Background Thread for a Tomcat servlet app [duplicate]

I am not very familiar with Tomcat, in my head it is basically abstracted as a cgi server that saves the JVM between calls -- I know it can do a lot more than that, though.

I am looking for a way to launch a background thread when a Tomcat server starts, which would periodically update the Server Context (in my particular case this is a thread that listens to heartbeats from some other services and updates availability information, but one can imagine a variety of uses for this).

Is there a standard way to do this? Both the launching, and the updating/querying of the Context?

Any pointers to the relevant documentation and/or code samples would be much appreciated.

Solution 1:

If you want to start a thread when your WAR is deployed, you can define a context listener within the web.xml:

<web-app>
    <listener>
       <listener-class>com.mypackage.MyServletContextListener</listener-class>
    </listener>
</web-app>

Then implement that class something like:

public class MyServletContextListener implements ServletContextListener {

    private MyThreadClass myThread = null;

    public void contextInitialized(ServletContextEvent sce) {
        if ((myThread == null) || (!myThread.isAlive())) {
            myThread = new MyThreadClass();
            myThread.start();
        }
    }

    public void contextDestroyed(ServletContextEvent sce){
        try {
            myThread.doShutdown();
            myThread.interrupt();
        } catch (Exception ex) {
        }
    }
}

Solution 2:

I am looking for a way to launch a background thread when a Tomcat server starts

I think you are looking for a way to launch a background thread when your web application is started by Tomcat.

This can be done using a ServletContextListener. It is registered in web.xml and will be called when your app is started or stopped. You can then created (and later stop) your Thread, using the normal Java ways to create a Thread (or ExecutionService).