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Moshi

Moshi is a modern JSON library for Android, Java and Kotlin. It makes it easy to parse JSON into Java and Kotlin classes:

Note: The Kotlin examples of this README assume use of either Kotlin code gen or KotlinJsonAdapterFactory for reflection. Plain Java-based reflection is unsupported on Kotlin classes.

String json = ...;

Moshi moshi = new Moshi.Builder().build();
JsonAdapter<BlackjackHand> jsonAdapter = moshi.adapter(BlackjackHand.class);

BlackjackHand blackjackHand = jsonAdapter.fromJson(json);
System.out.println(blackjackHand);

val json: String = ...

val moshi: Moshi = Moshi.Builder().build()
val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter<BlackjackHand>()

val blackjackHand = jsonAdapter.fromJson(json)
println(blackjackHand)

And it can just as easily serialize Java or Kotlin objects as JSON:

BlackjackHand blackjackHand = new BlackjackHand(
    new Card('6', SPADES),
    Arrays.asList(new Card('4', CLUBS), new Card('A', HEARTS)));

Moshi moshi = new Moshi.Builder().build();
JsonAdapter<BlackjackHand> jsonAdapter = moshi.adapter(BlackjackHand.class);

String json = jsonAdapter.toJson(blackjackHand);
System.out.println(json);

val blackjackHand = BlackjackHand(
    Card('6', SPADES),
    listOf(Card('4', CLUBS), Card('A', HEARTS))
  )

val moshi: Moshi = Moshi.Builder().build()
val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter<BlackjackHand>()

val json: String = jsonAdapter.toJson(blackjackHand)
println(json)

Built-in Type Adapters

Moshi has built-in support for reading and writing Java’s core data types:

• Primitives (int, float, char...) and their boxed counterparts (Integer, Float, Character...).
• Arrays, Collections, Lists, Sets, and Maps
• Strings
• Enums

It supports your model classes by writing them out field-by-field. In the example above Moshi uses these classes:

class BlackjackHand {
  public final Card hidden_card;
  public final List<Card> visible_cards;
  ...
}

class Card {
  public final char rank;
  public final Suit suit;
  ...
}

enum Suit {
  CLUBS, DIAMONDS, HEARTS, SPADES;
}

class BlackjackHand(
  val hidden_card: Card,
  val visible_cards: List<Card>,
  ...
)

class Card(
  val rank: Char,
  val suit: Suit
  ...
)

enum class Suit {
  CLUBS, DIAMONDS, HEARTS, SPADES;
}

to read and write this JSON:

{
  "hidden_card": {
    "rank": "6",
    "suit": "SPADES"
  },
  "visible_cards": [
    {
      "rank": "4",
      "suit": "CLUBS"
    },
    {
      "rank": "A",
      "suit": "HEARTS"
    }
  ]
}

The Javadoc catalogs the complete Moshi API, which we explore below.

Custom Type Adapters

With Moshi, it’s particularly easy to customize how values are converted to and from JSON. A type adapter is any class that has methods annotated @ToJson and @FromJson.

For example, Moshi’s default encoding of a playing card is verbose: the JSON defines the rank and suit in separate fields: {"rank":"A","suit":"HEARTS"}. With a type adapter, we can change the encoding to something more compact: "4H" for the four of hearts or "JD" for the jack of diamonds:

class CardAdapter {
  @ToJson String toJson(Card card) {
    return card.rank + card.suit.name().substring(0, 1);
  }

  @FromJson Card fromJson(String card) {
    if (card.length() != 2) throw new JsonDataException("Unknown card: " + card);

    char rank = card.charAt(0);
    switch (card.charAt(1)) {
      case 'C': return new Card(rank, Suit.CLUBS);
      case 'D': return new Card(rank, Suit.DIAMONDS);
      case 'H': return new Card(rank, Suit.HEARTS);
      case 'S': return new Card(rank, Suit.SPADES);
      default: throw new JsonDataException("unknown suit: " + card);
    }
  }
}

class CardAdapter {
  @ToJson fun toJson(card: Card): String {
    return card.rank + card.suit.name.substring(0, 1)
  }

  @FromJson fun fromJson(card: String): Card {
    if (card.length != 2) throw JsonDataException("Unknown card: $card")

    val rank = card[0]
    return when (card[1]) {
      'C' -> Card(rank, Suit.CLUBS)
      'D' -> Card(rank, Suit.DIAMONDS)
      'H' -> Card(rank, Suit.HEARTS)
      'S' -> Card(rank, Suit.SPADES)
      else -> throw JsonDataException("unknown suit: $card")
    }
  }
}

Register the type adapter with the Moshi.Builder and we’re good to go.

Moshi moshi = new Moshi.Builder()
    .add(new CardAdapter())
    .build();

val moshi = Moshi.Builder()
    .add(CardAdapter())
    .build()

Voilà:

{
  "hidden_card": "6S",
  "visible_cards": [
    "4C",
    "AH"
  ]
}

Another example

Note that the method annotated with @FromJson does not need to take a String as an argument. Rather it can take input of any type and Moshi will first parse the JSON to an object of that type and then use the @FromJson method to produce the desired final value. Conversely, the method annotated with @ToJson does not have to produce a String.

Assume, for example, that we have to parse a JSON in which the date and time of an event are represented as two separate strings.

{
  "title": "Blackjack tournament",
  "begin_date": "20151010",
  "begin_time": "17:04"
}

We would like to combine these two fields into one string to facilitate the date parsing at a later point. Also, we would like to have all variable names in CamelCase. Therefore, the Event class we want Moshi to produce like this:

class Event {
  String title;
  String beginDateAndTime;
}

class Event(
  val title: String,
  val beginDateAndTime: String
)

Instead of manually parsing the JSON line per line (which we could also do) we can have Moshi do the transformation automatically. We simply define another class EventJson that directly corresponds to the JSON structure:

class EventJson {
  String title;
  String begin_date;
  String begin_time;
}

class EventJson(
  val title: String,
  val begin_date: String,
  val begin_time: String
)

And another class with the appropriate @FromJson and @ToJson methods that are telling Moshi how to convert an EventJson to an Event and back. Now, whenever we are asking Moshi to parse a JSON to an Event it will first parse it to an EventJson as an intermediate step. Conversely, to serialize an Event Moshi will first create an EventJson object and then serialize that object as usual.

class EventJsonAdapter {
  @FromJson Event eventFromJson(EventJson eventJson) {
    Event event = new Event();
    event.title = eventJson.title;
    event.beginDateAndTime = eventJson.begin_date + " " + eventJson.begin_time;
    return event;
  }

  @ToJson EventJson eventToJson(Event event) {
    EventJson json = new EventJson();
    json.title = event.title;
    json.begin_date = event.beginDateAndTime.substring(0, 8);
    json.begin_time = event.beginDateAndTime.substring(9, 14);
    return json;
  }
}

class EventJsonAdapter {
  @FromJson
  fun eventFromJson(eventJson: EventJson): Event {
    return Event(
      title = eventJson.title,
      beginDateAndTime = "${eventJson.begin_date} ${eventJson.begin_time}"
    )
  }

  @ToJson
  fun eventToJson(event: Event): EventJson {
    return EventJson(
      title = event.title,
      begin_date = event.beginDateAndTime.substring(0, 8),
      begin_time = event.beginDateAndTime.substring(9, 14),
    )
  }
}

Again we register the adapter with Moshi.

Moshi moshi = new Moshi.Builder()
    .add(new EventJsonAdapter())
    .build();

val moshi = Moshi.Builder()
    .add(EventJsonAdapter())
    .build()

We can now use Moshi to parse the JSON directly to an Event.

JsonAdapter<Event> jsonAdapter = moshi.adapter(Event.class);
Event event = jsonAdapter.fromJson(json);

val jsonAdapter = moshi.adapter<Event>()
val event = jsonAdapter.fromJson(json)

Adapter convenience methods

Moshi provides a number of convenience methods for JsonAdapter objects:

• nullSafe()
• nonNull()
• lenient()
• failOnUnknown()
• indent()
• serializeNulls()

These factory methods wrap an existing JsonAdapter into additional functionality. For example, if you have an adapter that doesn't support nullable values, you can use nullSafe() to make it null safe:

String dateJson = "\"2018-11-26T11:04:19.342668Z\"";
String nullDateJson = "null";

// Hypothetical IsoDateDapter, doesn't support null by default
JsonAdapter<Date> adapter = new IsoDateDapter();

Date date = adapter.fromJson(dateJson);
System.out.println(date); // Mon Nov 26 12:04:19 CET 2018

Date nullDate = adapter.fromJson(nullDateJson);
// Exception, com.squareup.moshi.JsonDataException: Expected a string but was NULL at path $

Date nullDate = adapter.nullSafe().fromJson(nullDateJson);
System.out.println(nullDate); // null

val dateJson = "\"2018-11-26T11:04:19.342668Z\""
val nullDateJson = "null"

// Hypothetical IsoDateDapter, doesn't support null by default
val adapter: JsonAdapter<Date> = IsoDateDapter()

val date = adapter.fromJson(dateJson)
println(date) // Mon Nov 26 12:04:19 CET 2018

val nullDate = adapter.fromJson(nullDateJson)
// Exception, com.squareup.moshi.JsonDataException: Expected a string but was NULL at path $

val nullDate = adapter.nullSafe().fromJson(nullDateJson)
println(nullDate) // null

In contrast to nullSafe() there is nonNull() to make an adapter refuse null values. Refer to the Moshi JavaDoc for details on the various methods.

Parse JSON Arrays

Say we have a JSON string of this structure:

[
  {
    "rank": "4",
    "suit": "CLUBS"
  },
  {
    "rank": "A",
    "suit": "HEARTS"
  }
]

We can now use Moshi to parse the JSON string into a List<Card>.

String cardsJsonResponse = ...;
Type type = Types.newParameterizedType(List.class, Card.class);
JsonAdapter<List<Card>> adapter = moshi.adapter(type);
List<Card> cards = adapter.fromJson(cardsJsonResponse);

val cardsJsonResponse: String = ...
// We can just use a reified extension!
val adapter = moshi.adapter<List<Card>>()
val cards: List<Card> = adapter.fromJson(cardsJsonResponse)

Fails Gracefully

Automatic databinding almost feels like magic. But unlike the black magic that typically accompanies reflection, Moshi is designed to help you out when things go wrong.

JsonDataException: Expected one of [CLUBS, DIAMONDS, HEARTS, SPADES] but was ANCHOR at path $.visible_cards[2].suit
  at com.squareup.moshi.JsonAdapters$11.fromJson(JsonAdapters.java:188)
  at com.squareup.moshi.JsonAdapters$11.fromJson(JsonAdapters.java:180)
  ...

Moshi always throws a standard java.io.IOException if there is an error reading the JSON document, or if it is malformed. It throws a JsonDataException if the JSON document is well-formed, but doesn’t match the expected format.

Built on Okio

Moshi uses Okio for simple and powerful I/O. It’s a fine complement to OkHttp, which can share buffer segments for maximum efficiency.

Borrows from Gson

Moshi uses the same streaming and binding mechanisms as Gson. If you’re a Gson user you’ll find Moshi works similarly. If you try Moshi and don’t love it, you can even migrate to Gson without much violence!

But the two libraries have a few important differences:

• Moshi has fewer built-in type adapters. For example, you need to configure your own date adapter. Most binding libraries will encode whatever you throw at them. Moshi refuses to serialize platform types (java.*, javax.*, and android.*) without a user-provided type adapter. This is intended to prevent you from accidentally locking yourself to a specific JDK or Android release.
• Moshi is less configurable. There’s no field naming strategy, versioning, instance creators, or long serialization policy. Instead of naming a field visibleCards and using a policy class to convert that to visible_cards, Moshi wants you to just name the field visible_cards as it appears in the JSON.
• Moshi doesn’t have a JsonElement model. Instead it just uses built-in types like List and Map.
• No HTML-safe escaping. Gson encodes = as \u003d by default so that it can be safely encoded in HTML without additional escaping. Moshi encodes it naturally (as =) and assumes that the HTML encoder – if there is one – will do its job.

Custom field names with @Json

Moshi works best when your JSON objects and Java or Kotlin classes have the same structure. But when they don't, Moshi has annotations to customize data binding.

Use @Json to specify how Java fields or Kotlin properties map to JSON names. This is necessary when the JSON name contains spaces or other characters that aren’t permitted in Java field or Kotlin property names. For example, this JSON has a field name containing a space:

{
  "username": "jesse",
  "lucky number": 32
}

With @Json its corresponding Java or Kotlin class is easy:

class Player {
  String username;
  @Json(name = "lucky number") int luckyNumber;

  ...
}

class Player {
  val username: String
  @Json(name = "lucky number") val luckyNumber: Int

  ...
}

Because JSON field names are always defined with their Java or Kotlin fields, Moshi makes it easy to find fields when navigating between Java or Koltin and JSON.

Alternate type adapters with @JsonQualifier

Use @JsonQualifier to customize how a type is encoded for some fields without changing its encoding everywhere. This works similarly to the qualifier annotations in dependency injection tools like Dagger and Guice.

Here’s a JSON message with two integers and a color:

{
  "width": 1024,
  "height": 768,
  "color": "#ff0000"
}

By convention, Android programs also use int for colors:

class Rectangle {
  int width;
  int height;
  int color;
}

class Rectangle(
  val width: Int,
  val height: Int,
  val color: Int
)

But if we encoded the above Java or Kotlin class as JSON, the color isn't encoded properly!

{
  "width": 1024,
  "height": 768,
  "color": 16711680
}

The fix is to define a qualifier annotation, itself annotated @JsonQualifier:

@Retention(RUNTIME)
@JsonQualifier
public @interface HexColor {
}

@Retention(RUNTIME)
@JsonQualifier
annotation class HexColor

Next apply this @HexColor annotation to the appropriate field:

class Rectangle {
  int width;
  int height;
  @HexColor int color;
}

class Rectangle(
  val width: Int,
  val height: Int,
  @HexColor val color: Int
)

And finally define a type adapter to handle it:

/** Converts strings like #ff0000 to the corresponding color ints. */
class ColorAdapter {
  @ToJson String toJson(@HexColor int rgb) {
    return String.format("#%06x", rgb);
  }

  @FromJson @HexColor int fromJson(String rgb) {
    return Integer.parseInt(rgb.substring(1), 16);
  }
}

/** Converts strings like #ff0000 to the corresponding color ints.  */
class ColorAdapter {
  @ToJson fun toJson(@HexColor rgb: Int): String {
    return "#%06x".format(rgb)
  }

  @FromJson @HexColor fun fromJson(rgb: String): Int {
    return rgb.substring(1).toInt(16)
  }
}

Use @JsonQualifier when you need different JSON encodings for the same type. Most programs shouldn’t need this @JsonQualifier, but it’s very handy for those that do.

Omitting fields

Some models declare fields that shouldn’t be included in JSON. For example, suppose our blackjack hand has a total field with the sum of the cards:

public final class BlackjackHand {
  private int total;

  ...
}

class BlackjackHand(
  private val total: Int,

  ...
)

By default, all fields are emitted when encoding JSON, and all fields are accepted when decoding JSON. Prevent a field from being included by annotating them with @Json(ignore = true).

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