B.19. Visitor Framework

The API provides a language visitor framework in the org.teiid.language.visitor package. The framework provides utilities useful in navigating and extracting information from trees of language objects.

The visitor framework is a variant of the Visitor design pattern, which is documented in several popular design pattern references. The visitor pattern encompasses two primary operations: traversing the nodes of a graph (also known as iteration) and performing some action at each node of the graph. In this case, the nodes are language interface objects and the graph is really a tree rooted at some node. The provided framework allows for customization of both aspects of visiting.

The base AbstractLanguageVisitor class defines the visit methods for all leaf language interfaces that can exist in the tree. The LanguageObject interface defines an acceptVisitor() method. This method will call back on the visit method of the visitor to complete the contract. A base class with empty visit methods is provided as AbstractLanguageVisitor. The AbstractLanguageVisitor is a visitor shell - it performs no actions when visiting nodes and does not provide any iteration.

The HierarchyVisitor provides the basic code for walking a language object tree. The HierarchyVisitor performs no action as it walks the tree - it encapsulates the knowledge of how to walk it. If your translator wants to provide a custom iteration that walks the objects in a special order (to exclude nodes, include nodes multiple times, conditionally include nodes, and so forth) then you must either extend HierarchyVisitor or build your own iteration visitor. In general, that is not necessary.

The DelegatingHierarchyVisitor is a special subclass of the HierarchyVisitor that provides the ability to perform a different visitor's processing before and after iteration. This allows users of this class to implement either pre- or post-order processing based on the HierarchyVisitor. Two helper methods are provided on DelegatingHierarchyVisitor to aid in executing pre- and post-order visitors.