Property and interceptors

Since my last post, Cay Horstmann has recalled that properties are intended for tools and Hans Muller bid up by saying that property syntax is useless without a way to define bound properties.

Interceptors and bound properties

What Hans want is some kind of interceptor. An interceptor is an object or a method that can trap access to a property. In Java, these objects are implemented either using java.lang.reflect.Proxy or by bytecode enhancement using a VM agent or a special classloader.
It is interresting to know that bean interceptors are already defined in EJB 3 spec (JSR-220).

So for me, instead of trying to re-invent the wheel, we could perhaps transpose the concept of server side interceptors in Swing world. We 'just' need an equivalent to an EJB container for swing application. I think the concept of transaction can be borrow in the same time, it could liberate us from invokeLater and SwingWorker.

So in my opinion, adding bound properties to the language is a bad idea.
I prefer to let that job to Beans Binding experts group (JSR 295) and its leader Scott Violet.

So is property syntax usefull ?

I think that even if bound properties are not managed by the compiler, we need a property syntax at least to insert a special attribute in the bytecode that will be recognize by the reflection runtime to provide property objects at runtime.

Property interceptor by the compiler

But if you think that this kind of AOP must be done by the compiler, i propose to allow to declare two a special methods named set* and get* (the star is not a typo) that can be used instead of a particular getter or setter.

By example, a bean that declares two bound properties background and foreground in that way could be written like that :

  public class MyBean {
    public property Color background;
    public property Color foreground;

    public void set*(Property property,Object value) {
      Object oldValue=property.get(this);
      super();
      SwingPropertySupport.firePropertyChange(property,
        oldValue,property.get(this));
    }
  }

The compiler will generate foreground or background setters by duplicate the set* code for each setter.
java.lang.reflect.Property is the reflection counterpart of a property at runtime (I've just rename the Stephen Colebourne's property object to a more Java-like name), super() in that context means set the property value (pretty ugly isn't it) and SwingPropertySupport is a new class that allow to add/remove PropertyChangeListener for any property of any bean.

To Link a property to a listener, we can use

  MyBean bean=...
  SwingPropertySupport.addPropertyChangeListener(
    bean,"background",new PropertyChangeListener() {
      ...
    });

Cheers,
Rémi

P.S : to the little Peter, you can't return a gift :)