[jbossseam-issues] [JBoss JIRA] Created: (JBSEAM-3520) Handling Exception (not only) when using JPDL Pageflows

[ahus1 (JIRA)]

Handling Exception (not only) when using JPDL Pageflows
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                 Key: JBSEAM-3520
                 URL: https://jira.jboss.org/jira/browse/JBSEAM-3520
             Project: Seam
          Issue Type: Feature Request
          Components: BPM
    Affects Versions: 2.0.2.SP1
         Environment: seam with jpdl
            Reporter: ahus1


The problem: when there are exceptions in a jpdl pageflow , you will not be able to continue with the pageflow if you are stuck at a transition node or a decision node. There is no chance to display the error message and to return to the last page. 

The solution idea: Use an exception handler in the jpdl pageflow, store some information, reposition the pageflow on a page node in a seam exception handler. 

The other problem: When this can be done for pageflows, can this be done for conversations as well?

The other solution idea: This should work when there is an exception in the invoke application phase (in the POST-Phase), but when there is a GET request (rendering the view in the GET after POST) redirecting to the indentical view could lead to a infinite loop.

This is how I do it manually, maybe there is a chance to these things automatically in seam: 

(1) place an exception handler in every pageflow

  <exception-handler exception-class="javax.el.ELException">
    <action class="de.plusfinanzservice.common.bean.ExceptionAction" />
  </exception-handler>

In a jpdl pageflow with seam we usually use EL expressions, therefore all exceptions we get here are wrapped in an EL Exception. First we unwrap them.

We determine the source of the transition. If the source is a decision node (and not a page node), we will need to find the error handling page node. Our convention is to use a "failure" transition on every decision node to do exactly that. The data is saved on a new seam bean exception handler.

(2) pick up all exceptions in pages.xml

<exception class="java.lang.Exception">
    <redirect view-id="#{exceptionHandler.getViewId('/iss/error.jspx')}">
      <message severity="error">
        #{messages[exceptionHandler.getMessage(org.jboss.seam.handledException)]}
      </message>
    </redirect>
  </exception>

After collection the right information in the exception action, reposition in the exception handler is easy:

      if (node != null) {
        Pageflow.instance().reposition(node);
        retVal = Pageflow.instance().getPage().getViewId();
      } 

This exception handler tries to do "the right thing" even if you are not in a pageflow. For GET requests it assumes that the exception has been raised in the render view phase and no recovery is possible. If not, I tries to forward you to the last known view (using conversation information). The message to be displayed is extracted from the Exception. 

Final thoughts: maybe it would be a good idea to create a two phase solution. One basic with exception handling. And another one that extends it that adds jpdl handling. 

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