You can log request and responses by specifying-Djdk.httpclient.HttpClient.log=requests on the Java command line.
As for testing/mocking you might want to have a look at the offline test:
http://hg.openjdk.java.net/jdk/jdk/file/tip/test/jdk/java/net/httpclient/offline/
Depending on what you are looking to achieve you could use a “DelegatingHttpClient” to intercept and log requests and responses too.
Besides the Java API documentation there’s also some high level documentation at http://openjdk.java.net/groups/net/httpclient/index.html
Additional note:
The jdk.httpclient.HttpClient.log property is an implementation specific property whose value is a comma separated list which can be configured on the Java command line for diagnosis/debugging purposes with the following values:
-Djdk.httpclient.HttpClient.log=
errors,requests,headers,
frames[:control:data:window:all],content,ssl,trace,channel,all