Distributed computing JAVA/JSON

jhpc/gmi/00_ChangesFromBookVersion

This is the latest (improved) code for Generic Mailbox Invocations, a messaging framework for Remote Procedure Calls (RPC) with all of the benefits of RMI minus the hacks. This is not intended to be user documentation. This document will probably disappear at some point. This only identifies the recent changes to GMI and what you'll need to do to make GMI and your code coexist happily and merrily. 1. All code is now contained in a package named jhpc.gmi. This package will need to be imported in all code. It is recommended that you make this change immediately and keep the .jar file in your class path. 2. RemoteCallServerDispatcher had a bug. When a remote exception occurred, rather than dying gracefully, the client was left hanging. Needless to say, I should have documented this problem. The new version of this class includes full support for remote exceptions. If you have your own defined exceptions, you should make sure they are Serializable. Remote exceptions are wrapped in a CallMessageException instance, which is returned to the client. This makes it possible to have a remote call that can actually return (and not throw) an Exception or subclass thereof. 3. RemoteCallClient has also been cleaned up to accommodate the new remote exception capability. There are new GMI*Exception classes that will be thrown if a communication failure occurs during the call or the reply. There is also an exception generated if a communication failure occurs during the disconnect operation. 4. An unfortunate consequence of 3 (which you will later agree is a feature) is that you now must put try/catch around remote calls. I wanted to do this a long time ago, and this is just plugging a hole that I knew existed. In case you are wondering why this is a problem, consider the case where a remote call legitimately returns null. The original GMI used null as a flag to indicate general call failure (from the RemoteCallClient point of view.) Now you can rest assured that the return of null is clearly a result, not an exception or an error. I hope this makes sense. 5. The exception architecture for remote calls substantially differs from Java RMI. Regardless of whether your exceptions are local or remote, GMI simply does not care. Everything is an Exception. What I really despise about RMI are all of these kludges that impose an enormous burden on the programmer. GMI encapsulates the ugliness so programmers never have to see it in their code. Interestingly enough, the changes to accommodate exceptions were very straightforward. Since so little code had to be written (and some was even removed), I have taken the opportunity to comment the code, in case you want to understand how GMI works (the comments are placed strategically in the call() and run() methods in RemoteCallClient and RemoteCallServerDispatcher classes, respectively. Enjoy!