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# Friday, August 21, 2009

We always tacitly assumed that protected modifier in java permits member access from a class the member belongs to, or from an instance of class's descendant. Very like the C++ defines it, in fact.

In other words no external client of an instance can directly access a protected member of that instance or class the instance belongs to.

It would be very interesting to know how many people live with such a naivete, really!

Well, that's what java states:

The protected modifier specifies that the member can only be accessed within its own package (as with package-private) and, in addition, by a subclass of its class in another package.

If one'll  think, just a little, she'll see that this gorgeous definition is so different from C++'s and so meaningless that they would better dropped this modifier altogether.

The hole is so huge that I can easily build an example showing how to modify protected member of some other class in a perfectly valid way. Consider:

MyClass.java

package com.mypackage;

import javax.faces.component.Hack;
import javax.faces.component.UIComponentBase;

import javax.faces.event.FacesListener;

public class MyClass
{
   public void addFacesListener(
     UIComponentBase component,
     FacesListener listener)
   {
     Hack.addFacesListener(component, listener);
   }

   ...
}

Hack.java

package javax.faces.component;

import javax.faces.event.FacesListener;

public class Hack
{
   public static void addFacesListener(
     UIComponentBase component,
     FacesListener listener)
   {
     component.addFacesListener(listener);
   }
}

An example is about to how one adds custom listener to an arbitrary jsf component. Notice that this is not assumed  by design, as a method addFacesListener() is protected. But see how easy one can hack this dummy "protected" notion.

Update: for a proper implementation of protected please read Manifest file, a part about package sealing.

Friday, August 21, 2009 12:25:59 PM UTC  #    Comments [0] -
JSF and Facelets | Tips and tricks
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