Ruimo,
see below
Ruimo Uno schrieb:
Hi, thanks for your comment.
2007/8/17, Rene Gielen <[EMAIL PROTECTED]>:
It's no bug, it's a feature...
The policy for model access (e.g. property calls) via expression
evaluation is fail silent. It would cause tons of exceptions if ognl
expression evaluation / property access would not swallow them.
I feel it is overkill. The property getters in action classes may
throw RuntimeException because of programming bugs. Even in this case,
the current implementation just ignore the exception and the shown
page will be just corrupted. We cannot show user friendly 'sorry page'
because the Struts exception handler cannot get exception.
You would not be able to use the most common use patterns without having
to do exepction handling on the JSP page then! Think of a model object
foo with a getter getFoo() in your a FooCrudAction. You have an
editFoo.jsp. If you want to create a new foo entry, the sequence would be
- invoke edit/create action, triggering execute() in FooCrudAction
- render editFoo.jsp
- submit form
- invoke save action, triggering save() in FooCrudAction
- on success, render editFoo.jsp
if ognl would not swallow the exceptions, then placing simple tag like
<s:textfield label="'Name'" name="foo.name" />
would fail with NPE in first rendering if you did not provide a
(unnecessary) this.foo=new Foo() in your execute() method, or a lazy
getter. Just a simple example. But if you would change this behaviour,
the showcase app code would need approximately an additional 10-15% more
code just for doing unneeded initializations and exception handling.
What if the execption is thrown? It will be thrown within JSP page
handling, and swallowed there! Your OpenSessionInView Interceptor or any
higher level layer would take no notice of the exception. Instead you
would have to use the anti-pattern of exception handliing with in JSP
code to present useful information.
Most likely, you are doing business logic calls in your model access
domain and you should think of moving any call with the possibility to get
service relevant exceptions to the described business logic domain access
methods.
No, it is not 'business logc'. In 'open session in view' pattern, the
db transaction is kept open until the view rendering finishies. If you
call the property access method of the acction class, it calls the
getter method of tne entity class. It triggers O/R mapping layer
invocation and lazily accesses the database. As a result, some kind of
system exception (such as SQLException) may be thrown while view
rendering.
This pattern is widely used, I'm using it myself in many applications.
But where do you expect the exception? If you make sure that you have a
foo object loaded from persistence layer, the the foo.getBars() call to
a a lazy initialized collection of referenced objects should never fail
due to _runtime_ problems you have to deal with. If, for example, for
some reason your session is closed, you have to look for a design time
problem - most commonly people forget about holding the session open
(luckily we have OpenSessionInView).
The best practice pattern for a displaying a foo object looks like this:
- invoke showFoo.action?id=1
- prepare() method of your action: this.foo = fooService.getById(this.id)
- handle any exception you like, or let it pop to the upper layers
- on success, render showFoo.jsp
- iterate over foo.bars in view - will be lazy initialized, but should
not throw any exception is your design was proper
but if there was an exception, and OGNL would pop it, you still would
_never_ get it in your Interceptor - your container will swallow it when
rendering the JSP.
it's the same for saving etc as well.
Did you check out these?:
http://cwiki.apache.org/confluence/display/WW/CRUD+Demo+I
http://appfuse.org/display/APF/Demos+and+Videos
Regards,
Rene
Am Do, 16.08.2007, 17:01, schrieb Ruimo Uno:
Hi,
I have submitted this to user list a few hours ago but finally, I
found the root cause of this issue. So, here I report it to dev list.
If an exception is thrown while OGNL expression evaluation such as in
the following scenario:
XXXAction
public String getMessage() {
throw new NullPointerException();
}
XXX.jsp
<s:property value="message"/>
The exception is wrapped in an OgnlException and re-thrown. It is
caught in com.opensymphony.xwork2.util.CompoundRootAccessor.getProperty()
and wrapped in a XWorkException() and re-thrown.
} catch (OgnlException e) {
if (e.getReason() != null) {
final String msg = "Caught an Ognl exception while getting
property " + name;
throw new XWorkException(msg, e);
}
} catch (IntrospectionException e) {
Finally, it is caught in
com.opensymphony.xwork2.util.OgnlValueStack.findValue(String, Class)
and swallowed.
} catch (Exception e) {
logLookupFailure(expr, e);
return findInContext(expr);
} finally {
As a result, the upper layer does not receive any exception. So, no
exception handler is invoked even if it is registered. Moreover, in
'open session in view' pattern, since the upper layer are not notified
any error, the transaction is wrongly committed and it causes serious
problem.
I tried to add a catch clause for RuntimeException:
} catch (RuntimeException e) {
logLookupFailure(expr, e);
throw e;
} catch (Exception e) {
logLookupFailure(expr, e);
return findInContext(expr);
} finally {
Yes, the exception is successfully thrown to the upper layer with this
code change. However, Struts shows 'java.io.IOException: Stream
closed:' on the browser. This message seems a bit confusing. and
difficult to realize what is actually happening (NPE is thrown). Does
anybody have better solution on this?
Thanks,
--
Ruimo Uno
(Shisei Hanai)
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