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Servlet mapping specifies which servlet should be invoked when the client requests a particular URL. In Magnolia, servlet mappings are typically configured in /server/filters/servlets. This is a static way. While creating a component (Java class or Freemarker script), the path to a servlet is hard-coded. When the mapping changes, the hard-coded string literal must be changed too and the software must be re-deployed. To work around this limitation, use dynamic servlet mapping. (warning) Magnolia 5.3+

Dynamic mapping with SelfMappingServlet

The  SelfMappingServlet  interface makes dynamic servlet mapping possible. The interface has one method:

info.magnolia.cms.filters.SelfMappingServlet
String getSelfMappingPath();

Implement the interface in your own servlet. Instead of returning a hard-coded literal, return something originating in a configuration. Any other Java class such as a model class used in a Freemarker script can then read the value.

Example: DamDownloadServlet

Other components in the system may need to know about servlet mappings. This is the case when generating links for the DAM servlet.  DamDownloadServlet  implements the SelfMappingServlet interface:

info.magnolia.dam.core.download.DamDownloadServlet
public class DamDownloadServlet extends HttpServlet implements SelfMappingServlet {    
	// ... more code here (this is just a snippet)	

	@Inject
    public DamDownloadServlet(final DamCoreConfiguration configuration, final AssetProviderRegistry assetProviderRegistry) {
        this.configuration = configuration;
        this.assetProviderRegistry = assetProviderRegistry;
    }

	@Override
    public String getSelfMappingPath() {
        return configuration.getDownloadPath() + "/*";
    }
}

The getSelfMappingPath() method returns a value which comes from the DamCoreConfiguration - which could be injected into any other class, too. DamCoreConfiguration#getDownloadPath() is also used in JcrAssetProvider to construct paths for links:

info.magnolia.dam.jcr.JcrAssetProvider
// ... more code here (this is just a snippet)	    

	public String getLink(final Asset asset) {
        final String contextPath = MgnlContext.getContextPath();
        return contextPath + configuration.getDownloadPath() + asset.getItemKey().asString() + "/" + asset.getFileName();
    }

If for whatever reason the mapped path to the DamDownloadServlet must be changed, there is only one place where the path is adapted: /modules/dam/config/downloadPath in module configuration. The code of DamDownloadServlet and JcrAssetProvider remain the same. This means you don't need to re-deploy the application. You only need to change configuration on the running system.

(warning) You won't find anything at /modules/dam/config/downloadPath because DamCoreConfiguration#getDownloadPath() has a default value.

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