HTTP proxy cannot be configured for Apache HttpClient 4/5 using system properties

Issue

• I have a web application in JBoss trying to call an external web service through an HTTP proxy using Apache HttpClient 4.3.3.
• I have tried the following settings to no avail:

• In standalone.sh:

  JAVA_OPTS="$JAVA_OPTS -Dhttp.proxyHost=proxy.sample.com -Dhttp.proxyPort=7070"
  

• In standalone.xml:

  <system-properties>
      <property name="http.proxyHost" value="proxy.sample.com"></property>
      <property name="http.proxyPort" value="7070"></property>
      <property name="http.nonProxyHosts" value="localhost"></property> 
  </system-properties>
  

• And also in environment:

  HTTP_PROXY=proxy.sample.com:7070
  http_proxy=proxy.sample.com:7070
  

• After approximately 60 seconds I get "ERROR 500: Internal Server Error".

• My application code looks like this:

  CloseableHttpClient httpclient = HttpClients.createDefault();
  URI uri = ...
  HttpGet httpget = new HttpGet(uri);
  CloseableHttpResponse response = httpclient.execute(httpget);