Why does Spring 4 only allow one TaskScheduler in a context?

Question

We have a Spring web application we\'re porting from Spring 3.2 to Spring 4. Our application has several sub-contexts assembled into a single runtime context when the web appli

Answer 1

Just add bean to your config:

 @Bean()
 public ThreadPoolTaskScheduler taskScheduler() {
    return new ThreadPoolTaskScheduler();
 }

Answer 2

You need to explicitly specify which scheduler to use with @Scheduled annotations. Just add scheduler attribute:

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

Answer 3

Yeah, this is a change in behavior for sure. It appears that the ScheduledAnnotationBeanPostProcessor now has a limit of 2 schedulers per context. I ran into this with Spring 4's new WebSocket message broker as it allocates 2 schedulers for the Simple STOMP broker and the SockJS adapter. When I added my own, it totally dorked out with the same message you got. I found it rather annoying that I had to find out this limitation via errors rather documentation. This gotcha doesn't appear to be described in the Spring 4 documentation.

The solution is to create your own SchedulingConfigurer that manages its own TaskScheduler. I suspect that the reason is that there is only one and additional TaskScheduler implementations need to be added in order to isolate them from one another. I did something like so:

@Configuration
@EnableScheduling
public class MySchedulingConfigurer implements SchedulingConfigurer
{
   @Bean
   public TimedThingy timedThingy()
   {
      return new TimedThingy();
   }

  @Bean()
  public ThreadPoolTaskScheduler taskScheduler() {
     return new ThreadPoolTaskScheduler();
  }

  @Override
  public void configureTasks(ScheduledTaskRegistrar taskRegistrar)
  {
      taskRegistrar.setTaskScheduler(taskScheduler());
      taskRegistrar.addFixedRateTask(new Runnable()
      {
         public void run()
         {
            timedThingy().sendIt();
         }
      }, 1000);
  }
}

Once I did that, the issue went away and things worked as desired. The down side is that you can't use annotations like @Scheduled, etc.. But you do gain more control and things work. Hope this helps.