java - Getting InputStream with RestTemplate

Question

I am using URL class to read an InputStream from it. Is there any way I can use RestTemplate for this?

InputStream input = new URL(url).openStream();
JsonReader reader = new JsonReader(new InputStreamReader(input, StandardCharsets.UTF_8.displayName())); 

How can I get InputStream with RestTemplate instead of using URL?

Answer 1

The previous answers are not wrong, but they don't go into the depth that I like to see. There are cases when dealing with low level InputStream is not only desirable, but necessary, the most common example being streaming a large file from source (some web server) to destination (a database). If you try to use a ByteArrayInputStream, you will be, not so surprisingly, greeted with OutOfMemoryError. Yes, you can roll your own HTTP client code, but you'll have to deal with erroneous response codes, response converters etc. If you are already using Spring, looking to RestTemplate is a natural choice.

As of this writing, spring-web:5.0.2.RELEASE has a ResourceHttpMessageConverter that has a boolean supportsReadStreaming, which if set, and the response type is InputStreamResource, returns InputStreamResource; otherwise it returns a ByteArrayResource. So clearly, you're not the only one that asked for streaming support.

However, there is a problem: RestTemplate closes the response soon after the HttpMessageConverter runs. Thus, even if you asked for InputStreamResource, and got it, it's no good, because the response stream has been closed. I think this is a design flaw that they overlooked; it should've been dependent on the response type. So unfortunately, for reading, you must consume the response fully; you can't pass it around if using RestTemplate.

Writing is no problem though. If you want to stream an InputStream, ResourceHttpMessageConverter will do it for you. Under the hood, it uses org.springframework.util.StreamUtils to write 4096 bytes at a time from the InputStream to the OutputStream.

Some of the HttpMessageConverter support all media types, so depending on your requirement, you may have to remove the default ones from RestTemplate, and set the ones you need, being mindful of their relative ordering.

Last but not the least, implementations of ClientHttpRequestFactory has a boolean bufferRequestBody that you can, and should, set to false if you are uploading a large stream. Otherwise, you know, OutOfMemoryError. As of this writing, SimpleClientHttpRequestFactory (JDK client) and HttpComponentsClientHttpRequestFactory (Apache HTTP client) support this feature, but not OkHttp3ClientHttpRequestFactory. Again, design oversight.

Edit: Filed ticket SPR-16885.