Remove buffering in ClientHttpRequestFactory implementations

[poutsma]

The ClientHttpRequestFactory hierarchy provides an HTTP client libraries abstraction to the RestTemplate. This abstraction was introduced in Spring Framework 3.0, when memory constraints are different than they are today. In particular, most ClientHttpRequestFactory implementations buffer the entire request body in memory before sending it on the wire. Because the ClientHttpRequest exposes the request body through a OutputStream, and most HTTP client libraries do not expose such a stream, a ByteArrayOuputStream is used, converted to a byte array that is sent as request body.

Instead of exposing an output stream, which necessitates buffering the request body, we can store a callback that writes to the HTTP output stream, and use that callback when the HTTP connection has been made. In Spring Framework 4 we introduced the StreamingHttpOutputMessage to support this alternative, but this interface was only implemented by the Apache HttpClient request factory.

We should review request buffering in the ClientHttpRequestFactory hierarchy, and make sure that implementations can support the StreamingHttpOutputMessage do so.

[zaenk]

This change breaks implementations where RestTemplate is initialised with HttpClients.createMinimal(HttpClientConnectionManager).

These implementations would fail with this excpetion

Exception in thread "DefaultDispatcher-worker-1" java.lang.NullPointerException: Target host                                                                                                                                                         
    at java.base/java.util.Objects.requireNonNull(Unknown Source)                                                                                                                                                                                    
    at org.apache.hc.core5.util.Args.notNull(Args.java:169)                                                                                                                                                                                          
    at org.apache.hc.client5.http.impl.classic.MinimalHttpClient.doExecute(MinimalHttpClient.java:115)                                                                                                                                               
    at org.apache.hc.client5.http.impl.classic.CloseableHttpClient.execute(CloseableHttpClient.java:87)                                                                                                                                              
    at org.apache.hc.client5.http.impl.classic.CloseableHttpClient.execute(CloseableHttpClient.java:55)                                                                                                                                              
    at org.apache.hc.client5.http.classic.HttpClient.executeOpen(HttpClient.java:183)                                                                                                                                                                
    at org.springframework.http.client.HttpComponentsClientHttpRequest.executeInternal(HttpComponentsClientHttpRequest.java:95)                                                                                                                      
    at org.springframework.http.client.AbstractStreamingClientHttpRequest.executeInternal(AbstractStreamingClientHttpRequest.java:70)                                                                                                                
    at org.springframework.http.client.AbstractClientHttpRequest.execute(AbstractClientHttpRequest.java:66)                                                                                                                                          
    at org.springframework.web.client.RestTemplate.doExecute(RestTemplate.java:889)                                                                                                                                                                  
    at org.springframework.web.client.RestTemplate.execute(RestTemplate.java:790)                                                                                                                                                                    
    at org.springframework.web.client.RestTemplate.postForObject(RestTemplate.java:507)                                                                                                                                                              

The issue arised when updating from Spring Boot 3.0.x to 3.2.x.

The cause of this is that the MiminalHttpClient does not fall back to inspecting the request object for the target host as InternalHttpClient does.

The workaround is simple - instead fo MinimalHttpClient, use HttpClientBuilder:

HttpClientBuilder.create()
            .setConnectionManager(httpClientManager)
            .build()