Springs RestTemplate default connection pool

Question

Just wondering if RestTemplate out of the box uses connection pooling or does it simply establish a new connection each time ?

Answer 1

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

Answer 2

By default RestTemplate creates new Httpconnection every time and closes the connection once done.

If you need to have a connection pooling under rest template then you may use different implementation of the ClientHttpRequestFactory that pools the connections.

new RestTemplate(new HttpComponentsClientHttpRequestFactory())

Answer 3

Yes, Spring RestTemplateBuilder uses apache httpclient for pooling (usage). RestTemplateBuilder creates HttpComponentsClientHttpRequestFactory and uses HttpClientBuilder. HttpClientBuilder is the most interesting (source):

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

So, by default, pool size per route (host) is equal to 5. Total pool size = 10. To check connection pool logging set logging level as follows:

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

Answer 4

We can use okhttpclient underneath spring's rest template to use connection pooling. A detailed blog on this below

https://www.bytesville.com/changing-httpclient-in-spring-resttemplate/