Problem Statement:
What is the best way to write a Sling servlet?
Requirement:
The article aims to address the problem of finding the best way to write a Sling servlet and the recommended way to register it in Apache Sling.
Introduction:
Servlets and scripts are themselves resources in Sling and thus have a resource path: this is either the location in the resource repository, the resource type in a servlet component configuration or the “virtual” bundle resource path (if a script is provided inside a bundle without being installed into the JCR repository).
OSGi DS – SlingServletResourceTypes
OSGi DS 1.4 (R7) component property type annotations for Sling Servlets (recommended)
- Component as servlet based on OSGi R7/8
- Provide resourcetype, methods, and extensions
- ServiceDescription for the servlet
package com.mysite.core.servlets;
import com.day.cq.commons.jcr.JcrConstants;
import org.apache.sling.api.SlingHttpServletRequest
import org.apache.sling.api.SlingHttpServletResponse;
import org.apache.sling.api.resource.Resource;
import org.apache.sling.api.servlets.HttpConstants;
import org.apache.sling.api.servlets.SlingAllMethodsServlet;
import org.apache.sling.api.servlets.SlingSafeMethodsServlet;
import org.apache.sling.servlets.annotations.SlingServletResourceTypes;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.propertytypes.ServiceDescription;
import javax.servlet.Servlet;
import javax.servlet.ServletException;
import java.io.IOException;
@Component(service = { Servlet.class })
@SlingServletResourceTypes(resourceTypes = "mysite/components/page", methods = HttpConstants.METHOD_GET, extensions = "txt")
@ServiceDescription("Simple Demo Servlet")
public class SimpleServlet extends SlingSafeMethodsServlet {
private static final long serialVersionUID = 1L;
@Override
protected void doGet(final SlingHttpServletRequest req, final SlingHttpServletResponse resp)
throws ServletException, IOException {
final Resource resource = req.getResource();
resp.setContentType("text/plain");
resp.getWriter().write("Title = " + resource.getValueMap().get(JcrConstants.JCR_TITLE));
}
}
Simple OSGi DS 1.2 annotations
Registering Servlet by path
- Component as servlet based on OSGi R7/8
- Servlet paths, method, and other details
package com.mysite.core.servlets;
import com.drew.lang.annotations.NotNull;
import org.apache.sling.api.SlingHttpServletRequest;
import org.apache.sling.api.SlingHttpServletResponse;
import org.apache.sling.api.servlets.SlingSafeMethodsServlet;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.ConfigurationPolicy;
import javax.servlet.Servlet;
import javax.servlet.ServletException;
import java.io.IOException;
@Component(service = { Servlet.class }, configurationPolicy = ConfigurationPolicy.REQUIRE, property = {
"sling.servlet.paths=" + PathBased.RESOURCE_PATH, "sling.servlet.methods=GET" })
public class PathBased extends SlingSafeMethodsServlet {
static final String RESOURCE_PATH = "/bin/resourcepath";
@Override
protected void doGet(@NotNull SlingHttpServletRequest request, @NotNull SlingHttpServletResponse response)
throws ServletException, IOException {
}
}
The @SlingServlet annotation – Highly not recommended to use
@SlingServlet(
resourceTypes = "/apps/my/type",
selectors = "hello",
extensions = "html",
methods = "GET")
public class MyServlet extends SlingSafeMethodsServlet {
@Override
protected void doGet(SlingHttpServletRequest request, SlingHttpServletResponse response) throws ServletException, IOException {
...
}
}
Configuration Policy – Required
- configuration policy is mandatory to avoid calling any servlet or services outside a particular environment
- Its always recommended to run certain services like data sources to work only on authors
<strong>File Name - com.mysite.core.servlets.PathBased</strong>
<?xml version="1.0" encoding="UTF-8"?>
<jcr:root xmlns:sling="http://sling.apache.org/jcr/sling/1.0" xmlns:jcr="http://www.jcp.org/jcr/1.0"
jcr:primaryType="sling:OsgiConfig"/>
Project Structure
References:
https://sling.apache.org/documentation/the-sling-engine/servlets.html
AEM best practices and development guide 
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