Spring Multipart File – Can I Read InputStream Multiple Times?

The short answer is Yes.

Here is the long version and why I even asked myself this question.

If you are familiar with ServletRequest then you probably know that calling its getInputStream method only works once. If you need to read the body data multiple times then it is up to you to cache it in a buffer or employ workarounds such as a "caching servlet request". Unfortunately, this fact is not stated in the Javadoc of ServletRequest#getInputStream so it is no wonder this question gets asked.

Spring’s MultipartFile is a bit different here. It, too, has a getInputStream method, but this one can be called multiple times. Again, it is not obvious from the documentation which is why I am making this mental note for myself and others who are researching this question because they know about the behavior of ServletRequest and assume – as I did – it is the same for MultipartFile. Fortunately, it is not.

In my case I needed to compute a hash of an uploaded file and then move the file to Azure’s Blob Storage. The Azure API used an InputStream and I assumed, once I had consumed the multipart InputStream that I could not use that API anymore.

As a side note: Using DigestInputStream it is possible to do this in one go, move the data to storage and while doing that compute the hash. In my case, I needed the hash first to compare it with a value that was provided on upload. Only when they match can the data be transferred to storage.

Jules White Programming Cloud Services YouTube Video Series

In my search for information about what a web.xml exactly is and does, I ran across a video series on YouTube of Dr. Jules White who created over 70 videos explaining the basics and advanced topics of creating web services for mobile applications. The videos are roughly between 5 and 15 minutes long, so they are ideal for in-between watching, without sacrificing in content. You can binge them too, of course.

What I found most pleasing is that his presentation style is very informative and professional. There are no awkward pauses or anything else that would make me cringe. It’s very pleasant to watch and there’s a lot of good information in it, even for someone that already has a background in building web applications.

I created this list of links to all the individual videos because wanted to have more structure and information than a YouTube Playlist can provide in case I want to go back and watch something particular. Additionally, there’s a little sorting and numbering bug in the YouTube Playlist 😉

So, here you (or I) go.

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