How to increase Jetty threadpool size with jetty-runner on Heroku?

Question

I have a Java (Spring MVC) webapp running on Heroku. It uses the setup described in this article: Getting Started with Spring MVC Hibernate on Heroku

It looks like Jetty by default uses just one thread. Given this Heroku & jetty-runner setup, what is the simplest way to increase the threadpool size?

NB: I do not have any custom Jetty related code (so it's unclear how I'd apply the advice e.g. at: How to use setThreadPool() in Jetty). If possible, I'd prefer to keep it that way. Everything Jetty-related is now in Procfile and pom.xml (see below).

Can I set the threadpool size with some jetty-runner parameter or config option? If I need to create a Jetty config file, how do I make Heroku/jetty-runner use it?

Procfile:

web: java $JAVA_OPTS -jar target/dependency/jetty-runner.jar --port $PORT target/*.war

pom.xml:

<plugin>
    <groupId>org.apache.maven.plugins</groupId>
    <artifactId>maven-dependency-plugin</artifactId>
    <version>2.7</version>
    <executions>
        <execution>
            <phase>package</phase>
            <goals>
                <goal>copy</goal>
            </goals>
            <configuration>
                <artifactItems>
                    <artifactItem>
                        <groupId>org.mortbay.jetty</groupId>
                        <artifactId>jetty-runner</artifactId>
                        <version>8.1.10.v20130312</version>
                        <destFileName>jetty-runner.jar</destFileName>
                    </artifactItem>
                </artifactItems>
            </configuration>
        </execution>
    </executions>
</plugin>

Answer 1

What solved it for me (a Spring, not Jetty issue)

I was wrong when I wrote this in the question:

It looks like Jetty by default uses just one thread.

This turned out to be purely a Spring issue. In applicationContext.xml, I changed

<task:annotation-driven/>

into

<task:annotation-driven scheduler="scheduler-pool"/>
<task:scheduler id="scheduler-pool" pool-size="5"/>

... and different @Scheduled tasks now happily run in separate threads like scheduler-pool-1 or scheduler-pool-3.

How to tweak Jetty threadpool configs (with jetty-runner)

^{(Before I realised my problem was not a Jetty issue, I had already looked into how Jetty threadpools are configured. Documenting that here; maybe it is useful to someone.)}

Create a jetty.xml config file (somewhere like src/main/resources so that it gets copied to the compilation target dir), and customise it to your liking.

Example (probably a poor one):

<?xml version="1.0"?>

<!-- For some reason, must use org.eclipse classes, 
 even though we depend on org.mortbay Jetty... -->

<Configure id="Server" class="org.eclipse.jetty.server.Server">

    <Set name="ThreadPool">
        <New class="org.eclipse.jetty.util.thread.QueuedThreadPool">
            <Set name="minThreads">3</Set>
            <Set name="maxThreads">5</Set>
        </New>
    </Set>

</Configure>

Then, tell jetty-runner to use that config file with the --config switch. So for example in Heroku's Procfile, add --config target/classes/jetty.xml:

web: java $JAVA_OPTS -jar target/dependency/jetty-runner.jar --config target/classes/jetty.xml --port $PORT target/*.war 

---

If you also happen to be using jetty-maven-plugin, you can tell it to use your custom Jetty config by adding this under <configuration> in pom.xml:

<jettyConfig>target/classes/jetty.xml</jettyConfig>