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Ktor - File Upload and Download

The ability to perform file uploads and downloads is a staple part of any good web server framework. Ktor has support for both operations in just a few lines of code.

File Upload

File uploads are handled through multipart POST requests in standard HTTP - normally from form submissions where the file selector field would be just one item (another could be the title for example).

To handle this in Ktor, you can get hold of the multipart data through receiveMultipart and then loop over each part as required. In the below example, we are just interested in files (PartData.FileItem), although you could also look at the individual PartData.FormItem as well (which would be the other form fields in the submission).

A Ktor FileItem exposes an InputStream via streamProvider which can be used to access the raw bytes of the file which has been uploaded. As in the below example, you can then simply create the appropriate file and copy the bytes from one stream (input) to the other (output).

post("/upload") { _ ->
    // retrieve all multipart data (suspending)
    val multipart = call.receiveMultipart()
    multipart.forEachPart { part ->
        // if part is a file (could be form item)
        if(part is PartData.FileItem) {
            // retrieve file name of upload
            val name = part.originalFileName!!
            val file = File("/uploads/$name")

            // use InputStream from part to save file
            part.streamProvider().use { its ->
                // copy the stream to the file with buffering
                file.outputStream().buffered().use {
                    // note that this is blocking
                    its.copyTo(it)
                }
            }
        }
        // make sure to dispose of the part after use to prevent leaks
        part.dispose()
    }
}

The documentation also includes a suspending copyTo method which can be used to save the upload to a file in a non-blocking way.

File Download

File downloads are very straightforward in Ktor. You just have to create a handle to the specified File and use the respondFile method:

get("/{name}") {
    // get filename from request url
    val filename = call.parameters["name"]!!
    // construct reference to file
    // ideally this would use a different filename
    val file = File("/uploads/$filename")
    if(file.exists()) {
        call.respondFile(file)
    }
    else call.respond(HttpStatusCode.NotFound)
}

By default, if called from the browser, this will cause the file to be viewed inline. If you instead want to prompt the browser to download the file, you can include the Content-Disposition header:

call.response.header("Content-Disposition", "attachment; filename=\"${file.name}\"")

This is also helpful if you save the uploaded file with a different name (which is advisable), as you can override the filename with the original when it gets downloaded by users.

More information about the Content-Disposition header