I have recently come across a rather cool feature of gson. I like to call it as selective deserialization. It’s easier if I explain it with the help of an example. Please see the json of a Person object with age, name, gender and address fields.
"{age:20,name:john, address:{street:MainStreet,city:newyork},gender:male}"
If we plan to deserialze above json using gson, the first design to come in our mind will be creating Person with Address as follows.
public class Person {
int age;
String name;
Address address;
Gender gender;
}
class Address {
String street;
String city;
}
# Then deserialize it as
Person p = new Gson().fromJson(personJson, Person.class);
But I had a quirky requirement. I would like to deserialize Address as a String. Then I tried to modify Person class as follows.
public class Person {
int age;
String name;
String address;
Gender gender;
}
But when I tried to deserialize it as new Gson().fromJson(personJson, Person.class), it failed with following exception.
Exception in thread "main" com.google.gson.JsonSyntaxException: java.lang.IllegalStateException:
Expected a string but was BEGIN_OBJECT at line 1 column 29 path $.addres
As shown in the exception, gson expected a string but it saw a begining of Object ‘{‘. Therefore, we have to somehow let gson know that, we should treat address as differently. Luckily, we can control what field to serialize/deserialze in gson using Expose annotation. Here, we will annotate address field with @Expose(deserialize = false). Gson will ignore field with @Expose(deserialize = false) while deserializing and it will deserialize fields with
@Expose(deserialize = true). Now, we have to create Gson object which can ignore above fields as follows
Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation()
.create();
If we try to deserialize it as gson.fromJson(personJson, Person.class), then Person object will create successfully but address field will be null.
It’s expected as we told the gson to ignore address field. To get the string value of address, we have to write our own Deserilaizer, which will manually set the address field.
public Person deserialize(JsonElement paramJsonElement, Type paramType,
JsonDeserializationContext paramJsonDeserializationContext)
throws JsonParseException {
Person person = new GsonBuilder()
.excludeFieldsWithoutExposeAnnotation().create()
.fromJson(paramJsonElement, Person.class);
JsonElement adddressElement = paramJsonElement.getAsJsonObject()
.get("address");
if (adddressElement != null) {
person.setAddress(adddressElement.toString());
}
return person;
}
As another side note, if we want to deserilaize enum from it’s value, we can do that if we annotate the enum with it’s value using @SerializedName.
The whole example which I tried with gson(2.8.5) is shown below.
** Person.java **
package com.example.demo;
import com.google.gson.annotations.Expose;
public class Person {
@Expose(deserialize = true)
int age;
@Expose(deserialize = true)
String name;
@Expose(deserialize = false)
String address;
@Expose(deserialize = true)
Gender gender;
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
public Gender getGender() {
return gender;
}
public void setGender(Gender gender) {
this.gender = gender;
}
}
** Gender.java **
package com.example.demo;
import com.google.gson.annotations.SerializedName;
public enum Gender {
@SerializedName("male")
MALE("male"), @SerializedName("female")
FEMALE("female"), @SerializedName("other")
OTHER("other");
private String val;
Gender(String val) {
this.val = val;
}
public String getVal() {
return val;
}
}
** TestPersonDeserialize.java **
package com.example.demo;
import java.lang.reflect.Type;
import com.google.gson.Gson;
import com.google.gson.GsonBuilder;
import com.google.gson.JsonDeserializationContext;
import com.google.gson.JsonDeserializer;
import com.google.gson.JsonElement;
import com.google.gson.JsonParseException;
public class TestPersonDeserialize {
public static void main(String[] args) {
String personJson = "{age:20,name:john, address:{street:MainStreet,city:newyork},gender:male}";
Person p = toPerson(personJson);
System.out.println(p);
}
public static Person toPerson(String personJson) {
Gson gson = new GsonBuilder().excludeFieldsWithoutExposeAnnotation()
.registerTypeAdapter(Person.class, new PersonDeserializer())
.create();
Person person = gson.fromJson(personJson, Person.class);
return person;
}
static class PersonDeserializer implements JsonDeserializer<Person> {
@Override
public Person deserialize(JsonElement paramJsonElement, Type paramType,
JsonDeserializationContext paramJsonDeserializationContext)
throws JsonParseException {
Person person = new GsonBuilder()
.excludeFieldsWithoutExposeAnnotation().create()
.fromJson(paramJsonElement, Person.class);
JsonElement adddressElement = paramJsonElement.getAsJsonObject()
.get("address");
if (adddressElement != null) {
person.setAddress(adddressElement.toString());
}
return person;
}
}
}