COMP 310 Fall 2016	Lec44: Tips & Traps and Team Work

Trap:

Watch out for code that attempts to perform RMI actions before IRMIUtils.startRMI() has been run!   Symptoms can include strange RMI behavior such as remote dynamic class loading not working as expected.  Common culprit: constructors that try to do too much, too early.

Tip:

1. Separate pure construction operations (constructor) from start-up initialization operations ("start" method).   Put RMI operations with the start-up operations.
2.  Do not call the startup operations until after the RMI system has been started.

Trap:

Not waiting for network operations to complete can lead to race conditions such as concurrent modification errors.    Common culprit:  A method that sends the data to the a chat room on a new thread but provides no means of executing a process after the returned packets have been processed, i.e. after the data has been definitively sent.

Tips:

• Have the method that sends the data packet also take a command that the new thread can run after the data packet is sent and all returned data packets have been processeed.
• If needed, use a java.util.concurrent.CountDownLatch to wait for multiple threads to finish.

Trap:

Shutting down the application leaves objects still bound in the Registry, network connections hanging or disconnected, class file servers still running, etc.     Common culprit: A main GUI frame that is set to EXIT_ON_CLOSE and thus does not "clean-up" before it exits, e.g. gracefully exit all chat rooms, run IRMIUtils.stopRMI() to shut down the class server,  registry.unbind(name) to unbind the IHost stub, etc.

Tips:

• Leave the frame to its default HIDE_ON_CLOSE behavior and use a WindowListener to run the clean-up code, e.g.
  

  addWindowListener(new WindowAdapter(){
      public void windowClosing(WindowEvent e) {
         // call model to do clean up then call hide() and dispose() to kill this frame.
      }
  });

  This technique does not catch all shutdown scenarios, however.
• Install a "shutdown hook" into the Java Runtime object to run clean-up code on any shutdown by using its addShutdownHook() method., e.g.

  java.lang.Runtime.getRuntime().addShutdownHook(new Thread() {
      public void run() {
          // call method to do clean-up
      }
  }); 
  

© 2016 by Stephen Wong