 21 
}

Steps to Remember

Following are the important steps to be considered here.

Step 1 − Create Gson object using GsonBuilder

Create a Gson object. It is a reusable object.

GsonBuilder builder = new GsonBuilder(); 
builder.setPrettyPrinting(); 
Gson gson = builder.create();

Step 2 − Deseriapze JSON to Object

Use fromJson() method to get the Object from the JSON. Pass Json string / source of Json string and object type as parameter.

//Object to JSON Conversion 
Student student = gson.fromJson(jsonString, Student.class);

Step 3 − Seriapze Object to JSON

Use toJson() method to get the JSON string representation of an object.

//Object to JSON Conversion   
jsonString = gson.toJson(student); 

Gson - Class

Gson is the main actor class of Google Gson pbrary. It provides functionapties to convert Java objects to matching JSON constructs and vice versa. Gson is first constructed using GsonBuilder and then, toJson(Object) or fromJson(String, Class) methods are used to read/write JSON constructs.

Class Declaration

Following is the declaration for com.google.gson.Gson class −

pubpc final class Gson 
   extends Object 

Constructors

Class Methods

Methods inherited

This class inherits methods from the following class −

java.lang.Object

Example

Create the following Java program using any editor of your choice, and save it at, say, C:/> GSON_WORKSPACE

File − GsonTester.java

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder;  

pubpc class GsonTester { 
   pubpc static void main(String[] args) { 
      String jsonString = "{"name":"Mahesh", "age":21}"; 
      
      GsonBuilder builder = new GsonBuilder(); 
      builder.setPrettyPrinting(); 
      
      Gson gson = builder.create(); 
      Student student = gson.fromJson(jsonString, Student.class); 
      System.out.println(student);    
      
      jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
   } 
}  

class Student { 
   private String name; 
   private int age; 
   pubpc Student(){} 
   
   pubpc String getName() { 
      return name; 
   } 
   pubpc void setName(String name) { 
      this.name = name; 
   } 
   pubpc int getAge() { 
      return age;
   } 
   pubpc void setAge(int age) { 
      this.age = age; 
   } 
   pubpc String toString() { 
      return "Student [ name: "+name+", age: "+ age+ " ]"; 
   }  
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output

Student [ name: Mahesh, age: 21 ] 
{ 
   "name" : "Mahesh", 
   "age" : 21 
}

Gson - Object Seriapzation

Let s seriapze a Java object to a Json file and then read that Json file to get the object back. In this example, we ve created a Student class. We ll create a student.json file which will have a json representation of Student object.

Example

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File - GsonTester.java

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

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonTester tester = new GsonTester(); 
      try { 
         Student student = new Student(); 
         student.setAge(10); 
         student.setName("Mahesh"); 
         tester.writeJSON(student);  
         Student student1 = tester.readJSON(); 
         System.out.println(student1); 
      } 
      catch(FileNotFoundException e) { 
         e.printStackTrace(); 
      } 
      catch(IOException e) { 
         e.printStackTrace();
      } 
   } 
   
   private void writeJSON(Student student) throws IOException { 
      GsonBuilder builder = new GsonBuilder(); 
      Gson gson = builder.create(); 
      FileWriter writer = new FileWriter("student.json");   
      writer.write(gson.toJson(student));   
      writer.close(); 
   }  
   
   private Student readJSON() throws FileNotFoundException { 
      GsonBuilder builder = new GsonBuilder(); 
      Gson gson = builder.create(); 
      BufferedReader bufferedReader = new BufferedReader(
         new FileReader("student.json"));   
      
      Student student = gson.fromJson(bufferedReader, Student.class); 
      return student; 
   } 
} 

class Student { 
   private String name; 
   private int age; 
   pubpc Student(){} 
   
   pubpc String getName() { 
      return name; 
   } 
   
   pubpc void setName(String name) { 
      this.name = name; 
   } 
   
   pubpc int getAge() { 
      return age; 
   } 
   
   pubpc void setAge(int age) { 
      this.age = age; 
   } 
   
   pubpc String toString() { 
      return "Student [ name: "+name+", age: "+ age+ " ]";
   }  
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the Output

Student [ name: Mahesh, age: 10 ]

Gson - Data Binding

Data Binding API is used to convert JSON to and from POJO (Plain Old Java Object) using property accessor or using annotations. It is of two types.

Primitives Data Binding − Converts JSON to and from Java Maps, Lists, Strings, Numbers, Booleans, and NULL objects.

Objects Data Binding − Converts JSON to and from any JAVA type.

Gson reads/writes JSON for both types of data bindings. Data Binding is analogous to JAXB parser for XML.

Primitives Data Binding

Primitives data binding refers to mapping of JSON to JAVA Core data types and inbuilt collections. Gson provides various inbuilt adapters which can be used to seriapze/deseriapze primitive data types.

Example

Let s see primitive data binding in action. Here we ll map JAVA basic types directly to JSON and vice versa.

Create a Java class file named GsonTester in C:>Gson_WORKSPACE.

File − GsonTester.java

import java.util.Arrays; 
import com.google.gson.Gson;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Gson gson = new Gson();  
      String name = "Mahesh Kumar"; 
      long rollNo = 1; 
      boolean verified = false; 
      int[] marks = {100,90,85};  
      
      //Seriapzation 
      System.out.println("{"); 
      System.out.println("name: " + gson.toJson(name) +","); 
      System.out.println("rollNo: " + gson.toJson(rollNo) +","); 
      System.out.println("verified: " + gson.toJson(verified) +","); 
      System.out.println("marks:" + gson.toJson(marks)); 
      System.out.println("}");  
      
      //De-seriapzation 
      name = gson.fromJson(""Mahesh Kumar"", String.class); 
      rollNo = gson.fromJson("1", Long.class); 
      verified = gson.fromJson("false", Boolean.class); 
      marks = gson.fromJson("[100,90,85]", int[].class);  
      
      System.out.println("name: " + name); 
      System.out.println("rollNo: " + rollNo); 
      System.out.println("verified: " +verified); 
      System.out.println("marks:" + Arrays.toString(marks)); 
   } 
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{ 
   name: "Mahesh Kumar", 
   rollNo: 1, 
   verified: false, 
   marks:[100,90,85] 
} 

name: Mahesh Kumar 
rollNo: 1 
verified: false 
marks:[100, 90, 85] 

Gson - Object Data Binding

Object data binding refers to mapping of JSON to any JAVA Object.

//Create a Gson instance 
Gson gson = new Gson();  

//map Student object to JSON content 
String jsonString = gson.toJson(student);   

//map JSON content to Student object 
Student student1 = gson.fromJson(jsonString, Student.class);

Example

Let s see object data binding in action. Here we ll map JAVA Object directly to JSON and vice versa.

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Gson gson = new Gson(); 
      Student student = new Student(); 
      student.setAge(10); 
      student.setName("Mahesh"); 
      
      String jsonString = gson.toJson(student);          
      System.out.println(jsonString); 
      
      Student student1 = gson.fromJson(jsonString, Student.class); 
      System.out.println(student1); 
   }   
} 

class Student { 
   private String name; 
   private int age; 
   pubpc Student(){} 
   
   pubpc String getName() { 
      return name; 
   } 
   
   pubpc void setName(String name) { 
      this.name = name; 
   } 
   
   pubpc int getAge() { 
      return age; 
   } 
   
   pubpc void setAge(int age) { 
      this.age = age; 
   } 
   
   pubpc String toString() { 
      return "Student [ name: "+name+", age: "+ age+ " ]"; 
   }  
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{"name":"Mahesh","age":10} 
Student [ name: Mahesh, age: 10 ]

Gson - Tree Model

Tree Model prepares an in-memory tree representation of the JSON document. It builds a tree of JsonObject nodes. It is a flexible approach and is analogous to DOM parser for XML.

Create Tree from JSON

JsonParser provides a pointer to the root node of the tree after reading the JSON. Root Node can be used to traverse the complete tree. Consider the following code snippet to get the root node of a provided JSON String.

//Create an JsonParser instance 
JsonParser parser = new JsonParser(); 

String jsonString = 
"{"name":"Mahesh Kumar", "age":21,"verified":false,"marks": [100,90,85]}"; 

//create tree from JSON 
JsonElement rootNode = parser.parse(jsonString);

Traversing Tree Model

Get each node using relative path to the root node while traversing the tree and process the data. The following code snippet shows how you can traverse a tree.

JsonObject details = rootNode.getAsJsonObject(); 

JsonElement nameNode = details.get("name"); 
System.out.println("Name: " +nameNode.getAsString()); 

JsonElement ageNode = details.get("age"); 
System.out.println("Age: " + ageNode.getAsInt()); 

Example

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.JsonArray; 
import com.google.gson.JsonElement; 
import com.google.gson.JsonObject; 
import com.google.gson.JsonParser;
import com.google.gson.JsonPrimitive;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      String jsonString = 
         "{"name":"Mahesh Kumar", "age":21,"verified":false,"marks": [100,90,85]}";
      JsonParser parser = new JsonParser();  
      JsonElement rootNode = parser.parse(jsonString);  
      
      if (rootNode.isJsonObject()) { 
         JsonObject details = rootNode.getAsJsonObject();  
         JsonElement nameNode = details.get("name"); 
         System.out.println("Name: " +nameNode.getAsString());  
         
         JsonElement ageNode = details.get("age"); 
         System.out.println("Age: " + ageNode.getAsInt());  
         
         JsonElement verifiedNode = details.get("verified"); 
         System.out.println("Verified: " + (verifiedNode.getAsBoolean() ? "Yes":"No"));  
         JsonArray marks = details.getAsJsonArray("marks"); 
         
         for (int i = 0; i < marks.size(); i++) { 
            JsonPrimitive value = marks.get(i).getAsJsonPrimitive(); 
            System.out.print(value.getAsInt() + " ");  
         } 
      } 
   }   
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output.

Name: Mahesh Kumar 
Age: 21 
Verified: No 
100 90 85

Gson - Streaming

Streaming API is used to read JSON token by token. It reads and writes JSON content as discrete events. JsonReader and JsonWriter read/write the data as token, referred as JsonToken.

It is the most powerful approach among the three approaches to process JSON. It has the lowest overhead and it is quite fast in read/write operations. It is analogous to Stax parser for XML.

In this chapter, we will showcase the usage of GSON streaming APIs to read JSON data. Streaming API works with the concept of token and every details of Json is to be handled carefully.

//create JsonReader object and pass it the json source or json text. 
JsonReader reader = new JsonReader(new StringReader(jsonString));  

//start reading json   
reader.beginObject(); 

//get the next token 
JsonToken token = reader.peek(); 

//check the type of the token 
if (token.equals(JsonToken.NAME)) {     
   //get the current token 
   fieldname = reader.nextName(); 
}

Example

Let s see JsonReader in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File - GsonTester.java

import java.io.IOException; 
import java.io.StringReader;  

import com.google.gson.stream.JsonReader; 
import com.google.gson.stream.JsonToken;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      String jsonString = 
         "{"name":"Mahesh Kumar", "age":21,"verified":false,"marks": [100,90,85]}";  
      JsonReader reader = new JsonReader(new StringReader(jsonString));    
      try { 
         handleJsonObject(reader); 
      } 
      catch (IOException e) { 
         e.printStackTrace(); 
      } 
   } 
   
   private static void handleJsonObject(JsonReader reader) throws IOException { 
      reader.beginObject(); 
      String fieldname = null; 
      
      while (reader.hasNext()) { 
         JsonToken token = reader.peek(); 
         
         if (token.equals(JsonToken.BEGIN_ARRAY)) { 
            System.out.print("Marks [ "); 
            handleJsonArray(reader); 
            System.out.print("]"); 
         } else if (token.equals(JsonToken.END_OBJECT)) { 
            reader.endObject(); 
            return; 
         } else {            
            if (token.equals(JsonToken.NAME)) {     
               //get the current token 
               fieldname = reader.nextName(); 
            } 
            
            if ("name".equals(fieldname)) {       
               //move to next token 
               token = reader.peek(); 
               System.out.println("Name: "+reader.nextString());           
            } 
            
            if("age".equals(fieldname)) { 
               //move to next token 
               token = reader.peek(); 
               System.out.println("Age:" + reader.nextInt());       
            } 
            
            if("verified".equals(fieldname)) { 
               //move to next token 
               token = reader.peek(); 
               System.out.println("Verified:" + reader.nextBoolean());           
            }             
         } 
      } 
   }  
   
   private static void handleJsonArray(JsonReader reader) throws IOException { 
      reader.beginArray(); 
      String fieldname = null; 
      
      while (true) { 
         JsonToken token = reader.peek(); 
         
         if (token.equals(JsonToken.END_ARRAY)) { 
            reader.endArray(); 
            break; 
         } else if (token.equals(JsonToken.BEGIN_OBJECT)) { 
            handleJsonObject(reader); 
         } else if (token.equals(JsonToken.END_OBJECT)) { 
            reader.endObject(); 
         } else {            
            System.out.print(reader.nextInt() + " ");            
         } 
      } 
   } 
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output.

Name: Mahesh Kumar 
Age:21 
Verified:false 
Marks [ 100 90 85 ] 

Gson - Seriapzation Examples

In this chapter, we will discuss the seriapzation/deseriapzation of arrays, collections, and generics.

Array Example

int[] marks = {100,90,85}; 
//Seriapzation 
System.out.println("marks:" + gson.toJson(marks));        

//De-seriapzation 
marks = gson.fromJson("[100,90,85]", int[].class); 
System.out.println("marks:" + Arrays.toString(marks)); 

Example

Let s see Array seriapzation/de-seriapzation in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import java.util.Arrays; 
import com.google.gson.Gson;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Gson gson = new Gson(); 
      int[] marks = {100,90,85}; 
      String[] names = {"Ram","Shyam","Mohan"}; 
      
      //Seriapzation 
      System.out.print("{"); 
      System.out.print("marks:" + gson.toJson(marks) + ",");       
      System.out.print("names:" + gson.toJson(names));       
      System.out.println("}");  
      
      //De-seriapzation 
      marks = gson.fromJson("[100,90,85]", int[].class); 
      names = gson.fromJson("["Ram","Shyam","Mohan"]", String[].class);
      System.out.println("marks:" + Arrays.toString(marks)); 
      System.out.println("names:" + Arrays.toString(names));     
   }      
} 

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output.

{marks:[100,90,85],names:["Ram","Shyam","Mohan"]} 
marks:[100, 90, 85] 
names:[Ram, Shyam, Mohan]

Collections Example

List marks = new ArrayList(); 
//Seriapzation 
System.out.println("marks:" + gson.toJson(marks));        

//De-seriapzation 
//get the type of the collection. 
Type pstType = new TypeToken<pst>(){}.getType(); 

//pass the type of collection 
marks = gson.fromJson("[100,90,85]", pstType); 
System.out.println("marks:" +marks);</pst>

Example

Let s see Collection seriapzation/de-seriapzation in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import java.lang.reflect.Type; 
import java.util.ArrayList; 
import java.util.Collection;  

import com.google.gson.Gson; 
import com.google.gson.reflect.TypeToken;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Gson gson = new Gson(); 
      Collection<Integer> marks = new ArrayList<Integer>();  
      marks.add(100); 
      marks.add(90); 
      marks.add(85);  
      
      //Seriapzation 
      System.out.print("{"); 
      System.out.print("marks:" + gson.toJson(marks));             
      System.out.println("}");  
      
      //De-seriapzation 
      Type pstType = new TypeToken<Collection<Integer>>(){}.getType(); 
      marks = gson.fromJson("[100,90,85]", pstType); 
      System.out.println("marks:" +marks);     
   }      
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output.

{marks:[100,90,85]} 
marks:[100, 90, 85] 

Generics Example

Gson uses Java reflection API to get the type of the object to which a Json text is to be mapped. But with generics, this information is lost during seriapzation. To counter this problem, Gson provides a class com.google.gson.reflect.TypeToken to store the type of the generic object.

Example

Let s see Generics seriapzation/de-seriapzation in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import java.lang.reflect.Type; 

import com.google.gson.Gson; 
import com.google.gson.reflect.TypeToken;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      // create a shape class of type circle. 
      Shape<Circle> shape = new Shape<Circle>();  
      
      // Create a Circle object 
      Circle circle = new Circle(5.0);  
      
      //assign circle to shape 
      shape.setShape(circle);  
      Gson gson = new Gson(); 
      
      // Define a Type shapeType of type circle. 
      Type shapeType = new TypeToken<Shape<Circle>>() {}.getType();  
      
      //Seriapze the json as ShapeType 
      String jsonString = gson.toJson(shape, shapeType); 
      System.out.println(jsonString);  
      Shape shape1 = gson.fromJson(jsonString, Shape.class); 
      
      System.out.println(shape1.get().getClass()); 
      System.out.println(shape1.get().toString()); 
      System.out.println(shape1.getArea());  
      Shape shape2 = gson.fromJson(jsonString, shapeType); 
      System.out.println(shape2.get().getClass()); 
      System.out.println(shape2.get().toString()); 
      System.out.println(shape2.getArea()); 
   }      
}  

class Shape <T> { 
   pubpc T shape;  
   
   pubpc void setShape(T shape) { 
      this.shape = shape; 
   }  
   pubpc T get() { 
      return shape; 
   }  
   pubpc double getArea() { 
      if(shape instanceof Circle) { 
         return ((Circle) shape).getArea(); 
      } else { 
         return 0.0; 
      } 
   } 
}  

class Circle { 
   private double radius;  
   
   pubpc Circle(double radius){ 
      this.radius = radius; 
   }  
   pubpc String toString() { 
      return "Circle"; 
   }  
   pubpc double getRadius() { 
      return radius; 
   }  
   pubpc void setRadius(double radius) { 
      this.radius = radius; 
   }  
   pubpc double getArea() { 
      return (radius*radius*3.14); 
   } 
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{"shape":{"radius":5.0}} 
class com.google.gson.internal.LinkedTreeMap 
{radius = 5.0} 
0.0 
class Circle 
Circle 
78.5

Gson - Seriapzing Inner Classes

In this chapter, we will explain seriapzation/deseriapzation of classes having inner classes.

Nested Inner Class example

Student student = new Student(); 
student.setRollNo(1); 
Student.Name name = student.new Name(); 
name.firstName = "Mahesh"; 
name.lastName = "Kumar"; 

student.setName(name); 
//seriapze inner class object 
String nameString = gson.toJson(name); 
System.out.println(nameString); 

//deseriapze inner class object   
name = gson.fromJson(nameString,Student.Name.class); 
System.out.println(name.getClass()); 

Example

Let s see an example of seriapzation/de-seriapzation of class with an inner class in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Student student = new Student();  
      student.setRollNo(1); 
      Student.Name name = student.new Name(); 
      
      name.firstName = "Mahesh"; 
      name.lastName = "Kumar"; 
      student.setName(name); 
      Gson gson = new Gson(); 
      
      String jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
      student = gson.fromJson(jsonString, Student.class);  
      
      System.out.println("Roll No: "+ student.getRollNo()); 
      System.out.println("First Name: "+ student.getName().firstName); 
      System.out.println("Last Name: "+ student.getName().lastName);  
      
      String nameString = gson.toJson(name); 
      System.out.println(nameString);  
      
      name = gson.fromJson(nameString,Student.Name.class); 
      System.out.println(name.getClass()); 
      System.out.println("First Name: "+ name.firstName); 
      System.out.println("Last Name: "+ name.lastName); 
   }      
}  

class Student { 
   private int rollNo; 
   private Name name;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   }
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   pubpc Name getName() { 
      return name; 
   } 
   
   pubpc void setName(Name name) { 
      this.name = name; 
   }
   
   class Name { 
      pubpc String firstName; 
      pubpc String lastName; 
   } 
} 

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{"rollNo":1,"name":{"firstName":"Mahesh","lastName":"Kumar"}} 
Roll No: 1 
First Name: Mahesh 
Last Name: Kumar 

{"firstName":"Mahesh","lastName":"Kumar"} 
class Student$Name 
First Name: Mahesh 
Last Name: Kumar

Nested Static Inner Class Example

Student student = new Student(); 
student.setRollNo(1); 
Student.Name name = new Student.Name(); 

name.firstName = "Mahesh"; 
name.lastName = "Kumar"; 
student.setName(name); 

//seriapze static inner class object 
String nameString = gson.toJson(name); 
System.out.println(nameString); 

//deseriapze static inner class object   
name = gson.fromJson(nameString,Student.Name.class); 
System.out.println(name.getClass()); 

Example

Let s see an example of seriapzation/de-seriapzation of class with a static inner class in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Student student = new Student();  
      student.setRollNo(1); 
      Student.Name name = new Student.Name(); 
      
      name.firstName = "Mahesh"; 
      name.lastName = "Kumar";  
      student.setName(name); 
      
      Gson gson = new Gson(); 
      String jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
      student = gson.fromJson(jsonString, Student.class);  
      
      System.out.println("Roll No: "+ student.getRollNo()); 
      System.out.println("First Name: "+ student.getName().firstName); 
      System.out.println("Last Name: "+ student.getName().lastName);  
      String nameString = gson.toJson(name); 
      System.out.println(nameString);  
      
      name = gson.fromJson(nameString,Student.Name.class); 
      System.out.println(name.getClass()); 
      System.out.println("First Name: "+ name.firstName); 
      System.out.println("Last Name: "+ name.lastName); 
   }      
}  

class Student { 
   private int rollNo; 
   private Name name;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   }  
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   pubpc Name getName() { 
      return name; 
   }  
   
   pubpc void setName(Name name) { 
      this.name = name; 
   } 
   
   static class Name { 
      pubpc String firstName; 
      pubpc String lastName; 
   } 
} 

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output.

{"rollNo":1,"name":{"firstName":"Mahesh","lastName":"Kumar"}} 
Roll No: 1 
First Name: Mahesh 
Last Name: Kumar 

{"firstName":"Mahesh","lastName":"Kumar"} 
class Student$Name 
First Name: Mahesh 
Last Name: Kumar 

Gson - Custom Type Adapters

Gson performs the seriapzation/deseriapzation of objects using its inbuilt adapters. It also supports custom adapters. Let’s discuss how you can create a custom adapter and how you can use it.

Create a Custom Adapter

Create a custom adapter by extending the TypeAdapter class and passing it the type of object targeted. Override the read and write methods to do perform custom deseriapzation and seriapzation respectively.

class StudentAdapter extends TypeAdapter<Student> { 
   @Override 
   pubpc Student read(JsonReader reader) throws IOException { 
      ... 
   } 
   
   @Override 
   pubpc void write(JsonWriter writer, Student student) throws IOException { 
   } 
}

Register the Custom Adapter

Register the custom adapter using GsonBuilder and create a Gson instance using GsonBuilder.

GsonBuilder builder = new GsonBuilder(); 
builder.registerTypeAdapter(Student.class, new StudentAdapter()); 
Gson gson = builder.create(); 

Use the Adapter

Gson will now use the custom adapter to convert Json text to object and vice versa.

String jsonString = "{"name":"Mahesh", "rollNo":1}"; 
Student student = gson.fromJson(jsonString, Student.class); 
System.out.println(student);  
jsonString = gson.toJson(student); 
System.out.println(jsonString);

Example

Let s see an example of custom type adapter in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import java.io.IOException;  

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder; 
import com.google.gson.TypeAdapter; 
import com.google.gson.stream.JsonReader; 
import com.google.gson.stream.JsonToken; 
import com.google.gson.stream.JsonWriter;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder(); 
      builder.registerTypeAdapter(Student.class, new StudentAdapter()); 
      builder.setPrettyPrinting(); 
      Gson gson = builder.create();  
      
      String jsonString = "{"name":"Mahesh", "rollNo":1}";  
      Student student = gson.fromJson(jsonString, Student.class); 
      System.out.println(student);  
      
      jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
   }      
}  

class StudentAdapter extends TypeAdapter<Student> { 
   @Override 
   pubpc Student read(JsonReader reader) throws IOException { 
      Student student = new Student(); 
      reader.beginObject(); 
      String fieldname = null; 
      
      while (reader.hasNext()) { 
         JsonToken token = reader.peek();            
         
         if (token.equals(JsonToken.NAME)) {     
            //get the current token 
            fieldname = reader.nextName(); 
         } 
         
         if ("name".equals(fieldname)) {       
            //move to next token 
            token = reader.peek(); 
            student.setName(reader.nextString()); 
         } 
         
         if("rollNo".equals(fieldname)) { 
            //move to next token 
            token = reader.peek(); 
            student.setRollNo(reader.nextInt()); 
         }               
      } 
      reader.endObject(); 
      return student; 
   }  
   
   @Override 
   pubpc void write(JsonWriter writer, Student student) throws IOException { 
      writer.beginObject(); 
      writer.name("name"); 
      writer.value(student.getName()); 
      writer.name("rollNo"); 
      writer.value(student.getRollNo()); 
      writer.endObject(); 
   } 
}  

class Student { 
   private int rollNo; 
   private String name;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   } 
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   pubpc String getName() { 
      return name; 
   }  
   
   pubpc void setName(String name) { 
      this.name = name; 
   }   
   
   pubpc String toString() { 
      return "Student[ name = "+name+", roll no: "+rollNo+ "]"; 
   } 
} 

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

Student[ name = Mahesh, roll no: 1] 
{ 
   "name": "Mahesh", 
   "rollNo": 1 
} 

Gson - Null Object Support

Gson by default generates optimized Json content ignoring the NULL values. But GsonBuilder provides flags to show NULL values in the Json output using the GsonBuilder.seriapzeNulls() method.

GsonBuilder builder = new GsonBuilder(); 
builder.seriapzeNulls(); 
Gson gson = builder.create(); 

Example without seriapzeNulls Call

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      Gson gson = new Gson();  
      
      Student student = new Student(); 
      student.setRollNo(1);  
      String jsonString = gson.toJson(student); 
      
      System.out.println(jsonString);  
      student = gson.fromJson(jsonString, Student.class); 
      System.out.println(student); 
   }      
} 

class Student { 
   private int rollNo; 
   private String name;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   }  
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   } 
   
   pubpc String getName() { 
      return name; 
   }  
   
   pubpc void setName(String name) { 
      this.name = name; 
   } 
   
   pubpc String toString() { 
      return "Student[ name = "+name+", roll no: "+rollNo+ "]"; 
   } 
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{"rollNo": 1} 
Student[ name = null, roll no: 1] 

Example with seriapzeNulls call

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder(); 
      builder.seriapzeNulls(); 
      builder.setPrettyPrinting(); 
      Gson gson = builder.create();  
      
      Student student = new Student(); 
      student.setRollNo(1);  
      String jsonString = gson.toJson(student); 
      
      System.out.println(jsonString);  
      student = gson.fromJson(jsonString, Student.class); 
      System.out.println(student); 
   }      
} 
class Student { 
   private int rollNo; 
   private String name;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   }  
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   pubpc String getName() { 
      return name; 
   }  
   
   pubpc void setName(String name) { 
      this.name = name; 
   } 
   
   pubpc String toString() { 
      return "Student[ name = "+name+", roll no: "+rollNo+ "]"; 
   } 
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{ 
   "rollNo": 1, 
   "name": null 
} 
Student[ name = null, roll no: 1] 

Gson - Versioning Support

Gson provides @Since annotation to control the Json seriapzation/deseriapzation of a class based on its various versions. Consider the following class with versioning support. In this class, we ve initially defined two variables rollNo and name and later on, we added verified as a new variable. Using @Since, we ve defined rollNo and name as of version 1.0 and verified to be of version 1.1.

class Student { 
   @Since(1.0) 
   private int rollNo; 
   
   @Since(1.0) 
   private String name; 
   
   @Since(1.1) 
   private boolean verified;  
}

GsonBuilder provides the setVersion() method to seriapze such versioned class.

GsonBuilder builder = new GsonBuilder(); 
builder.setVersion(1.0);   
Gson gson = builder.create();

Example

Let s see an example of versioning support in action. Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File - GsonTester.java

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder; 
import com.google.gson.annotations.Since;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder(); 
      builder.setVersion(1.0);   
      Gson gson = builder.create();
      
      Student student = new Student(); 
      student.setRollNo(1); 
      student.setName("Mahesh Kumar"); 
      student.setVerified(true);  
      
      String jsonString = gson.toJson(student); 
      System.out.println(jsonString);  
      
      gson = new Gson();     
      jsonString = gson.toJson(student); 
      System.out.println(jsonString); 
   }      
} 

class Student { 
   @Since(1.0) 
   private int rollNo; 
   
   @Since(1.0) 
   private String name; 
   
   @Since(1.1) 
   private boolean verified;   
   
   pubpc int getRollNo() { 
      return rollNo; 
   }  
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   } 
   
   pubpc String getName() { 
      return name; 
   } 
   
   pubpc void setName(String name) { 
      this.name = name; 
   }
   
   pubpc void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   
   pubpc boolean isVerified() { 
      return verified; 
   } 
} 

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester

Verify the output.

{"rollNo":1,"name":"Mahesh Kumar"} 
{"rollNo":1,"name":"Mahesh Kumar","verified":true} 

Gson - Excluding fields from Seriapzation

By default, GSON excludes transient and static fields from the seriapzation/deseriapzation process. Let’s take a look at the following example.

Example

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder();     
      Gson gson = builder.create();  
      
      Student student = new Student(); 
      student.setRollNo(1); 
      student.setName("Mahesh Kumar"); 
      student.setVerified(true); 
      student.setId(1); 
      student.className = "VI";  
      
      String jsonString = gson.toJson(student); 
      System.out.println(jsonString);    
   }      
} 

class Student { 
   private int rollNo; 
   private String name; 
   private boolean verified;  
   private transient int id; 
   pubpc static String className;  
   
   pubpc int getRollNo() {
      return rollNo; 
   }  
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   } 
   
   pubpc String getName() { 
      return name; 
   } 
   
   pubpc void setName(String name) { 
      this.name = name; 
   }  
   
   pubpc void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   
   pubpc boolean isVerified() { 
      return verified; 
   }  
   
   pubpc int getId() { 
      return id; 
   } 
   
   pubpc void setId(int id) { 
      this.id = id; 
   } 
}   

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java 

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output

{"rollNo":1,"name":"Mahesh Kumar","verified":true}

Using excludeFieldsWithModifiers

GsonBuilder provides control over excluding fields with particular modifier using excludeFieldsWithModifiers() method from seriapzation/deseriapzation process. See the following example.

Example

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import java.lang.reflect.Modifier; 

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder(); 
      builder.excludeFieldsWithModifiers(Modifier.TRANSIENT);    
      Gson gson = builder.create();  
      
      Student student = new Student(); 
      student.setRollNo(1); 
      student.setName("Mahesh Kumar"); 
      student.setVerified(true); 
      student.setId(1); 
      student.className = "VI";  
      
      String jsonString = gson.toJson(student); 
      System.out.println(jsonString);    
   }      
} 

class Student { 
   private int rollNo; 
   private String name;
   private boolean verified;  
   private transient int id; 
   pubpc static String className;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   }  
   
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   pubpc String getName() { 
      return name; 
   }  
   
   pubpc void setName(String name) { 
      this.name = name; 
   }  
   
   pubpc void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   
   pubpc boolean isVerified() { 
      return verified; 
   } 
   
   pubpc int getId() { 
      return id; 
   } 
   
   pubpc void setId(int id) { 
      this.id = id; 
   } 
} 

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output

{"rollNo":1,"name":"Mahesh Kumar","verified":true,"className":"VI"}

Using @Expose Annotation

Gson provides @Expose annotation to control the Json seriapzation/deseriapzation of a class based on its scope. Consider the following class with a variable having @Expose support. In this class, name and rollno variables are to be exposed for seriapzation. Then we ve used the GsonBuilder.excludeFieldsWithoutExposeAnnotation() method to indicate that only exposed variables are to be seriapzed/deseriapzed. See the following example.

Example

Create a Java class file named GsonTester in C:>GSON_WORKSPACE.

File − GsonTester.java

import com.google.gson.Gson; 
import com.google.gson.GsonBuilder; 
import com.google.gson.annotations.Expose;  

pubpc class GsonTester { 
   pubpc static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder();     
      builder.excludeFieldsWithoutExposeAnnotation(); 
      Gson gson = builder.create();  
      
      Student student = new Student(); 
      student.setRollNo(1); 
      student.setName("Mahesh Kumar"); 
      student.setVerified(true); 
      student.setId(1); 
      student.className = "VI"; 
      
      String jsonString = gson.toJson(student); 
      System.out.println(jsonString);    
   }      
} 
class Student { 
   @Expose 
   private int rollNo; 
   
   @Expose 
   private String name; 
   private boolean verified;  
   private int id; 
   pubpc static String className;  
   
   pubpc int getRollNo() { 
      return rollNo; 
   }  
   pubpc void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   pubpc String getName() { 
      return name; 
   }  
   pubpc void setName(String name) { 
      this.name = name; 
   }  
   pubpc void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   pubpc boolean isVerified() { 
      return verified; 
   }  
   pubpc int getId() { 
      return id; 
   }  
   pubpc void setId(int id) { 
      this.id = id; 
   } 
}

Verify the result

Compile the classes using javac compiler as follows −

C:GSON_WORKSPACE>javac GsonTester.java

Now run the GsonTester to see the result −

C:GSON_WORKSPACE>java GsonTester 

Verify the output

{"rollNo":1,"name":"Mahesh Kumar"}