• Subject: Feedback Solicited: Java Beans Change Notification and Vetoable Change Support
  • From: dawall@xxxxxxxxxx
  • Date: Mon, 13 Dec 1999 15:32:46 -0600

Most public interfaces of the AS/400 Toolbox for Java are Java Beans.  The
Toolbox team will continue to produce Java Beans for its public interface,
but the requirement for change notification and vetoable change support is
being questioned.  The purpose of this append is to find out if anyone uses
change notification or vetoable change support in the Toolbox.

With change notification, code can register itself with a bean to be
notified when the value of a property changes.  The old and new value of
the property are passed to the code that asked to be notified of the
change.  With vetoable change support, code can register itself with a bean
to be notified when the value of a property is about to be changed.  The
code can veto (reject) the change before it is applied to the bean.

The Toolbox team willl not support these two bean features for new
properties for the following reasons:
   1. Code size -- If no application code uses these features,  why add
   code in the Toolbox to implement them?  It just makes our objects
   bigger.
   2. Exceptions -- Application code that sets a property must catch the
   property veto exception even though the feature is not used.  If you use
   the Toolbox you probably have code like:
      ToolboxClass toolboxObject = new ToolboxClass();
          try {
              toolboxObject.setProperty(properyValue);
          }
          catch (PropertyVetoException e) {}                 // keep the
     compiler happy
   3. Performance -- Every time a property is changed the Toolbox must
   check the list of listeners just in case someone registered  to be
   notified of the change.

Even if this change is made, note that
     No existing properties will be affected.  If a property of a Toolbox
     object supports these bean features today, it will continue to support
     them.
     The Toolbox interfaces will continue to be beans.  The change is to
     stop supporting just these two features in new properties.

If you use these Java Bean features, please e-mail us at
rchjt400@us.ibm.com.  If we don't have any users of these features, we will
stop adding them to new properties.  Thank you for your feedback!

David Wall
AS/400 Toolbox for Java


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