Subj : Unable to access protected fields in compiled script that extends another class
To   : netscape.public.mozilla.jseng
From : vwine@yahoo.com (Vaughn)
Date : Fri Apr 25 2003 06:21 pm

Hello,

I've just discovered that I cannot access protected fields in compiled
script that extends another class. It took me quite a while to
discover the problem, as all I was getting was a ConversionError. I
wrote a very simplified version of our architecture, started tweaking
and comparing, and finally figured out what the problem was.

Before I begin, let me explain a bit about our project (at least a
simplified explanation). First, we have an engine that performs
actions. Originally, all of these actions were defined in a UI and
saved as XML, but recently an extension mechanism has been developed
that allows developers to create their own Actions that plug-in to the
framework. Our hope is that we can further the ease-of-use of this
process and allow developers to use JavaScript to extend the
framework. The idea is that we would expose a TextArea in our UI where
the user can simply write JavaScript for the key function bodies in
our Action interface. Here's an important facet of our situation: In
each function body, the user needs to access some fields that are
inherited from the base class. And it is this access that was causing
me grief.

To illustrate this, I've create a simple example. I've got an Engine,
which calls  init  and  perform  on an Action. The Engine can be given
any custom Action written by the user. In the example below, the
Engine sets a value on the Action's  vectorTool  in the init method.
Then, the custom Action takes that value and modifies it in the
perform method/function.  In the example below, I've got a Java
version and a scripted version. They both work, but the script version
only works if  vectorTool  is public. If it's protected, I get the
following ConversionError:

Exception in thread "main" ConversionError:
  The undefined value has no properties.
  (CustomScriptAction.js; line 5)
        at org.mozilla.javascript.NativeGlobal.constructError(NativeGlobal.java:597)
        at org.mozilla.javascript.NativeGlobal.constructError(NativeGlobal.java:557)
        at org.mozilla.javascript.ScriptRuntime.getProp(ScriptRuntime.java:712)
        at CustomScriptAction$perform.call(CustomScriptAction.js:5)
        at org.mozilla.javascript.ScriptRuntime.call(ScriptRuntime.java:1191)
        at org.mozilla.javascript.JavaAdapter.callMethod(JavaAdapter.java:366)
        at CustomScriptAction.perform(<adapter>)
        at Engine.main(Engine.java:23)

Is this a bug? In our project, I can change the field to public, but
it's very inconvenient. Is there another way to work around the issue?

Thanks for your time! Rhinoscript is great!

Vaughn



-- Engine.java -----------------------------------

public class Engine
{
  // Run either the java version:
  //   java -cp ".;js.jar" Engine CustomJavaAction
  // or the script version:
  //   java -cp ".;js.jar" Engine CustomScriptAction
  //   
  // When I run the script version and the vectorTool field on
  // Action is changed to protected, I get a ConversionError.
  public static void main(String[] args)
  {
    if (args.length == 0)
    {
      System.out.print("Please provide a class name. ");
      System.out.println("(The class must extend Action.)");
      return;
    }
    
    Engine engine = new Engine();

    try
    {
      Class actionClass = Class.forName(args[0]);
      
      if (Action.class.isAssignableFrom(actionClass))
      {   
        Action action = (Action) actionClass.newInstance();

        action.init(23);        
        action.perform();

        System.out.println(action);  // Should print out the number 30
      }
      else
      {
        System.out.println(args[0] + " does not extend Action!");
      }
    }
    catch (Exception failure)
    {
      failure.printStackTrace();
    }
  }
}


-- Action.java -----------------------------------

import java.util.*;

public abstract class Action
{
  // When I Change this field to protected, I get a ConversionError.
  public Vector vectorTool = new Vector();
  
  public void init(int initVectorToolValue)
  {
    vectorTool.add(new Integer(initVectorToolValue));
  }
  
  public abstract void perform();

  public String toString()
  {
    StringBuffer sb = new StringBuffer();
    
    for (Iterator it = vectorTool.iterator(); it.hasNext(); )
    {
      sb.append(((Integer) it.next()).toString());
    }
    
    return sb.toString();
  }
}


-- CustomJavaAction.java -------------------------

public class CustomJavaAction
extends Action
{
  public void perform()
  {
    int initVal = ((Integer) vectorTool.get(0)).intValue();
    initVal += 7;
    vectorTool.set(0, new Integer(initVal));
  }
}


-- CustomScriptAction.js -------------------------

// I compile this with -extends Action
function perform()
{
  // The next line causes the ConversionError if vectorTool is
protected
  var initVal = Number(this.vectorTool.get(0));
  initVal += 7;
  this.vectorTool.set(0, new java.lang.Integer(initVal));
}

.