Download a file using Spring RestTemplate

Carvia Tech | October 17, 2020 | | 122 views

In this article we will explore 2 different ways to download a file over Http using Spring RestTemplate.

RestTemplate provides the following two ways to download a file from a remote Http url:

Using byte array (holding everything in memory)
Using ResponseExtractor (stream the response instead of loading it to memory)

We will cover both in details, with example java code.

Option 1. Using byte array

This is quite straight forward approach where we will use getForObject method to get a byte array from the remote service. This approach shall work fine for smaller file sizes, as entire file content is first loaded into memory.

RestTemplate getForObject for download a file

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.http.*;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RequestCallback;
import org.springframework.web.client.ResponseExtractor;
import org.springframework.web.client.RestTemplate;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.Arrays;
import java.util.Collections;

@RestController
public class DownloadController {

    private final RestTemplate restTemplate;

    @Autowired
    public DownloadController(RestTemplateBuilder builder) {
        this.restTemplate = builder.build();
    }

    @GetMapping("/test-download-v1")
    public void download() throws IOException {
        String url = "http://img.championat.com/news/big/l/c/ujejn-runi_1439911080563855663.jpg";
        byte[] imageBytes = restTemplate.getForObject(url, byte[].class);
        Files.write(Paths.get("image.jpg"), imageBytes);
    }

}

Optionally, you can also use RestTemplate.exchange() method to achieve the same results, as shown in below code:

Using exchange method

@GetMapping("/test-download-v2")
public void downloadFile() throws IOException {
    String url = "http://img.championat.com/news/big/l/c/ujejn-runi_1439911080563855663.jpg";
    HttpHeaders headers = new HttpHeaders();
    headers.setAccept(Collections.singletonList(MediaType.APPLICATION_OCTET_STREAM));
    HttpEntity<String> entity = new HttpEntity<>(headers);
    ResponseEntity<byte[]> response = restTemplate.exchange(url, HttpMethod.GET, entity, byte[].class);
    Files.write(Paths.get("demo1.jpg"), response.getBody());
}

Option 2. Using stream approach

If the file size is large, then previous approach might cause OOM error and its better to use ResponseExtractor to stream the results from remote service without loading the file content into memory.

Using RestTemplate with ResponseExtractor

@GetMapping("/test-download-v3")
public void downloadFile() throws IOException {
    String url = "http://img.championat.com/news/big/l/c/ujejn-runi_1439911080563855663.jpg";
    // 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 you can write the inputstream to a file or any other place
        Path path = Paths.get("downloadv3.jpg");
        Files.copy(response.getBody(), path);
        return null;
    };
    restTemplate.execute(url, HttpMethod.GET, requestCallback, responseExtractor);
}

That’s all.