The Sling Engine includes support for handling uncaught
Throwable as well as rendering custom HTTP status code pages. This is implemented by expecting a (single)
org.apache.sling.api.servlets.ErrorHandler service to which handling of uncaught
Throwable and HTTP status responses are delegated.
The Sling Servlet Resolver bundle implements this interface by providing an elaborate mechanism to find the correct error handling script or servlet using the same algorithms as are used to select the scripts or servlets to handle regular requests.
This page provides more information on how error handler scripts are selected and what is provided out of the box.
The ErrorHandlingTest in our integration tests suite provides working examples of various error handling scenarios.
Before an ErrorHandler is invoked by the Sling Engine, the response is resetted. This removes all headers and potentially written content. If the response can't be reset, e.g. when it is already partially committed to the client, no ErrorHandler will be invoked. In this case the response will be committed as-is.
As the response is reset by Sling Engine, an ErrorHandler must not reset the response again. Especially as there might be error filters being invoked before the ErrorHandler.
Note: This is only relevant when you are using an older version of Sling Engine before release 2.15.0
If you are using an older Sling Engine version, an ErrorHandler must reset the response and do the following:
IllegalStateException. Also the writer may not be available.
The Sling engine implements the
HttpServletResponse.sendError methods by calling the
ErrorHandler.handleError(int status, String message, SlingHttpServletRequest request, SlingHttpServletResponse response) method.
The Servlet Resolver bundle implementation looks up a script to handle the status code as follows:
sling/servlet/errorhandleris used as the implied base resource type (as opposed to
sling/servlet/defaultfor regular script resolution.
To handle uncaught Throwables the simple name (
Class.getSimpleName()) of the
Throwable class is used as request extension. Similarly to the Java try-catch clauses the class hierarchy is supported. That is to handle an uncaught
FileNotFoundException, the names
Throwable are checked for a Servlet and the first one found is then used. Again, the Serlvet may be a Servlet registered as an OSGi service or may be a plain script stored in the JCR repository or provided through some custom Resource provider.
Example: To register a catch-all handler for any uncaught Throwables you might create a script
Note: If no script or servlet to handle an uncaught
Throwable is registered, the default handler kicks in, which sends back a 500/INTERNAL SERVER ERROR response containing the
Throwable and the stack trace. This response is not handled by the HTTP Status Code handling described above because the response status is sent using
HttpServletResponse.setStatus(int, String). To prevent this default response you have to implement a catch-all handler for the
Throwable class as shown in the example.
The Sling Servlet Resolver bundle provides a default error handler servlet which is used if the algorithms described above do not resolve to a handler script or servlet. The provided error handler servlet does the following:
javax.servlet.error.messagerequest attribute by default
Starting with Sling Servlet Resolver version 2.0.10 the default error handler servlet is looked up using the string
default as the request extension and the provided default servlet is registered as
/libs by default.
Thus to overwrite the default error handler servlet provide a servlet or script for the
default extension, for example