Springs RestTemplate default connection pool

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

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())

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](

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)

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

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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

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**EDIT**

If you want to use micrometer metrics you should also use a [RestTemplateBuilder][1] for constructing the RestTemplate.


[1]:

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Yes, Spring `RestTemplateBuilder` uses Apache HttpClient for pooling ([usage][1]).
`RestTemplateBuilder` creates `HttpComponentsClientHttpRequestFactory` and uses `HttpClientBuilder`.

`HttpClientBuilder`, by default, sets pool size per route (host) to 5 and total pool size to 10 ([source][2]):

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

[1]:

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[2]:

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### 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

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):

> 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.