How to resume download with Spring RestTemplate?

lucidMonkey :

I'm using RestTemplate to download a file from a Nexus Server (about 350 mb). The code provided in this post works well for that purpose:

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);

I'd like to check if file exists, then attempt to resume download:

...
if(Files.exists(path)) {
    log.info("{} exists. Attempting to resume download", path);
    Files.write(path, StreamUtils.copyToByteArray(response.getBody()), StandardOpenOption.APPEND);
} else {
    Files.copy(response.getBody(), path);
}

But this just just results in OOM error:

java.lang.OutOfMemoryError: Java heap space
    at java.util.Arrays.copyOf(Unknown Source) ~[na:1.8.0_191]
    at java.io.ByteArrayOutputStream.grow(Unknown Source) ~[na:1.8.0_191]
    at java.io.ByteArrayOutputStream.ensureCapacity(Unknown Source) ~[na:1.8.0_191]
    at java.io.ByteArrayOutputStream.write(Unknown Source) ~[na:1.8.0_191]
    ...

I have tested the call using the Range header with curl and am certain Nexus supports that. I am setting it this way:

long bytes = path.toFile().length();
...
request.getHeaders().setRange(Arrays.asList(HttpRange.createByteRange(bytes)));

I'm guessing the memory error above occurs because InputStream blocks. So I try to use Channel/Buffer instead:

...
try {
    if(Files.exists(path)) {
        log.info("{} exists. Attempting to resume download", path);
        ReadableByteChannel channel = Channels.newChannel(response.getBody());

        FileChannel fileChannel = FileChannel.open(path, StandardOpenOption.APPEND);
        fileChannel.tryLock();
        ByteBuffer buffer = ByteBuffer.allocate(4096);
        int bytesRead = 0;
        while((bytesRead = channel.read(buffer)) != -1) {
            fileChannel.write(buffer);
            buffer.clear();
        }

        fileChannel.close();    
    } else {
        Files.copy(response.getBody(), path);
    }
...

This at least writes some data to the file but still fails. I don't have much experience with java.nio facilities so any help is appreciated.

*edit: Any answers are appreciated but I am forced to use JDK 8 for this project.

Alexandar Petrov :

You need the following lines of code if you are working with large files to ensure the stream is not read in memory:

SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
requestFactory.setBufferRequestBody(false);     
restTemplate.setRequestFactory(requestFactory);