Download Large file from server using REST template Java Spring MVC

As @bernie mentioned you can use WebClient to achieve this:

public Flux<DataBuffer> downloadFileUrl( ) throws IOException {

    WebClient webClient = WebClient.create();

    // Request service to get file data
    return Flux<DataBuffer> fileDataStream = webClient.get()
            .uri( this.fileUrl )
            .accept( MediaType.APPLICATION_OCTET_STREAM )
            .retrieve()
            .bodyToFlux( DataBuffer.class );
}

@GetMapping( produces = MediaType.APPLICATION_OCTET_STREAM_VALUE )
public void downloadFile( HttpServletResponse response ) throws IOException
{
    Flux<DataBuffer> dataStream = this.downloadFileUrl( );

    // Streams the stream from response instead of loading it all in memory
    DataBufferUtils.write( dataStream, response.getOutputStream() )
            .map( DataBufferUtils::release )
            .blockLast();
}

You can still use WebClient even if you don't have Reactive Server stack - Rossen Stoyanchev (a member of Spring Framework team) explains it quite well in the Guide to "Reactive" for Spring MVC Developers presentation. During this presentation, Rossen Stoyanchev mentioned that they thought about deprecating RestTemplate, but they have decided to postpone it after all, but it may still happen in the future!

The main disadvantage of using WebClient so far it's a quite steep learning curve (reactive programming), but I think there is no way to avoid in the future, so better to take a look on it sooner than latter.

Here is how I do it. Based on hints from this Spring Jira issue.

RestTemplate restTemplate // = ...;

// Optional Accept header
RequestCallback requestCallback = request -> request.getHeaders()
        .setAccept(Arrays.asList(MediaType.APPLICATION_OCTET_STREAM, MediaType.ALL));

// Streams the response instead of loading it all in memory
ResponseExtractor<Void> responseExtractor = response -> {
    // Here I write the response to a file but do what you like
    Path path = Paths.get("some/path");
    Files.copy(response.getBody(), path);
    return null;
};
restTemplate.execute(URI.create("www.something.com"), HttpMethod.GET, requestCallback, responseExtractor);

From the aforementioned Jira issue:

Note that you cannot simply return the InputStream from the extractor, because by the time the execute method returns, the underlying connection and stream are already closed.

Update for Spring 5

Spring 5 introduced the WebClient class which allows asynchronous (e.g. non-blocking) http requests. From the doc:

By comparison to the RestTemplate, the WebClient is:

• non-blocking, reactive, and supports higher concurrency with less hardware resources.
• provides a functional API that takes advantage of Java 8 lambdas.
• supports both synchronous and asynchronous scenarios.
• supports streaming up or down from a server.

To get WebClient in Spring Boot, you need this dependency:

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-webflux</artifactId>
</dependency>

For the moment, I'm sticking with RestTemplate because I don't want to pull in another dependency only to get access to WebClient.