Step by Step Procedure of Spring Webflux multipart file upload and read each line without saving it

Spring Webflux

I’ve been working on spring webflux for a while. And in my experience, uploading and reading files in this framework is quite a hassle.

Today I am going to tell about uploading a file using spring webflux. And the most amazing part is that I am not going to save the file but will read it. I will also be going to check whether all the data of the file match my regex criteria or not using powerful Java stream API.

The real-life problem description I’ve faced:
Have to upload any type of file but the condition is lines of the file have to be separated by a new line. There is no way you can save the file on the server. Create a list of String by reading the file, where each item of the list will be a single line of the file. Each item must have to match a validation rule otherwise, discard the whole file as it is corrupted. So, the summary is: upload -> read -> check -> list of string from the file without saving it.

So, the rough steps are:

• Controller to consume the multipart file into Flux FilePart
• Converting the file parts into Flux of String using dataBuffer
• Collect all the data parts string and process them
• Check validity using Java Stream API and regex
• And tons of magic

Sounds scary? Well, I will explain to you step by step. So, what are we waiting for? Let’s dig in.

1. Controller

FileUploadController

This part is easy. This is a post endpoint that is able to accept multiple files. URL part is upload-flux and must have to use consumes = MediaType.MULTIPART_FORM_DATA_VALUE . As we can see I have used:

... upload(@RequestPart("files") Flux<FilePart> filePartFux)

Here, part of the request files will be automatically injected as Flux<FilePart> into the method by spring webflux.

N.B: Remember that,
1. To upload multiple file you must have to use Flux<FilePart> .
2. To upload single file you have to use Mono<FilePart> or FilePart .
3. Mono<MultiValueMap<String, Part>> can be used for both case. But in that case you have to find out the FilePart(s) from the map by key. Like for this tutorial the key is files for both single and multiple file.

For this tutorial I am going to use Flux<FilePart>

2. Service

From the controller layer,filePartFlux is now passed to the service layer. I have divided the work of this service into two methods. Let’s try to understand these methods one by one.

i. First method:

getLines() method

In this method the filePartFlux is directly passed from the controller layer. Then we flatmap filePartFlux and get a new Flux<String> stream.

filePartFlux.flatMap(filePart ->
    filePart.content().map(dataBuffer -> {
        byte[] bytes = new byte[dataBuffer.readableByteCount()];
        dataBuffer.read(bytes);
        DataBufferUtils.release(dataBuffer);

        return new String(bytes, StandardCharsets.UTF_8);
    }))

filePartFlux will emit filepart into the flatmap. Then we access the content of filepart and map to create a Flux of String. Inside the map, we get dataBuffer , which is emitted from the content(). Here we have to keep in mind that a certain amount of bytes are readable from this dataBuffer. So, we take a byte array variable bytes with a length of dataBuffer.readableByteCount()
Then we fill the bytes array by reading data from dataBuffer like dataBuffer.read(bytes) . Then we free the dataBuffer by releasing it like DataBufferUtils.release(dataBuffer) . Then we convert the bytes into String and return it. So, when this full process will be completed we will get a new Flux<String> stream. Now let's see the rest of the method.

.map(this::processAndGetLinesAsList)
.flatMapIterable(Function.identity());

Now, we get every String from the Flux<String> stream and by processing them via processAndGetLinesAsList method we generate another Flux<String> stream from flatMapIterable(Function.identity()). processAndGetLinesAsList method is described in the next section. For processing and validation checks we need another method. After validation check, if the file is corrupted then an empty Flux<String> will be returned from here.

ii. Second Method:

processAndGetLinesAsList() method

This is not so scary as it looks. Just read and translate as it is written here.

In this method, we have added some validation over our data. To do that at first we have to split each string like string::lines

At this point, you might ask why we are doing this? Well, we need every line from the file. But, you know what, as we are getting this string variable’s value from FilePart & DataBuffer , it is not guaranteed that every lines variable from the Flux stream will be a single line from the file. Because we have generated this list from FilePart & dataBuffer , so each string will contain multiple lines from the file as the file is read part by part and the strings are generated from each part respectively.

So, what we have done here is, we have created a Supplier that will supply a Stream of string. In the Lambda function, we have made a stream from the splitted string .

The next statement is our validation checkpoint. Here we are checking every string of the stream (eventually that means every line of the uploaded file) against our REGEX rules using java’s stream API.

streamSupplier.get().allMatch(line -> line.matches(Util.YOUR_REGEX))

In short form, this is equivalent to:

stream.allMatch(value -> condition)

And this will return true only if all the value of the stream meets the condition successfully. What a fantabulous magic it is. Mind-blowing, isn’t it?

So, in our code, if all is well, then the stream will be converted into a list and returned. Otherwise, that means the file’s value(s) is/are against our rules, in other words, it is a corrupted file. And thus empty list will be returned.

And that’s it, the Flux of String is returned all the way through to the client. Enjoy the List of String. Phewww, I think we can take a break and have a drink now. Oh, man.

But wait, what on earth anybody will want to upload a file through a RestAPI and get the lines of the file as a response? That doesn’t make sense, right? Maybe s/he wants to trigger some other operation by this list of strings. Or, one could publish these strings to a message broker queue. Or, what if one wants to save these tremendous numbers of lines in a database like DynamoDB?

Well, that’s a story for another step-by-step tutorial.

N.B:
Please share and leave a comment for any questions or feedback.
To see the full tutorial in action with much more ingredients, browse the project on GitHub:
https://github.com/eaiman-shoshi/MultipartFileUpload