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.

A related question on initializing quartz: http://stackoverflow.com/questions/3257074/whats-the-best-way-to-initalize-quartz — Gnanam, Sep 08 '10 at 13:56

Alexander Pogrebnyak · Answer 1 · 2010-09-08T16:21:25.243

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( )
{
    ...
}

Assuming I don't manipulate anything with scheduler apart from just getting scheduler related information, is it still should be made thread-safe? Hope you meant "not thread-safe" in this context, am I right? Also, I'm not able to clearly understand your recommendation mentioned. Can you explain with some simple code sample? — Gnanam, Sep 08 '10 at 14:10
@Gnanam. If you intend to call your original getScheduler( ) method directly from JSP ( not recommended ) it will most definitely be called from multiple threads. Check this post for correct way to initialize singleton: http://stackoverflow.com/questions/3635396/pattern-for-lazy-thread-safe-singleton-instantiation-in-java/3635619#3635619 — Alexander Pogrebnyak, Sep 08 '10 at 16:41

score 0 · Answer 2 · answered Sep 08 '10 at 13:55

0

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();
    ...
}

answered Sep 08 '10 at 13:55

gpeche

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1

No, I'm getting a handle to the scheduler which was initialized before from `web.xml`. I can verify this using `scheduler.getMetaData().getSummary()` and check for this line `Running since: xxxxx` – Gnanam Sep 08 '10 at 14:02
I believe that this is already taken care by Quartz scheduler itself. – Gnanam Sep 08 '10 at 14:04
1

Well, then there is no need to use a singleton as Quartz is taking care of that. Just use option 1. – gpeche Sep 08 '10 at 14:21

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

2 Answers2