[TAPESTRY-131] NullPointerException when initialising page property

• Type: Bug
• Status: Resolved
• Resolution: Won't Fix
• Affects Version/s: 3.0
• Component/s: Framework
• Environment: Operating System: Other; Platform: Other
• Bugzilla Id: 27608
• Assignee: Unassigned
• Reporter: Petter Måhlén
• Created: 11/Mar/04 23:18
• Updated: 10/Mar/06 10:03
• Resolved: 10/Mar/06 10:03

Description

I got a null pointer exception with the following (partial) stack trace:

org.apache.tapestry.Tapestry.fireObservedChange(Tapestry.java:1373)
org.apache.tapestry.AbstractComponent.fireObservedChange
(AbstractComponent.java:345)
se.elevance.timer.web.EditTaskPage$Enhance_0.setTask
(EditTaskPage$Enhance_0.java)
se.elevance.timer.web.EditTaskPage.initialize(EditTaskPage.java:35)
org.apache.tapestry.AbstractPage.<init>(AbstractPage.java:165)
org.apache.tapestry.html.BasePage.<init>(BasePage.java:73)
se.elevance.timer.web.EditTaskPage.<init>(EditTaskPage.java:31)
se.elevance.timer.web.EditTaskPage$Enhance_0.<init>
(EditTaskPage$Enhance_0.java)
sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
sun.reflect.NativeConstructorAccessorImpl.newInstance
(NativeConstructorAccessorImpl.java:39)

It appears that the following line in Tapestry.java is the problem:

ChangeObserver observer = component.getPage().getChangeObserver();

Looking a bit into the code, it would seem that component.getPage() returns
null for a page. I can see two possible ways to solve this:

1. modify the AbstractPage() constructor to:
public AbstractPage()

{ setPage(this); initialize(); }

2. Modify the fireObservedChange() methods along the following lines:
public static void fireObservedChange(
IComponent component,
String propertyName,
Object newValue)
{
IPage page = component.getPage(); // will be null for a page

if (component instanceof IPage)

{ // maybe only overwrite page if it is in fact null? page = (IPage) component; }

ChangeObserver observer = page.getChangeObserver();

if (observer == null)
return;

ObservedChangeEvent event = new ObservedChangeEvent(component,
propertyName, newValue);

observer.observeChange(event);
}

I don't know what the side effects would be of either alternative. In my case,
the page specification contains the following and the line in
EditTaskPage.initialize() that is the cause for the exception is a call to
setTask(null) - which I think should be no problem.

<property-specification name="task"
type="se.elevance.timer.data.Task" persistent="yes" />