Springs RestTemplate default connection pool

I believe RestTemplate doesn’t use a connection pool to send requests, it uses a SimpleClientHttpRequestFactory that wraps a standard JDK’s HttpURLConnection opening and closing the connection.

Indeed you can configure RestTemplate to use a pooled implementation such as HttpComponentsClientHttpRequestFactory but most-likely you might also need to configure some settings to prevent requests from timing out.

I have blogged about this issue at Troubleshooting Spring's RestTemplate Requests Timeout

Yes, Spring RestTemplateBuilder uses Apache HttpClient for pooling (usage). RestTemplateBuilder creates HttpComponentsClientHttpRequestFactory and uses HttpClientBuilder.

HttpClientBuilder, by default, sets pool size per route (host) to 5 and total pool size to 10 (source):

s = System.getProperty("http.maxConnections", "5"); 
int max = Integer.parseInt(s); 
poolingmgr.setDefaultMaxPerRoute(max); 
poolingmgr.setMaxTotal(2 * max); 
            

To check connection pool logging set logging level as follows:

org.apache.http.impl.conn.PoolingHttpClientConnectionManager=TRACE