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Letters to the Editor

JavaWorld.com, 03/28/03

"Manage Users with JMS"
Amit Goel and David Marshall

ERP system updates

Amit,

How do you handle transactionality in the internals of the subscribing apps, like the ERP (enterprise resource planning) subscriber and the CRM (customer relationship management) subscriber? What if the ERP app fails (for some reason) to update its internal database app after consuming the published message? How will you failover?

Sanjaya Ganesh

/* Update ERP system with new user data */
updateUserData(String username, String email, ...)
{
  // Interface with ERP app.
  // ...
  // ...
  // Also handle any errors and alert notifications.
  // ...
  // ...
}

What about single sign-on?

Amit,

This article solves the problem of keeping user registrations in sync. But how would you achieve single sign-on?

Suppose instead of ERP, CRM, and XYZ apps, you have WebApp1, WebApp2, and WebApp3 apps, and wanted to ensure that a user could sign on only once to any of these apps. Of course there's no need to maintain multiple user registrations with each app.

Is there a solution that could achieve single sign-on for these applications?

Jitender Bhatia

Java Q&A
"Breaking Java Exception-Handling Rules is Easy "
Vladimir Roubtsov

Throw and catch

Vladimir,

There is another way for a checked exception to propagate up the call chain without the compiler noticing. Consider the following, which is modeled after a real-world situation I recently encountered:

import java.io.*;
public class InvisibleThrow
 {
  public static void main(String[] av)
   {
    try
     {
       InvisibleThrow.class.newInstance();
     }
    catch(InstantiationException e) { }
    catch(IllegalAccessException e) { }
   }
  InvisibleThrow()
   throws IOException
   {
     throw new IOException("SURPRISE!");
   }
}

The IOException gets thrown and crashes the program. What's more, you can't just try to catch the IOException directly! The compiler complains if you just add a catch(IOException e) in main, since newInstance() isn't declared to throw such an exception.

So, you have two options: you can cast a wide net and catch(Throwable e), or you can trick the compiler by rewriting main as follows:

   try
    {
     InvisibleThrow.class.newInstance();
     if(false)
        throw new IOException("NEVER THROWN");
    }
    catch(InstantiationException e) { }
    catch(IllegalAccessException e) { }
    catch(IOException e) { System.out.prinln("Gotcha!"); }

This works with my compiler, but I suppose your mileage may vary since the extra throw is technically unreachable code.

I'm puzzled as to why Class.newInstance() doesn't throw an InvocationTargetException, like Constructor.newInstance() does. Perhaps someone can provide a justification for this; it seems like a significant oversight in the API.

Michael Weiss-Malik

Java Q&A
"The Thread Threat"
Vladimir Roubtsov

Thread safety

Vladimir,

Several questions arise from this article:

1. Why the emphasis on static fields and accessors? Are the semantics different for instance fields and accessors? Do monitors not work the same on objects of class Class as they do on other objects? Or do thread-specific memory views have their own "independent copies" of static data only?
2. In a threaded app, when is it safe not to declare a (static?) field volatile and/or its accessors synchronized? Only when you can assert that concurrent access is impossible by design?

Thanks for the warning. It's very useful information.