A Java JSON Library intended to be easy to learn and simple to teach.

Requires Java 21+.

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The Data Model

JSON is a data format. It looks like the following sample.

{
    "name": "kermit",
    "wife": null,
    "girlfriend": "Ms. Piggy",
    "age": 22,
    "children": [
        {
            "species": "frog",
            "gender": "male"
        },
        {
            "species": "pig",
            "gender": "female"
        }
    ],
    "commitmentIssues": true
}

In JSON you represent data using a combination of objects (maps from strings to JSON), arrays (ordered sequences of JSON), strings, numbers, true, false, and null.

Therefore, one "natural" way to think about the data stored in a JSON document is as the union of those possibilities.

JSON is one of
- a map of string to JSON
- a list of JSON
- a string
- a number
- true
- false
- null

The way to represent this in Java is using a sealed interface, which provides an explicit list of types which are allowed to implement it.

public sealed interface Json
        permits
            JsonObject,
            JsonArray,
            JsonString,
            JsonNumber,
            JsonBoolean,
            JsonNull {
}

This means that if you have a field or variable which has the type Json, you know that it is either a JsonObject, JsonArray, JsonString, JsonNumber, JsonBoolean, or JsonNull.

That is the first thing provided by my library. There is a Json type and subtypes representing those different cases.

import dev.mccue.json.*;
public class Main {
    static Json greeting() {
        return JsonString.of("hello");
    }
    public static void main(String[] args) {
        Json json = greeting();
        switch (json) {
            case JsonObject object ->
                    System.out.println("An object");
            case JsonArray array ->
                    System.out.println("An array");
            case JsonString str ->
                    System.out.println("A string");
            case JsonNumber number ->
                    System.out.println("A number");
            case JsonBoolean bool ->
                    System.out.println("A boolean");
            case JsonNull __ ->
                    System.out.println("A json null");
        }
    }
}

You can create instances of these subtypes using factory methods on the types themselves.

import dev.mccue.json.*;
import java.util.List;
import java.util.Map;
public class Main {
    public static void main(String[] args) {
        JsonObject kermit = JsonObject.of(Map.of(
                "name", JsonString.of("kermit"),
                "age", JsonNumber.of(22),
                "commitmentIssues", JsonBoolean.of(true),
                "wife", JsonNull.instance(),
                "children", JsonArray.of(List.of(
                        JsonString.of("Tiny Tim")
                ))
        ));
        System.out.println(kermit);
    }
}

Or by using factory methods on Json, which aren't guaranteed to give you any specific subtype but in exchange will handle converting any stray nulls to JsonNull.

import dev.mccue.json.*;
import java.util.List;
import java.util.Map;
public class Main {
    public static void main(String[] args) {
        Json kermit = Json.of(Map.of(
                "name", Json.of("kermit"),
                "age", Json.of(22),
                "commitmentIssues", Json.of(true),
                "wife", Json.ofNull(),
                "children", Json.of(List.of(
                        JsonString.of("Tiny Tim")
                ))
        ));
        System.out.println(kermit);
    }
}

For JsonObject and JsonArray, there also use builders available which can make it so that you don't need to write Json.of on every value.

import dev.mccue.json.Json;
public class Main {
    public static void main(String[] args) {
        Json kermit = Json.objectBuilder()
                .put("name", "kermit")
                .put("age", 22)
                .putTrue("commitmentIssues")
                .putNull("wife")
                .put("children", Json.arrayBuilder()
                        .add("Tiny Tim"))
                .build();
        System.out.println(kermit);
    }
}

Writing

Once you have some Json you can write it out to a String using Json.writeString

import dev.mccue.json.Json;
public class Main {
    public static void main(String[] args) {
        Json songJson = Json.objectBuilder()
                .put("title", "Rainbow Connection")
                .put("year", 1979)
                .build();
        String song = Json.writeString(songJson);
        System.out.println(song);
    }
}

{"title":"Rainbow Connection","year":1979}

If output is meant to be consumed by humans then whitespace can be added using a customized instance of JsonWriteOptions.

import dev.mccue.json.Json;
import dev.mccue.json.JsonWriteOptions;
public class Main {
    public static void main(String[] args) {
        Json songJson = Json.objectBuilder()
                .put("title", "Rainbow Connection")
                .put("year", 1979)
                .build();
        String song = Json.writeString(
                songJson,
                new JsonWriteOptions()
                        .withIndentation(4)
        );
        System.out.println(song);
    }
}

{
    "title": "Rainbow Connection",
    "year": 1979
}

If you want to write JSON to something other than a String, you need to obtain a Writer and use Json.write.

import dev.mccue.json.Json;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
public class Main {
    public static void main(String[] args) throws IOException {
        Json songJson = Json.objectBuilder()
                .put("title", "Rainbow Connection")
                .put("year", 1979)
                .build();
        try (var fileWriter = Files.newBufferedWriter(
                Path.of("song.json"))
        ) {
            Json.write(songJson, fileWriter);
        }
    }
}

Encoding

To turn a class you have defined into JSON, you just need to make a method which creates an instance of Json from the data stored in your class.

import dev.mccue.json.Json;
record Muppet(String name) {
    Json toJson() {
        return Json.objectBuilder()
                .put("name", name)
                .build();
    }
}
public class Main {
    public static void main(String[] args) {
        var beaker = new Muppet("beaker");
        Json beakerJson = beaker.toJson();
        System.out.println(Json.writeString(beakerJson));
    }
}

This process is "encoding." You "encode" your data into JSON and then "write" that JSON to some output.

For classes that you did not define, the logic for the conversion just needs to live somewhere. Dealer's choice where, but static methods are generally a good call.

import dev.mccue.json.Json;
import java.time.Month;
import java.time.MonthDay;
import java.time.format.DateTimeFormatter;
final class TimeEncoders {
    private TimeEncoders() {}
    static Json monthDayToJson(MonthDay monthDay) {
        return Json.of(
                DateTimeFormatter.ofPattern("MM-dd")
                        .format(monthDay)
        );
    }
}
record Muppet(String name, MonthDay birthday) {
    Json toJson() {
        return Json.objectBuilder()
                .put("name", name)
                .put(
                        "birthday",
                        TimeEncoders.monthDayToJson(birthday)
                )
                .build();
    }
}
public class Main {
    public static void main(String[] args) {
        var elmo = new Muppet(
                "Elmo",
                MonthDay.of(Month.FEBRUARY, 3)
        );
        Json elmoJson = elmo.toJson();
        System.out.println(Json.writeString(elmoJson));
    }
}

{"name":"Elmo","birthday":"02-03"}

If a class you define has a JSON representation that could be considered "canonical", the interface JsonEncodable can be implemented. This will let you pass an instance of the class directly to Json.writeString or Json.write.

import dev.mccue.json.Json;
import dev.mccue.json.JsonEncodable;
record Muppet(String name, boolean great)
        implements JsonEncodable {
    @Override
    public Json toJson() {
        return Json.objectBuilder()
                .put("name", name)
                .put("great", great)
                .build();
    }
}
public class Main {
    public static void main(String[] args) {
        var gonzo = new Muppet("Gonzo", true);
        System.out.println(Json.writeString(gonzo));
    }
}

Reading

The inverse of writing JSON is reading it.

If you have some JSON stored in a String you can read it into Json using Json.readString.

import dev.mccue.json.Json;
public class Main {
    public static void main(String[] args) {
        Json movie = Json.readString("""
                {
                    "title": "Treasure Island",
                    "cast": [
                        {
                            "name": "Kermit",
                            "role": "The Captain",
                            "muppet": true
                        },
                        {
                            "name": "Tim Curry",
                            "role": "Long John Silver",
                            "muppet": false
                        }
                    ]
                
                }
                """);
        System.out.println(movie);
    }
}

If that JSON is coming from another source, you need to obtain a Reader and use Json.read.

import dev.mccue.json.Json;
import java.io.IOException;
import java.io.Reader;
import java.nio.file.Files;
import java.nio.file.Path;
public class Main {
    public static void main(String[] args) throws IOException {
        // If you were following along, we created this earlier!
        Json song;
        try (Reader fileReader = Files.newBufferedReader(
                Path.of("song.json"))
        ) {
            song = Json.read(fileReader);
        }
        System.out.println(song);
    }
}

If the JSON you provide is malformed in some way, a JsonReadException will be thrown.

import dev.mccue.json.Json;
public class Main {
    public static void main(String[] args) {
        // Should be in quotes
        Json.readString("fozzie");
    }
}

Exception in thread "main" dev.mccue.json.JsonReadException: JSON error (unexpected character): f
	at dev.mccue.json.JsonReadException.unexpectedCharacter(JsonReadException.java:33)
	at dev.mccue.json.internal.JsonReaderMethods.readStream(JsonReaderMethods.java:525)
	at dev.mccue.json.internal.JsonReaderMethods.read(JsonReaderMethods.java:533)
	at dev.mccue.json.internal.JsonReaderMethods.readFullyConsume(JsonReaderMethods.java:543)
	at dev.mccue.json.Json.readString(Json.java:369)
	at dev.mccue.json.Json.readString(Json.java:364)
	at dev.mccue.example.Main.main(Main.java:9)

Decoding

Up to this point, everything has been more or less the same as it is for other "tree-based" JSON libraries like org.json or json-simple.

This is where that will start to change.

To take some Json and turn it into a user defined class, a basic approach would be to use instanceof checks to see if the Json is a particular subtype and navigate from there.

import dev.mccue.json.*;
record Muppet(String name, boolean canSpeak) {
    static Muppet fromJson(Json json) {
        if (json instanceof JsonObject object &&
            object.get("name") instanceof JsonString name &&
            object.get("canSpeak") instanceof JsonBoolean canSpeak) {
            return new Muppet(name.toString(), canSpeak.value());
        }
        else {
            throw new RuntimeException("Invalid Muppet");
        }
    }
}
public class Main {
    public static void main(String[] args) {
        var json = Json.readString("""
                {
                    "name": "animal",
                    "canSpeak": false
                }
                """);
        var animal = Muppet.fromJson(json);
        System.out.println(animal);
    }
}

This process is "decoding." You "read" your data into JSON and then "decode" it to some type you define.

The problem with the instanceof approach is that you will end up with bad error messages on unexpected data. In this case the error message would just be "Invalid Muppet". The code to get better errors is tedious to write and I haven't seen many folks in the wild do it.

To get good errors, you should use the static methods defined in JsonDecoder.

package dev.mccue.example;
import dev.mccue.json.*;
record Muppet(String name, boolean canSpeak) {
    static Muppet fromJson(Json json) {
        return new Muppet(
                JsonDecoder.field(
                        json,
                        "name",
                        JsonDecoder::string
                ),
                JsonDecoder.field(
                        json,
                        "canSpeak",
                        JsonDecoder::boolean_
                )
        );
    }
}
public class Main {
    public static void main(String[] args) {
        var json = Json.readString("""
                {
                    "name": "animal",
                    "canSpeak": false
                }
                """);
        var animal = Muppet.fromJson(json);
        System.out.println(animal);
    }
}

These handle the fiddly process of checking whether the JSON matches the structure you expect and throwing an appropriate error.

You should read this declaration as "at the field name I expect a string."

JsonDecoder.field(json, "name", JsonDecoder::string)

If the JSON is not an object, or doesn't have a value for name, or that value is not a string, you will get a JsonDecodeException.

public class Main {
    public static void main(String[] args) {
        var json = Json.readString("""
                {
                    "canSpeak": false
                }
                """);
        var animal = JsonDecoder.field(
                json,
                "name",
                JsonDecoder::string
        );
        System.out.println(animal);
    }
}

Which will have a message indicating exactly what went wrong and where.

Problem with the value at json.name:
    {
        "canSpeak": false
    }
no value for field

The last argument to JsonDecoder.field is the JsonDecoder you want to use to interpret the value at that field. In this case a method reference to JsonDecoder.string, which is a method that asserts JSON is a string and throws if it isn't.

For the methods which take more than one argument, there are overloads which can be used to get an instance of JsonDecoder.

// This will actually decode the json into a list of strings
List<String> items = JsonDecoder.array(json, JsonDecoder::string);
// This will just return a decoder
Decoder<List<String>> decoder =
        JsonDecoder.array(JsonDecoder::string);

This, in conjunction with JsonDecoder.field is how you are intended to explore nested paths.

public class Main {
    public static void main(String[] args) {
        var json = Json.readString("""
                {
                    "villains": ["constantine", "doc hopper"]
                }
                """);
        List<String> villains = JsonDecoder.field(
                json,
                "villains",
                JsonDecoder.array(JsonDecoder::string)
        );
        System.out.println(villains);
    }
}

To decode JSON into your custom classes, you should add either a constructor or a static factory method which takes in Json and use these decoders to make your objects.

import dev.mccue.json.*;
import java.util.List;
record Actor(String name, String role, boolean muppet) {
    static Actor fromJson(Json json) {
        return new Actor(
                JsonDecoder.field(json, "name", JsonDecoder::string),
                JsonDecoder.field(json, "role", JsonDecoder::string),
                JsonDecoder.optionalField(
                        json,
                        "muppet",
                        JsonDecoder::boolean_,
                        true
                )
        );
    }
}
record Movie(String title, List<Actor> cast) {
    static Movie fromJson(Json json) {
        return new Movie(
                JsonDecoder.field(json, "title", JsonDecoder::string),
                JsonDecoder.field(
                        json,
                        "cast",
                        JsonDecoder.array(Actor::fromJson)
                )
        );
    }
}
public class Main {
    public static void main(String[] args) {
        var json = Json.readString("""
                 {
                     "title": "Treasure Island",
                     "cast": [
                         {
                             "name": "Kermit",
                             "role": "The Captain"
                         },
                         {
                             "name": "Tim Curry",
                             "role": "Long John Silver",
                             "muppet": false
                         }
                     ]
                 }
                 """);
        var movie = Movie.fromJson(json);
        System.out.println(movie);
    }
}

Full Round-Trip

With all of that out of the way, here is how you might define a model, write it to json, and read it back in.