GSON.md

toJson

toJson: Object -> String, takes an object and returns a string json representation of the same.

By default all fields of object are included for serialization.

class BagOfPrims {
    private int val1 = 1;
    private String val2 = "abc";
    BagOfPrims(){
        // no args constructor
    }
}

null handling in toJson

1. If any fields within the object have null value for a key, they are ignored in the serialization.

2. while deserialization, missing entry in JSON results in setting corresponding field in object to its default value: a. null for object types b. zero for numeric types c. false for booleans

fromJson

Takes a string and a class, and returns a deserialized instance of string according to the class. e.g.

BagOfPrims obj2 = gson.fromJson(jsonStr, BagOfPrims.class);

If the json-string itself contains null as a value for a field, Gson will override default value in the POJO class and set it to null.

if json-string has a field missing that is present POJO class it will be given default values: null for Object type, 0 for numeric types, false for booleans.

GsonBuilder

In order to configure gson, you must create Gson instance using GsonBuilder e.g.

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

Deserialization of parametric types

class Foo<T> { // Foo is a parametric type e.g. say List
    T value;
}
Gson gson = new Gson();
Foo<Bar> foo = new Foo<Bar>();// foo.value type is Bar
gson.toJson(foo); // may not serialize foo.value correctly

gson.fromJson(json, Foo.getClass()); // fails to deserialize foo.value as Bar

To solve this problem, you must use TypeToken.

Type fooType = new TypeToken<Foo<Bar>>(){}.getType();
gson.fromJson(jsonStr, fooType);

e.g. for deserializing a list of users

TypeToken userListType = new TypeToken<List<User>>(){}.getType();
List<User> allUsers = new Gson().fromJson(jsonStr, userListType);

This TypeToken trick is only needed when toplevel JSON is an array of values, not needed when top level is an object value, where a simple class will suffice.

SerializedName

It is a gson annotation, that should be used like @SerializedName on a field when you want different name in JSON compared to your POJOs.

e.g.

class User {
    @SerializedName("name")
    String fullName;

    @SerializedName("id")
    String mId;
}

Custom Serialization and Deserialization