Caused by: org.apache.sling.api.resource.PersistenceException: Resource is not modifiable

The below exception will be thrown most of the cases while accessing the servlet with post from AEM.

The actual issue is Adobe Experience Manager(AEM) could not ale to resolve the servlet and assumes it as a resource path but this is not a valid resource path, the below exception will be thrown when AEM tries to set the property values.

Caused by: org.apache.sling.api.resource.PersistenceException: Resource at '/services/formservlet' is not modifiable.
at org.apache.sling.servlets.post.impl.helper.SlingPropertyValueHandler.setProperty(SlingPropertyValueHandler.java:153)
at org.apache.sling.servlets.post.impl.operations.ModifyOperation.writeContent(ModifyOperation.java:411)
at org.apache.sling.servlets.post.impl.operations.ModifyOperation.doRun(ModifyOperation.java:101)

The steps to narrow down the issue:
Resolve the Servlet: Go to http://localhost:4502/system/console/configMgr - Sling--> Sling Servlet Resolver and resolve the servlet path, if the servlet path is resolved then there is no issue if not the servlet is not configured properly.

Path resolved to a Servlet



Path not resolved a Servlet



Verify the Servlet configuration:Cross verify and make sure the servlet is configured properly

e.g servlet

@Component(
immediate = true,
label = "Servlet",
description = "Servlet",
metatype = true)
@SlingServlet(
name = "FormServlet",
description = "FormServlet",
methods = "POST",
generateComponent = false,
paths = "/services/formservlet")
@Service(value = Servlet.class)

public class FormServlet extends SlingAllMethodsServlet implements Serializable {

@Override
protected void doPost(SlingHttpServletRequest request,
SlingHttpServletResponse response) throws ServletException,
IOException {
}
protected void doGet(SlingHttpServletRequest request,
SlingHttpServletResponse response) throws ServletException,
IOException {
doPost(request, response);
}

}

Verify the servlet resolver configuration: Go to http://localhost:4502/system/console/configMgr - Sling--> Apache Sling Servlet/Script Resolver and Error Handler and make sure the servlet base patch is added under Execution paths.



Apache Sling Referrer Filter:If the servlet is invoked externally then make sure the host name of the source system is added to the Allow Hosts section of the Apache Sling Referrer Filter



CSRF Filter(above AEM 6.1):Adobe added CSRF (Cross-Site Request Forgery) protection with version 6.1 and we need to ensure that the CSRF token is included while submitting the form.

This should be handled automatically if we are using AEM's version of jQuery. This library has the code to get the token and add it to all XHR and forms.

If we are using a different version of jQuery or not using the jQuery, then we have to include the 'granite.csrf.standalone' client library and it will do the same functionality.

Refer the following link for more details - https://helpx.adobe.com/experience-manager/using/custom-sling-servlets6_1.html

Also, if the servlet is working properly from publisher/author and not only working from dispatcher then the issue might be with some of the cached scripts in dispatcher is not updated, try to clear the dispatcher cache for JS scripts and try the scenario again.