The class com.atlassian.jira.event.issue.AbstractIssueEventListener requirs a default constructor.
Case:
I have implemented a CustomListener which has the dependency injection for a reference to 'IssueMailQueueItemFactory'
public CustomtListener(IssueMailQueueItemFactory issueMailQueueItemFactory)

{ this.issueMailQueueItemFactory = issueMailQueueItemFactory; }

The CustomListener also has has several properties. Thus the following line gives a 'ClassNotFoundError' as the EditListener.java cannot instantiate the class
as it does not have a default Constructor
public String[] getAcceptedParams() {
try

{ return getJiraListener().getAcceptedParams(); }

catch (Exception e)

{ log.error("Error getting accepted params: " + e.getMessage(), e); return new String[0]; }

}
public JiraListener getJiraListener() throws ClassNotFoundException, IllegalAccessException, InstantiationException

{ return (JiraListener) Class.forName(getListener().getString("clazz")).newInstance(); }