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

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

Answer 2

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 3

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 4

You can create a Bean for RestTemplate and config there :

    @Bean
    public RestTemplate restTemplate() {
        PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();
        connectionManager.setMaxTotal(100);
        connectionManager.setDefaultMaxPerRoute(20);

    RequestConfig requestConfig = RequestConfig
        .custom()
        .setConnectionRequestTimeout(5000) // timeout to get connection from pool
        .setSocketTimeout(5000) // standard connection timeout
        .setConnectTimeout(5000) // standard connection timeout
        .build();

    HttpClient httpClient = HttpClientBuilder.create()
                                             .setConnectionManager(connectionManager)
                                             .setDefaultRequestConfig(requestConfig).build();

    ClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);


    return new RestTemplate(requestFactory);
}

And there are a lot config you can do. Refer to https://hc.apache.org/httpcomponents-client-ga/httpclient/apidocs/org/apache/http/impl/client/HttpClientBuilder.html

EDIT

If you want to use micrometer metrics you should also use a RestTemplateBuilder for constructing the RestTemplate.

Answer 5

In case of using Spring Boot configured with Apache HttpClient (having org.apache.httpcomponents:httpclient library in dependencies)

There is a default connection pool configured by PoolingHttpClientConnectionManager

Default concurrent settings for connections (you can find more about default settings here https://hc.apache.org/httpcomponents-client-4.5.x/current/tutorial/html/connmgmt.html):

PoolingHttpClientConnectionManager maintains a maximum limit of connections on a per route basis and in total. Per default this implementation will create no more than 2 concurrent connections per given route and no more 20 connections in total. For many real-world applications these limits may prove too constraining, especially if they use HTTP as a transport protocol for their services.

Answer 6

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/