From the design of the API, it appears that you should be able to use HttpOp#setDefaultHttpClient()
to modify the default HttpClient
utilized within Jena.
HttpOp#execHttpGet(String,String)
is delegated to by LocatorURL#open(String)
, which is called by the StreamManager
used by RDFDataMgr
. Elsewhere, HttpOp#ensureClient(...) substitutes in the default client, if it is present.