Why “FormDataMultiPart” type parameter is treated

问题:

I am writing little web app uploading the file to a web server. I got everything working, but I became puzzled by the fact that almost all parameters send from the client (browser) must be on the server side injected with the word @FormDataParam except FormDataMultiPart type parameter. Can somebody explain that to me, please?

Regards, Janusz

回答1:

Generally, all entity body parameters are the parameter without any annotation. Such as with JSON or XML, you would see

@POST
@Consumes({"application/json", "application/xml"})
public Response post(RequestEntity entity) {
}

Here, the MessageBodyReader that handles JSON or XML would be used to handle deserializing the entity stream into the ResponseEntity.

This is the same with FormDataMultiPart. There is a MessageBodyReader to handle deserializing the entire multipart stream and creating the FormDataBodyPart for the JAX-RS runtime to pass to the resource method when it is called.

@FormDataParam is treated more like @FormParam, @PathParam, @QueryParam, etc. We can have multiple parameters. Each parameter for this annotation represent a single part of the multipart request.

So we have the option to either get the entire request into a single entity with FormDataMultiPart or we can get them pre-separated into individual parts. If we used FormDataMultiPart, we would have to extract all the parts manually from it. We would use this in such a case where the name of each part is not known. Or there are many parts, where declaring each one is not pleasing to the coder.