Java HTTP Keep-Alive Summary

gistfile1.txt

* HTTP Keep alives are enabled by default and hold a connection up to a couple of minutes.
* By default up to 5 connections per destination are hold up
* You can disable it by calling explicitly disconnect
   (Note: For android you should call disconnect always)
* For re-use the following has to be true
    * share similar settings like same **instance** of SSLFactory
    * Server has to accept keep-alive (no close)
    * Fully consume inputstream, outputstream and errorstream and close it
      (Since JDK 6: spawns a extra thread that consume some remaining data if the length is longer it's not used for keep-alives)

With a Debugger you can see it this example will spawn a Keep-Alive thread:
   
try (OutputStream out = con.getOutputStream()) {
   // write
} catch (IOException ioEx) {
   //handle
}

// if no errrors

try {
   try (InputStream in = con.getInputStream()) {
       // have to be inside, because it calls getInputStream too
       con.getResponse()
       // read fully
       System.out.println(in.getClass());
   } catch (IOException ioEx) {
       ioEx.printStackTrace();
       try (InputStream errStream = con.getErrorStream()) {
           if (errStream != null)
               // read fully
               errStream.skip(Integer.MAX_VALUE);
       } catch (IOException errIoEx) {
         // re-throw only original exception
       }

       throw ioEx;
   }
} catch (IOException ioEx) {
   //
}

Sources:
* Java Source-Code: KeepAliveCleaner
* https://stackoverflow.com/questions/9943351/httpsurlconnection-and-keep-alive
* https://docs.oracle.com/javase/7/docs/technotes/guides/net/http-keepalive.html
* https://en.wikipedia.org/wiki/Keepalive