How to Download Large Files Efficiently with Spring RestTemplate and Resume Support

Overview

In this tutorial we demonstrate several techniques for downloading large files with Spring's RestTemplate.

RestTemplate

RestTemplate is a synchronous, blocking HTTP client introduced in Spring 3. It may be deprecated in future versions in favor of the non‑blocking WebClient introduced in Spring 5.

Pitfalls

Downloading a file directly into memory can cause OutOfMemoryError. Using ResourceHttpMessageConverter loads the whole response into a ByteArrayInputStream, still causing memory pressure. Configuring ResourceHttpMessageConverter#supportsReadStreaming to return InputStreamResource fails because the underlying execute method closes the response stream before the input stream can be read.

Solution

Two possible solutions are:

Write a custom HttpMessageConverter that supports File as the return type.

Use RestTemplate.execute together with a custom ResponseExtractor to stream the response directly to a file.

We focus on the second approach.

Implementation of a non‑resumable download

File file = restTemplate.execute(
    FILE_URL,
    HttpMethod.GET,
    null,
    clientHttpResponse -> {
        File ret = File.createTempFile("download", "tmp");
        StreamUtils.copy(clientHttpResponse.getBody(),
                         new FileOutputStream(ret));
        return ret;
    });
Assertions.assertNotNull(file);
Assertions.assertThat(file.length()).isEqualTo(contentLength);

Checking server support for range requests

HttpHeaders headers = restTemplate.headForHeaders(FILE_URL);
Assertions.assertThat(headers.get("Accept-Ranges")).contains("bytes");
Assertions.assertThat(headers.getContentLength()).isGreaterThan(0);

Implementation of a resumable download

File file = /* previously downloaded part */;
restTemplate.execute(
    FILE_URL,
    HttpMethod.GET,
    request -> request.getHeaders().set(
        "Range",
        String.format("bytes=%d-%d", file.length(), contentLength)),
    response -> {
        StreamUtils.copy(response.getBody(),
                         new FileOutputStream(file, true));
        return file;
    });
Assertions.assertThat(file.length()).isLessThanOrEqualTo(contentLength);

When content length is unknown

String rangeHeader = String.format("bytes=%d-", file.length());
request.getHeaders().set("Range", rangeHeader);

Conclusion

We discussed common problems when downloading large files and provided a RestTemplate‑based solution, including a method for resumable (range) downloads. The full source code is available in the linked GitHub repository.