How to enable Async support for Sling Servlet - Felix - osgi

Question

I have this code
#Component(service = Servlet.class, scope = ServiceScope.PROTOTYPE, property={
HttpWhiteboardConstants.HTTP_WHITEBOARD_SERVLET_PATTERN+"=/content/*",
HttpWhiteboardConstants.HTTP_WHITEBOARD_FILTER_ASYNC_SUPPORTED+"=true",
HttpWhiteboardConstants.HTTP_WHITEBOARD_SERVLET_ASYNC_SUPPORTED+"=true"}
)
#SlingServletResourceTypes(
resourceTypes="cq/Page",
methods=HttpConstants.METHOD_GET,
selectors = "asynctest",
extensions="json")
public class ReactiveServlet extends HttpServlet {
#Override
protected void doGet(
HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
AsyncContext async = request.startAsync();
// Some logic
}
}
When calling this servlet /content/mypage.asynctest.json then getting this error
java.lang.IllegalStateException: null
at org.apache.felix.http.base.internal.dispatch.ServletRequestWrapper.startAsync(ServletRequestWrapper.java:338) [org.apache.felix.http.jetty:4.1.10]
at javax.servlet.ServletRequestWrapper.startAsync(ServletRequestWrapper.java:369) [org.apache.felix.http.servlet-api:1.1.2]
at javax.servlet.ServletRequestWrapper.startAsync(ServletRequestWrapper.java:369) [org.apache.felix.http.servlet-api:1.1.2]

Answer 1

The short answer is that async is NOT supported by Sling-Servlets. Your exception is thrown in this class:
https://github.com/apache/felix-dev/blob/master/http/base/src/main/java/org/apache/felix/http/base/internal/dispatch/ServletRequestWrapper.java
#Override
public AsyncContext startAsync() throws IllegalStateException
{
if ( !this.asyncSupported )
{
throw new IllegalStateException();
}
return super.startAsync();
}
But you are mixing OSGi Http-Whiteboard Pattern with Sling-Servlets. I'm not sure, what you wanted to do.
Sling/AEM is a technology-stack, where layer is built on layer. Unfortunately multiple of these layers allow registering a servlet.
Sling Servlets = Apache Sling (recommended, default)
OSGi HTTP Whiteboard Pattern = Apache Felix (only for special cases)
JEE Servlet = Jetty Servlet Container (NOT recommended)
Sling-Servlet
The Sling-Servlet you registered with #SlingServletResourceTypes doesn't support async. The output of the following servlet is: Async is not supported by an Sling-Servlet! (http://localhost:4502/content/we-retail.asynctest.json)
import static org.apache.sling.api.servlets.ServletResolverConstants.*;
import javax.servlet.AsyncContext;
import javax.servlet.Servlet;
import javax.servlet.ServletException;
import java.io.IOException;
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;
#Component(service = Servlet.class, property = {
SLING_SERVLET_RESOURCE_TYPES + "=cq:Page",
SLING_SERVLET_SELECTORS + "=asynctest",
SLING_SERVLET_EXTENSIONS + "=json",
SLING_SERVLET_METHODS + "=GET"
})
public class AsyncSlingServlet extends SlingSafeMethodsServlet {
#Override
protected void doGet(
SlingHttpServletRequest request,
SlingHttpServletResponse response
) throws ServletException, IOException {
if (request.isAsyncSupported() /* false */) {
final AsyncContext async = request.startAsync();
async.start(() -> {
async.getResponse().setContentType("text/plain");
async.getResponse().setCharacterEncoding("utf-8");
try {
async.getResponse().getWriter().println(
"Hello from the Sling-Servlet!");
} catch (IOException e) {
// ignore
}
async.complete();
});
} else {
response.setContentType("text/plain");
response.setCharacterEncoding("utf-8");
response.getWriter().println(
"Async is not supported by an Sling-Servlet!");
}
}
}
OSGi HTTP Whiteboard Pattern
The almost same servlet registered via the OSGi HTTP Whiteboard pattern supports async. It returns Hello from the OSGi Http-Whiteboard Servlet! (http://localhost:4502/my-project/hello). But such servlets are aside to Sling, so they are "rivals" or "competitors". You have to be careful, not to negatively impact Sling. So the /content path should be avoided.
import javax.servlet.AsyncContext;
import javax.servlet.Servlet;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import org.osgi.service.component.annotations.Component;
import org.osgi.service.component.annotations.ServiceScope;
import org.osgi.service.http.whiteboard.HttpWhiteboardConstants;
#Component(
service = Servlet.class,
scope = ServiceScope.PROTOTYPE,
property = {
HttpWhiteboardConstants.HTTP_WHITEBOARD_SERVLET_PATTERN
+ "=/my-project/*",
HttpWhiteboardConstants.HTTP_WHITEBOARD_CONTEXT_SELECT
+ "=("
+ HttpWhiteboardConstants.HTTP_WHITEBOARD_CONTEXT_NAME
+ "=org.apache.sling)",
HttpWhiteboardConstants.HTTP_WHITEBOARD_FILTER_ASYNC_SUPPORTED
+ "=true",
HttpWhiteboardConstants.HTTP_WHITEBOARD_SERVLET_ASYNC_SUPPORTED
+ "=true" }
)
public class AsyncOSGiServlet extends HttpServlet {
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
if (request.isAsyncSupported() /* true */) {
final AsyncContext async = request.startAsync();
async.start(() -> {
async.getResponse().setContentType("text/plain");
async.getResponse().setCharacterEncoding("utf-8");
try {
async.getResponse().getWriter().println(
"Hello from the OSGi Http-Whiteboard Servlet!");
} catch (IOException e) {
// ignore
}
async.complete();
});
} else {
response.setContentType("text/plain");
response.setCharacterEncoding("utf-8");
response.getWriter().println(
"Async is not supported by an OSGi Http-Whiteboard Servlet!");
}
}
}