Spring Cloud: Feign and Http Connection Pooling

From investigation I will try to answer my own question:

Spring Cloud Feign uses Netflix Feign. Netflix Feign in turn creates connections using java.net.HttpURLConnection which makes use of 'persistent connections' but not a connection pool.

It is possible to override the Client, for example using Apache HttpClient instead, and Netflix provide a library for this (feign-httpclient). When using this approach the connection pool size can be set using SystemProperties.

In Spring Cloud Brixton it seems that if Apache HttpClient or OkHttpClient are available (via @ConditionalOnClass) then they are automatically used.