title: Retrieving action output in oozie with wf:actionData property
body: <p>I'm writing out some messages to <code>oozie.action.output.properties</code> file in java action called through oozie workflow. That message is retrieved in next workflow using <code>${wf:actionData('&lt;action&gt;')['&lt;messageKey&gt;']}</code>. But, sometimes I need not write any message to properties file. In such case, oozie gives error message when retrieving the message as <code>variable [&lt;messageKey&gt;] cannot be resolved</code>. 
  If it returns null or empty, I can continue with the workflow but if fails with the mentioned error. or How can I verify if that variable exists or no message exists for that messageKey.
  Example snippet:</p>
  
  <pre><code>&lt;action name="fail-check"&gt;
        &lt;sub-workflow&gt;
           &lt;app-path&gt;${wf:appPath()}/../fail-check&lt;/app-path&gt;
           &lt;propagate-configuration /&gt;
           &lt;configuration&gt;
             &lt;property&gt;
               &lt;name&gt;dataPrepSetupMsg&lt;/name&gt;
               &lt;value&gt;${wf:actionData('setup')['errMsg']}&lt;/value&gt;
  </code></pre>
  
  <p>
             
               
            
   </p>
  
  <p>Inside JAVA action:</p>
  
  <pre><code>if(message != null){
      final String filePath = System.getProperty("oozie.action.output.properties");
      properties.put(errorMessage, message); 
      OutputStream outputStream = new FileOutputStream(filePath);
      properties.store(outputStream, null);
  }
  </code></pre>