Streaming upload via #Bean-provided RestTemplateBuilder buffers full file - spring

Question

I'm building a reverse-proxy for uploading large files (multiple gigabytes), and therefore want to use a streaming model that does not buffer entire files. Large buffers would introduce latency and, more importantly, they could result in out-of-memory errors.
My client class contains
#Autowired private RestTemplate restTemplate;
#Bean
public RestTemplate restTemplate(RestTemplateBuilder restTemplateBuilder) {
int REST_TEMPLATE_MODE = 1; // 1=streams, 2=streams, 3=buffers
return
REST_TEMPLATE_MODE == 1 ? new RestTemplate() :
REST_TEMPLATE_MODE == 2 ? (new RestTemplateBuilder()).build() :
REST_TEMPLATE_MODE == 3 ? restTemplateBuilder.build() : null;
}
and
public void upload_via_streaming(InputStream inputStream, String originalname) {
SimpleClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory();
requestFactory.setBufferRequestBody(false);
restTemplate.setRequestFactory(requestFactory);
InputStreamResource inputStreamResource = new InputStreamResource(inputStream) {
#Override public String getFilename() { return originalname; }
#Override public long contentLength() { return -1; }
};
MultiValueMap<String, Object> body = new LinkedMultiValueMap<String, Object>();
body.add("myfile", inputStreamResource);
HttpHeaders headers = new HttpHeaders();
headers.setContentType(MediaType.MULTIPART_FORM_DATA);
HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(body,headers);
String response = restTemplate.postForObject(UPLOAD_URL, requestEntity, String.class);
System.out.println("response: "+response);
}
This is working, but notice my REST_TEMPLATE_MODE value controls whether or not it meets my streaming requirement.
Question: Why does REST_TEMPLATE_MODE == 3 result in full-file buffering?
References:
How to forward large files with RestTemplate?
How to send Multipart form data with restTemplate Spring-mvc
Spring - How to stream large multipart file uploads to database without storing on local file system -- establishing the InputStream
How to autowire RestTemplate using annotations
Design notes and usage caveats, also: restTemplate does not support streaming downloads

Answer 1

In short, the instance of RestTemplateBuilder provided as an #Bean by Spring Boot includes an interceptor (filter) associated with actuator/metrics -- and the interceptor interface requires buffering of the request body into a simple byte[].
If you instantiate your own RestTemplateBuilder or RestTemplate from scratch, it won't include this by default.
I seem to be the only person visiting this post, but just in case it helps someone before I get around to posting a complete solution, I've found a big clue:
restTemplate.getInterceptors().forEach(item->System.out.println(item));
displays...
org.SF.boot.actuate.metrics.web.client.MetricsClientHttpRequestInterceptor
If I clear the interceptor list via setInterceptors, it solves the problem. Furthermore, I found that any interceptor, even if it only performs a NOP, will introduce full-file buffering.
public class SimpleClientHttpRequestFactory { ...
I have explicitly set bufferRequestBody = false, but apparently this code is bypassed if interceptors are used. This would have been nice to know earlier...
#Override
public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
HttpURLConnection connection = openConnection(uri.toURL(), this.proxy);
prepareConnection(connection, httpMethod.name());
if (this.bufferRequestBody) {
return new SimpleBufferingClientHttpRequest(connection, this.outputStreaming);
}
else {
return new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming);
}
}
public abstract class InterceptingHttpAccessor extends HttpAccessor { ...
This shows that the InterceptingClientHttpRequestFactory is used if the list of interceptors is not empty.
/**
* Overridden to expose an {#link InterceptingClientHttpRequestFactory}
* if necessary.
* #see #getInterceptors()
*/
#Override
public ClientHttpRequestFactory getRequestFactory() {
List<ClientHttpRequestInterceptor> interceptors = getInterceptors();
if (!CollectionUtils.isEmpty(interceptors)) {
ClientHttpRequestFactory factory = this.interceptingRequestFactory;
if (factory == null) {
factory = new InterceptingClientHttpRequestFactory(super.getRequestFactory(), interceptors);
this.interceptingRequestFactory = factory;
}
return factory;
}
else {
return super.getRequestFactory();
}
}
class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest { ...
The interfaces make it clear that using InterceptingClientHttpRequest requires buffering body to a byte[]. There is not an option to use a streaming interface.
#Override
public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {