Debugging a standalone jetty server - how to specify single threaded mode? - scala

Question

I have successfully created a standalone Scalatra / Jetty server, using the official instructions from Scalatra ( http://www.scalatra.org/2.3/guides/deployment/standalone.html )
I am debugging it under Ensime, and would like to limit the number of threads handling messages to a single one - so that single-stepping through the servlet methods will be easier.
I used this code to achieve it:
package ...
import org.eclipse.jetty.server.Server
import org.eclipse.jetty.servlet.{DefaultServlet, ServletContextHandler}
import org.eclipse.jetty.webapp.WebAppContext
import org.scalatra.servlet.ScalatraListener
import org.eclipse.jetty.util.thread.QueuedThreadPool
import org.eclipse.jetty.server.ServerConnector
object JettyLauncher {
def main(args: Array[String]) {
val port =
if (System.getenv("PORT") != null)
System.getenv("PORT").toInt
else
4080
// DEBUGGING MODE BEGINS
val threadPool = new QueuedThreadPool()
threadPool.setMaxThreads(8)
val server = new Server(threadPool)
val connector = new ServerConnector(server)
connector.setPort(port)
server.setConnectors(Array(connector))
// DEBUGGING MODE ENDS
val context = new WebAppContext()
context setContextPath "/"
context.setResourceBase("src/main/webapp")
context.addEventListener(new ScalatraListener)
context.addServlet(classOf[DefaultServlet], "/")
server.setHandler(context)
server.start
server.join
}
}
It works fine - except for one minor detail...
I can't tell Jetty to use 1 thread - the minimum value is 8!
If I do, this is what happens:
$ sbt assembly
...
$ java -jar ./target/scala-2.11/CurrentVersions-assembly-0.1.0-SNAPSHOT.jar
18:13:27.059 [main] INFO org.eclipse.jetty.util.log - Logging initialized #41ms
18:13:27.206 [main] INFO org.eclipse.jetty.server.Server - jetty-9.1.z-SNAPSHOT
18:13:27.220 [main] WARN o.e.j.u.component.AbstractLifeCycle - FAILED org.eclipse.jetty.server.Server#1ac539f: java.lang.IllegalStateException: Insufficient max threads in ThreadPool: max=1 < needed=8
java.lang.IllegalStateException: Insufficient max threads in ThreadPool: max=1 < needed=8
...which is why you see setMaxThreads(8) instead of setMaxThreads(1) in my code above.
Any ideas why this happens?

Answer 1

The reason is that the size of the threadpool also depends on th enumber of connectors you've got defined. If you look at the source code of the jetty server you'll see this:
// check size of thread pool
SizedThreadPool pool = getBean(SizedThreadPool.class);
int max=pool==null?-1:pool.getMaxThreads();
int selectors=0;
int acceptors=0;
if (mex.size()==0)
{
for (Connector connector : _connectors)
{
if (connector instanceof AbstractConnector)
acceptors+=((AbstractConnector)connector).getAcceptors();
if (connector instanceof ServerConnector)
selectors+=((ServerConnector)connector).getSelectorManager().getSelectorCount();
}
}
int needed=1+selectors+acceptors;
if (max>0 && needed>max)
throw new IllegalStateException(String.format("Insufficient threads: max=%d < needed(acceptors=%d + selectors=%d + request=1)",max,acceptors,selectors));
So the minimum with a single serverconnector is 2. It looks like you've got a couple of other default connectors or selectors running.