Package io.permazen.annotation

Annotation Interface ReferencePath

@Retention(RUNTIME)
@Target({ANNOTATION_TYPE,METHOD})
@Documented
public @interface ReferencePath

Annotates Java methods that should return all objects found at the far end of a ReferencePath.

This annotation can be used as a convenience to let Permazen auto-generate reference path traversal code. A common use case is inverting references, e.g., from a parent back to a child in a one-to-many relationship.

Annotating an abstract method with @ReferencePath("->some->path") is equivalent to providing an implementation that returns all objects reachable via the reference path "->some->path" when starting at the current instance. References can be traversed in either the forward or inverse directions; see ReferencePath for details on reference paths.

For example:

  @PermazenType
  public interface TreeNode extends PermazenObject {

      /**
       * Get the parent of this node, or null if node is a root.
       */
      TreeNode getParent();
      void setParent(TreeNode parent);

      /**
       * Get the children of this node.
       */
      @ReferencePath("<-TreeNode.parent")
      NavigableSet<TreeNode> getChildren();

      /**
       * Get this node's grandparent, if any.
       */
      @ReferencePath("->parent->parent")
      Optional<TreeNode> getGrandparent();
  }
 

In the example above, the generated getChildren() implementation will be functionally equivalent to (and slightly more efficient than) this one:

      /**
       * Get the children of this node.
       */
      NavigableSet<TreeNode> getChildren() {
          final PermazenTransaction jtx = this.getTransaction();
          final ReferencePath path = jtx.getPermazen().parseReferencePath(this.getClass(), "<-TreeNode.parent");
          return jtx.followReferencePath(path, Collections.singleton(this));
      }
  }
 

Method Return Type

The annotated method must be an instance method and return either NavigableSet<T> or Optional<T>, where T is any super-type of the target object type(s) at the end of the reference path.

If the reference path contains only forward traversals of many-to-one relationships (i.e., simple reference fields), then only one object can be returned and the return type must be Optional<T>.

Otherwise, either return type is valid, and you can use Optional<T> if you only care to retrieve at most a single instance.

More examples:

  @PermazenType
  public interface TreeNode extends PermazenObject {

      /**
       * Get the parent of this node, or null if node is a root.
       */
      TreeNode getParent();
      void setParent(TreeNode parent);

      /**
       * Get the children of this node.
       */
      @ReferencePath("<-TreeNode.parent")
      NavigableSet<TreeNode> getChildren();

      /**
       * Get the siblings of this node (including this node).
       */
      @ReferencePath("->parent<-TreeNode.parent")
      NavigableSet<TreeNode> getSiblings();

      /**
       * Get second cousins once removed (and parents), but only the first.
       */
      @ReferencePath("->parent->parent->parent<-TreeNode.parent<-TreeNode.parent")
      Optional<TreeNode> getSecondCousinOnceRemoved();
  }
 

Meta-Annotations

This annotation may be configured indirectly as a Spring meta-annotation when spring-core is on the classpath.

Optional Element Summary

Optional Elements

Modifier and Type	Optional Element	Description
String	value	The reference path to follow.

Element Details

value

String value

The reference path to follow.

The starting type for the reference path is the class containing the annotated method.

Returns:

the reference path to traverse

See Also:

• ReferencePath

Default:

""