Default Content Mapping and Request Rendering

One of the big obstacles in quick adoption of Sling might arguably be the requirement for multiple developments, such as...

1. Creating a Content implementation (or decide on reusing an existing implementation)
2. Defining the mapping descriptor to map the repository contents to the Content object and vice versa
3. Optionally create a node type defintion file in CND format
4. Creating a Component implementation (or decide on reusing an existing implementation)
5. Package this all up into an OSGi Bundle for deployment

While these steps make sense in an ideal world we all know does not exist (with the exception of Utopia, but there are no computers in Utopia), helpers for rapid development are needed. These helpers come in the form of usefull defaults on various levels.

Default Content Mapping

When a request is processed by Sling, one step is to resolve the request URL into a Content object. This works by checking the request URL for the longest match with an existing JCR repository node. The path of this node is then used to load the Content object through the ContentManager.load(String) method. If no mapping exists for the given node, an exception is thrown and the request fails.

In such a case of missing content mapping, a default Content mapping is defined in the form of the org.apache.sling.content.jcr.DefaultContent class. This mapping has the following features:

• The DefaultContent class is a java.util.Map. Thus all properties may be accessed using the familiar Map API.
• All non-protected properties of the node are loaded. Single value properties become scalar objects, while multi value properties become java.util.List objects.
• The types of the repository values are mapped according to the JCR specification for mapping between Property types and Java types.
• A few properties have special significance. See below.
• Creating new instances of this class and inserting these into the repository creates nodes of type nt:unstructured. When loading instances of this class the actual primary type of the node does not matter.

Default Component Selection

After having mapped the JCR repository node into the Content object the Component to actually handle the request must be resolved. This is done by calling the Content.getComponentId() method and looking up this component ID in an internal table. If either the Content.getComponentId() method returns null or no component is registered with the requested component ID a default resolution processing takes place as follows:

1. Let cid be the result of calling Content.getComponentId()
2. If cid is null, let cid be the result of calling Content.getPath() (this is never null}
3. Check for a component with the given cid and use it if existing
4. Otherwise, remove any leading slash from cid and replace slashes by dots and check for a component with this modified cid and use it if existing
5. Otherwise, let cid be the fully qualified name of the Content object class and check for a component with this modified cid and use it if existing
6. Otherwise and if cid ends with the string Content, remove that suffix and check for a component with this modified cid and use it if existing
7. Otherwise, append Component to the end of cid and and check for a component with this modified cid and use it if existing
8. Otherwise, let cid be the value of the org.apache.sling.components.DefaultComponent.ID field and check for a component with this modified cid and use it if existing
9. Finally, fail without having found a component to use - this is highly unlikely, though, because the default component is part of the Sling Core bundle and should always be available.

DefaultComponent

The default component first checks whether the request is sent with parameters and will update the Content object with the parameters as follows:

• If the Content object is a java.util.Map (such as is the case for the DefaultContent) the properties will directly accessed through the Map} API. Otherwise, the Content object is wrapped inside a org.apache.commons.collections.BeanMap to access the fields through the Map API.
• Any properties listed in the _delete parameter are removed. The _delete parameter may contain a comma-separated list of property names and may occurr multiple times.
• All other parameters are used to set property values, where any existing properties will be replaced and all properties not listed in the parameters remain unmodified. If a parameter occurrs only once a single value property is set, if parameter occurrs multiple times, a multi value property is set as a list of strings. Note, that any data type conversion may happen only by the BeanMap as required and thus lead to failure to update a single proeperty.

After the optional update phase, the fields of the Content object are written back. Again, the Content object is either accessed as a Map directly if it is a Map or packed in a BeanMap otherwise. The format of the output is deduced from the request URL's extension as returned by the ComponentRequest.getExtension() method:

Default Script

The easiest way to develop and deploy a component is to create a scripted component in the repository by just creating a node of type sling:scriptedComponent and creating a single JSP script at jsp/start.jsp below the component node. After that you can refer to that component by the path of the component node and get the start.jsp script called.

For more more elaborate script selection you may of course create more scripts and refer to them below the sling:scripts node of the component node.

Rapid Development Primer

To summarize, for rapid development you will have to execute the following steps:

1. Create a sling:ScriptedComponent node, for example at /some/sample/component
2. Create a JSP script file at jsp/start.jsp below that node; that would be /some/sample/component/jsp/start.jsp in the example
3. Create one or more nodes of any type, for example nt:unstructured, which have a single value string property named sling:componentId referring to the component via its path
4. Request the node by typing its path in your browser's address field