AEM error - Usage of org.apache.sling.api.resource.ResourceResolver as a field is not thread safe.

Hi ​@fionas76543059 

The issue is that 

ResourceResolver

 is being stored as a field in 

ExternalLinkTransformerFactory

, which is a singleton service. Since 

Transformer

 instances can be used concurrently across multiple threads, this creates a thread-safety problem.

Solution: Store the 

ResourceResolver

 in the 

ExternalLinkTransformer

 instance (not the factory), since a new transformer is created per request.

Here's the corrected code:

@Component(
        label = "External Link Transformer Factory",
        description = "Rewrites external links by ...")
@Service(value = TransformerFactory.class)
public class ExternalLinkTransformerFactory implements TransformerFactory {

    @Property(value = "extlink", propertyPrivate = true)
    private static final String PROPERTY_PIPELINE_TYPE = "pipeline.type";

    @Override
    public Transformer createTransformer() {
        return new ExternalLinkTransformer();
    }

    private class ExternalLinkTransformer implements Transformer {

        private static final String TAG_ANCHOR = "a";
        private static final String ATTR_HREF = "href";

        private ContentHandler contentHandler;
        private ResourceResolver resourceResolver; // Move here

        @Override
        public void init(ProcessingContext processingContext,
                         ProcessingComponentConfiguration processingComponentConfiguration)
                throws IOException {
            resourceResolver = processingContext.getRequest().getResourceResolver();
        }

        @Override
        public void setContentHandler(ContentHandler contentHandler) {
            this.contentHandler = contentHandler;
        }

        @Override
        public void startElement(String uri, String localName, String qName, Attributes attributes) throws SAXException {
            contentHandler.startElement(uri, localName, qName, rebuildAttributes(localName, attributes));
        }

        private Attributes rebuildAttributes(String elementName, Attributes currentAttrs) {
            if(!TAG_ANCHOR.equalsIgnoreCase(elementName)) {
                return currentAttrs;
            }

            String url = currentAttrs.getValue(ATTR_HREF);
            try {
                final AttributesImpl newAttrs = new AttributesImpl(currentAttrs);
                if (resourceResolver != null && resourceResolver.isLive()) {
                    String modifiedHref = resourceResolver.map(pathname);
// modifiedHref = modifiedHref + other stuff...
                    newAttrs.setValue(currentAttrs.getIndex(ATTR_HREF), modifiedHref);
                    return newAttrs;
                }
            } catch (Exception e) {
// Handle exception
            }
            return currentAttrs;
        }

// Implement other Transformer methods...
    }
}

Key change: Moved 

resourceResolver

 from the factory class to the 

ExternalLinkTransformer

 inner class. Each transformer instance now has its own 

ResourceResolver

, making it thread-safe.

​@fionas76543059  Do not declare ResourceResolver at class level field/variable. Either get it wherever required or pass it as a parameter.