Which is the recommended way as a client to get handle to the scheduler?

Question

I'm using Quartz Scheduler v1.8.3 and is integrated into Apache Tomcat v6.0.10, and hence scheduler is initialized from within servlet container. Jobs are also added/scheduled to scheduler from within Apache Tomcat.

I'm designing a JSP web front end page (quartzAdmin.jsp) from where I only want to see scheduler information like meta-data values, all job details along with its triggers, currently executing jobs, etc.

My question is, in order to get a handle to the scheduler, which one of the below 2 option is recommended:

Option 1: Getting handle by directly calling schedulerFactory.getScheduler()

SchedulerFactory sf = new StdSchedulerFactory();
Scheduler scheduler = sf.getScheduler();

Option 2: Getting handle by implementing singleton pattern

public class Quartz {

    private Quartz() {
    }

    private static Scheduler scheduler = null;

    public static Scheduler getScheduler() {
        if(scheduler == null) {
            SchedulerFactory sf = new StdSchedulerFactory();
            scheduler = sf.getScheduler();
        }
        return scheduler;
    }
}

Then in my quartzAdmin.jsp, just a call Quartz.getScheduler() would return a handle to the scheduler.

Answer 1

What you are describing here is a singleton pattern, and your singleton initialization is not thread-safe.

I would recommend not using singleton here, but instead add a reference to Quartz to ServletContext, possibly initializing it in ServletContextListener.

web.xml

<listener>
  <listener-class>my.package.MyServletContextListener</listener-class>
</listener>

MyServletContextListener.java

package my.package;

public class MyServletContextListener implements ServletContextListener
{
    @Override
    public final void contextInitialized (
            final ServletContextEvent event
        )
    {
        final ServletContext servlet_ctx = event.getServletContext( );

        Quartz quartz = new Quartz( );

        servlet_ctx.addAttribute( Quartz.ATTRIBUTE_NAME, quartz );
    }

    @Override
    public void contextDestroyed (
            final ServletContextEvent event
        )
    {
        final ServletContext servlet_ctx = event.getServletContext( );

        Quartz.fromContext( servlet_ctx ).shutdown( );

        servlet_ctx.removeAttribute( Quartz.ATTRIBUTE_NAME );
    }
}

Quartz.java

public final static String ATTRIBUTE_NAME = Quartz.class.getName( );

public Quartz( )
{
    ...
}

public static Quartz fromContext( final ServletContext ctx )
{
  return ctx.getAttribute( ATTRIBUTE_NAME );
}

public void shutdown( )
{
    ...
}

Answer 2

I think what both options do is create a new Scheduler, not getting the one you have configured in web.xml.

Currently I am doing this (similar to Option 1):

...configure a QuartzInitializerListener for my webapp...
// my servlet's init()
public void init(ServletConfig config) throws ServletException {
    ...
    SchedulerFactory factory =
            (SchedulerFactory) config.getServletContext().getAttribute(
                QuartzInitializerListener.QUARTZ_FACTORY_KEY
            );
    Scheduler factory.getScheduler();
    ...
}