The Spring Framework provides abstractions for asynchronous
execution and scheduling of tasks with the
TaskExecutor
and
TaskScheduler
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.
Spring also features integration classes for supporting scheduling
with the Timer
, part of the JDK since 1.3, and the
Quartz Scheduler (http://quartz-scheduler.org). Both of those
schedulers are set up using a FactoryBean
with optional references to Timer
or
Trigger
instances, respectively. Furthermore, a
convenience class for both the Quartz Scheduler and the
Timer
is available that allows you to invoke a
method of an existing target object (analogous to the normal
MethodInvokingFactoryBean
operation).
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.
Spring's TaskExecutor
interface is
identical to the java.util.concurrent.Executor
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 execute(Runnable task)
that accepts a task
for execution based on the semantics and configuration of the thread
pool.
The TaskExecutor
was originally
created to give other Spring components an abstraction for thread pooling
where needed. Components such as the
ApplicationEventMulticaster
, JMS's
AbstractMessageListenerContainer
, and Quartz
integration all use the TaskExecutor
abstraction to pool threads. However, if your beans need thread pooling
behavior, it is possible to use this abstraction for your own
needs.
There are a number of pre-built implementations of
TaskExecutor
included with the Spring
distribution. In all likelihood, you shouldn't ever need to implement
your own.
SimpleAsyncTaskExecutor
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.
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.
This implementation is a wrapper for a Java 5
java.util.concurrent.Executor
. There is an
alternative, ThreadPoolTaskExecutor
, that
exposes the Executor
configuration parameters
as bean properties. It is rare to need to use the
ConcurrentTaskExecutor
but if the ThreadPoolTaskExecutor
isn't robust enough for your needs, the
ConcurrentTaskExecutor
is an
alternative.
This implementation is actually a subclass of Quartz's
SimpleThreadPool
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.
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
java.util.concurrent.ThreadPoolExecutor
and
wraps it in a TaskExecutor
. If you
need something advanced such as a
ScheduledThreadPoolExecutor
, it is
recommended that you use a ConcurrentTaskExecutor
instead.
TimerTaskExecutor
This implementation uses a single
TimerTask
as its backing implementation. It's
different from the SyncTaskExecutor
in that the method invocations are executed in a separate thread,
although they are synchronous in that thread.
WorkManagerTaskExecutor
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 SimpleThreadPoolTaskExecutor
,
this class implements the WorkManager interface and therefore can be
used directly as a WorkManager as well.
Spring's TaskExecutor
implementations are used as simple JavaBeans. In the example below, we
define a bean that uses the
ThreadPoolTaskExecutor
to asynchronously print
out a set of messages.
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 < 25; i++) { taskExecutor.execute(new MessagePrinterTask("Message" + i)); } } }
As you can see, rather than retrieving a thread from the pool and
executing yourself, you add your Runnable
to the
queue and the TaskExecutor
uses its
internal rules to decide when the task gets executed.
To configure the rules that the
TaskExecutor
will use, simple bean
properties have been exposed.
<bean id="taskExecutor" class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor"> <property name="corePoolSize" value="5" /> <property name="maxPoolSize" value="10" /> <property name="queueCapacity" value="25" /> </bean> <bean id="taskExecutorExample" class="TaskExecutorExample"> <constructor-arg ref="taskExecutor" /> </bean>
In addition to the TaskExecutor
abstraction, Spring 3.0 introduces a
TaskScheduler
with a variety of methods for
scheduling tasks to run at some point in the future.
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); }
The simplest method is the one named 'schedule' that takes a
Runnable
and Date
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.
The Trigger
interface is
essentially inspired by JSR-236, which, as of Spring 3.0, has not yet
been officially implemented. The basic idea of the
Trigger
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
TriggerContext
. The
Trigger
interface itself is quite
simple:
public interface Trigger { Date nextExecutionTime(TriggerContext triggerContext); }
As you can see, the TriggerContext
is the most important part. It encapsulates all of the relevant data,
and is open for extension in the future if necessary. The
TriggerContext
is an interface (a
SimpleTriggerContext
implementation is used by
default). Here you can see what methods are available for
Trigger
implementations.
public interface TriggerContext { Date lastScheduledExecutionTime(); Date lastActualExecutionTime(); Date lastCompletionTime(); }
Spring provides two implementations of the
Trigger
interface. The most interesting
one is the CronTrigger
. 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.
scheduler.schedule(task, new CronTrigger("* 15 9-17 * * MON-FRI"));
The other out-of-the-box implementation is a
PeriodicTrigger
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
TaskScheduler
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 PeriodicTrigger
implementation is that it can
be used within components that rely on the
Trigger
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
Triggers
could be configured
externally.
As with Spring's TaskExecutor
abstraction, the primary benefit of the
TaskScheduler
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 TimerManagerTaskScheduler
that
delegates to a CommonJ TimerManager instance, typically configured with
a JNDI-lookup.
A simpler alternative, the
ThreadPoolTaskScheduler
, can be used whenever
external thread management is not a requirement. Internally, it
delegates to a ScheduledExecutorService
instance. ThreadPoolTaskScheduler
actually
implements Spring's TaskExecutor
interface as well, so that a single instance can be used for
asynchronous execution as soon as possible as well
as scheduled, and potentially recurring, executions.
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 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 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 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"/>
Spring 3.0 also adds annotation support for both task scheduling and asynchronous method execution.
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.
@Scheduled(fixedDelay=5000) public void doSomething() { // something that should execute periodically }
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.
@Scheduled(fixedRate=5000) public void doSomething() { // something that should execute periodically }
If simple periodic scheduling is not expressive enough, then a cron expression may be provided. For example, the following will only execute on weekdays.
@Scheduled(cron="*/5 * * * * MON-FRI") public void doSomething() { // something that should execute on weekdays only }
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.
Note | |
---|---|
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. |
The @Async
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
TaskExecutor
. In the simplest case, the
annotation may be applied to a void
-returning
method.
@Async void doSomething() { // this will be executed asynchronously }
Unlike the methods annotated with the
@Scheduled
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 @Async
annotation.
@Async void doSomething(String s) { // this will be executed asynchronously }
Even methods that return a value can be invoked asynchronously.
However, such methods are required to have a
Future
typed return value. This still
provides the benefit of asynchronous execution so that the caller can
perform other tasks prior to calling get()
on
that Future.
@Async Future<String> returnSomething(int i) { // this will be executed asynchronously }
@Async
can not be used in
conjunction with lifecycle callbacks such as
@PostConstruct
. To asynchronously
initialize Spring beans you currently have to use a separate
initializing Spring bean that invokes the
@Async
annotated method on the target
then.
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(); } }
To enable both @Scheduled and @Async annotations, simply include the 'annotation-driven' element from the task namespace in your configuration.
<task:annotation-driven executor="myExecutor" scheduler="myScheduler"/> <task:executor id="myExecutor" pool-size="5"/> <task:scheduler id="myScheduler" pool-size="10"/>}
Notice that an executor reference is provided for handling those tasks that correspond to methods with the @Async annotation, and the scheduler reference is provided for managing those methods annotated with @Scheduled.
By default when specifying @Async
on
a method, the executor that will be used is the one supplied to the
'annotation-driven' element as described above. However, the
value
attribute of the
@Async
annotation can be used when needing
to indicate that an executor other than the default should be used when
executing a given method.
@Async("otherExecutor") void doSomething(String s) { // this will be executed asynchronously by "otherExecutor" }
In this case, "otherExecutor" may be the name of any
Executor
bean in the Spring container, or
may be the name of a qualifier associated with any
Executor
, e.g. as specified with the
<qualifier>
element or Spring's
@Qualifier
annotation.
Quartz uses Trigger
,
Job
and JobDetail
objects to
realize scheduling of all kinds of jobs. For the basic concepts behind
Quartz, have a look at http://quartz-scheduler.org. For convenience
purposes, Spring offers a couple of classes that simplify the usage of
Quartz within Spring-based applications.
JobDetail
objects contain all information
needed to run a job. The Spring Framework provides a
JobDetailBean
that makes the
JobDetail
more of an actual JavaBean with
sensible defaults. Let's have a look at an example:
<bean name="exampleJob" class="org.springframework.scheduling.quartz.JobDetailBean"> <property name="jobClass" value="example.ExampleJob" /> <property name="jobDataAsMap"> <map> <entry key="timeout" value="5" /> </map> </property> </bean>
The job detail bean has all information it needs to run the job
(ExampleJob
). The timeout is specified in the job
data map. The job data map is available through the
JobExecutionContext
(passed to you at execution
time), but the JobDetailBean
also maps the
properties from the job data map to properties of the actual job. So in
this case, if the ExampleJob
contains a property
named timeout
, the
JobDetailBean
will automatically apply it:
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 { // do the actual work } }
All additional settings from the job detail bean are of course available to you as well.
Note: Using the name
and
group
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
exampleJob
).
Often you just need to invoke a method on a specific object. Using
the MethodInvokingJobDetailFactoryBean
you can do
exactly this:
<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"> <property name="targetObject" ref="exampleBusinessObject" /> <property name="targetMethod" value="doIt" /> </bean>
The above example will result in the doIt
method being called on the exampleBusinessObject
method (see below):
public class ExampleBusinessObject { // properties and collaborators public void doIt() { // do the actual work } }
<bean id="exampleBusinessObject" class="examples.ExampleBusinessObject"/>
Using the
MethodInvokingJobDetailFactoryBean
, 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.
By default, Quartz Jobs are stateless, resulting in the
possibility of jobs interfering with each other. If you specify two
triggers for the same JobDetail
, it might be
possible that before the first job has finished, the second one will
start. If JobDetail
classes implement the
Stateful
interface, this won't happen.
The second job will not start before the first one has finished. To make
jobs resulting from the
MethodInvokingJobDetailFactoryBean
non-concurrent, set the concurrent
flag to
false
.
<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean"> <property name="targetObject" ref="exampleBusinessObject" /> <property name="targetMethod" value="doIt" /> <property name="concurrent" value="false" /> </bean>
Note | |
---|---|
By default, jobs will run in a concurrent fashion. |
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
SchedulerFactoryBean
. Several triggers are
available within Quartz. Spring offers two subclassed triggers with
convenient defaults: CronTriggerBean
and
SimpleTriggerBean
.
Triggers need to be scheduled. Spring offers a
SchedulerFactoryBean
that exposes triggers to be
set as properties. SchedulerFactoryBean
schedules
the actual jobs with those triggers.
Find below a couple of examples:
<bean id="simpleTrigger" class="org.springframework.scheduling.quartz.SimpleTriggerBean"> <!-- see the example of method invoking job above --> <property name="jobDetail" ref="jobDetail" /> <!-- 10 seconds --> <property name="startDelay" value="10000" /> <!-- repeat every 50 seconds --> <property name="repeatInterval" value="50000" /> </bean> <bean id="cronTrigger" class="org.springframework.scheduling.quartz.CronTriggerBean"> <property name="jobDetail" ref="exampleJob" /> <!-- run every morning at 6 AM --> <property name="cronExpression" value="0 0 6 * * ?" /> </bean>
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
SchedulerFactoryBean
:
<bean class="org.springframework.scheduling.quartz.SchedulerFactoryBean"> <property name="triggers"> <list> <ref bean="cronTrigger" /> <ref bean="simpleTrigger" /> </list> </property> </bean>
More properties are available for the
SchedulerFactoryBean
for you to set, such as the
calendars used by the job details, properties to customize Quartz with,
etc. Have a look at the SchedulerFactoryBean
Javadoc for more information.
The other way to schedule jobs in Spring is to use JDK
Timer
objects. You can create custom timers or use
the timer that invokes methods. Wiring timers is done using the
TimerFactoryBean
.
Using the TimerTask
you can create customer
timer tasks, similar to Quartz jobs:
public class CheckEmailAddresses extends TimerTask { private List emailAddresses; public void setEmailAddresses(List emailAddresses) { this.emailAddresses = emailAddresses; } public void run() { // iterate over all email addresses and archive them } }
Wiring it up is simple:
<bean id="checkEmail" class="examples.CheckEmailAddress"> <property name="emailAddresses"> <list> <value>[email protected]</value> <value>[email protected]</value> <value>[email protected]</value> </list> </property> </bean> <bean id="scheduledTask" class="org.springframework.scheduling.timer.ScheduledTimerTask"> <!-- wait 10 seconds before starting repeated execution --> <property name="delay" value="10000" /> <!-- run every 50 seconds --> <property name="period" value="50000" /> <property name="timerTask" ref="checkEmail" /> </bean>
Note that letting the task only run once can be done by
changing the period
property to 0 (or a negative
value).
Similar to the Quartz support, the Timer
support also features a component that allows you to periodically invoke
a method:
<bean id="doIt" class="org.springframework.scheduling.timer.MethodInvokingTimerTaskFactoryBean"> <property name="targetObject" ref="exampleBusinessObject" /> <property name="targetMethod" value="doIt" /> </bean>
The above example will result in the doIt
method being called on the exampleBusinessObject
(see
below):
public class BusinessObject { // properties and collaborators public void doIt() { // do the actual work } }
Changing the timerTask
reference of the
ScheduledTimerTask
example to the bean
doIt
will result in the doIt
method being executed on a fixed schedule.
The TimerFactoryBean
is similar to the
Quartz SchedulerFactoryBean
in that it serves the
same purpose: setting up the actual scheduling. The
TimerFactoryBean
sets up an actual
Timer
and schedules the tasks it has references
to. You can specify whether or not daemon threads should be used.
<bean id="timerFactory" class="org.springframework.scheduling.timer.TimerFactoryBean"> <property name="scheduledTimerTasks"> <list> <!-- see the example above --> <ref bean="scheduledTask" /> </list> </property> </bean>