Download Large file from server using REST template Java Spring MVC

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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.

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