Spring Boot Controller export an Excel

Question

I have a java/spring boot application where I want to build an API endpoint that creates and returns a downloadable excel file. Here is my controller endpoint:

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Answer 1

Basically , there are few points that you first need to understand & then decide what you want to do ,

1.Is excel creation on disk needed or can you stream it from memory?

If its a download pop up, user might keep it open for long time & memory be occupied during that period ( disadvantage of in memory approach ) .

Secondly, if generated file has to be new for each request ( i.e. data to be exported is different ) then there is no point in keeping it at disk ( disadvantage of in disk approach ) .

Thirdly, it will be hard for an API code to do disk clean up because you never know in advance as when user will finish up his down load ( disadvantage of in disk approach ) .

Answer by Fizik26 is this In - Memory approach where you don't create a file on disk. . Only thing from that answer is that you need to keep track of length of array out.toByteArray() & that can easily be done via a wrapper class.

2.While downloading a file , your code needs to stream a file chunk by chunk - thats what Java streams are for. Code like below does that.

return ResponseEntity.ok().contentLength(inputStreamWrapper.getByteCount())
            .contentType(MediaType.parseMediaType("application/vnd.ms-excel"))
            .cacheControl(CacheControl.noCache())
            .header("Content-Disposition", "attachment; filename=" + "SYSTEM_GENERATED_FILE_NM")
            .body(new InputStreamResource(inputStreamWrapper.getByteArrayInputStream()));

and inputStreamWrapper is like ,

public class ByteArrayInputStreamWrapper {
    private ByteArrayInputStream byteArrayInputStream;
    private int byteCount;


    public ByteArrayInputStream getByteArrayInputStream() {
    return byteArrayInputStream;
    }


    public void setByteArrayInputStream(ByteArrayInputStream byteArrayInputStream) {
    this.byteArrayInputStream = byteArrayInputStream;
    }


    public int getByteCount() {
    return byteCount;
    }


    public void setByteCount(int byteCount) {
    this.byteCount = byteCount;
    }

}

Regarding file name, if file name is not an input to end point - that means ..its system generated ( a combination of constant string plus a variable part per user ). I am not sure why you need to get that from resource.

You won't need this wrapper if use - org.springframework.core.io.ByteArrayResource