Alternative to massive if/else block for examining different exception conditions?

Is there a more elegant way of examining all of the possible exception types besides a massive if/else block like this?

public ModelAndView resolveException(HttpServletRequest request, 
HttpServletResponse response, Object obj, Exception e) {

    if (e instanceof BadException)
    {
      displayMessage("That was bad.");
    }

    else if (e instanceof ReallyBadException)
  开发者_JAVA百科  {
      displayMessage("That was really bad.");
    }

    else if (e instanceof ReallyReallyBadException)
    {
      displayMessage("That was really really bad.");
    }

    // ...
    // and so on
    // ...

    return null;
}

If you control the exceptions that end up in this method, you could make use of the message embedded in them:

public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object obj, Exception e) {
    displayMessage(e.getMessage());
    return null;
}

But you should only do this, if the messages are meaningful to an enduser. Normally you should handle exceptions in a way that the user doesn't notice anything is wrong.

public ModelAndView resolveException(HttpServletRequest request, 
        HttpServletResponse response, Object obj, Exception exception)
{
    try
    { 
        throw exception;
    }
    catch(BadException e)
    {
        displayMessage("That was bad.");
    } catch (ReallyBadException e)
    {
      displayMessage("That was really bad.");
    } catch (ReallyReallyBadException e)
    {
      displayMessage("That was really really bad.");
    }

    // ...
    // and so on
    // ...

    return null;
}

Use multiple catch blocks, see here.

You could create a pre-defined Map of specific Exception type to the message you want to display:

private Map<Class<?>, String> exceptionMessages = ...;

public ModelAndView resolveException(HttpServletRequest request, HttpServletResponse response, Object obj, Exception e) {

    if exceptionMessages.containsKey(e.getClass()) {
        displayMessage(exceptionMessages.get(e.getClass()));
    }
    else {
         // what to do by default?
    }
    return ...;
}

Or even better, you could externalize the Map to a properties file or ResourceBundle so you could load the user-visible string from some place other than code (and/or globalize it).

This code sample isn't capable of dealing with using the same message for a hierarchy of Exception types, but it should be pretty simple to walk the inheritance chain if the direct class is not found in the map.