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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 }
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