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.

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