KTor Gson DataConversion 带接口答案

【问题标题】：KTor Gson DataConversion with interfaceKTor Gson DataConversion 带接口
【发布时间】：2018-11-17 11:47:37
【问题描述】：

我正在尝试在 KTor 中注册 PubicKey 接口进行数据转换，以便我可以轻松地接收这样的公钥：

data class StoreRequest(
    val publicKey: PublicKey
)
...
val publicKey: PublicKey = call.receive<StoreRequest>().publicKey

为了实现这一点，我使用了这个页面：https://ktor.io/servers/features/data-conversion.html
我注册了这个数据转换器：

convert<PublicKey> {
    decode { values, _ ->
        // When I add a breakpoint here it won't be reached.
        values.singleOrNull()?.let { key ->
            val encryptedKey = Base64.getDecoder().decode(key.split(" ")[1])
            val inputStream = DataInputStream(ByteArrayInputStream(encryptedKey))

            val format = String(ByteArray(inputStream.readInt()).also(inputStream::readFully))

            if (format != "ssh-rsa") throw RuntimeException("Unsupported format")

            val publicExponent = ByteArray(inputStream.readInt()).also(inputStream::readFully)
            val modulus = ByteArray(inputStream.readInt()).also(inputStream::readFully)

            val spec = RSAPublicKeySpec(BigInteger(modulus), BigInteger(publicExponent))
            val keyFactory = KeyFactory.getInstance("RSA")

            keyFactory.generatePublic(spec)
        }
    }
}

但由于某种原因，Gson 抱怨，因为我正在使用接口：

java.lang.RuntimeException: Unable to invoke no-args constructor for interface java.security.PublicKey. Register an InstanceCreator with Gson for this type may fix this problem`

所以我猜我需要为初始值创建一个InstanceCreator。这是我创建的初始 PublicKey 类：

class PkTest : PublicKey {
    override fun getAlgorithm(): String = ""
    override fun getEncoded(): ByteArray = ByteArray(0)
    override fun getFormat(): String = ""
}
...
install(ContentNegotiation) {
    gson {
        setPrettyPrinting()
        registerTypeAdapter(PublicKey::class.java, InstanceCreator<PublicKey> { PkTest() // This is called when I add a breakpoint })
    }
}

但这也行不通！这是我得到的例外：

com.google.gson.JsonSyntaxException: java.lang.IllegalStateException: Expected BEGIN_OBJECT but was STRING at line 2 column 16 path $.publicKey

由于某种原因，它期望提供的密钥是一个 JSON 对象，而我正在提供这个：

{
    "publicKey": "ssh-rsa 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 dynmem@memmen.frl"
}

如何“欺骗”GSON 接受 PublicKey 的字符串？还是我做错了什么？

我认为 GSON 想要将 JSON 对象序列化为 PublicKey。但我希望它接受String。我认为这应该是可能的，因为像 UUID 和 Date 这样的类工作得很好......

【问题讨论】：

标签： java kotlin gson ktor

【解决方案1】：

我解决了！我没有使用InstanceCreator，而是使用了JsonDeserializer：

install(ContentNegotiation) {
    gson {
        setPrettyPrinting()
        registerTypeAdapter(PublicKey::class.java, JsonDeserializer<PublicKey> { json, _, _ ->
            // TODO some type checking.
            val key = json.asString
            val encryptedKey = Base64.getDecoder().decode(key.split(" ")[1])
            val inputStream = DataInputStream(ByteArrayInputStream(encryptedKey))

            val format = String(ByteArray(inputStream.readInt()).also(inputStream::readFully))

            if (format != "ssh-rsa") throw RuntimeException("Unsupported format")

            val publicExponent = ByteArray(inputStream.readInt()).also(inputStream::readFully)
            val modulus = ByteArray(inputStream.readInt()).also(inputStream::readFully)

            val spec = RSAPublicKeySpec(BigInteger(modulus), BigInteger(publicExponent))
            val keyFactory = KeyFactory.getInstance("RSA")

            keyFactory.generatePublic(spec)
        })
    }
}

这部分可以完全去掉：

convert<PublicKey> {
    decode { values, _ ->
        ...
    }
}

【讨论】：