AEM Iterating Node – Best Practices

Problem statement:

How can I iterate child nodes and get certain properties?

Requirement:

get child resources of the current resource and get all image component filerefernce property into a list

Can I use Java 8 Streams?

Introduction: Using while or for loop:

@PostConstruct
private void initModel() {
  List < String > imagePath = new ArrayList < > ();
  Iterator < Resource > children = resource.listChildren();
  while (children.hasNext()) {
    final Resource child = children.next();
    if (StringUtils.equalsIgnoreCase(child.getResourceType(), "wknd/components/image")) {
      Image image = modelFactory.getModelFromWrappedRequest(request, child, Image.class);
      imagePath.add(image.getFileReference());
    }
  }
}

Introduction Abstract Resource Visitor:

Sling provides AbstractResourceVisitor API, which performs traversal through a resource tree, which helps in getting child properties.

Create the class which extends AbstractResourceVisitor abstract class

Override accept, traverseChildren and visit methods as shown below

Call visit inside accept method instead of super. visit, I have observed it was traversing twice if I use super hence keep this in mind

package utils;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import org.apache.commons.lang3.StringUtils;
import org.apache.sling.api.resource.AbstractResourceVisitor;
import org.apache.sling.api.resource.Resource;
import org.apache.sling.api.resource.ResourceResolver;
import org.apache.sling.api.resource.ValueMap;
import com.day.cq.wcm.foundation.Image;
import com.drew.lang.annotations.NotNull;

public class ExampleResourceVisitor extends AbstractResourceVisitor {
	
	private static final String IMAGE_RESOURCE_TYPE = "wknd/components/image";
	private static final String TEXT_RESOURCE_TYPE = "wknd/components/text";
	
	private static final ArrayList<String> ACCEPTED_PRIMARY_TYPES = new ArrayList<>();
	static {
		ACCEPTED_PRIMARY_TYPES.add(IMAGE_RESOURCE_TYPE);
		ACCEPTED_PRIMARY_TYPES.add(TEXT_RESOURCE_TYPE);
	}
	
	private final List<String> imagepaths = new ArrayList<>();	
	
	public List<String> getImagepaths() {
		return imagepaths;
	}

	@Override
	public final void accept(final Resource resource) {
		if (null != resource) {
			final ValueMap properties = resource.adaptTo(ValueMap.class);
			final String primaryType = properties.get(ResourceResolver.PROPERTY_RESOURCE_TYPE, StringUtils.EMPTY);
			if(ACCEPTED_PRIMARY_TYPES.contains(primaryType)){
				visit(resource);
			}
			this.traverseChildren(resource.listChildren());
		}
	}

	@Override
	protected void traverseChildren(final @NotNull Iterator<Resource> children) {
		while (children.hasNext()) {
			final Resource child = children.next();
			accept(child);
		}
	}

	@Override
	protected void visit(@NotNull Resource resource) {
		final ValueMap properties = resource.adaptTo(ValueMap.class);
		final String primaryType = properties.get(ResourceResolver.PROPERTY_RESOURCE_TYPE, StringUtils.EMPTY);
		if (StringUtils.equalsIgnoreCase(primaryType, IMAGE_RESOURCE_TYPE)) {
			imagepaths.add(properties.get(Image.PN_REFERENCE, StringUtils.EMPTY));
		}
	}
}

Call the ExampleResourceVisitor and pass the resource and call the getImagepaths() to get the list of image paths

@PostConstruct
private void initModel() {
  ExampleResourceVisitor exampleResourceVisitor = new ExampleResourceVisitor();
  exampleResourceVisitor.accept(resource);
  List < String > imageVisitorPaths = exampleResourceVisitor.getImagepaths();
}

Introduction Resource Filter Stream:

Resource Filter bundle provides a number of services and utilities to identify and filter resources in a resource tree.

Resource Filter Stream:

ResourceFilterStream combines the ResourceStream functionality with the ResourcePredicates service to provide an ability to define a Stream<Resource> that follows specific child pages and looks for specific Resources as defined by the resources filter script. The ResourceStreamFilter is accessed by adaptation.

ResourceFilterStream rfs = resource.adaptTo(ResourceFilterStream.class);
rfs.stream().collect(Collectors.toList());

Example code for our problem statement would like this:

@PostConstruct
private void initModel() {
  ResourceFilterStream rfs = resource.adaptTo(ResourceFilterStream.class);
  List < String > imagePaths = rfs.stream()
    .filter(r -> StringUtils.equalsIgnoreCase(r.getResourceType(), "wknd/components/image"))
    .map(r -> modelFactory.getModelFromWrappedRequest(request, r, Image.class).getFileReference())
    .collect(Collectors.toList());
}

Optimizing Traversals

Similar to indexing in a query there are strategies that you can do within a tree traversal so that traversals can be done in an efficient manner across a large number of resources. The following strategies will assist in traversal optimization.


Limit traversal paths

In a naive implementation of a tree traversal, the traversal occurs across all nodes in the tree regardless of the ability of the tree structure to support the nodes that are being looked for. An example of this is a tree of Page resources that have a child node of jcr:content which contains a subtree of data to define the page structure. If the jcr:content node is not capable of having a child resource of type Page and the goal of the traversal is to identify Page resources that match specific criteria then the traversal of the jcr:content node can not lead to additional matches. Using this knowledge of the resource structure, you can improve performance by adding a branch selector that prevents the traversal from proceeding down a non productive path


Limit memory consumption

The instantiation of a Resource object from the underlying ResourceResolver is a non trivial consumption of memory. When the focus of a tree traversal is obtaining information from thousands of Resources, an effective method is to extract the information as part of the stream processing or utilizing the forEach method of the ResourceStream object which allows the resource to be garbage collected in an efficient manner.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s