Gson is unable to serialize LocalDateTime data in Android (Java)

[UPDATED:02.20.2023] I was misused between LocalDateTime and Instant data type in my example so I adjusted it. Sorry for this inconvenient. Thank you

I am practicing coding and facing an issue relating to Gson and LocalDateTime in Java Android.
For example, I created a Java object

public class Person {
private String name;
private LocalDateTime createdAt;

// Constructors
// Getters and Setters
}

Then I used Gson to convert this Person object in to Json

Person person = new Person("John Doe", LocalDateTime.now()); //For example: 2022-02-02T11:11:045701100
String personJsonAsString = new Gson().toJson(person);

I expect the Json should be

{
"name": "John Doe",
"createdAt": "2022-02-02T11:11:045701100"
}

But the result I got was

{
"name": "John Doe",
"createdAt": {} // <- It's empty here
}

After that, I researched and found that I can add custom serializer class into GsonBuilder instance so I tried again

Person person = new Person("John Doe", LocalDateTime.now()); //For example: 2022-02-02T11:11:510Z
GsonBuilder builder = new GsonBuilder();
Gson gson = builder.registerTypeAdapter(LocalDateTime.class, new LocalDateTimeJsonSerializer()).create();
String personJsonAsString = gson.toJson(person);

While LocalDateTimeJsonSerializer is a class that I wrote

public class LocalDateTimeJsonSerializer implements JsonSerializer<LocalDateTime> {
    @Override
    public JsonElement serialize(LocalDateTime src, Type typeOfSrc, JsonSerializationContext context) {
        return new JsonPrimitive(src.format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
    }
}

The exception was throw when I try to persist the data on the Android app like this:

For now, I cannot figure out what makes LocalDateTime cannot be serialized in Android Platform by Gson. Please help me. Thank you so much for your time.

What I did try:
Researching other questions on Stackoverflow.
Try out any available solutions.
What I was expecting:
I was expecting to know the actual root cause of the exception instead of fixing the bug and making it run only

Square peg in a round hole

Wrong class. Use Instant, not LocalDateTime.

public class Person {
    private String name;
    private Instant createdAt;
    …
}

Your input "2022-02-02T11:11:510Z" has a Z on the end. That letter indicates an offset from UTC of zero hours-minutes-seconds.

Instant instant = Instant.parse( "2022-02-02T11:11:510Z" ) ;

The LocalDateTime lacks any concept of offset or time zone. So that class cannot represent a moment, is not a specific point on the timeline. That class cannot represent the meaning intended by your input. That class cannot be used to record the moment something was created.

Your input string complies with ISO 8601 standard. See Wikipedia.

You showed line of code:

>Person person = new Person("John Doe", LocalDateTime.now());

I cannot imagine a case where calling LocalDateTime.now is the right thing to do.

Change to:

Person person = new Person( "John Doe" , Instant.now() ) ;

Example code

I am no expert on Gson. But I seem to have gotten the following to work.

For brevity, let's define your Person class as a record. I am pleased to see that the current version 2.10.1 of Gson seems to be working well with records.

package work.basil.example.gson;

import java.time.Instant;

public record Person( String name , Instant createdAt ) { }

Apparently the only date-time type adapter bundled with GSON is for java.util.Date. That terrible legacy class should be avoided. Use only java.time classes.

We are using java.time.Instant. So we need to write our own type adapter. You can find 3rd party open-source type adapter implementations. But writing your own for Instant is simple enough.

The java.time classes by default use ISO 8601 standard formats when parsing/generating text. So we will use that standard.

package work.basil.example.gson;

import com.google.gson.TypeAdapter;
import com.google.gson.stream.JsonReader;
import com.google.gson.stream.JsonWriter;

import java.io.IOException;
import java.time.Instant;

public class Gson_InstantTypeAdapter extends TypeAdapter < Instant >
{
    @Override
    public void write ( JsonWriter jsonWriter , Instant instant ) throws IOException
    {
        jsonWriter.value( instant.toString() );  // Writes in standard ISO 8601 format.
    }

    @Override
    public Instant read ( JsonReader jsonReader ) throws IOException
    {
        return Instant.parse( jsonReader.nextString() );   // Parses standard ISO 8601 format.
    }
}

Now we write an app to exercise these classes.

First, define a Gson object to be using for generating and parsing JSON text.

Gson gson =
        new GsonBuilder()
                .registerTypeAdapter( Instant.class , new Gson_InstantTypeAdapter() )
                .create();

Write some JSON.

// Write to JSON.
Person person = new Person( "John Doe" , Instant.now() ); //For example: 2022-02-02T11:11:510Z
String personAsJson = gson.toJson( person );

>personAsJson = {"name":"John Doe","createdAt":"2023-02-19T22:47:42.566132Z"}

And read some JSON.

// Parse JSON.
Person p = gson.fromJson( personAsJson , Person.class );

>p.toString() = Person[name=John Doe, createdAt=2023-02-19T22:47:42.566132Z]

Bring that code together.

package work.basil.example.gson;

import com.google.gson.*;

import java.time.Instant;

public class App
{
    public static void main ( String[] args )
    {
        App app = new App();
        app.demo();
    }

    private void demo ( )
    {
        Gson gson =
                new GsonBuilder()
                        .registerTypeAdapter( Instant.class , new Gson_InstantTypeAdapter() )
                        .create();

        // Write to JSON.
        Person person = new Person( "John Doe" , Instant.now() ); //For example: 2022-02-02T11:11:510Z
        String personAsJson = gson.toJson( person );

        System.out.println( "personAsJson = " + personAsJson );

        // Parse JSON.
        Person p = gson.fromJson( personAsJson , Person.class );

        System.out.println( "p.toString() = " + p );
    }
}