Starting External Processes from Vert.x

Sometimes it’s necessary to run a script or command line application from your JVM applications. In my case, I wanted to generate my thumbnails using the ImageMagick CLI tools. Luckily the convert utility has support for reading images from standard input and writing the thumbnails to standard output.

This is the code I ended up with:

private val QUALITY_PRESET = 85

private suspend fun InputStream.createThumbnail(px: Int): ByteArray {
  val command = listOf("convert", "-thumbnail", "${px}x${px}>", "-quality", "$QUALITY_PRESET", "-", "webp:-")

  return awaitBlocking {
    val convertProcess = ProcessBuilder(command)
      .redirectError(ProcessBuilder.Redirect.INHERIT)
      .start()

    convertProcess.outputStream.use(this::transferTo)

    convertProcess.inputStream.use(InputStream::readAllBytes)
  }
}

Here we are starting convert and giving it the desired resolution. The > in the third list entry means that we don’t want to scale up images that are smaller than our target resolution. The last two entries specify the input and output methods for the tool. The - signals standard IO and the webp: specifies the output format for the thumbnails. Once the process is started, we can feed it our image input stream and then read the thumbnail from the process.