
=== Markup loading and parsing

Before rendering any component Wicket must retrieve its markup calling method _getMarkup()_ of class _org.apache.wicket.Component_. This markup is an instance of interface _org.apache.wicket.markup.IMarkupFragment_. Markup is lazy loaded the first time we render the relative component and is cached at application level. The internal class that actually loads the markup is _org.apache.wicket.markup.MarkupFactory_ and is part of application's markup settings:

[source,java]
----
	//get current markup factory
	Application.get().getMarkupSettings().getMarkupFactory()
----
 
After the markup has been loaded by _MarkupFactory_, it's parsed with class _org.apache.wicket.markup.MarkupParser_. _MarkupFactory_ creates a new _MarkupParser_ with method _newMarkupParser(MarkupResourceStream resource)_. The effective markup parsing is performed with a chain of entities implementing interface _org.apache.wicket.markup.parser.IMarkupFilter_. The default set of _IMarkupFilters_ used by _MarkupParser_ takes care of different tasks such as HTML validation, comments removing, Wicket tags handling, etc... 

To customize the set of _IMarkupFiltersS_ used in our application we can create a subclass of _MarkupFactory_ overriding method _newMarkupParser(MarkupResourceStream resource)_:

[source,java]
----
public MyMarkupFactory 
{
...
  public MarkupParser newMarkupParser(final MarkupResourceStream resource) 
  {
       MarkupParser parser = super.newMarkupParser(resource);
       parser.add(new MyFilter());
       return parser;
  }
}
----

This custom class must be registered in the markup settings during application's initialization:

[source,java]
----
@Override
public void init()
{
	super.init();
	getMarkupSettings().setMarkupFactory(myMarkupFactory)
}
----

Usually we won't need to change the default configuration of _IMarkupFiltersS_, but it's important to be aware of this internal mechanism before we talk about another advanced feature, which is building auto components resolvers.

=== Auto components resolvers

Even if Wicket encourages developers to use just standard HTML in their markup code, in this guide we have seen a number of "special" tags (those starting with _wicket:_) that help us for specific tasks (e.g. _wicket:enclosure_ tag). Wicket handles most of these tags creating a corresponding special component called _auto_ component. This kind of components are resolved in two steps:

1. first their tag is identified by a _IMarkupFilters_ which also takes care of assigning a unique tag id.
2. then during rendering phase when an auto component is found a new component is created for it using one of the registered _org.apache.wicket.markup.resolver.IComponentResolver_:

[source,java]
----
public interface IComponentResolver extends IClusterable
{
	/**
	 * Try to resolve a component.
	 * 
	 * @param container
	 *            The container parsing its markup
	 * @param markupStream
	 *            The current markupStream
	 * @param tag
	 *            The current component tag while parsing the markup
	 * @return component or {@code null} if not found
	 */
	public Component resolve(final MarkupContainer container, final MarkupStream markupStream,
		final ComponentTag tag);
}
----

Registered _IComponentResolverS_ can be retrieved through Application's settings:

[source,java]
----
Application.get()
	.getPageSettings()
	.getComponentResolvers()
----

NOTE: An internal utility class named _org.apache.wicket.markup.resolver.ComponentResolvers_ is also available to resolve auto components for the current markup tag.

