Custom RestTemplate using requestFactory of RestTemplateBuilder in SpringBoot 2.1.x is not backward compatible with version 1.5.x

After digging deeper into the source code of RestTemplateBuilder of Spring Boot 2.1.x, I found that they have removed the method requestFactory(ClientHttpRequestFactory requestFactory). That means you can no longer inject the ClientHttpRequestFactory object into requestFactory method.

But, it accept a Supplier<ClientHttpRequestFactory> as the input now. So if you have only one restTemplate and one requestFactory, all you need to do is register a HttpComponentsClientHttpRequestFactory bean in Spring context and pass a ClientHttpRequestFactorySupplier to requestFactory method. The supplier will automatically detect your HttpComponentsClientHttpRequestFactory and return you the required RestTemplate.

Refer the below code for that:

@Bean
  public HttpComponentsClientHttpRequestFactory httpComponentsClientHttpRequestFactory() {
    PoolingHttpClientConnectionManager poolingConnectionManager = new PoolingHttpClientConnectionManager();
    poolingConnectionManager.setMaxTotal(restTemplateProps.getMaxConnectionsPerPool());
    poolingConnectionManager.setDefaultMaxPerRoute(restTemplateProps.getMaxDefaultConnectionPerRoute());
    CloseableHttpClient client = HttpClientBuilder.create().setConnectionManager(poolingConnectionManager).build();
    HttpComponentsClientHttpRequestFactory clientHttpRequestFactory =
        new HttpComponentsClientHttpRequestFactory(client);
    clientHttpRequestFactory.setConnectTimeout(restTemplateProps.getConnectionTimeout());
    clientHttpRequestFactory.setReadTimeout(restTemplateProps.getSocketTimeout());
    return clientHttpRequestFactory;
  }

  @Bean
  public RestTemplate authRestTemplate(RestTemplateBuilder restTemplateBuilder) {
    return restTemplateBuilder.requestFactory(new ClientHttpRequestFactorySupplier()).build();
  }

For those who are interested in all the changes made to RestTemplateBuilder in SpringBoot 2.1.x, please refer this.

Here is a more general solution if you have multiple restTemplates which need different requestFactories (and not an auto-injection of the first one found) :

@Configuration
public class Config {

  @Bean
  public RestTemplate restTemplate(RestTemplateBuilder builder, ApplicationContext ctx) {
    return builder.requestFactory(new MyRequestFactorySupplier()).build();
  }

  class MyRequestFactorySupplier implements Supplier<ClientHttpRequestFactory> {

    @Override
    public ClientHttpRequestFactory get() {

      // Using Apache HTTP client.
      HttpClientBuilder clientBuilder = HttpClientBuilder.create();
      HttpClient httpClient = clientBuilder.build();
      HttpComponentsClientHttpRequestFactory requestFactory = new HttpComponentsClientHttpRequestFactory(httpClient);
      requestFactory.setBufferRequestBody(false); // When sending large amounts of data via POST or PUT, it is recommended to change this property to false, so as not to run out of memory.
      return requestFactory;
    }

  }

}

Or even easier, just:

 restTemplatebuilder.requestFactory(() -> new 
    HttpComponentsClientHttpRequestFactory(httpClient)).build();