The Task Namespace - Java-Springs

Beginning with Spring 3.0, there is an XML namespace for configuring TaskExecutor and TaskScheduler instances. It also provides a convenient way to configure tasks to be scheduled with a trigger.

The 'scheduler' element

The following element will create a ThreadPoolTaskScheduler instance with the specified thread pool size.

<task:scheduler id="scheduler" pool-size="10"/>

The value provided for the 'id' attribute will be used as the prefix for thread names within the pool. The 'scheduler' element is relatively straightforward. If you do not provide a 'pool-size' attribute, the default thread pool will only have a single thread. There are no other configuration options for the scheduler.

The 'executor' element

The following will create a ThreadPoolTaskExecutor instance:

<task:executor id="executor" pool-size="10"/>

As with the scheduler above, the value provided for the 'id' attribute will be used as the prefix for thread names within the pool. As far as the pool size is concerned, the 'executor' element supports more configuration options than the 'scheduler' element. For one thing, the thread pool for a ThreadPoolTaskExecutor is itself more configurable. Rather than just a single size, an executor's thread pool may have different values for the core and the max size. If a single value is provided then the executor will have a fixed-size thread pool (the core and max sizes are the same). However, the 'executor' element's 'pool-size' attribute also accepts a range in the form of "min-max".

<task:executor id="executorWithPoolSizeRange"
pool-size="5-25"
queue-capacity="100"/>

As you can see from that configuration, a 'queue-capacity' value has also been provided. The configuration of the thread pool should also be considered in light of the executor's queue capacity. For the full description of the relationship between pool size and queue capacity, consult the documentation for ThreadPoolExecutor. The main idea is that when a task is submitted, the executor will first try to use a free thread if the number of active threads is currently less than the core size. If the core size has been reached, then the task will be added to the queue as long as its capacity has not yet been reached. Only then, if the queue's capacity has been reached, will the executor create a new thread beyond the core size. If the max size has also been reached, then the executor will reject the task.

By default, the queue is unbounded, but this is rarely the desired configuration, because it can lead to OutOfMemoryErrors if enough tasks are added to that queue while all pool threads are busy. Furthermore, if the queue is unbounded, then the max size has no effect at all. Since the executor will always try the queue before creating a new thread beyond the core size, a queue must have a finite capacity for the thread pool to grow beyond the core size (this is why a fixed size pool is the only sensible case when using an unbounded queue).

In a moment, we will review the effects of the keep-alive setting which adds yet another factor to consider when providing a pool size configuration. First, let's consider the case, as mentioned above, when a task is rejected. By default, when a task is rejected, a thread pool executor will throw a TaskRejectedException. However, the rejection policy is actually configurable. The exception is thrown when using the default rejection policy which is the AbortPolicy implementation. For applications where some tasks can be skipped under heavy load, either the DiscardPolicy or DiscardOldestPolicy may be configured instead. Another option that works well for applications that need to throttle the submitted tasks under heavy load is the CallerRunsPolicy. Instead of throwing an exception or discarding tasks, that policy will simply force the thread that is calling the submit method to run the task itself. The idea is that such a caller will be busy while running that task and not able to submit other tasks immediately. Therefore it provides a simple way to throttle the incoming load while maintaining the limits of the thread pool and queue. Typically this allows the executor to "catch up" on the tasks it is handling and thereby frees up some capacity on the queue, in the pool, or both. Any of these options can be chosen from an enumeration of values available for the 'rejection-policy' attribute on the 'executor' element.

<task:executor id="executorWithCallerRunsPolicy"
pool-size="5-25"
queue-capacity="100"
rejection-policy="CALLER_RUNS"/>

The 'scheduled-tasks' element

The most powerful feature of Spring's task namespace is the support for configuring tasks to be scheduled within a Spring Application Context. This follows an approach similar to other "method-invokers" in Spring, such as that provided by the JMS namespace for configuring Message-driven POJOs. Basically a "ref" attribute can point to any Spring-managed object, and the "method" attribute provides the name of a method to be invoked on that object. Here is a simple example.

<task:scheduled-tasks scheduler="myScheduler">
<task:scheduled ref="someObject" method="someMethod" fixed-delay="5000"/>
</task:scheduled-tasks>
<task:scheduler id="myScheduler" pool-size="10"/>

As you can see, the scheduler is referenced by the outer element, and each individual task includes the configuration of its trigger metadata. In the preceding example, that metadata defines a periodic trigger with a fixed delay. It could also be configured with a "fixed-rate", or for more control, a "cron" attribute could be provided instead. Here's an example featuring these other options.

<task:scheduled-tasks scheduler="myScheduler">
<task:scheduled ref="someObject" method="someMethod" fixed-rate="5000"/>
<task:scheduled ref="anotherObject" method="anotherMethod" cron="*/5 * * * * MON-FRI"/>
</task:scheduled-tasks>
<task:scheduler id="myScheduler" pool-size="10"/>

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