vs Gson

Gson的仓库介绍：Gson is a Java library that can be used to convert Java Objects into their JSON representation. It can also be used to convert a JSON string to an equivalent Java object.

Gson专门用于Java。

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

在开始学习之前，有理由推测 Moshi针对Kotlin做了一些提升，可以做到Gson对Kotlin所做不到的功能。

依赖

// kts
plugins {
    kotlin("jvm") version "1.8.10"
    java
    kotlin("kapt") version "1.8.10"
}  
// Moshi
implementation("com.squareup.moshi:moshi:1.14.0")
implementation("com.squareup.moshi:moshi-kotlin:1.14.0")// 包含KotlinJsonAdapterFactory()，可使用反射的方式序列化Kotlin 类
kapt("com.squareup.moshi:moshi-kotlin-codegen:1.14.0") // 使用codegen的方式序列化 Kotlin 类

反射和codegen必须使用其一或都使用。不然会报错。

使用反射添加KotlinJsonAdapterFactory
使用codegen在类添加 @JsonClass(generateAdapter = true)

使用反射或codegen的不同

基本使用

基本代码

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

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

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

val json = """
    {
      "hidden_card": {
        "rank": "6",
        "suit": "SPADES"
      },
      "visible_cards": [
        {
          "rank": "4",
          "suit": "CLUBS"
        },
        {
          "rank": "A",
          "suit": "HEARTS"
        }
      ]
    }
""".trimIndent()

val moshi: Moshi = Moshi.Builder()
    .addLast(KotlinJsonAdapterFactory())
    .build()

from json

// from json
fun main() {
    val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter(BlackjackHand::class.java)
    // val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter() 实验性
    val blackjackHand = jsonAdapter.fromJson(json)
    println(blackjackHand)
}

to json

// to json
fun main() {
    val blackjackHand = BlackjackHand(
        Card('6', Suit.SPADES),
        listOf(Card('4', Suit.CLUBS), Card('A', Suit.HEARTS))
    )

    val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter(BlackjackHand::class.java)
	// val jsonAdapter: JsonAdapter<BlackjackHand> = moshi.adapter() 实验性
    val json: String = jsonAdapter.toJson(blackjackHand)
    println(json)
}

parse array

@OptIn(ExperimentalStdlibApi::class)
fun main() {
    val arrayJson = """
        [
          {
            "rank": "4",
            "suit": "CLUBS"
          },
          {
            "rank": "A",
            "suit": "HEARTS"
          }
        ]
    """.trimIndent()

    val adapter: JsonAdapter<List<Card>> = moshi.adapter()// 实验性方法
    val cards: List<Card>? = adapter.fromJson(arrayJson)
    print(cards)
}

报错规则

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.

自定义字段名

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

  ...
}

忽略字段

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).

class BlackjackHand(...) {
  @Json(ignore = true)
  var total: Int = 0

  ...
}

These 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. In Kotlin, these fields must have a default value if they are in the primary constructor.

Note that you can also use Java’s transient keyword or Kotlin’s @Transient annotation on these fields for the same effect.