Deserialization Json With Field-Type-Container By Gson New update

To deserialize JSON with a field-type-container using GSON, you can define a Java class that maps to the JSON structure, and annotate the field that contains the container with the @JsonAdapter annotation, passing in a custom TypeAdapter that knows how to deserialize the container.

For example, suppose you have the following JSON structure:

json
{
  "name": "Alice",
  "age": 30,
  "hobbies": ["reading", "cooking", "hiking"]
}

You could define a Java class like this:

java
import com.google.gson.annotations.JsonAdapter;
import java.util.List;

public class Person {
  public String name;
  public int age;

  @JsonAdapter(HobbiesTypeAdapter.class)
  public List<String> hobbies;
}

Note the @JsonAdapter(HobbiesTypeAdapter.class) annotation on the hobbies field. This tells GSON to use the HobbiesTypeAdapter class to deserialize the hobbies field.

You would then need to define the HobbiesTypeAdapter class, which would implement the TypeAdapter interface and provide custom deserialization logic for the List<String> type:

java
import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

public class HobbiesTypeAdapter extends TypeAdapter<List<String>> {
  @Override
  public void write(JsonWriter out, List<String> value) throws IOException {
    // Not needed for deserialization
  }

  @Override
  public List<String> read(JsonReader in) throws IOException {
    List<String> hobbies = new ArrayList<>();
    in.beginArray();
    while (in.hasNext()) {
      hobbies.add(in.nextString());
    }
    in.endArray();
    return hobbies;
  }
}

In this example, the HobbiesTypeAdapter class reads the JSON array of hobby strings and returns a List<String> object.

To deserialize the JSON into a Person object, you would simply call Gson.fromJson(json, Person.class), where json is a string containing the JSON structure. GSON would automatically use the HobbiesTypeAdapter class to deserialize the hobbies field.

Yes, Gson by default ignores extra fields present in a JSON string that do not match any fields in the Java object being deserialized. This means that if you have a Java class with a set of fields, and you try to deserialize a JSON string that contains additional fields not present in the Java class, Gson will not throw an exception but will simply ignore those extra fields.

For example, let’s say you have a Java class like this:

java
public class Person {
    private String name;
    private int age;

    // getters and setters
}

And you try to deserialize the following JSON string:

json
{
    "name": "John",
    "age": 30,
    "gender": "Male"
}

Gson will successfully deserialize the JSON string into a Person object and ignore the “gender” field because it does not match any field in the Person class.

However, if you want to configure Gson to throw an exception when extra fields are present in the JSON string, you can use the setLenient(false) method when creating the Gson instance. This will cause Gson to throw a JsonSyntaxException if there are extra fields in the JSON string that do not match any fields in the Java class being deserialized.

By default, Gson does serialize static fields. When you serialize an object using Gson, it will serialize all fields, including static fields, by default.

For example, consider the following class:

java
public class MyClass {
    public static int myStaticField = 42;
    public int myField = 123;
}

If you serialize an instance of this class using Gson, like this:

java
MyClass myObject = new MyClass();
Gson gson = new Gson();
String json = gson.toJson(myObject);

The resulting JSON string will include both the static field myStaticField and the non-static field myField.

json
{
  "myStaticField": 42,
  "myField": 123
}

If you want to exclude static fields from serialization, you can create a custom ExclusionStrategy and register it with Gson. For example:

java
public class StaticFieldExclusionStrategy implements ExclusionStrategy {
    @Override
    public boolean shouldSkipField(FieldAttributes f) {
        return Modifier.isStatic(f.getModifiers());
    }

    @Override
    public boolean shouldSkipClass(Class<?> clazz) {
        return false;
    }
}

This ExclusionStrategy skips any fields that are static. You can use it when creating your Gson instance:

java
MyClass myObject = new MyClass();
Gson gson = new GsonBuilder()
        .setExclusionStrategies(new StaticFieldExclusionStrategy())
        .create();
String json = gson.toJson(myObject);

Now, the resulting JSON string will only include the non-static field myField.

json
{
  "myField": 123
}

To convert a JSON string to an object in Java using Gson, you can follow the following steps:

1. Create a Gson object. The Gson class is provided by the Gson library and can be used to serialize and deserialize JSON objects.

java
Gson gson = new Gson();

1. Call the fromJson() method of the Gson object and pass the JSON string as the first argument and the class of the object you want to convert it to as the second argument.

java
MyClass obj = gson.fromJson(jsonString, MyClass.class);

Here, MyClass is the class of the object you want to convert the JSON string to.

1. Use the object as needed.

Here’s an example of converting a JSON string to an object:

java
import com.google.gson.Gson;

public class Example {
    public static void main(String[] args) {
        String jsonString = "{\"name\":\"John\", \"age\":30, \"city\":\"New York\"}";

        Gson gson = new Gson();

        Person person = gson.fromJson(jsonString, Person.class);

        System.out.println(person.getName());
        System.out.println(person.getAge());
        System.out.println(person.getCity());
    }
}

class Person {
    private String name;
    private int age;
    private String city;

    public String getName() {
        return name;
    }

    public int getAge() {
        return age;
    }

    public String getCity() {
        return city;
    }
}

In this example, the JSON string {"name":"John", "age":30, "city":"New York"} is converted to a Person object using the fromJson() method of the Gson object. The Person object is then used to print the name, age, and city of the person.