Random bits of information by a developer

28 February 2010

Seam In Depth: Exception Handling Part II

I would like to thank those who provided feedback for Part I of this exception handling series. A special thanks goes out to Dan Allen and Dan Hinojosa for the wonderful constructive feedback. I'm sure over time these posts will improve and become better.

This entry was originally going to be talking about custom handlers and provide some code; however, due to family and work factors I have not been able to finish what I wanted for this. Instead we'll be diving even further into the built-in handlers and providing examples. At the end I'll also give a sneak peek of the exception handling framework that will make up Part III of this series, hopefully complete in March. To cut down on the length of this post I've linked to Seam's Fisheye install of the 2.2.0.GA release. With that, let the journey into org.jboss.seam.exception begin!


This class (org.jboss.seam.exception.Exceptions) is a built in Seam class, all you need to do is obtain a reference to it (either via injection @In("exceptions") Exceptions or Exceptions.getInstance()) and call the getHandlers() method to add your own exception handlers. Typical interaction is done by adding exception handlers via the annotations or pages.xml. Let's begin the exploration with the initialize() method where all the initial setup and creation of the built-in handlers happen. Handlers are added in the following order:
  1. AnnotationRedirectHandler
  2. AnnotationErrorHandler
  3. Handlers declared in pages.xml
  4. User defined custom handlers (more on this later)
At first glance, it appears that an old deprecated way of adding exceptions is first added, then the exception handlers added via page(s).xml; however, this is not the case as each of those ExceptionHandlers is added to a list, then added at the end. Just before any handlers that the user specifies with pages.xml or custom handlers the Seam Debug Page is added.
It's important to know that while in debug (typically only development mode) your annotated exceptions will take precedence over the Seam Debug Page, and nothing after the debug page will be triggered (the debug page is a global catch all handler).

The parsing of the XML files happens in the parse() method which does some basic XML parsing looking for the exception elements. The class and logging level (if configured) are pulled from the exception element, and a new ConfigRedirectHandler or ConfigErrorHandler is created (more on these handlers later). If there is no class attribute specified then a catch-all handler, that handles Exception.class is created.

The private createHandler() method does the work of pulling additional information about the configured handler such as

  • ending conversations
  • redirecting
  • adding FacesMessages
  • finding the http error code, etc.
The correct handler is then created and returned to be added into the list.

The only other part of this class that has any importance for the developer/user is the handle() method, which is really just an iterator over the handlers looking for a handler for the exception which was thrown. Though there is a catch: Nested exceptions are unwrapped and handled from the bottom up (though in the Seam Debug Page the exception is recreated so you may need to scroll to the bottom to actually find the real exception). If a long running conversation is active then the "org.jboss.seam.handledException" object, which is the exception object, is added to the conversation context. If logging is configured for the exception handler a log message is added then two events are raised (regardless of logging): "org.jboss.seam.exceptionHandled." + cause.getClass().getName() and org.jboss.seam.exceptionHandled allowing a further user extension point if so desired. If no handler is found then an event is raised ("org.jboss.seam.exceptionNotHandled") and the exception is rethrown. Even if this event is observed, (the exception is passed as a parameter to the observer's method) it will not be able to influence the exception being rethrown.

Any handlers the developer creates and adds to the Exceptions list will be added at the end of the list (unless the list is manually modified and rearranged, which is allowed). Depending on the specific needs of the application this is a great place to add a general catch all handler for the application (unless a general handler in pages.xml is sufficient). When adding custom handlers or any handler really, it is important not to create multiple handlers for the same exception as the first in the list will be called and the list of handlers will terminate with that handler.

AnnotationErrorHandler and ConfigErrorHandler

ErrorHandlers (AnnotationErrorHandler and ConfigErrorHandler) are a specific type of ExceptionHandler, they're used to send an HTTP Error code to the browser, probably more useful when combined with Seam's Web Service or REST integration. There is not a lot of code to these handlers, in fact, they are only data containers used by the parent class ErrorHandler which handles sending the error to the browser. However, they are great examples of reuse for exception handling, and make for good templates to follow in abstracting logic in custom handlers. The two sub classes hold
  • the message (if configured)
  • the error code number
  • and the flag to end the currently conversation.
All of the logic to make it happen is in the handle() method.

AnnotationRedirectHandler and ConfigRedirectHandler

The redirect handlers (AnnotationRedirectHandler and ConfigRedirectHandler) are in the same category as the error handlers: there's not much to them, and most of the interesting code is in the parent class RedirectHandler. The first bit of code in the handle() method is finding the viewId if none was specified in the handler instance, but it really isn't related to the actual exception handling. Next is the addition of a defined FacesMessage, ending the conversation (again, if configured), and finally the redirect.


This is a great handler (source) along with the associated code in the jboss-seam-debug.jar to fully understand the potential of exception handlers in Seam. The handler is really the entry point into the debug system in Seam. Java code wise, there's not much to it, it's a redirect handler that sets some additional parameters for the redirect. The whole process of Seam Debug really could be it's own blog entry or series of entries. It's pretty involved: it includes a PhaseListener, a SerializationProvider, and some custom Introspection. Because it's such a large, involved piece of code, I'll cover it more fully in a future post, or perhaps a whole series.



Consistent with the rest of the framework, Seam provides a few Annotations to help with exception handling, these were briefly explored in Part I. Here are some examples on how they're applied to application code.
@Redirect(message="This item is out of stock")
public class OutOfStockException extends Exception {
The @ApplicationException annotation is a mirror of javax.ejb.ApplicationException with additional support for ending a conversation. It is also the entry point Seam uses to configure annotation based exception handlers and must be used if the @HttpError or @Redirect annotations are to be used. In this particular example some of the defaults are used such as not ending the conversation (with the end attribute) and there is no redirect happening, but the message (which can also take an EL expression for i18n, or substitution purposes) will be displayed on the page where the exception occurred.

This next example might be useful with Seam's WebService support, it will return an HTTP error code 500 (probably not the most useful for the clients, but it's an example, right?).

public class OutOfStockWsException extends Exception {
As noted earlier, these two exception handlers (AnnotationRedirectHandler and AnnotationErrorHandler) are the first two handlers in the list and will be the first called to handle the exception, if they are capable, as defined by boolean isHandler(Exception). The classes use the meta data from the annotations to determine if the exception can be handled by one of these two classes. Next up is a closer look at defining the same exception handlers in pages.xml.
Exception handlers can only be defined in the pages.xml, not in the individual page.xml files, which in my opinion is a bit of a short coming. There could be times when the same exception should be handled differently for different pages.


Because the handlers for pages.xml are roughly the same handlers, different sub classes but they do the same as the annotation sub classes, the above handlers could be declared via pages.xml with the following XML:
<exception class="OutOfStockException">
    <message>This item is out of stock 
<exception class="OutofStockWsException">
    <http-error code="501" />
Pretty simple stuff. These exceptions will follow the annotations in the list (it's worth noting that each exception that is declared in the XML creates a new instance of the associated handler, unlike the annotation counterpart). If both were declared in the same application the annotation ones would be used. Both ways get the job done, and for your own exceptions it's really just a matter of preference.
The pages.xml declaration really lends itself better for exceptions that cannot be annotated, such as form Hibernate or JPA, please see Part I for examples.

Part III of this series will demonstrate an exception handler framework that could be plugged into Seam and will allow chaining for handling (for example send an email or IRC message and create a bug report for the exception). It should be generic enough to work with Seam, JSF2 (Ed Burns talked about JSF2 Exception handling on his blog), or even the base JRE (yep, 1.5 added Exception handling). It will be hosted on GitHub.

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