The document provides an overview of Apache Sling filters within Adobe CQ5/AEM, explaining their capabilities, registration, and use cases. It emphasizes the importance of understanding the Sling request invocation chain and highlights potential pitfalls when manipulating data. Various use cases such as logging, measuring, decoration, and preprocessing are discussed to showcase the practical applications of Sling filters.

1. Understanding & Utilizing Apache Sling Filters in Adobe CQ5 / AEM
Dominik Süß, Senior Developer, pro!vision GmbH
Sling Component Filters in CQ5
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2. • The Sling Request Invocation Chain
• Implementing own Sling Filters
• Use Cases and Samples
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Agenda

3. !!!WARNING!!!
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Sling Filters are a very generic and mighty extensionpoint. When
utilizing them you should always have in mind that AEM/CQ Core
functionality relies on the data provided by Sling, manipulation
of this data therefore may break CQ.

4. The Sling Request Invocation Chain
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How the Sling Engine processes HTTP Requests

5. FilterManager
Sling Request Invocation Chain
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SlingMainServlet
FilterChain
Scope: REQUEST
FilterChain
Scope: COMPONENT
Filter A
Filter B
Filter C
Registered as Services
(preferably via SCR Annotation)
Component Servlet
processRequest()dispatchRequest()
INCLUDE/FORWARD
SlingRequestProcessor

6. Implementing Sling Filters
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How to use Filters as Extensionpoint

7. Registration of Filters
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• Standard Servlet Filter (javax.servlet.Filter)
– Need to be registered as Service („manually“ or SlingFilter SCR Annotation)
• Order and FilterScopes can be defined
• Order defined by Serviceranking
• Filterscopes
– REQUEST
– COMPONENT
– ERROR
– INCLUDE *
– FORWARD *
* FilterChain executes COMPONENT scope for these scopes as well.

8. SCR Annotation
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@SlingFilter(scope = SlingFilterScope.COMPONENT, order = -10000, metatype =
false)
public class MyComponentFilter implements javax.servlet.Filter {
@Override
public void doFilter(ServletRequest pRequest, ServletResponse pResponse,
FilterChain pChain) throws IOException, ServletException {
// Implement Filter
// continue with filter chaining
pChain.doFilter(pRequest, pResponse);
}
}

9. Filter Order
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• Existing Filters might use the deprecated „filter.order“ instead
„service.ranking“
• NOTE: SLING-2920 was identified while verifying this behavior – look up the
mailing list for further information how to use it for now
• Existing Filters and their execution ORDER can be inspected via
– http://<instance>:<port>/system/console/status-slingfilter

10. Check Filterconditions at first place
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public void doFilter(ServletRequest pRequest, ServletResponse pResponse,
FilterChain pChain) throws IOException, ServletException {
if (!(pRequest instanceof SlingHttpServletRequest)) {
throw new ServletException("Request is not a Sling HTTP request.");
}
if (isFilterEnabled(slingRequest)) {
// Implement Filter-Logic
} else {
// continue with filter chaining
pChain.doFilter(pRequest, pResponse);
}
}
• Check available Information from slingRequest if Filter should do anything.
• Be aware that these checks may be performed a lot especially for COMPONENT scope

11. Accessing and Using Data in Filter
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public void doFilter(ServletRequest pRequest, ServletResponse pResponse,
FilterChain pChain) throws IOException, ServletException {
if (!(pRequest instanceof SlingHttpServletRequest)) {
throw new ServletException("Request is not a Sling HTTP request.");
}
if (isFilterEnabled(slingRequest)) {
Resource resource = slingRequest.getResource(); // already resolved
if (null != resource) {
PrintWriter writer = pResponse.getWriter();
writer.write("<component resourceType="" + resource.getResourceType()
"">");
pChain.doFilter(pRequest, pResponse);
writer.write("</component>");
return;
}
}
pChain.doFilter(pRequest, pResponse);
}

12. Useful Wrapper Objects in Filters
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• SlingHttpServletRequestWrapper
– Implement to override parts of the request
• E.g. Inject impersonated ResourceResolver*
• SlingHttpServletResponseWrapper
– Implement to inject additional logic to response
• E.g. inject own writer to buffer the response to perform some postprocessing
* Security-Note: Make sure you don’t create security holes this way – the request
should always respect the permissions of the current user!

13. Dispatch to other Resource
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RequestDispatcher dispatcher =
slingRequest.getRequestDispatcher(otherResource);
dispatcher.forward(request, response);
Required in any situation where you need to redirect the request.
Note: You should be aware that a Dispatcher Forward or Include will retrigger
the FilterChain. Place such filters early in the Filterchain to prevent other
Filters to be executed twice.

14. Usecases & Samples
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15. Usecases
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• Logging
• Measuring
• Decoration
• Pre-Processing
• Post-Processing
• Manipulating Inclusions

16. Logging
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• Allows to hook in Requestcentric logging at Sling Level
• It is possible to log even requests/includes of components which are not
part of the own application (like foundation components)
• Sample
– org.apache.sling.engine.impl.debug.RequestProgressTrackerLogFilter

17. Measuring
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• Since the Filterchain gets consulted at each call of a component they
provide a natural location to measure the actual usage of the system
• The concrete usecases depend on the goal of the application
• You should just add additional measuring if no other techniques like
loganalysis or available reports are not sufficient.

18. Decoration
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• Adding generic but usecasespecific decoration to components
• Sample
– WCMDebugFilter ( for Requestparameter debug=layout)
• Further Usecase
– For specific selector: Metadata for third party systems like
• Search Parsers
• Integrating Systems (e.g. Portals)

19. Preprocessing
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• Injecting data that needs to be available within the components
• Sample:
– WCMComponentFilter – generic logic that links CQ Authoring logic with Sling
Components
– I18nFilter – Injecting I18n Resource Bundles
• Further Usecases
– Build Java Bean and place in Request to be used via jsp:useBean and EL within
JSPs (Sepearation of businesslogic)*
* Just a proof of concept, impact of preprocessing on performance yet untested

20. Postprocessing
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• Injecting custom writer to postprocess output
• Sample:
– RewriterFilter
• Further Usecases:
– Integration of thirt party markup (e.g. via Velocity or Freemarker)
– Injection of a generic Placeholderlogic (e.g. Personalization)

21. Manipulating Inclusions
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• Redirecting , Skipping or manipulating further processing
• Samples:
– Mobile RedirectFilter (trigger Redirect with matching selector for device)
– TimeWarp Filter ( replacing ResourceData & corresponding Objects with Objects at
given Timestamp)
• Further Usecases
– Controlled Impersonation in Author Mode for Permissionpreview
(Publishpermissions)
– Component Level Caching (Record response to cache and answer subsequent
requests from cache)*
* Not yet verified, but might be a solution for partial caching where dispatcher caches
cannot be used

22. Some last words…
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• The Apache Sling Stack is the core of CQ5 / AEM
• Apache Sling is an open source project that relies on an active community
• So – be part the community!
– Join the User and/or Dev Mailinglist
– File Bugs and contribute Patches
– Participate in community events to drive evolution of Apache Sling …
… like adaptTo() 2013 in Berlin (http://www.adaptto.org)
;)

23. Berlin
pro!vision GmbH
Wilmersdorfer Str. 50-51
10627 Berlin
Germany
Tel. +49 (30) 818828-50
E-Mail info@pro-vision.de
Frankfurt
pro!vision GmbH
Löwengasse 27 A
60385 Frankfurt am Main
Germany
Tel. +49 (69) 8700328-0
E-Mail info@pro-vision.de
Karim Khan
Tel. +49 (69) 8700328-0
E-Mail kkhan@pro-vision.de
Contact
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