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  <title>Task Execution and Scheduling</title>

  <section xml:id="scheduling-introduction">
    <title>Introduction</title>

    <para>The Spring Framework provides abstractions for asynchronous
    execution and scheduling of tasks with the
    <interfacename>TaskExecutor</interfacename> and
    <interfacename>TaskScheduler</interfacename> interfaces, respectively.
    Spring also features implementations of those interfaces that support
    thread pools or delegation to CommonJ within an application server
    environment. Ultimately the use of these implementations behind the common
    interfaces abstracts away the differences between Java SE 5, Java SE 6 and
    Java EE environments.</para>

    <para>Spring also features integration classes for supporting scheduling
    with the <classname>Timer</classname>, part of the JDK since 1.3, and the
    Quartz Scheduler (<link
    xl:href="http://quartz-scheduler.org"></link>). Both of those
    schedulers are set up using a <interfacename>FactoryBean</interfacename>
    with optional references to <classname>Timer</classname> or
    <classname>Trigger</classname> instances, respectively. Furthermore, a
    convenience class for both the Quartz Scheduler and the
    <classname>Timer</classname> is available that allows you to invoke a
    method of an existing target object (analogous to the normal
    <classname>MethodInvokingFactoryBean</classname> operation).</para>
  </section>

  <section xml:id="scheduling-task-executor">
    <title>The Spring <interfacename>TaskExecutor</interfacename>
    abstraction</title>

    <para>Spring 2.0 introduces a new abstraction for dealing with executors.
    Executors are the Java 5 name for the concept of thread pools. The
    "executor" naming is due to the fact that there is no guarantee that the
    underlying implementation is actually a pool; an executor may be
    single-threaded or even synchronous. Spring's abstraction hides
    implementation details between Java SE 1.4, Java SE 5 and Java EE
    environments.</para>

    <para>Spring's <interfacename>TaskExecutor</interfacename> interface is
    identical to the <classname>java.util.concurrent.Executor</classname>
    interface. In fact, its primary reason for existence is to abstract away
    the need for Java 5 when using thread pools. The interface has a single
    method <classname>execute(Runnable task)</classname> that accepts a task
    for execution based on the semantics and configuration of the thread
    pool.</para>

    <para>The <interfacename>TaskExecutor</interfacename> was originally
    created to give other Spring components an abstraction for thread pooling
    where needed. Components such as the
    <classname>ApplicationEventMulticaster</classname>, JMS's
    <classname>AbstractMessageListenerContainer</classname>, and Quartz
    integration all use the <interfacename>TaskExecutor</interfacename>
    abstraction to pool threads. However, if your beans need thread pooling
    behavior, it is possible to use this abstraction for your own
    needs.</para>

    <section xml:id="scheduling-task-executor-types">
      <title><interfacename>TaskExecutor</interfacename> types</title>

      <para>There are a number of pre-built implementations of
      <interfacename>TaskExecutor</interfacename> included with the Spring
      distribution. In all likelihood, you shouldn't ever need to implement
      your own.</para>

      <itemizedlist>
        <listitem>
          <para><classname>SimpleAsyncTaskExecutor</classname></para>

          <para>This implementation does not reuse any threads, rather it
          starts up a new thread for each invocation. However, it does support
          a concurrency limit which will block any invocations that are over
          the limit until a slot has been freed up. If you're looking for true
          pooling, keep scrolling further down the page.</para>
        </listitem>

        <listitem xml:id="syncTaskExecutor">
          <para><classname>SyncTaskExecutor</classname></para>

          <para>This implementation doesn't execute invocations
          asynchronously. Instead, each invocation takes place in the calling
          thread. It is primarily used in situations where multithreading
          isn't necessary such as simple test cases.</para>
        </listitem>

        <listitem xml:id="concurrentTaskExecutor">
          <para><classname>ConcurrentTaskExecutor</classname></para>

          <para>This implementation is a wrapper for a Java 5
          <classname>java.util.concurrent.Executor</classname>. There is an
          alternative, <classname>ThreadPoolTaskExecutor</classname>, that
          exposes the <classname>Executor</classname> configuration parameters
          as bean properties. It is rare to need to use the
          <classname>ConcurrentTaskExecutor</classname> but if the <link
          linkend="threadPoolTaskExecutor"><classname>ThreadPoolTaskExecutor</classname></link>
          isn't robust enough for your needs, the
          <classname>ConcurrentTaskExecutor</classname> is an
          alternative.</para>
        </listitem>

        <listitem xml:id="simpleThreadPoolTaskExecutor">
          <para><classname>SimpleThreadPoolTaskExecutor</classname></para>

          <para>This implementation is actually a subclass of Quartz's
          <classname>SimpleThreadPool</classname> which listens to Spring's
          lifecycle callbacks. This is typically used when you have a thread
          pool that may need to be shared by both Quartz and non-Quartz
          components.</para>
        </listitem>

        <listitem xml:id="threadPoolTaskExecutor">
          <para><classname>ThreadPoolTaskExecutor</classname></para>

          <sidebar>
            <para>It is not possible to use any backport or alternate versions
            of the <classname>java.util.concurrent</classname> package with
            this implementation. Both Doug Lea's and Dawid Kurzyniec's
            implementations use different package structures which will
            prevent them from working correctly.</para>
          </sidebar>

          <para>This implementation can only be used in a Java 5 environment
          but is also the most commonly used one in that environment. It
          exposes bean properties for configuring a
          <classname>java.util.concurrent.ThreadPoolExecutor</classname> and
          wraps it in a <interfacename>TaskExecutor</interfacename>. If you
          need something advanced such as a
          <classname>ScheduledThreadPoolExecutor</classname>, it is
          recommended that you use a <link
          linkend="concurrentTaskExecutor"><classname>ConcurrentTaskExecutor</classname></link>
          instead.</para>
        </listitem>

        <listitem>
          <para><classname>TimerTaskExecutor</classname></para>

          <para>This implementation uses a single
          <classname>TimerTask</classname> as its backing implementation. It's
          different from the <link
          linkend="syncTaskExecutor"><classname>SyncTaskExecutor</classname></link>
          in that the method invocations are executed in a separate thread,
          although they are synchronous in that thread.</para>
        </listitem>

        <listitem>
          <para><classname>WorkManagerTaskExecutor</classname></para>

          <sidebar>
            <para>CommonJ is a set of specifications jointly developed between
            BEA and IBM. These specifications are not Java EE standards, but
            are standard across BEA's and IBM's Application Server
            implementations.</para>
          </sidebar>

          <para>This implementation uses the CommonJ WorkManager as its
          backing implementation and is the central convenience class for
          setting up a CommonJ WorkManager reference in a Spring context.
          Similar to the <link
          linkend="simpleThreadPoolTaskExecutor"><classname>SimpleThreadPoolTaskExecutor</classname></link>,
          this class implements the WorkManager interface and therefore can be
          used directly as a WorkManager as well.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section xml:id="scheduling-task-executor-usage">
      <title>Using a <interfacename>TaskExecutor</interfacename></title>

      <para>Spring's <interfacename>TaskExecutor</interfacename>
      implementations are used as simple JavaBeans. In the example below, we
      define a bean that uses the
      <classname>ThreadPoolTaskExecutor</classname> to asynchronously print
      out a set of messages.</para>

      <programlisting language="java">import org.springframework.core.task.TaskExecutor;

public class TaskExecutorExample {

  private class MessagePrinterTask implements Runnable {

    private String message;

    public MessagePrinterTask(String message) {
      this.message = message;
    }

    public void run() {
      System.out.println(message);
    }

  }

  private TaskExecutor taskExecutor;

  public TaskExecutorExample(TaskExecutor taskExecutor) {
    this.taskExecutor = taskExecutor;
  }

  public void printMessages() {
    for(int i = 0; i &lt; 25; i++) {
      taskExecutor.execute(new MessagePrinterTask("Message" + i));
    }
  }
}</programlisting>

      <para>As you can see, rather than retrieving a thread from the pool and
      executing yourself, you add your <classname>Runnable</classname> to the
      queue and the <interfacename>TaskExecutor</interfacename> uses its
      internal rules to decide when the task gets executed.</para>

      <para>To configure the rules that the
      <interfacename>TaskExecutor</interfacename> will use, simple bean
      properties have been exposed.</para>

      <programlisting language="xml">&lt;bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor"&gt;
  &lt;property name="corePoolSize" value="5" /&gt;
  &lt;property name="maxPoolSize" value="10" /&gt;
  &lt;property name="queueCapacity" value="25" /&gt;
&lt;/bean&gt;

&lt;bean id="taskExecutorExample" class="TaskExecutorExample"&gt;
  &lt;constructor-arg ref="taskExecutor" /&gt;
&lt;/bean&gt;</programlisting>
    </section>
  </section>

  <section xml:id="scheduling-task-scheduler">
    <title>The Spring <interfacename>TaskScheduler</interfacename>
    abstraction</title>

    <para>In addition to the <interfacename>TaskExecutor</interfacename>
    abstraction, Spring 3.0 introduces a
    <interfacename>TaskScheduler</interfacename> with a variety of methods for
    scheduling tasks to run at some point in the future.</para>

    <programlisting language="java">public interface TaskScheduler {

    ScheduledFuture schedule(Runnable task, Trigger trigger);

    ScheduledFuture schedule(Runnable task, Date startTime);

    ScheduledFuture scheduleAtFixedRate(Runnable task, Date startTime, long period);

    ScheduledFuture scheduleAtFixedRate(Runnable task, long period);

    ScheduledFuture scheduleWithFixedDelay(Runnable task, Date startTime, long delay);

    ScheduledFuture scheduleWithFixedDelay(Runnable task, long delay);

}</programlisting>

    <para>The simplest method is the one named 'schedule' that takes a
    <interfacename>Runnable</interfacename> and <classname>Date</classname>
    only. That will cause the task to run once after the specified time. All
    of the other methods are capable of scheduling tasks to run repeatedly.
    The fixed-rate and fixed-delay methods are for simple, periodic execution,
    but the method that accepts a Trigger is much more flexible.</para>

    <section xml:id="scheduling-trigger-interface">
      <title>The <interfacename>Trigger</interfacename> interface</title>

      <para>The <interfacename>Trigger</interfacename> interface is
      essentially inspired by JSR-236, which, as of Spring 3.0, has not yet
      been officially implemented. The basic idea of the
      <interfacename>Trigger</interfacename> is that execution times may be
      determined based on past execution outcomes or even arbitrary
      conditions. If these determinations do take into account the outcome of
      the preceding execution, that information is available within a
      <interfacename>TriggerContext</interfacename>. The
      <interfacename>Trigger</interfacename> interface itself is quite
      simple:</para>

      <programlisting language="java">public interface Trigger {

    Date nextExecutionTime(TriggerContext triggerContext);

}</programlisting>

      <para>As you can see, the <interfacename>TriggerContext</interfacename>
      is the most important part. It encapsulates all of the relevant data,
      and is open for extension in the future if necessary. The
      <interfacename>TriggerContext</interfacename> is an interface (a
      <classname>SimpleTriggerContext</classname> implementation is used by
      default). Here you can see what methods are available for
      <interfacename>Trigger</interfacename> implementations.</para>

      <programlisting language="java">public interface TriggerContext {

    Date lastScheduledExecutionTime();

    Date lastActualExecutionTime();

    Date lastCompletionTime();

}</programlisting>
    </section>

    <section xml:id="scheduling-trigger-implementations">
      <title><interfacename>Trigger</interfacename> implementations</title>

      <para>Spring provides two implementations of the
      <interfacename>Trigger</interfacename> interface. The most interesting
      one is the <classname>CronTrigger</classname>. It enables the scheduling
      of tasks based on cron expressions. For example the following task is
      being scheduled to run 15 minutes past each hour but only during the
      9-to-5 "business hours" on weekdays.</para>

      <programlisting language="java">scheduler.schedule(task, new CronTrigger("* 15 9-17 * * MON-FRI"));</programlisting>

      <para>The other out-of-the-box implementation is a
      <classname>PeriodicTrigger</classname> that accepts a fixed period, an
      optional initial delay value, and a boolean to indicate whether the
      period should be interpreted as a fixed-rate or a fixed-delay. Since the
      <interfacename>TaskScheduler</interfacename> interface already defines
      methods for scheduling tasks at a fixed-rate or with a fixed-delay,
      those methods should be used directly whenever possible. The value of
      the <classname>PeriodicTrigger</classname> implementation is that it can
      be used within components that rely on the
      <interfacename>Trigger</interfacename> abstraction. For example, it may
      be convenient to allow periodic triggers, cron-based triggers, and even
      custom trigger implementations to be used interchangeably. Such a
      component could take advantage of dependency injection so that such
      <interfacename>Triggers</interfacename> could be configured
      externally.</para>
    </section>

    <section xml:id="scheduling-task-scheduler-implementations">
      <title><interfacename>TaskScheduler</interfacename>
      implementations</title>

      <para>As with Spring's <interfacename>TaskExecutor</interfacename>
      abstraction, the primary benefit of the
      <interfacename>TaskScheduler</interfacename> is that code relying on
      scheduling behavior need not be coupled to a particular scheduler
      implementation. The flexibility this provides is particularly relevant
      when running within Application Server environments where threads should
      not be created directly by the application itself. For such cases,
      Spring provides a <classname>TimerManagerTaskScheduler</classname> that
      delegates to a CommonJ TimerManager instance, typically configured with
      a JNDI-lookup.</para>

      <para>A simpler alternative, the
      <classname>ThreadPoolTaskScheduler</classname>, can be used whenever
      external thread management is not a requirement. Internally, it
      delegates to a <interfacename>ScheduledExecutorService</interfacename>
      instance. <classname>ThreadPoolTaskScheduler</classname> actually
      implements Spring's <interfacename>TaskExecutor</interfacename>
      interface as well, so that a single instance can be used for
      asynchronous execution <emphasis>as soon as possible</emphasis> as well
      as scheduled, and potentially recurring, executions.</para>
    </section>
  </section>

  <section xml:id="scheduling-annotation-support">
    <title>Annotation Support for Scheduling and Asynchronous
    Execution</title>

    <para>Spring provides annotation support for both task scheduling and
    asynchronous method execution.</para>

    <section xml:id="scheduling-enable-annotation-support">

      <title>Enable scheduling annotations</title>

      <para>To enable support for <interfacename>@Scheduled</interfacename> and
      <interfacename>@Async</interfacename> annotations add
      <interfacename>@EnableScheduling</interfacename> and
      <interfacename>@EnableAsync</interfacename> to one of your
      <interfacename>@Configuration</interfacename> classes:</para>

      <programlisting language="java">@Configuration
@EnableAsync
@EnableScheduling
public class AppConfig {
}</programlisting>

      <para>You are free to pick and choose the relevant annotations
      for your application. For example, if you only need support
      for <interfacename>@Scheduled</interfacename>, simply omit
      <interfacename>@EnableAsync</interfacename>. For more fine-grained
      control you can additionally implement the
      <interfacename>SchedulingConfigurer</interfacename> and/or
      <interfacename>AsyncConfigurer</interfacename> interfaces. See
      the Javadoc for full details.</para>

      <para>If you prefer XML configuration use the
      <literal>&lt;task:annotation-driven&gt;</literal> element.</para>

      <programlisting language="xml">&lt;task:annotation-driven executor="myExecutor" scheduler="myScheduler"/&gt;
&lt;task:executor id="myExecutor" pool-size="5"/&gt;
&lt;task:scheduler id="myScheduler" pool-size="10"/&gt;}</programlisting>

      <para>Notice with the above XML that an executor reference is provided
      for handling those tasks that correspond to methods with the
      <interfacename>@Async</interfacename> annotation, and the scheduler
      reference is provided for managing those methods annotated
      with <interfacename>@Scheduled</interfacename>.</para>
    </section>

    <section xml:id="scheduling-annotation-support-scheduled">
      <title>The @Scheduled Annotation</title>

      <para>The @Scheduled annotation can be added to a method along with
      trigger metadata. For example, the following method would be invoked
      every 5 seconds with a fixed delay, meaning that the period will be
      measured from the completion time of each preceding invocation.</para>

      <programlisting language="java">@Scheduled(fixedDelay=5000)
public void doSomething() {
    // something that should execute periodically
}</programlisting>

      <para>If a fixed rate execution is desired, simply change the property
      name specified within the annotation. The following would be executed
      every 5 seconds measured between the successive start times of each
      invocation.</para>

      <programlisting language="java">@Scheduled(fixedRate=5000)
public void doSomething() {
    // something that should execute periodically
}</programlisting>

      <para>For fixed-delay and fixed-rate tasks, an initial delay may be
      specified indicating the number of milliseconds to wait before the first
      execution of the method.
      </para>

      <programlisting language="java">@Scheduled(initialDelay=1000, fixedRate=5000)
public void doSomething() {
    // something that should execute periodically
}</programlisting>

      <para>If simple periodic scheduling is not expressive enough, then a
      cron expression may be provided. For example, the following will only
      execute on weekdays.</para>

      <programlisting language="java">@Scheduled(cron="*/5 * * * * MON-FRI")
public void doSomething() {
    // something that should execute on weekdays only
}</programlisting>

      <para>Notice that the methods to be scheduled must have void returns and
      must not expect any arguments. If the method needs to interact with
      other objects from the Application Context, then those would typically
      have been provided through dependency injection.</para>

      <note>
        <para>Make sure that you are not initializing multiple instances of
        the same @Scheduled annotation class at runtime, unless you do want to
        schedule callbacks to each such instance. Related to this, make sure
        that you do not use @Configurable on bean classes which are annotated
        with @Scheduled and registered as regular Spring beans with the
        container: You would get double initialization otherwise, once through
        the container and once through the @Configurable aspect, with the
        consequence of each @Scheduled method being invoked twice.</para>
      </note>
    </section>

    <section xml:id="scheduling-annotation-support-async">
      <title>The @Async Annotation</title>

      <para>The <interfacename>@Async</interfacename> annotation can be
      provided on a method so that invocation of that method will occur
      asynchronously. In other words, the caller will return immediately upon
      invocation and the actual execution of the method will occur in a task
      that has been submitted to a Spring
      <interfacename>TaskExecutor</interfacename>. In the simplest case, the
      annotation may be applied to a <literal>void</literal>-returning
      method.</para>

      <programlisting language="java">@Async
void doSomething() {
    // this will be executed asynchronously
}</programlisting>

      <para>Unlike the methods annotated with the
      <interfacename>@Scheduled</interfacename> annotation, these methods can
      expect arguments, because they will be invoked in the "normal" way by
      callers at runtime rather than from a scheduled task being managed by
      the container. For example, the following is a legitimate application of
      the <interfacename>@Async</interfacename> annotation.</para>

      <programlisting language="java">@Async
void doSomething(String s) {
    // this will be executed asynchronously
}</programlisting>

      <para>Even methods that return a value can be invoked asynchronously.
      However, such methods are required to have a
      <interfacename>Future</interfacename> typed return value. This still
      provides the benefit of asynchronous execution so that the caller can
      perform other tasks prior to calling <methodname>get()</methodname> on
      that Future.</para>

      <programlisting language="java">@Async
Future&lt;String&gt; returnSomething(int i) {
    // this will be executed asynchronously
}</programlisting>

      <para><interfacename>@Async</interfacename> can not be used in
      conjunction with lifecycle callbacks such as
      <interfacename>@PostConstruct</interfacename>. To asynchronously
      initialize Spring beans you currently have to use a separate
      initializing Spring bean that invokes the
      <interfacename>@Async</interfacename> annotated method on the target
      then.</para>

      <programlisting language="java">public class SampleBeanImpl implements SampleBean {

  @Async
  void doSomething() { … }
}


public class SampleBeanInititalizer {

  private final SampleBean bean;

  public SampleBeanInitializer(SampleBean bean) {
    this.bean = bean;
  }

  @PostConstruct
  public void initialize() {
    bean.doSomething();
  }
}</programlisting>
    </section>

    <section xml:id="scheduling-annotation-support-qualification">
      <title>Executor qualification with @Async</title>

      <para>By default when specifying <interfacename>@Async</interfacename> on
      a method, the executor that will be used is the one supplied to the
      'annotation-driven' element as described above. However, the
      <literal>value</literal> attribute of the
      <interfacename>@Async</interfacename> annotation can be used when needing
      to indicate that an executor other than the default should be used when
      executing a given method.</para>
      <programlisting language="java">@Async("otherExecutor")
void doSomething(String s) {
    // this will be executed asynchronously by "otherExecutor"
}</programlisting>

      <para>In this case, "otherExecutor" may be the name of any
      <interfacename>Executor</interfacename> bean in the Spring container, or
      may be the name of a <emphasis>qualifier</emphasis> associated with any
      <interfacename>Executor</interfacename>, e.g. as specified with the
      <literal>&lt;qualifier&gt;</literal> element or Spring's
      <interfacename>@Qualifier</interfacename> annotation.</para>
    </section>
  </section>

  <section xml:id="scheduling-task-namespace">
    <title>The Task Namespace</title>

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

    <section xml:id="scheduling-task-namespace-scheduler">
      <title>The 'scheduler' element</title>

      <para>The following element will create a
      <classname>ThreadPoolTaskScheduler</classname> instance with the
      specified thread pool size.</para>

      <programlisting language="xml">&lt;task:scheduler id="scheduler" pool-size="10"/&gt;</programlisting>

      <para>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.</para>
    </section>

    <section xml:id="scheduling-task-namespace-executor">
      <title>The 'executor' element</title>

      <para>The following will create a
      <classname>ThreadPoolTaskExecutor</classname> instance: <programlisting
      language="xml">&lt;task:executor id="executor" pool-size="10"/&gt;</programlisting></para>

      <para>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 <classname>ThreadPoolTaskExecutor</classname> is
      itself more configurable. Rather than just a single size, an executor's
      thread pool may have different values for the <emphasis>core</emphasis>
      and the <emphasis>max</emphasis> 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". <programlisting
      language="xml">&lt;task:executor id="executorWithPoolSizeRange"
                 pool-size="5-25"
                 queue-capacity="100"/&gt;</programlisting></para>

      <para>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 <link
      xl:href="http://java.sun.com/javase/6/docs/api/java/util/concurrent/ThreadPoolExecutor.html">ThreadPoolExecutor</link>.
      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 <emphasis>has</emphasis>
      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.</para>

      <para>By default, the queue is <emphasis>unbounded</emphasis>, but this
      is rarely the desired configuration, because it can lead to
      <classname>OutOfMemoryErrors</classname> 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 <emphasis>fixed size</emphasis> pool
      is the only sensible case when using an unbounded queue).</para>

      <para>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 <classname>TaskRejectedException</classname>.
      However, the rejection policy is actually configurable. The exception is
      thrown when using the default rejection policy which is the
      <classname>AbortPolicy</classname> implementation. For applications
      where some tasks can be skipped under heavy load, either the
      <classname>DiscardPolicy</classname> or
      <classname>DiscardOldestPolicy</classname> may be configured instead.
      Another option that works well for applications that need to throttle
      the submitted tasks under heavy load is the
      <classname>CallerRunsPolicy</classname>. 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.</para>

      <programlisting language="xml">&lt;task:executor id="executorWithCallerRunsPolicy"
               pool-size="5-25"
               queue-capacity="100"
               rejection-policy="CALLER_RUNS"/&gt;</programlisting>
    </section>

    <section xml:id="scheduling-task-namespace-scheduled-tasks">
      <title>The 'scheduled-tasks' element</title>

      <para>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.</para>

      <programlisting language="xml">&lt;task:scheduled-tasks scheduler="myScheduler"&gt;
    &lt;task:scheduled ref="beanA" method="methodA" fixed-delay="5000"/&gt;
&lt;/task:scheduled-tasks&gt;

&lt;task:scheduler id="myScheduler" pool-size="10"/&gt;</programlisting>

      <para>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.</para>

      <programlisting language="xml">&lt;task:scheduled-tasks scheduler="myScheduler"&gt;
    &lt;task:scheduled ref="beanA" method="methodA" fixed-delay="5000" initial-delay="1000"/&gt;
    &lt;task:scheduled ref="beanB" method="methodB" fixed-rate="5000"/&gt;
    &lt;task:scheduled ref="beanC" method="methodC" cron="*/5 * * * * MON-FRI"/&gt;
&lt;/task:scheduled-tasks&gt;

&lt;task:scheduler id="myScheduler" pool-size="10"/&gt;</programlisting>
    </section>
  </section>

  <section xml:id="scheduling-quartz">
    <title>Using the Quartz Scheduler</title>

    <para>Quartz uses <classname>Trigger</classname>,
    <classname>Job</classname> and <classname>JobDetail</classname> objects to
    realize scheduling of all kinds of jobs. For the basic concepts behind
    Quartz, have a look at <link
    xl:href="http://quartz-scheduler.org"></link>. For convenience
    purposes, Spring offers a couple of classes that simplify the usage of
    Quartz within Spring-based applications.</para>

    <section xml:id="scheduling-quartz-jobdetail">
      <title>Using the JobDetailBean</title>

      <para><classname>JobDetail</classname> objects contain all information
      needed to run a job. The Spring Framework provides a
      <classname>JobDetailBean</classname> that makes the
      <classname>JobDetail</classname> more of an actual JavaBean with
      sensible defaults. Let's have a look at an example:</para>

      <programlisting language="xml">
&lt;bean name="exampleJob" class="org.springframework.scheduling.quartz.JobDetailBean"&gt;
  &lt;property name="jobClass" value="example.ExampleJob" /&gt;
  &lt;property name="jobDataAsMap"&gt;
    &lt;map&gt;
      &lt;entry key="timeout" value="5" /&gt;
    &lt;/map&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting>

      <para>The job detail bean has all information it needs to run the job
      (<classname>ExampleJob</classname>). The timeout is specified in the job
      data map. The job data map is available through the
      <classname>JobExecutionContext</classname> (passed to you at execution
      time), but the <classname>JobDetailBean</classname> also maps the
      properties from the job data map to properties of the actual job. So in
      this case, if the <classname>ExampleJob</classname> contains a property
      named <literal>timeout</literal>, the
      <classname>JobDetailBean</classname> will automatically apply it:</para>

      <programlisting language="java">package example;

public class ExampleJob extends QuartzJobBean {

  private int timeout;

  /**
   * Setter called after the ExampleJob is instantiated
   * with the value from the JobDetailBean (5)
   */
  public void setTimeout(int timeout) {
    this.timeout = timeout;
  }

  protected void executeInternal(JobExecutionContext ctx) throws JobExecutionException {
      <lineannotation>// do the actual work</lineannotation>
  }
}</programlisting>

      <para>All additional settings from the job detail bean are of course
      available to you as well.</para>

      <para><emphasis>Note: Using the <literal>name</literal> and
      <literal>group</literal> properties, you can modify the name and the
      group of the job, respectively. By default, the name of the job matches
      the bean name of the job detail bean (in the example above, this is
      <literal>exampleJob</literal>).</emphasis></para>
    </section>

    <section xml:id="scheduling-quartz-method-invoking-job">
      <title>Using the
      <classname>MethodInvokingJobDetailFactoryBean</classname></title>

      <para>Often you just need to invoke a method on a specific object. Using
      the <classname>MethodInvokingJobDetailFactoryBean</classname> you can do
      exactly this:</para>

      <programlisting language="xml">&lt;bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"&gt;
  &lt;property name="targetObject" ref="exampleBusinessObject" /&gt;
  &lt;property name="targetMethod" value="doIt" /&gt;
&lt;/bean&gt;</programlisting>

      <para>The above example will result in the <literal>doIt</literal>
      method being called on the <literal>exampleBusinessObject</literal>
      method (see below):</para>

      <programlisting language="java">public class ExampleBusinessObject {

  <lineannotation>// properties and collaborators</lineannotation>

  public void doIt() {
    <lineannotation>// do the actual work</lineannotation>
  }
}</programlisting>

      <programlisting language="xml">
&lt;bean id="exampleBusinessObject" class="examples.ExampleBusinessObject"/&gt;</programlisting>

      <para>Using the
      <classname>MethodInvokingJobDetailFactoryBean</classname>, you don't
      need to create one-line jobs that just invoke a method, and you only
      need to create the actual business object and wire up the detail
      object.</para>

      <para>By default, Quartz Jobs are stateless, resulting in the
      possibility of jobs interfering with each other. If you specify two
      triggers for the same <classname>JobDetail</classname>, it might be
      possible that before the first job has finished, the second one will
      start. If <classname>JobDetail</classname> classes implement the
      <interfacename>Stateful</interfacename> interface, this won't happen.
      The second job will not start before the first one has finished. To make
      jobs resulting from the
      <classname>MethodInvokingJobDetailFactoryBean</classname>
      non-concurrent, set the <literal>concurrent</literal> flag to
      <literal>false</literal>.</para>

      <programlisting language="xml">
&lt;bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"&gt;
  &lt;property name="targetObject" ref="exampleBusinessObject" /&gt;
  &lt;property name="targetMethod" value="doIt" /&gt;
  &lt;property name="concurrent" value="false" /&gt;
&lt;/bean&gt;</programlisting>

      <note>
        <para>By default, jobs will run in a concurrent fashion.</para>
      </note>
    </section>

    <section xml:id="scheduling-quartz-cron">
      <title>Wiring up jobs using triggers and the
      <classname>SchedulerFactoryBean</classname></title>

      <para>We've created job details and jobs. We've also reviewed the
      convenience bean that allows you to invoke a method on a specific
      object. Of course, we still need to schedule the jobs themselves. This
      is done using triggers and a
      <classname>SchedulerFactoryBean</classname>. Several triggers are
      available within Quartz and Spring offers two Quartz <interfacename>FactoryBean</interfacename>
      implementations with convenient defaults: <classname>CronTriggerFactoryBean</classname> and
      <classname>SimpleTriggerFactoryBean</classname>.</para>

      <para>Triggers need to be scheduled. Spring offers a
      <classname>SchedulerFactoryBean</classname> that exposes triggers to be
      set as properties. <classname>SchedulerFactoryBean</classname> schedules
      the actual jobs with those triggers.</para>

      <para>Find below a couple of examples:</para>

      <programlisting language="xml">&lt;bean id="simpleTrigger" class="org.springframework.scheduling.quartz.SimpleTriggerFactoryBean"&gt;
    &lt;!-- see the example of method invoking job above --&gt;
    &lt;property name="jobDetail" ref="jobDetail" /&gt;
    &lt;!-- 10 seconds --&gt;
    &lt;property name="startDelay" value="10000" /&gt;
    &lt;!-- repeat every 50 seconds --&gt;
    &lt;property name="repeatInterval" value="50000" /&gt;
&lt;/bean&gt;

&lt;bean id="cronTrigger" class="org.springframework.scheduling.quartz.CronTriggerFactoryBean"&gt;
    &lt;property name="jobDetail" ref="exampleJob" /&gt;
    &lt;!-- run every morning at 6 AM --&gt;
    &lt;property name="cronExpression" value="0 0 6 * * ?" /&gt;
&lt;/bean&gt;</programlisting>

      <para>Now we've set up two triggers, one running every 50 seconds with a
      starting delay of 10 seconds and one every morning at 6 AM. To finalize
      everything, we need to set up the
      <classname>SchedulerFactoryBean</classname>:</para>

      <programlisting language="xml">&lt;bean class="org.springframework.scheduling.quartz.SchedulerFactoryBean"&gt;
    &lt;property name="triggers"&gt;
        &lt;list&gt;
            &lt;ref bean="cronTrigger" /&gt;
            &lt;ref bean="simpleTrigger" /&gt;
        &lt;/list&gt;
    &lt;/property&gt;
&lt;/bean&gt;</programlisting>

      <para>More properties are available for the
      <classname>SchedulerFactoryBean</classname> for you to set, such as the
      calendars used by the job details, properties to customize Quartz with,
      etc. Have a look at the <link
      xl:href="http://static.springsource.org/spring-framework/docs/current/javadoc-api/org/springframework/scheduling/quartz/SchedulerFactoryBean.html">SchedulerFactoryBean
      Javadoc</link> for more information.</para>
    </section>
  </section>
</chapter>
