The task namespace Task Execution and Scheduling
34.5 The task namespace
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. 4.3.9.RELEASE Spring Framework 766 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 Finally, the keep-alive setting determines the time limit in seconds for which threads may remain idle before being terminated. If there are more than the core number of threads currently in the pool, after waiting this amount of time without processing a task, excess threads will get terminated. A time value of zero will cause excess threads to terminate immediately after executing a task without remaining follow-up work in the task queue. task:executor id = executorWithKeepAlive pool-size = 5-25 keep-alive = 120 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 = beanA method = methodA 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 indicating the number of milliseconds to wait after each task execution has completed. Another option is fixed-rate, indicating how often the method should be executed regardless of how long any previous execution takes. Additionally, for both fixed-delay and fixed-rate tasks an initial-delay parameter may be specified indicating the number of milliseconds to wait before the first execution of the method. For more control, a cron attribute may be provided instead. Here is an example demonstrating these other options. 4.3.9.RELEASE Spring Framework 767 task:scheduled-tasks scheduler = myScheduler task:scheduled ref = beanA method = methodA fixed-delay = 5000 initial-delay = 1000 task:scheduled ref = beanB method = methodB fixed-rate = 5000 task:scheduled ref = beanC method = methodC cron = 5 MON-FRI task:scheduled-tasks task:scheduler id = myScheduler pool-size = 1034.6 Using the Quartz Scheduler
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