AbstractValidatorServlet.java

import java.io.IOException;
import java.util.Collections;

import javax.servlet.ServletException;

import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.api.SlingHttpServletResponse;
import org.apache.sling.api.servlets.SlingSafeMethodsServlet;
import org.codehaus.jackson.JsonFactory;
import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonGenerator;
import org.codehaus.jackson.JsonGenerator.Feature;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public abstract class AbstractValidatorServlet extends SlingSafeMethodsServlet {

    private static final Logger LOG = LoggerFactory.getLogger(AbstractValidatorServlet.class);

    private static final JsonFactory FACTORY = new JsonFactory().disable(Feature.AUTO_CLOSE_TARGET);

    private static final ObjectMapper MAPPER = new ObjectMapper();

    @Override
    protected final void doGet(final SlingHttpServletRequest request, final SlingHttpServletResponse response)
        throws ServletException, IOException {
        final String value = request.getRequestParameter("value").getString();
        final String path = request.getResource().getPath();

        final boolean valid = isValid(request, path, value);

        response.setContentType("application/json");
        response.setCharacterEncoding("utf-8");

        try {
            final JsonGenerator generator = FACTORY.createJsonGenerator(response.getWriter());

            MAPPER.writeValue(generator, Collections.singletonMap("valid", valid));
        } catch (final JsonGenerationException jge) {
            LOG.error("error generating JSON response", jge);
        } catch (final JsonMappingException jme) {
            LOG.error("error mapping JSON response", jme);
        } catch (final IOException ioe) {
            LOG.error("error writing JSON response", ioe);
        }
    }

    /**
     * Validate the given value for this request and path.
     *
     * @param request servlet request
     * @param path path to current component being validated
     * @param value input value to validate
     * @return true if value is valid, false otherwise
     */
    protected abstract boolean isValid(final SlingHttpServletRequest request, final String path, final String value);
}

dialog.xml

<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:cq="http://www.day.com/jcr/cq/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0" xmlns:nt="http://www.jcp.org/jcr/nt/1.0"
    jcr:primaryType="cq:Dialog" xtype="dialog">
    <items jcr:primaryType="cq:WidgetCollection">
        <tabs jcr:primaryType="cq:TabPanel">
            <items jcr:primaryType="cq:WidgetCollection">
                <first jcr:primaryType="nt:unstructured" title="Form" xtype="panel">
                    <items jcr:primaryType="cq:WidgetCollection">
                        <name jcr:primaryType="cq:Widget" fieldLabel="Name" name="./name" xtype="textfield" allowBlank="{Boolean}false"
                            validator="function(value) {
                                var dialog = this.findParentByType('dialog');

                                var url = CQ.HTTP.addParameter(dialog.path + '.validator.json', 'value', value);

                                var result = CQ.HTTP.eval(url);

                                return result.valid ? true : 'Form name already exists on this page.';
                            }" />
                    </items>
                </first>
            </items>
        </tabs>
    </items>
</jcr:root>

FormValidatorServlet.java

import java.util.HashMap;
import java.util.Map;

import javax.jcr.Node;
import javax.jcr.NodeIterator;
import javax.jcr.RepositoryException;

import org.apache.felix.scr.annotations.Component;
import org.apache.felix.scr.annotations.Properties;
import org.apache.felix.scr.annotations.Property;
import org.apache.felix.scr.annotations.Service;
import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.jcr.resource.JcrResourceConstants;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import com.day.cq.wcm.foundation.forms.FormsConstants;

@Component
@Service
@Properties({
    @Property(name = "sling.servlet.resourceTypes", value = FormsConstants.RT_FORM_BEGIN),
    @Property(name = "sling.servlet.extensions", value = "json"),
    @Property(name = "sling.servlet.methods", value = "GET"),
    @Property(name = "sling.servlet.selectors", value = "validator"),
    @Property(name = "service.description", value = "Form Validator Servlet")
})
public final class FormValidatorServlet extends AbstractValidatorServlet {

    private static final Logger LOG = LoggerFactory.getLogger(FormValidatorServlet.class);

    @Override
    protected boolean isValid(final SlingHttpServletRequest request, final String path, final String value) {
        final Map<String, String> names = getFormNames(request);

        // ensure that form name is unique among all forms defined on this page
        // (except for itself)
        return !names.containsKey(value) || names.get(value).equals(path);
    }

    private Map<String, String> getFormNames(final SlingHttpServletRequest request) {
        final Map<String, String> names = new HashMap<String, String>();

        final Node currentNode = request.getResource().adaptTo(Node.class);

        try {
            final Node par = currentNode.getParent();

            final NodeIterator nodes = par.getNodes();

            // get all form names for the current paragraph system
            while (nodes.hasNext()) {
                final Node node = nodes.nextNode();

                if (isFormStart(node)) {
                    final String name = node.getProperty(FormsConstants.ELEMENT_PROPERTY_NAME).getString();

                    names.put(name, node.getPath());
                }
            }
        } catch (final RepositoryException re) {
            LOG.error("error getting form names", re);
        }

        return names;
    }

    private boolean isFormStart(final Node node) throws RepositoryException {
        return node.hasProperty(JcrResourceConstants.SLING_RESOURCE_TYPE_PROPERTY)
            && FormsConstants.RT_FORM_BEGIN.equals(node.getProperty(JcrResourceConstants.SLING_RESOURCE_TYPE_PROPERTY).getString())
            && node.hasProperty(FormsConstants.ELEMENT_PROPERTY_NAME);
    }
}