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Update README.md to include Kotlin Examples (#1355)

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Sangeet Narayan
2021-11-01 01:55:55 +05:30
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@@ -1,8 +1,13 @@
Moshi Moshi
===== =====
Moshi is a modern JSON library for Android and Java. It makes it easy to parse JSON into Java Moshi is a modern JSON library for Android, Java and Kotlin. It makes it easy to parse JSON into Java and Kotlin
objects: 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._
<details open>
<summary>Java</summary>
```java ```java
String json = ...; String json = ...;
@@ -13,8 +18,26 @@ JsonAdapter<BlackjackHand> jsonAdapter = moshi.adapter(BlackjackHand.class);
BlackjackHand blackjackHand = jsonAdapter.fromJson(json); BlackjackHand blackjackHand = jsonAdapter.fromJson(json);
System.out.println(blackjackHand); System.out.println(blackjackHand);
``` ```
</details>
And it can just as easily serialize Java objects as JSON: <details>
<summary>Kotlin</summary>
```kotlin
val json: String = ...
val moshi: Moshi = Moshi.Builder().build()
val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter<BlackjackHand>()
val blackjackHand = jsonAdapter.fromJson(json)
println(blackjackHand)
```
</details>
And it can just as easily serialize Java or Kotlin objects as JSON:
<details open>
<summary>Java</summary>
```java ```java
BlackjackHand blackjackHand = new BlackjackHand( BlackjackHand blackjackHand = new BlackjackHand(
@@ -27,6 +50,24 @@ JsonAdapter<BlackjackHand> jsonAdapter = moshi.adapter(BlackjackHand.class);
String json = jsonAdapter.toJson(blackjackHand); String json = jsonAdapter.toJson(blackjackHand);
System.out.println(json); System.out.println(json);
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
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)
```
</details>
### Built-in Type Adapters ### Built-in Type Adapters
@@ -40,6 +81,9 @@ Moshi has built-in support for reading and writing Java’s core data types:
It supports your model classes by writing them out field-by-field. In the example above Moshi uses It supports your model classes by writing them out field-by-field. In the example above Moshi uses
these classes: these classes:
<details open>
<summary>Java</summary>
```java ```java
class BlackjackHand { class BlackjackHand {
public final Card hidden_card; public final Card hidden_card;
@@ -57,6 +101,30 @@ enum Suit {
CLUBS, DIAMONDS, HEARTS, SPADES; CLUBS, DIAMONDS, HEARTS, SPADES;
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
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;
}
```
</details>
to read and write this JSON: to read and write this JSON:
@@ -91,6 +159,9 @@ suit in separate fields: `{"rank":"A","suit":"HEARTS"}`. With a type adapter, we
encoding to something more compact: `"4H"` for the four of hearts or `"JD"` for the jack of encoding to something more compact: `"4H"` for the four of hearts or `"JD"` for the jack of
diamonds: diamonds:
<details open>
<summary>Java</summary>
```java ```java
class CardAdapter { class CardAdapter {
@ToJson String toJson(Card card) { @ToJson String toJson(Card card) {
@@ -111,14 +182,54 @@ class CardAdapter {
} }
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
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")
}
}
}
```
</details>
Register the type adapter with the `Moshi.Builder` and we’re good to go. Register the type adapter with the `Moshi.Builder` and we’re good to go.
<details open>
<summary>Java</summary>
```java ```java
Moshi moshi = new Moshi.Builder() Moshi moshi = new Moshi.Builder()
.add(new CardAdapter()) .add(new CardAdapter())
.build(); .build();
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
val moshi = Moshi.Builder()
.add(CardAdapter())
.build()
```
</details>
Voilà: Voilà:
@@ -154,17 +265,35 @@ We would like to combine these two fields into one string to facilitate the date
later point. Also, we would like to have all variable names in CamelCase. Therefore, the `Event` 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 we want Moshi to produce like this:
<details open>
<summary>Java</summary>
```java ```java
class Event { class Event {
String title; String title;
String beginDateAndTime; String beginDateAndTime;
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class Event(
val title: String,
val beginDateAndTime: String
)
```
</details>
Instead of manually parsing the JSON line per line (which we could also do) we can have Moshi do the 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 transformation automatically. We simply define another class `EventJson` that directly corresponds
to the JSON structure: to the JSON structure:
<details open>
<summary>Java</summary>
```java ```java
class EventJson { class EventJson {
String title; String title;
@@ -172,6 +301,19 @@ class EventJson {
String begin_time; String begin_time;
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class EventJson(
val title: String,
val begin_date: String,
val begin_time: String
)
```
</details>
And another class with the appropriate `@FromJson` and `@ToJson` methods that are telling Moshi how 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 convert an `EventJson` to an `Event` and back. Now, whenever we are asking Moshi to parse a JSON
@@ -179,6 +321,9 @@ to an `Event` it will first parse it to an `EventJson` as an intermediate step.
serialize an `Event` Moshi will first create an `EventJson` object and then serialize that object as serialize an `Event` Moshi will first create an `EventJson` object and then serialize that object as
usual. usual.
<details open>
<summary>Java</summary>
```java ```java
class EventJsonAdapter { class EventJsonAdapter {
@FromJson Event eventFromJson(EventJson eventJson) { @FromJson Event eventFromJson(EventJson eventJson) {
@@ -197,21 +342,72 @@ class EventJsonAdapter {
} }
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class EventJsonAdapter {
@FromJson fun eventFromJson(eventJson: EventJson): Event {
val event = Event()
event.title = eventJson.title
event.beginDateAndTime = "${eventJson.begin_date} ${eventJson.begin_time}"
return event
}
@ToJson fun eventToJson(event: Event): EventJson {
val json = EventJson()
json.title = event.title
json.begin_date = event.beginDateAndTime.substring(0, 8)
json.begin_time = event.beginDateAndTime.substring(9, 14)
return json
}
}
```
</details>
Again we register the adapter with Moshi. Again we register the adapter with Moshi.
<details open>
<summary>Java</summary>
```java ```java
Moshi moshi = new Moshi.Builder() Moshi moshi = new Moshi.Builder()
.add(new EventJsonAdapter()) .add(new EventJsonAdapter())
.build(); .build();
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
val moshi = Moshi.Builder()
.add(EventJsonAdapter())
.builder
```
</details>
We can now use Moshi to parse the JSON directly to an `Event`. We can now use Moshi to parse the JSON directly to an `Event`.
<details open>
<summary>Java</summary>
```java ```java
JsonAdapter<Event> jsonAdapter = moshi.adapter(Event.class); JsonAdapter<Event> jsonAdapter = moshi.adapter(Event.class);
Event event = jsonAdapter.fromJson(json); Event event = jsonAdapter.fromJson(json);
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
val jsonAdapter = moshi.adapter<Event>()
val event = jsonAdapter.fromJson(json)
```
</details>
### Adapter convenience methods ### Adapter convenience methods
@@ -226,6 +422,9 @@ Moshi provides a number of convenience methods for `JsonAdapter` objects:
These factory methods wrap an existing `JsonAdapter` into additional functionality. 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: For example, if you have an adapter that doesn't support nullable values, you can use `nullSafe()` to make it null safe:
<details open>
<summary>Java</summary>
```java ```java
String dateJson = "\"2018-11-26T11:04:19.342668Z\""; String dateJson = "\"2018-11-26T11:04:19.342668Z\"";
String nullDateJson = "null"; String nullDateJson = "null";
@@ -242,6 +441,28 @@ Date nullDate = adapter.fromJson(nullDateJson);
Date nullDate = adapter.nullSafe().fromJson(nullDateJson); Date nullDate = adapter.nullSafe().fromJson(nullDateJson);
System.out.println(nullDate); // null System.out.println(nullDate); // null
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
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
```
</details>
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. 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.
@@ -264,12 +485,27 @@ Say we have a JSON string of this structure:
We can now use Moshi to parse the JSON string into a `List<Card>`. We can now use Moshi to parse the JSON string into a `List<Card>`.
<details open>
<summary>Java</summary>
```java ```java
String cardsJsonResponse = ...; String cardsJsonResponse = ...;
Type type = Types.newParameterizedType(List.class, Card.class); Type type = Types.newParameterizedType(List.class, Card.class);
JsonAdapter<List<Card>> adapter = moshi.adapter(type); JsonAdapter<List<Card>> adapter = moshi.adapter(type);
List<Card> cards = adapter.fromJson(cardsJsonResponse); List<Card> cards = adapter.fromJson(cardsJsonResponse);
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
val cardsJsonResponse: String = ...
// We can just use a reified extension!
val adapter = moshi.adapter<List<Card>>()
val cards: List<Card> = adapter.fromJson(cardsJsonResponse)
```
</details>
### Fails Gracefully ### Fails Gracefully
@@ -317,11 +553,11 @@ But the two libraries have a few important differences:
### Custom field names with @Json ### Custom field names with @Json
Moshi works best when your JSON objects and Java objects have the same structure. But when they 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. don't, Moshi has annotations to customize data binding.
Use `@Json` to specify how Java fields map to JSON names. This is necessary when the JSON name 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 names. For example, this 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: JSON has a field name containing a space:
```json ```json
@@ -331,7 +567,10 @@ JSON has a field name containing a space:
} }
``` ```
With `@Json` its corresponding Java class is easy: With `@Json` its corresponding Java or Kotlin class is easy:
<details open>
<summary>Java</summary>
```java ```java
class Player { class Player {
@@ -341,9 +580,23 @@ class Player {
... ...
} }
``` ```
</details>
Because JSON field names are always defined with their Java fields, Moshi makes it easy to find <details>
fields when navigating between Java and JSON. <summary>Kotlin</summary>
```kotlin
class Player {
val username: String
@Json(name = "lucky number") val luckyNumber: Int
...
}
```
</details>
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 ### Alternate type adapters with @JsonQualifier
@@ -363,6 +616,9 @@ Here’s a JSON message with two integers and a color:
By convention, Android programs also use `int` for colors: By convention, Android programs also use `int` for colors:
<details open>
<summary>Java</summary>
```java ```java
class Rectangle { class Rectangle {
int width; int width;
@@ -370,8 +626,21 @@ class Rectangle {
int color; int color;
} }
``` ```
</details>
But if we encoded the above Java class as JSON, the color isn't encoded properly! <details>
<summary>Kotlin</summary>
```kotlin
class Rectangle(
val width: Int,
val height: Int,
val color: Int
)
```
</details>
But if we encoded the above Java or Kotlin class as JSON, the color isn't encoded properly!
```json ```json
{ {
@@ -383,15 +652,33 @@ But if we encoded the above Java class as JSON, the color isn't encoded properly
The fix is to define a qualifier annotation, itself annotated `@JsonQualifier`: The fix is to define a qualifier annotation, itself annotated `@JsonQualifier`:
<details open>
<summary>Java</summary>
```java ```java
@Retention(RUNTIME) @Retention(RUNTIME)
@JsonQualifier @JsonQualifier
public @interface HexColor { public @interface HexColor {
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
@Retention(RUNTIME)
@JsonQualifier
annotation class HexColor
```
</details>
Next apply this `@HexColor` annotation to the appropriate field: Next apply this `@HexColor` annotation to the appropriate field:
<details open>
<summary>Java</summary>
```java ```java
class Rectangle { class Rectangle {
int width; int width;
@@ -399,9 +686,25 @@ class Rectangle {
@HexColor int color; @HexColor int color;
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class Rectangle(
val width: Int,
val height: Int,
@HexColor val color: Int
)
```
</details>
And finally define a type adapter to handle it: And finally define a type adapter to handle it:
<details open>
<summary>Java</summary>
```java ```java
/** Converts strings like #ff0000 to the corresponding color ints. */ /** Converts strings like #ff0000 to the corresponding color ints. */
class ColorAdapter { class ColorAdapter {
@@ -414,6 +717,24 @@ class ColorAdapter {
} }
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
/** 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)
}
}
```
</details>
Use `@JsonQualifier` when you need different JSON encodings for the same type. Most programs 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. shouldn’t need this `@JsonQualifier`, but it’s very handy for those that do.
@@ -423,6 +744,9 @@ shouldn’t need this `@JsonQualifier`, but it’s very handy for those that do.
Some models declare fields that shouldn’t be included in JSON. For example, suppose our blackjack 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: hand has a `total` field with the sum of the cards:
<details open>
<summary>Java</summary>
```java ```java
public final class BlackjackHand { public final class BlackjackHand {
private int total; private int total;
@@ -430,9 +754,25 @@ public final class BlackjackHand {
... ...
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class BlackjackHand(
private val total: Int,
...
)
```
</details>
By default, all fields are emitted when encoding JSON, and all fields are accepted when decoding By default, all fields are emitted when encoding JSON, and all fields are accepted when decoding
JSON. Prevent a field from being included by adding Java’s `transient` keyword: JSON. Prevent a field from being included by adding Java’s `transient` keyword or Kotlin's `@Transient` annotation:
<details open>
<summary>Java</summary>
```java ```java
public final class BlackjackHand { public final class BlackjackHand {
@@ -441,6 +781,19 @@ public final class BlackjackHand {
... ...
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class BlackjackHand(...) {
@Transient var total: Int
...
}
```
</details>
Transient fields are omitted when writing JSON. When reading JSON, the field is skipped even if the Transient fields are omitted when writing JSON. When reading JSON, the field is skipped even if the
JSON contains a value for the field. Instead, it will get a default value. JSON contains a value for the field. Instead, it will get a default value.
@@ -448,13 +801,15 @@ JSON contains a value for the field. Instead, it will get a default value.
### Default Values & Constructors ### Default Values & Constructors
When reading JSON that is missing a field, Moshi relies on the Java or Android runtime to assign When reading JSON that is missing a field, Moshi relies on the Java or Kotlin or Android runtime to assign
the field’s value. Which value it uses depends on whether the class has a no-arguments constructor. the field’s value. Which value it uses depends on whether the class has a no-arguments constructor.
If the class has a no-arguments constructor, Moshi will call that constructor and whatever value If the class has a no-arguments constructor, Moshi will call that constructor and whatever value
it assigns will be used. For example, because this class has a no-arguments constructor the `total` it assigns will be used. For example, because this class has a no-arguments constructor the `total`
field is initialized to `-1`. field is initialized to `-1`.
Note: This section only applies to Java reflections.
```java ```java
public final class BlackjackHand { public final class BlackjackHand {
private int total = -1; private int total = -1;
@@ -474,6 +829,7 @@ If the class doesn’t have a no-arguments constructor, Moshi can’t assign the
numbers, `false` for booleans, and `null` for references. In this example, the default value of numbers, `false` for booleans, and `null` for references. In this example, the default value of
`total` is `0`! `total` is `0`!
```java ```java
public final class BlackjackHand { public final class BlackjackHand {
private int total = -1; private int total = -1;
@@ -489,6 +845,7 @@ This is surprising and is a potential source of bugs! For this reason consider d
no-arguments constructor in classes that you use with Moshi, using `@SuppressWarnings("unused")` to no-arguments constructor in classes that you use with Moshi, using `@SuppressWarnings("unused")` to
prevent it from being inadvertently deleted later: prevent it from being inadvertently deleted later:
```java ```java
public final class BlackjackHand { public final class BlackjackHand {
private int total = -1; private int total = -1;
@@ -511,6 +868,9 @@ adapters to build upon the standard conversion.
In this example, we turn serialize nulls, then delegate to the built-in adapter: In this example, we turn serialize nulls, then delegate to the built-in adapter:
<details open>
<summary>Java</summary>
```java ```java
class TournamentWithNullsAdapter { class TournamentWithNullsAdapter {
@ToJson void toJson(JsonWriter writer, Tournament tournament, @ToJson void toJson(JsonWriter writer, Tournament tournament,
@@ -525,6 +885,27 @@ class TournamentWithNullsAdapter {
} }
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class TournamentWithNullsAdapter {
@ToJson fun toJson(writer: JsonWriter, tournament: Tournament?,
delegate: JsonAdapter<Tournament?>) {
val wasSerializeNulls: Boolean = writer.getSerializeNulls()
writer.setSerializeNulls(true)
try {
delegate.toJson(writer, tournament)
} finally {
writer.setLenient(wasSerializeNulls)
}
}
}
```
</details>
When we use this to serialize a tournament, nulls are written! But nulls elsewhere in our JSON When we use this to serialize a tournament, nulls are written! But nulls elsewhere in our JSON
document are skipped as usual. document are skipped as usual.
@@ -534,11 +915,28 @@ the encoding and decoding process for any type, even without knowing about the t
this example, we customize types annotated `@AlwaysSerializeNulls`, which an annotation we create, this example, we customize types annotated `@AlwaysSerializeNulls`, which an annotation we create,
not built-in to Moshi: not built-in to Moshi:
<details open>
<summary>Java</summary>
```java ```java
@Target(TYPE) @Target(TYPE)
@Retention(RUNTIME) @Retention(RUNTIME)
public @interface AlwaysSerializeNulls {} public @interface AlwaysSerializeNulls {}
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
@Target(TYPE)
@Retention(RUNTIME)
annotation class AlwaysSerializeNulls
```
</details>
<details open>
<summary>Java</summary>
```java ```java
@AlwaysSerializeNulls @AlwaysSerializeNulls
@@ -548,11 +946,28 @@ static class Car {
String color; String color;
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
@AlwaysSerializeNulls
class Car(
val make: String?,
val model: String?,
val color: String?
)
```
</details>
Each `JsonAdapter.Factory` interface is invoked by `Moshi` when it needs to build an adapter for a Each `JsonAdapter.Factory` interface is invoked by `Moshi` when it needs to build an adapter for a
user's type. The factory either returns an adapter to use, or null if it doesn't apply to the user's type. The factory either returns an adapter to use, or null if it doesn't apply to the
requested type. In our case we match all classes that have our annotation. requested type. In our case we match all classes that have our annotation.
<details open>
<summary>Java</summary>
```java ```java
static class AlwaysSerializeNullsFactory implements JsonAdapter.Factory { static class AlwaysSerializeNullsFactory implements JsonAdapter.Factory {
@Override public JsonAdapter<?> create( @Override public JsonAdapter<?> create(
@@ -567,6 +982,24 @@ static class AlwaysSerializeNullsFactory implements JsonAdapter.Factory {
} }
} }
``` ```
</details>
<details>
<summary>Kotlin</summary>
```kotlin
class AlwaysSerializeNullsFactory : JsonAdapter.Factory {
override fun create(type: Type, annotations: Set<Annotation>, moshi: Moshi): JsonAdapter<*>? {
val rawType: Class<*> = type.rawType
if (!rawType.isAnnotationPresent(AlwaysSerializeNulls::class.java)) {
return null
}
val delegate: JsonAdapter<Any> = moshi.nextAdapter(this, type, annotations)
return delegate.serializeNulls()
}
}
```
</details>
After determining that it applies, the factory looks up Moshi's built-in adapter by calling After determining that it applies, the factory looks up Moshi's built-in adapter by calling
`Moshi.nextAdapter()`. This is key to the composition mechanism: adapters delegate to each other! `Moshi.nextAdapter()`. This is key to the composition mechanism: adapters delegate to each other!
@@ -643,8 +1076,8 @@ encode as JSON:
```kotlin ```kotlin
@JsonClass(generateAdapter = true) @JsonClass(generateAdapter = true)
data class BlackjackHand( data class BlackjackHand(
val hidden_card: Card, val hidden_card: Card,
val visible_cards: List<Card> val visible_cards: List<Card>
) )
``` ```