Error Handling

Correct error handling is an important concern when developing web applications. Because there are typically several layers a web request must proceed through before a response is returned, there are many places where an error can occur. Moreover, the error message the user should see when something goes wrong, compared to what the developer would like to see, are almost certainly bound to be different. In traditional servlet/JSP environments, the developer would have to take certain measures to ensure that the end user would not see a stack trace, for example, and would instead see a friendlier error screen. This process, however, is not built in to the servlet/JSP environment, and inevitably the same soultion would need to be re-implemented every time a new project was started.

Mojave addresses this problem by providing a number of safe mechanisms for introducing correct error handling at the web application level. The error handling functionality of Mojave can be found in the org.mojavemvc.exception package. At the core is the ErrorHandler interface, which simply defines a single handlerError method. The handleError method returns a View to be rendered in place of the View that was not able to be rendered. A developer is able to provide their own ErrorHandler implementation to define what to render when an error occurs.

Mojave makes no assumptions about how an ErrorHandler behaves internally, and so a new instance of an ErrorHandler is created per request. Therefore, for a developer to specify their own ErrorHandler, they must specify an ErrorHandlerFactory. An ErrorHandlerFactory is specified to the application in the web.xml file, through the error-handler-factory init parameter. The value consists of the custom ErrorHandlerFactory's fully qualified class name.

By default, if no ErrorHandlerFactory is specified, then a default implementation is provided that simply prints the stack trace in the response. Another init parameter, jsp-error-file, exists to allow a developer to quickly specify a JSP page that should be rendered in case of an exception. If this init parameter is specified, then no error-handler-factory need be specified (though it can be).

As an example of an application that specifies an ErrorHandlerFactory, consider the following web.xml snippet:

<servlet>
    <servlet-name>FrontController</servlet-name>
    <servlet-class>org.mojavemvc.FrontController</servlet-class>
    <init-param>
      <param-name>controller-classes</param-name>
      <param-value>helloworld.controllers</param-value>
    </init-param>
    <init-param>
      <param-name>error-handler-factory</param-name>
      <param-value>helloworld.exception.MyErrorHandlerFactory</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>FrontController</servlet-name>
    <url-pattern>/serv/*</url-pattern>
  </servlet-mapping>

The following might be an implementation of MyErrorHandlerFactory, and a corresponding MyErrorHandler:

public class MyErrorHandlerFactory implements ErrorHandlerFactory {
  public ErrorHandler createErrorHandler() {
    return new MyErrorHandler();
  }
}

public class MyErrorHandler implements ErrorHandler {
  public PlainText handleError(Throwable e) {
    return new PlainText("Error!");
  }
}

Alternatively, if the application is JSP-based, supplying simply the name of a JSP file to render in place of errors would require no extra work:

<servlet>
    <servlet-name>FrontController</servlet-name>
    <servlet-class>org.mojavemvc.FrontController</servlet-class>
    <init-param>
      <param-name>controller-classes</param-name>
      <param-value>helloworld.controllers</param-value>
    </init-param>
    <init-param>
      <param-name>jsp-path</param-name>
      <param-value>/WEB-INF/jsp/</param-value>
    </init-param>
	<init-param>
      <param-name>jsp-error-file</param-name>
      <param-value>error.jsp</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
  </servlet>
  <servlet-mapping>
    <servlet-name>FrontController</servlet-name>
    <url-pattern>/serv/*</url-pattern>
  </servlet-mapping>