Handling file upload

Uploading files in a form using multipart/form-data

The standard way to upload files in a web application is to use a form with a special multipart/form-data encoding, which lets you mix standard form data with file attachment data.

Note: The HTTP method used to submit the form must be POST (not GET).

Start by writing an HTML form:

@file-upload-form

Now define the upload action:

@syncUpload

The getRef() method gives you a reference to a TemporaryFile. This is the default way Play handles file uploads.

And finally, add a POST route:

@application-upload-routes

Note: An empty file will be treated just like no file was uploaded at all. The same applies if the filename header of a multipart/form-data file upload part is empty - even when the file itself would not empty.

Testing the file upload

You can also write an automated JUnit test to your upload action:

@testSyncUpload

Basically, we are creating a Http.MultipartFormData.FilePart that is required by RequestBuilder method bodyMultipart. Besides that, everything else is just like [[unit testing controllers|JavaTest#Unit-testing-controllers]].

Direct file upload

Another way to send files to the server is to use Ajax to upload files asynchronously from a form. In this case, the request body will not be encoded as multipart/form-data, but will just contain the plain file contents.

@asyncUpload

Writing a custom multipart file part body parser

The multipart upload specified by MultipartFormData takes uploaded data from the request and puts into a TemporaryFile object. It is possible to override this behavior so that Multipart.FileInfo information is streamed to another class, using the DelegatingMultipartFormDataBodyParser class:

@customfileparthandler

Here, pekko.stream.javadsl.FileIO class is used to create a sink that sends the ByteString from the Accumulator into a java.io.File object, rather than a TemporaryFile object.

Using a custom file part handler also means that behavior can be injected, so a running count of uploaded bytes can be sent elsewhere in the system.

Cleaning up temporary files

Uploading files uses a TemporaryFile API which relies on storing files in a temporary filesystem, accessible through the getRef() method. All TemporaryFile references come from a TemporaryFileCreator trait, and the implementation can be swapped out as necessary, and there's now an atomicMoveWithFallback method that uses StandardCopyOption.ATOMIC_MOVE if available.

Uploading files is an inherently dangerous operation, because unbounded file upload can cause the filesystem to fill up -- as such, the idea behind TemporaryFile is that it's only in scope at completion and should be moved out of the temporary file system as soon as possible. Any temporary files that are not moved are deleted.

However, under certain conditions, garbage collection does not occur in a timely fashion. As such, there's also a play.api.libs.Files.TemporaryFileReaper that can be enabled to delete temporary files on a scheduled basis using the Pekko scheduler, distinct from the garbage collection method.

The reaper is disabled by default, and is enabled through configuration of application.conf:

play.temporaryFile {
  reaper {
    enabled = true
    initialDelay = "5 minutes"
    interval = "30 seconds"
    olderThan = "30 minutes"
  }
}

The above configuration will delete files that are more than 30 minutes old, using the "olderThan" property. It will start the reaper five minutes after the application starts, and will check the filesystem every 30 seconds thereafter. The reaper is not aware of any existing file uploads, so protracted file uploads may run into the reaper if the system is not carefully configured.