Gson – Quick Guide

Gson – Quick Guide

Gson – Overview

Google Gson is a simple Java-based library to serialize Java
objects to JSON and vice versa. It is an open-source library
developed by Google.
The following points highlight why you should be using this
library −
  • Standardized − Gson is a standardized library that is
    managed by Google.
  • Efficient − It is a reliable, fast, and efficient
    extension to the Java standard library.
  • Optimized − The library 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 list of some of the most prominent features of Gson −
  • Easy to use − Gson API provides a high-level facade to
    simplify commonly used use-cases.
  • No need to create mapping − Gson API provides default
    mapping for most of the objects to be serialized.
  • 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 library does not require any
    other library apart from JDK.
  • Open Source − Gson library 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

<!–

Try it Option Online

You do not have to set up your own environment to start learning Gson. We have already set up an online Java Programming environment for you. This platform allows you to compile and execute all the available examples online and analyze the output with different options. We believe it will boost your confidence and make your learning an enjoyable activity. Feel free to modify any example and execute it online.
Try the following example using the Try it option available at the top right corner of the following sample code box −
public class MyFirstJavaProgram {  
   public static void main(String []args) { 
      System.out.println("Hello World"); 
   } 
}
For most of the examples given in this tutorial, you will find a Try it option. Please do use this option and enjoy your learning.
–>

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 link . 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-click on ‘My Computer’ and select ‘Properties’.
  • Click 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 Filesjavajdkbin’.

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 line at
    the end: ‘SET PATH=%PATH%;C:Program Filesjavajdkbin’

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 line 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 like Notepad (Recommended for this tutorial) or
    TextPad.
  • Netbeans − It is a Java IDE that is open-source and
    free which can be downloaded from .
  • Eclipse − It is also a Java IDE developed by the
    Eclipse open-source community and can be downloaded from .

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.
OSARCHIVE NAME
Windowsgson-2.3.1.jar
Linuxgson-2.3.1.jar
Macgson-2.3.1.jar

Set Gson Environment

Set the GSON_HOME environment variable to point to the
base directory location where Gson jar is stored on your machine.
OSOUTPUT
WindowsSet the environment variable GSON_HOME to C:gson
Linuxexport GSON_HOME=/usr/local/gson
Macexport GSON_HOME=/Library/gson

Set CLASSPATH variable

Set the CLASSPATH environment variable to point to the
Gson jar location.
OSOUTPUT
WindowsSet the environment variable CLASSPATH to
%CLASSPATH%;%GSON_HOME%gson-2.3.1.jar;.;
Linuxexport CLASSPATH=$CLASSPATH:$GSON_HOME/gson-2.3.1.jar:.
Macexport CLASSPATH=$CLASSPATH:$GSON_HOME/gson-2.3.1.jar:.

Gson – First Application

Before going into the details of the Google Gson library, let’s
see an application in action. In this example, we’ve created a
Student class. We’ll create a JSON string with student
details and deserialize it to student object and then
serialize 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;  

public class GsonTester { 
   public 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; 
   public Student(){} 
   
   public String getName() { 
      return name; 
   }
   
   public void setName(String name) { 
      this.name = name; 
   } 
   
   public int getAge() { 
      return age; 
   }
   
   public void setAge(int age) { 
      this.age = age; 
   }
   
   public 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 − Deserialize 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 − Serialize 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 library. It provides
functionalities 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 −
public final class Gson 
   extends Object

Constructors

SR.NOCONSTRUCTOR & DESCRIPTION
1
Gson()
Constructs a Gson object with default configuration.

Class Methods

SR.NOMETHOD & DESCRIPTION
1
<T> T fromJson(JsonElement json, Class<T>
classOfT)
This method deserializes the Json read from the specified
parse tree into an object of the specified type.
2
<T> T fromJson(JsonElement json, Type typeOfT)
This method deserializes the Json read from the specified
parse tree into an object of the specified type.
3
<T> T fromJson(JsonReader reader, Type
typeOfT)
Reads the next JSON value from reader and convert it to an
object of type typeOfT.
4
<T> T fromJson(Reader json, Class<T>
classOfT)
This method deserializes the Json read from the specified
reader into an object of the specified class.
5
<T> T fromJson(Reader json, Type typeOfT)
This method deserializes the Json read from the specified
reader into an object of the specified type.
6
<T> T fromJson(String json, Class<T>
classOfT)
This method deserializes the specified Json into an object
of the specified class.
7
<T> T fromJson(String json, Type typeOfT)
This method deserializes the specified Json into an object
of the specified type.
8
<T> TypeAdapter<T> getAdapter(Class<T>
type)
Returns the type adapter for type.
9
<T> TypeAdapter<T>
getAdapter(TypeToken<T> type)
Returns the type adapter for type.
10
<T> TypeAdapter<T>
getDelegateAdapter(TypeAdapterFactory skipPast,
TypeToken<T> type)
This method is used to get an alternate type adapter for
the specified type.
11
String toJson(JsonElement jsonElement)
Converts a tree of JsonElements into its equivalent JSON
representation.
12
void toJson(JsonElement jsonElement, Appendable
writer)
Writes out the equivalent JSON for a tree of JsonElements.
13
void toJson(JsonElement jsonElement, JsonWriter
writer)
Writes the JSON for jsonElement to writer.
14
String toJson(Object src)
This method serializes the specified object into its
equivalent Json representation.
15
void toJson(Object src, Appendable writer)
This method serializes the specified object into its
equivalent Json representation.
16
String toJson(Object src, Type typeOfSrc)
This method serializes the specified object, including
those of generic types, into its equivalent Json
representation.
17
void toJson(Object src, Type typeOfSrc, Appendable
writer)
This method serializes the specified object, including
those of generic types, into its equivalent Json
representation.
18
void toJson(Object src, Type typeOfSrc, JsonWriter
writer)
Writes the JSON representation of src of type typeOfSrc to
writer.
19
JsonElement toJsonTree(Object src)
This method serializes the specified object into its
equivalent representation as a tree of JsonElements.
20
JsonElement toJsonTree(Object src, Type typeOfSrc)
This method serializes the specified object, including
those of generic types, into its equivalent representation
as a tree of JsonElements.
21
String toString()

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;  

public class GsonTester { 
   public 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; 
   public Student(){} 
   
   public String getName() { 
      return name; 
   } 
   public void setName(String name) { 
      this.name = name; 
   } 
   public int getAge() { 
      return age;
   } 
   public void setAge(int age) { 
      this.age = age; 
   } 
   public 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 Serialization

Let’s serialize 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;  

public class GsonTester { 
   public 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; 
   public Student(){} 
   
   public String getName() { 
      return name; 
   } 
   
   public void setName(String name) { 
      this.name = name; 
   } 
   
   public int getAge() { 
      return age; 
   } 
   
   public void setAge(int age) { 
      this.age = age; 
   } 
   
   public 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 serialize/deserialize 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;  

public class GsonTester { 
   public static void main(String args[]) { 
   
      Gson gson = new Gson();  
      String name = "Mahesh Kumar"; 
      long rollNo = 1; 
      boolean verified = false; 
      int[] marks = {100,90,85};  
      
      //Serialization 
      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-serialization 
      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;  

public class GsonTester { 
   public 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; 
   public Student(){} 
   
   public String getName() { 
      return name; 
   } 
   
   public void setName(String name) { 
      this.name = name; 
   } 
   
   public int getAge() { 
      return age; 
   } 
   
   public void setAge(int age) { 
      this.age = age; 
   } 
   
   public 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;  

public class GsonTester { 
   public 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;  

public class GsonTester { 
   public 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 – Serialization Examples

In this chapter, we will discuss the
serialization/deserialization of arrays, collections, and
generics.

Array Example

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

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

Example

Let’s see Array serialization/de-serialization 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;  

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

//De-serialization 
//get the type of the collection. 
Type listType = new TypeToken<list>(){}.getType(); 

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

Example

Let’s see Collection serialization/de-serialization 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;  

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

public class GsonTester { 
   public 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();  
      
      //Serialize 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> { 
   public T shape;  
   
   public void setShape(T shape) { 
      this.shape = shape; 
   }  
   public T get() { 
      return shape; 
   }  
   public double getArea() { 
      if(shape instanceof Circle) { 
         return ((Circle) shape).getArea(); 
      } else { 
         return 0.0; 
      } 
   } 
}  

class Circle { 
   private double radius;  
   
   public Circle(double radius){ 
      this.radius = radius; 
   }  
   public String toString() { 
      return "Circle"; 
   }  
   public double getRadius() { 
      return radius; 
   }  
   public void setRadius(double radius) { 
      this.radius = radius; 
   }  
   public 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 – Serializing Inner Classes

In this chapter, we will explain serialization/deserialization 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); 
//serialize inner class object 
String nameString = gson.toJson(name); 
System.out.println(nameString); 

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

Example

Let’s see an example of serialization/de-serialization 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;  

public class GsonTester { 
   public 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;  
   
   public int getRollNo() { 
      return rollNo; 
   }
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   public Name getName() { 
      return name; 
   } 
   
   public void setName(Name name) { 
      this.name = name; 
   }
   
   class Name { 
      public String firstName; 
      public 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); 

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

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

Example

Let’s see an example of serialization/de-serialization 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;  

public class GsonTester { 
   public 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;  
   
   public int getRollNo() { 
      return rollNo; 
   }  
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   public Name getName() { 
      return name; 
   }  
   
   public void setName(Name name) { 
      this.name = name; 
   } 
   
   static class Name { 
      public String firstName; 
      public 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 serialization/deserialization 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
deserialization and serialization respectively.
class StudentAdapter extends TypeAdapter<Student> { 
   @Override 
   public Student read(JsonReader reader) throws IOException { 
      ... 
   } 
   
   @Override 
   public 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;  

public class GsonTester { 
   public 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 
   public 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 
   public 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;  
   
   public int getRollNo() { 
      return rollNo; 
   } 
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   public String getName() { 
      return name; 
   }  
   
   public void setName(String name) { 
      this.name = name; 
   }   
   
   public 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.serializeNulls()
method.
GsonBuilder builder = new GsonBuilder(); 
builder.serializeNulls(); 
Gson gson = builder.create();

Example without serializeNulls Call

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

File – GsonTester.java

import com.google.gson.Gson;  

public class GsonTester { 
   public 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;  
   
   public int getRollNo() { 
      return rollNo; 
   }  
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   } 
   
   public String getName() { 
      return name; 
   }  
   
   public void setName(String name) { 
      this.name = name; 
   } 
   
   public 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 serializeNulls 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;  

public class GsonTester { 
   public static void main(String args[]) { 
   
      GsonBuilder builder = new GsonBuilder(); 
      builder.serializeNulls(); 
      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;  
   
   public int getRollNo() { 
      return rollNo; 
   }  
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   public String getName() { 
      return name; 
   }  
   
   public void setName(String name) { 
      this.name = name; 
   } 
   
   public 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
serialization/deserialization 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 serialize
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;  

public class GsonTester { 
   public 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;   
   
   public int getRollNo() { 
      return rollNo; 
   }  
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   } 
   
   public String getName() { 
      return name; 
   } 
   
   public void setName(String name) { 
      this.name = name; 
   }
   
   public void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   
   public 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 Serialization

By default, GSON excludes transient and static fields from the
serialization/deserialization 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;  

public class GsonTester { 
   public 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; 
   public static String className;  
   
   public int getRollNo() {
      return rollNo; 
   }  
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   } 
   
   public String getName() { 
      return name; 
   } 
   
   public void setName(String name) { 
      this.name = name; 
   }  
   
   public void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   
   public boolean isVerified() { 
      return verified; 
   }  
   
   public int getId() { 
      return id; 
   } 
   
   public 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 serialization/deserialization 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;  

public class GsonTester { 
   public 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; 
   public static String className;  
   
   public int getRollNo() { 
      return rollNo; 
   }  
   
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   
   public String getName() { 
      return name; 
   }  
   
   public void setName(String name) { 
      this.name = name; 
   }  
   
   public void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   
   public boolean isVerified() { 
      return verified; 
   } 
   
   public int getId() { 
      return id; 
   } 
   
   public 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
serialization/deserialization 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 serialization. Then
we’ve used the
GsonBuilder.excludeFieldsWithoutExposeAnnotation() method
to indicate that only exposed variables are to be
serialized/deserialized. 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;  

public class GsonTester { 
   public 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; 
   public static String className;  
   
   public int getRollNo() { 
      return rollNo; 
   }  
   public void setRollNo(int rollNo) { 
      this.rollNo = rollNo; 
   }  
   public String getName() { 
      return name; 
   }  
   public void setName(String name) { 
      this.name = name; 
   }  
   public void setVerified(boolean verified) { 
      this.verified = verified; 
   }  
   public boolean isVerified() { 
      return verified; 
   }  
   public int getId() { 
      return id; 
   }  
   public 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"}

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