I would like to thank those who provided feedback for Part 1 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!

Exceptions

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 thegetHandlers() 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 classRedirectHandler. 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.

DebugPageHandler

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.

Examples

Annotations

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.

@ApplicationException(rollback=true)
@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?).

@ApplicationException(rollback=true)
@HttpError(code=501)
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.

Pages.xml

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">
<redirect>
    <message>This item is out of stock 
</redirect>
</exception>
<exception class="OutofStockWsException">
    <http-error code="501" />
</exception>

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.

This three part entry will focus on the mechanisms Seam provides for handling exceptions gracefully -- both out of the box and extensible. An in depth look at where this happens in the Seam code base and user programmable extension points will be covered in Part I. Part II will dive even further and provide examples, and Part III will cover a custom ExceptionHandler implementation. Exception handling in any large application is something that's typically debated at least once: What does it mean, is it simply logging, what do we show the user, can our framework help etc. etc. All software developers will run into this debate at least once in their career, even if it's as simple as "what should I put in that catch block?" It's pretty safe to say swallowing exceptions (as a general rule of thumb) is a bad idea. Of course System.out.println(...) or log.error(...) really isn't much better. Typically the user should be informed that something they have done has resulted in an error on the server, and at the very least ask them to try again. We've all seen pages like the following: An error has occurred, please try again. If the error continues, please contact the webmaster / admin / whatever. This is a cop out and typically a sign of poor exception handling. At least the user didn't see that nasty stack trace or the 500 page from the server. If it's being left up to the user to make contact about an error, chances are it won't happen and the development team probably won't hear about it. Enter the debate about handling those exceptions, and being more pro-active about knowing about errors. Covering each of those questions is beyond the scope of this series, but the question about framework help certainly applies!

Built-in Handling

Seam proveds a simple, yet robust solution for integrating your exception handling strategy. Two methods exist out of the box for rudimentary handling: <exception> in pages.xml and annotations directly on application specific exceptions. Dan Allen covers these two options nicely in his book Seam in Action section 3.6, (if you don't already own a copy, go get one. It's a life saver) so these will only be briefly reviewed. A third option exists (and the before mentioned options, as well as the powerful Seam Debug page are all built on top of this mechanism) to allow complete customization of display to the end user and actions on the back-end: ExceptionHandler classes.

pages.xml

In a basic seam-gen application one will find the following stanzas of XML relating to exception handling:

<exception class="org.jboss.seam.framework.EntityNotFoundException">
    <redirect view-id="/error.xhtml">
        <message severity="warn">Record not found</message>
    </redirect>
</exception>

<exception class="javax.persistence.EntityNotFoundException">
    <redirect view-id="/error.xhtml">
        <message severity="warn">Record not found</message>
    </redirect>
</exception>

<exception class="javax.persistence.EntityExistsException">
    <redirect view-id="/error.xhtml">
        <message severity="warn">Duplicate record</message>
    </redirect>
</exception>

<exception class="javax.persistence.OptimisticLockException">
    <end-conversation/>
    <redirect view-id="/error.xhtml">
        <message severity="warn">Another user changed the same data, please try again</message>
    </redirect>
</exception>

<exception class="org.jboss.seam.security.AuthorizationException">
    <redirect view-id="/error.xhtml">
        <message severity="error">You don't have permission to access this resource</message>
    </redirect>
</exception>

<exception class="org.jboss.seam.security.NotLoggedInException">
    <redirect view-id="/login.xhtml">
        <message severity="warn">#{messages['org.jboss.seam.NotLoggedIn']}</message>
    </redirect>
</exception>

<exception class="javax.faces.application.ViewExpiredException">
    <redirect view-id="/error.xhtml">
        <message severity="warn">Your session has timed out, please try again</message>
    </redirect>
</exception>

<exception class="org.jboss.seam.ConcurrentRequestTimeoutException" log-level="trace">
  <http-error error-code="503" />
</exception>
  
<exception>
    <redirect view-id="/error.xhtml">
        <message severity="error">Unexpected error, please try again</message>
    </redirect>
</exception>

The format is pretty straight forward, a fully qualified class is given in the optional class attribute (leaving this out creates a catch-all handler) and the body is as follows from the XSD:

<xs:element name="exception">
	<xs:annotation>
		<xs:documentation>A Seam exception handler</xs:documentation>
	</xs:annotation>
	<xs:complexType>
		<xs:sequence>
			<xs:element minOccurs="0" ref="pages:end-conversation"/>
			<xs:choice>
				<xs:element ref="pages:http-error"/>
				<xs:element ref="pages:redirect"/>
			</xs:choice>
		</xs:sequence>
		<xs:attributeGroup ref="pages:attlist.exception"/>
	</xs:complexType>
</xs:element>

<xs:attributeGroup name="attlist.exception">
	<xs:attribute name="class" type="xs:token"/>
	<xs:attribute name="log" type="pages:tf-boolean"/>
	<xs:attribute name="log-level" type="pages:loglevel-values"/>
</xs:attributeGroup>

Redirects are the typical body of these XML blocks. FacesMessages can be added to the redirect as well as ending conversations. Typically these are used for exceptions the developer does not create, and therefore cannot be annotated; however, there may be uses for this style of handling with application exceptions. Please note that order does matter. If the catch-all handler is first, none of the other handlers will be used. Each of these entries creates an ExceptionHandler instance which is then added to the chain of handlers, which is covered later in the entry.

Annotations

Seam has three annotations to help with exception handling: @Redirect, @HttpError, and @ApplicationException. Only one of @Redirect or @HttpError may be used on an exception. Both allow for an info level FacesMessage to be added via the message attribute. The @Redirect annotation allows input for a view-id to which the user will be redirected. @HttpError simply returns the specified HTTP Error code. The last annotation, @ApplicationException provides the ability to rollback the transaction and end the existing conversation. These annotations are added directly on the Exception class that is created. Both of these built-in methods are built on top of two classes in Seam: org.jboss.seam.exception.Exceptions and org.jboss.seam.exception.ExceptionHandler. These two classes and their usage make up Seam's infrastructure for exception handling in an application, and provide customization to the developer.

Extensible Handling

The above two methods work well for simple cases. What if a team wants to be more pro-active about addressing errors? Could the application send the team emails about exceptions, or create bug tickets, or perhaps create a contextual log entry? This is where the exception handling capabilities of Seam really shine, and the answer to the above questions are all yes!

ExceptionHandler

package org.jboss.seam.exception;

import org.jboss.seam.faces.Navigator;

/**
 * An element of the chain that knows how to handle a
 * specific exception type.
 *
 * @author Gavin King
 *
 */
public abstract class ExceptionHandler extends Navigator
{ 
   public enum LogLevel { fatal, error, warn, info, debug, trace }

   private boolean logEnabled;
   private LogLevel logLevel;

   public abstract void handle(Exception e) throws Exception;
   public abstract boolean isHandler(Exception e);

   public boolean isLogEnabled()
   { 
      return logEnabled;
   }

   public void setLogEnabled(boolean logEnabled)
   { 
      this.logEnabled = logEnabled;
   }

   public LogLevel getLogLevel()
   {
      return logLevel;
   }

   public void setLogLevel(LogLevel logLevel)
   {
      this.logLevel = logLevel;
   }      
}

The contract for this class is pretty straight forward. The only methods that need to be implemented are isHandler and handleException. Of course the isHandler implementation is trivial, and based on the needs of the handler the handleException implementation could be fairly simple as well. Take note that ExceptionHandler is a subclass of Navigator. There are methods defined in that class which may be helpful in some exception handlers.

Exceptions

This class is used in the org.jboss.seam.jsf.SeamPhaseListener before and after each phase of the JSF lifecycle and also in the org.jboss.seam.web.ExceptionFilter, wrapping the filter chain, and used again after any redirects that may happen after the filter chain has completed. As Dan Allen described it in Seam in Action: "A try-catch around the [whole request]" (Seam in Action pg. 126). There's only one method that needs any kind of attention: getHandlers. It returns a list that is used to add or remove ExceptionHandlers as needed. This method should be called in the @Create method, or constructor of any ExceptionHandler. ExceptionHandler classes should also be marked with @BypassInterceptors as there's no need to do bi-jection. A simple POJO, though, will typically suffice. Exception handling in Seam can be very powerful if used to it's full potential and should be an excellant tool in crafting an exception handler strategy for your application. The next entry in this series will demonstrate a custom exception handler in action.

Over the past few days I've been working on getting HornetQ (http://www.hornetq.org) to work on Jetty (http://www.mortbay.org/jetty/) -- specifically the embedded Jetty that Gradle (http://www.gradle.org) uses. I would first like to thank those on the HornetQ irc channel (irc://freenode.net:6667/hornetq) for helping me out and offering advice. Specifically I would like to thank Clebert Suconic and Andy Taylor for looking at some of my code and making suggestions that ultimately led me to this solution. It should be noted that the two solutions I will be discussing here are probably not the only roads to success. I'm quite confident the same could be achieved programmatically by setting up HornetQ in a bean that's loaded at application deploy time or similar fashion.

HornetQ setup using jetty-env.xml

Both of my two solutions are using jetty-env.xml to call the needed setup functions for HornetQ. The same could be achieved with jetty.xml if deploying to a standard install of Jetty. More information about these configuration files can be found at http://docs.codehaus.org/display/JETTY/Configuring+Jetty. In short these files act as an XML means of working with Java objects.  Objects can be instantiated, methods called, etc. All that's being done here is calling the methods in the HornetQ API to startup HornetQ in Embedded mode. Below is the jetty-env.xml I used:

<?xml version="1.0"?>
<!DOCTYPE Configure PUBLIC "-//Mort Bay Consulting//DTD Configure//EN" "http://jetty.mortbay.org/configure.dtd">

<Configure id="wac" class="org.mortbay.jetty.webapp.WebAppContext">
  <New id="jmsServerManager" class="org.mortbay.jetty.plus.naming.Resource">
    <Arg><Ref id="wac"/></Arg>
    <Arg>jms/serverManager</Arg>
    <Arg>
      <New class="org.hornetq.jms.server.impl.JMSServerManagerImpl">
        <Arg>
          <Call class="org.hornetq.core.server.HornetQ" name="newHornetQServer">
            <Arg>
              <New class="org.hornetq.core.config.impl.ConfigurationImpl">
                <Set name="persistenceEnabled">false</Set>
                <Set name="securityEnabled">false</Set>
                <Get name="AcceptorConfigurations">
                  <Call name="add">
                    <Arg>
                      <New class="org.hornetq.core.config.TransportConfiguration">
                        <Arg>org.hornetq.core.remoting.impl.invm.InVMAcceptorFactory</Arg>
                      </New>
                    </Arg>
                  </Call>
                </Get>
              </New>
            </Arg>
          </Call>
        </Arg>
        <Call name="start" />
        <Call name="createConnectionFactory">
          <Arg>HornetQConnectionFactory</Arg>
          <Arg>
            <New class="org.hornetq.core.config.TransportConfiguration">
              <Arg>org.hornetq.core.remoting.impl.invm.InVMConnectorFactory</Arg>
            </New>
          </Arg>
          <Arg>
            <New class="java.util.ArrayList">
              <Call name="add">
                <Arg>java:comp/env/jms/connectionFactory</Arg>
              </Call>
            </New>
          </Arg>
        </Call>
        <Set name="context">
          <New class="javax.naming.InitialContext">
          </New>
        </Set>
        <Call name="createQueue">
          <Arg>testQueue</Arg>
          <Arg>java:comp/env/jms/queues/testQueue</Arg>
          <Arg></Arg>
          <Arg type="boolean">false</Arg>
        </Call>
      </New>
    </Arg>
  </New>
</Configure> 

An instance of JMSServerManager is being created and the needed configuration objects are being created inline in this example. It's very important to make the start call on the JMSServerManager before any other calls on the object to create queues, topics or connection factories are made, otherwise you'll get exceptions in your log saying the server hasn't been started yet. It's also important to make sure the full JNDI names are used. jetty-env.xml is used to bind elements to JNDI for you but because the JMSServerManager will be doing the binding for us I found that the full JNDI name must be used. If programmatic access to the JMSServerManager is needed this is also bound to JNDI for us by Jetty under jms/serverManager -- which doesn't use the full JNDI name because Jetty is handling the binding, a slight bit confusing I know. Work with JMS can now proceed using standard JNDI lookups and JMS code to send and receive messages. All of this code is available at my GitHub repo: http://github.com/LightGuard/Research---Development--JEE-/tree/hornetq feel free to check it out, fork it, whatever :)

HornetQ setup using jetty-env.xml and HornetQ configuration files

This setup is very similar to one above, only using the HornetQ configuration files. Code is located at http://github.com/LightGuard/Research---Development--JEE-/tree/hornetq-with-config-file. Here's the jetty-env.xml file:

<?xml version="1.0"?>
<!DOCTYPE Configure PUBLIC "-//Mort Bay Consulting//DTD Configure//EN" "http://jetty.mortbay.org/configure.dtd">

<Configure id="wac" class="org.mortbay.jetty.webapp.WebAppContext">
	<New id="jmsServerManager" class="org.mortbay.jetty.plus.naming.Resource">
		<Arg><Ref id="wac"/></Arg>
		<Arg>jms/serverManager</Arg>
		<Arg>
			<New class="org.hornetq.jms.server.impl.JMSServerManagerImpl">
				<Arg>
					<Call class="org.hornetq.core.server.HornetQ" name="newHornetQServer">
						<Arg>
							<New class="org.hornetq.core.config.impl.FileConfiguration">
								<Set name="configurationUrl">hornetq-configuration.xml</Set>
								<Call name="start" />
							</New>
						</Arg>
					</Call>
				</Arg>
				<Arg>hornetq-jms.xml</Arg>
				<Call name="start" />
				<Set name="context">
					<New class="javax.naming.InitialContext">
					</New>
				</Set>
			</New>
		</Arg>
	</New>
</Configure> 

It's much smaller this time as most of the configuration has been moved into the hornetq-configuration.xml and hornetq-jms.xml files (both of which are on GitHub). That's really all there is to it! My next steps will be to get logging of hornetq (and hopefully Jetty) using log4j and same log file as the rest of the application.

I recently saw a blog post that listed out performance benchmarks of two presentation layer technologies for Java.  The posting probably wasn't as non-biased as it could have been, but that's besides the point, and not the issue of this post.  The performance stats were measured in milliseconds from the time a request hit the server to the time it left the server to go back to the user.  A stat worth keeping an eye on, right?  We want our applications to be fast, of course we need to be focused on how long that request sits on the server processing, don't we?

As developers most of us want to squeeze out the last bit of speed we can get from our applications and technologies.  We want ours to be the fastest, naturally; and most developers have a tendency to be perfectionists to some degree or another.  But is it really necessary?

Back to the post I read.  As I stated the results were measured in milliseconds.  Most of recorded data (even in  multi-user scenarios, the author did up to 25 concurrent users) points for both technologies were sub second.  That's less than one second for a request sitting on the server processing, sounds pretty darn good to me.  The author's conclusion (assuming I read it correctly) was essentially technology X should be used instead of technology A because it's faster.  Now to the heart of this post: do your users really care about that much of a speed difference??

When we're dealing with web applications we have to remember that the users are not running this application on their local machines.   Each request has to go over through the Interet, which is definitely slower than seeing something on our local development machines, or even on a local network.  We also have to keep in mind the user's perception of "slow."  That's really what this boils down to.  If our users feel the application is sluggish then we should probably look into it.  Users of the web don't expect requests to come back instantaneously.  Most users are happy with a 5 - 10 second return of their request.  If a couple seconds of that is spent processing the request on the server, you're probably good.  There are also other considerations on the client's side for perceived web performance, namely the browser being used.  How fast does it render HTML, does it wait to display the full page or does it load incrementally, etc ?

Basically there are a lot of variables that make up the perception of a slow web site.  Your best source of knowing if something is slow is your user base, and I'm not just talking about one or two but a sizable chunk of users.  If they're saying it's slow, go look into it.  If you don't hear any rumblings, don't worry about it.  Your site is "fast enough."  Also remember web front end technologies that are mainstream are all "fast enough" otherwise no one would use them.