Reactive way to read and parse file from resources using WebFlux?

Question

I wonder what is the correct way to read, parse and serve a file from resources.

Currently, I do something like this:

fun getFile(request: ServerRequest): Mono<ServerResponse> {
    val parsedJson =
        objectMapper.readValue(readFile("fileName.json"), JsonModel::class.java)

    // modify parsed json

    return ok().contentType(APPLICATION_JSON).bodyValue(parsedJson)
}

private fun readFile(fileName: String) =
    DefaultResourceLoader()
        .getResource(fileName)
        .inputStream.bufferedReader().use { it.readText() }

I've noticed JsonObjectDecoder class in Netty, but I don't know if can be applied to my use case.

What is the reactive way to do read/parse resource file then?

All i/o operations are blocking. The reading of the file should by wrapped in a Mono#fromCallable and placed on its own scheduler thats bounded to avoid thread starvation. You can read more about blocking calls here projectreactor.io/docs/core/release/reference/… — Toerktumlare
– Toerktumlare, Commented Apr 10, 2020 at 15:00

pixel · Accepted Answer · 2020-04-13 15:38:05Z

4

After expanding @vins answer, I've came to following solution:

Jackson2JsonDecoder()
    .decodeToMono(
        DataBufferUtils.read(
            DefaultResourceLoader()
                .getResource("$fileName.json"),
            DefaultDataBufferFactory(),
            4096
        ),
        ResolvableType.forClass(JsonModel::class.java), null, null
    )
    .map { it as JsonModel }

answered Apr 13, 2020 at 15:38

pixel

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vins · Accepted Answer · 2020-04-12 23:15:47Z

2

You can take a look at Flux.using here for file read.

As you are using Spring framework, You can also take a look at the DataBufferUtils.

This DataBufferUtils uses AsynchronousFileChannel to read the file and it also internally uses Flux.using to release the file once the file is read / cancelled by the subscriber.

@Value("classpath:somefile.json")
private Resource resource;


@GetMapping("/resource")
public Flux<DataBuffer> serve(){
   return DataBufferUtils.read(
            this.resource,
            new DefaultDataBufferFactory(),
            4096
    );
}

answered Apr 12, 2020 at 23:15

vins

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