"No getter method error for property" caused by java.beans.Introspector feature/bug

[Davide Cavallari <d....@ilte.com>]

I recently had a problem with a bean which causes the "No getter method 
for property..." error when used with the <bean:write> tag.

Surfing the web, I found a message posted October 27, 2004 09:41 AM in 
the JavaRanch Struts forum by Erick Reid. The topic was "No getter 
method for property... (but not the usual suspects)". 
(http://saloon.javaranch.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=58&t=004112)

Interestingly, Erick suggested using a bean dump util 
(http://wiki.apache.org/struts/ExposedBeanProperties) to get insight 
about a bean's exposed properties.

Using it I discovered that the problem wasn't Struts Tag's 
responsability, but Introspector's. Basically, it's due to the the 
Introspector's default way of finding the BeanInfo.

I've found the following post by Dawid Weiss in 
comp.lang.java.programmer. He reports the very same problem and clearly 
explains what's going on.


xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Newsgroup: comp.lang.java.programmer
From: dawid.we...@cs.put.poznan.pl (Dawid Weiss)
Date: 20 Feb 2003.02.20 03:41:41 -0800
Subject: JDK 'feature' in java.beans.Introspector prevents accessing 
properties of classes named Component


I've just spent 3 hours trying to find a bug in my STRUTS application.
As it turned out, it was neither my fault, nor STRUTS. It's just 
undocumented (or maybe documented, but I'm not aware of it) 'feature' of 
java.beans.Introspector class, used by STRUTS to access properties of 
java beans.

The bug (?) only shows up when your class is named "Component", no 
matter what package it is in and does not have any associated beaninfo 
class. Then the 'discovered' set of properties will NOT be that of your 
class, but java.awt.Component...

Is this a bug? A feature? This should at least have a mention in the 
docs, right?

Dawid

-- 

Try this: create a java bean in you own package and name it "Component", 
for example:

com.bubu.dede.Component

Create several setters and getters, as you wish. Now, try to use it
with STRUTS and use automatic introspection (JSP tags). It won't work 
and you'll get a message saying "no property xxx in bean yyy". So you 
dig in STRUTS code and see that it's using standard Java Introspection 
class. Amazed, you type the following code:

BeanInfo beanInfo = Introspector.getBeanInfo(myBean.getClass());
PropertyDescriptor [] descriptors = beanInfo.getPropertyDescriptors();
for (int i=0;i<descriptors.length;i++)
System.out.println("property: " + descriptors[i].getName());

And the properties you see discovered are not at all those, which you 
declared.

Why is that? Because Introspector's getBeanInfo uses the default: 
USE_ALL_BEANINFO, which should check also BeanInfo classes from up the 
hierarchy of your bean's classes. Unfortunately, which is mentioned in 
the documentation, but quite vague, Introspector also uses "search path" 
to look for BeanInfos:

"We first look for the BeanInfo class by taking the full 
package-qualified name of the target bean class and appending "BeanInfo" 
to form a new class name. If this fails, then we take the final 
classname component of this name, and look for that class in each of the 
packages specified in the BeanInfo package search path."

This search path is declared as (JDK1.4.0_03):

private static String[] searchPath = { "sun.beans.infos" };

A quick look into the rt.jar reveals, that there is a class 
ComponentBeanInfo, which describes java.awt.Component java Bean. This 
way, your component's real properties are never revealed to the 
introspection mechanism.

xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

-- 
Cheers,

       Davide

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