Error handling
DRAFT: May need corrections!
Error handling is a fundamental part of system and application management. Commands you write that govern a key operational process should take that into account. Depending on the error and surrounding conditions, there are alternate courses to take. You might want to fail immediatly, create a log and send a notification and then fail, take some user input and have them decide what to do next. You might even have a corrective procedure which should be run when an error is encountered. Some combination of these choices might also be used.
This page provides a few recipes to handling errors.
Error handling via Trycatch
Ant-contrib's Trycatch task is a well known solution to catching and handling errors. Between its <try></try> tags insert the steps which might throw the error. The error handling code resides between the <catch></catch> tags.
In the example, below the catch block prompts the user via Ant's Input task. If the user responds with a "no" answer then the code will carry out steps to clean up and then error out.
<trycatch property="msg">
<try>
<fail>Tada!</fail>
</try>
<catch>
<input message="keep going?" validargs="yes,no" addproperty="answer" />
<if>
<equals arg1="${answer}" arg2="no"/>
<then>
<!--
** clean up steps
<!--
...
<fail message="user opted out after failure: ${msg}" />
</then>
</if>
</catch>
</trycatch>
<!--
** keep doing some stuff
<!--
...
The Trycatch task is a good starting point to create your own custom error handling logic, allowing you a great deal of freedom.
Error handling via Workflow
CTL workflows implicitly behave like a try/catch block.
The CTL workflow construct accepts a data type called errorhandler that will be executed if a BuildException is thrown from any of the tasks in the workflow's task sequence. The errorhandler datatype inherits from Ant's TaskContainer class so can accept any task.
<controller>
<execute>
<context depot="${context.depot}"/>
<workflow name="onetwothree" threadcount="1">
<!--
** set the error handler to execute the "echo" task
-->
<errorhandler errorProperty="workflow.error">
<echo message="caught an error: ${workflow.error}"/>
</errorhandler>
<tasksequence>
<echo message="one"/>
<echo message="two"/>
<echo message="three"/>
<fail>Tada!</fail>
</tasksquence>
</workflow>
</execute>
</controller>
A nice advanage to using CTL's workflow is how you can define a sequence of defined command, as well as, the error-handling logic succinctly, in the type.xml file.
<command name="mycommand" command-type="WorkflowCommand"
error-handler-type="HANDLER">
<!--
** set the error handler to execute the "echo" task
-->
<errorhandler>
<report>caught an error: ${workflow.error}"</report>
<fail/>
</errorhandler>
<workflow>
<command name="step-one"/>
<command name="step-two"/>
<command name="step-three"/>
<fail>Tada!</fail>
</workflow>
</command>