Ktor中如何向FormData传递非字符串类型数据？

方案1：将Attachment列表序列化为JSON字符串传递 #

如果后端支持接收JSON格式字符串作为attachments参数值，直接把列表转成JSON字符串就能满足需求，前提是你的项目引入了序列化库（比如kotlinx.serialization）。

操作步骤： #

1. 给Attachment类添加序列化注解：

import kotlinx.serialization.Serializable

@Serializable
data class Attachment(
    val url: String,
    val title: String,
    val mimetype: String,
    val size: Int,
    val signedUrl: String
)

2. 修改请求代码，把列表序列化为JSON后传入：

import kotlinx.serialization.encodeToString
import kotlinx.serialization.json.Json

suspend fun postGov(
        number: Int,
        closeLocation: String,
        closeTime: String,
        openLocation: String,
        openTime: String,
        attachments: List<Attachment>
    ): Gov = httpClient.submitForm(
        url = "$baseUrl/$GOVS_TABLE",
        formParameters = parameters {
            append("number", number.toString())
            append("close location", closeLocation)
            append("close time", closeTime)
            append("open location", openLocation)
            append("open time", openTime)
            // 将Attachment列表序列化为JSON字符串
            append("attachments", Json.encodeToString(attachments))
        }
    ) {
        apiKeyHeader()
    }.body()

方案2：用Multipart表单传递嵌套结构 #

如果后端要求严格遵循传统表单数组格式（比如attachments[0][url]这种参数名），可以改用submitMultipartForm构建请求：

suspend fun postGov(
        number: Int,
        closeLocation: String,
        closeTime: String,
        openLocation: String,
        openTime: String,
        attachments: List<Attachment>
    ): Gov = httpClient.submitMultipartForm(
        url = "$baseUrl/$GOVS_TABLE"
    ) {
        // 添加普通字段
        append("number", number.toString())
        append("close location", closeLocation)
        append("close time", closeTime)
        append("open location", openLocation)
        append("open time", openTime)
        
        // 遍历列表，逐个添加嵌套参数
        attachments.forEachIndexed { index, attachment ->
            append("attachments[$index][url]", attachment.url)
            append("attachments[$index][title]", attachment.title)
            append("attachments[$index][mimetype]", attachment.mimetype)
            append("attachments[$index][size]", attachment.size.toString())
            append("attachments[$index][signedUrl]", attachment.signedUrl)
        }
    } {
        apiKeyHeader()
    }.body()

补充说明 #

• 方案1代码更简洁，只要后端能解析JSON字符串就可以用，适合大多数场景；
• 方案2兼容性更强，符合传统表单提交的数组格式规范；
• 如果项目用Gson替代kotlinx.serialization，把Json.encodeToString替换成Gson().toJson(attachments)即可。

内容的提问来源于stack exchange，提问作者Tus Die