Planned lessonJNR-023-L03

Streaming downloads

Lesson 3 of 6
Junior

On This Lesson 4 topics

In this lesson
Planned topics
Draft outline details
Practice

This lesson is planned. The outline below lists the topics it will cover. The full lesson has not been published yet.

In this lesson

send big files without loading them into memory.

Planned topics

Reading a whole file into a byte[] uses memory per request; ten parallel downloads can crash the server.
Return a Resource (for example FileSystemResource) or a StreamingResponseBody; Spring copies it in chunks.
Set Content-Type from your stored record, and Content-Length when known.
Content-Disposition: attachment; filename="..." makes browsers download; inline displays.
Return 404 if the file record or file is missing.

Draft outline details

Demonstration

java

@GetMapping("/images/{id}")
ResponseEntity<Resource> download(@PathVariable String id) {
    Resource r = storage.load(id);
    return ResponseEntity.ok().contentType(MediaType.IMAGE_PNG).body(r);
}

Check your understanding

Why not return a byte[]?

Show answer

The whole file sits in memory for each request.

Practice

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