spring-framework/framework-docs/modules/ROOT/pages/integration/scheduling.adoc

[[scheduling]]
= Task Execution and Scheduling

The Spring Framework provides abstractions for the 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 and Jakarta EE environments.

Spring also features integration classes to support scheduling with the
https://www.quartz-scheduler.org/[Quartz Scheduler].



[[scheduling-task-executor]]
== The Spring `TaskExecutor` Abstraction

Executors are the JDK 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 the Java SE and Jakarta EE environments.

Spring's `TaskExecutor` interface is identical to the `java.util.concurrent.Executor`
interface. In fact, originally, its primary reason for existence was 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, you can also use this abstraction for your own needs.


[[scheduling-task-executor-types]]
=== `TaskExecutor` Types

Spring includes a number of pre-built implementations of `TaskExecutor`.
In all likelihood, you should never need to implement your own.
The variants that Spring provides are as follows:

* `SyncTaskExecutor`:
  This implementation does not run invocations asynchronously. Instead, each
  invocation takes place in the calling thread. It is primarily used in situations
  where multi-threading is not necessary, such as in simple test cases.
* `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 that blocks
  any invocations that are over the limit until a slot has been freed up. If you
  are looking for true pooling, see `ThreadPoolTaskExecutor`, later in this list.
  This will use JDK 21's Virtual Threads, when the "virtualThreads"
  option is enabled. This implementation also supports graceful shutdown through
  Spring's lifecycle management.
* `ConcurrentTaskExecutor`:
  This implementation is an adapter for a `java.util.concurrent.Executor` instance.
  There is an alternative (`ThreadPoolTaskExecutor`) that exposes the `Executor`
  configuration parameters as bean properties. There is rarely a need to use
  `ConcurrentTaskExecutor` directly. However, if the `ThreadPoolTaskExecutor` is not
  flexible enough for your needs, `ConcurrentTaskExecutor` is an alternative.
* `ThreadPoolTaskExecutor`:
  This implementation is most commonly used. It exposes bean properties for configuring
  a `java.util.concurrent.ThreadPoolExecutor` and wraps it in a `TaskExecutor`.
  If you need to adapt to a different kind of `java.util.concurrent.Executor`,
  we recommend that you use a `ConcurrentTaskExecutor` instead.
  It also provides a pause/resume capability and graceful shutdown through
  Spring's lifecycle management.
* `DefaultManagedTaskExecutor`:
  This implementation uses a JNDI-obtained `ManagedExecutorService` in a JSR-236
  compatible runtime environment (such as a Jakarta EE application server),
  replacing a CommonJ WorkManager for that purpose.



[[scheduling-task-executor-usage]]
=== Using a `TaskExecutor`

Spring's `TaskExecutor` implementations are commonly used with dependency injection.
In the following example, we define a bean that uses the `ThreadPoolTaskExecutor`
to asynchronously print out a set of messages:

include-code::./TaskExecutorExample[tag=snippet,indent=0]

As you can see, rather than retrieving a thread from the pool and executing it yourself,
you add your `Runnable` to the queue. Then the `TaskExecutor` uses its internal rules to
decide when the task gets run.

To configure the rules that the `TaskExecutor` uses, we expose simple bean properties:

include-code::./TaskExecutorConfiguration[tag=snippet,indent=0]

Most `TaskExecutor` implementations provide a way to automatically wrap tasks submitted
with a `TaskDecorator`. Decorators should delegate to the task it is wrapping, possibly
implementing custom behavior before/after the execution of the task.

Let's consider a simple implementation that will log messages before and after the execution
or our tasks:

include-code::./LoggingTaskDecorator[indent=0]

We can then configure our decorator on a `TaskExecutor` instance:

include-code::./TaskExecutorConfiguration[tag=decorator,indent=0]

In case multiple decorators are needed, the `org.springframework.core.task.support.CompositeTaskDecorator`
can be used to execute sequentially multiple decorators.


[[scheduling-task-scheduler]]
== The Spring `TaskScheduler` Abstraction

In addition to the `TaskExecutor` abstraction, Spring has a `TaskScheduler` SPI with a
variety of methods for scheduling tasks to run at some point in the future. The following
listing shows the `TaskScheduler` interface definition:

[source,java,indent=0,subs="verbatim,quotes"]
----
	public interface TaskScheduler {

		Clock getClock();

		ScheduledFuture schedule(Runnable task, Trigger trigger);

		ScheduledFuture schedule(Runnable task, Instant startTime);

		ScheduledFuture scheduleAtFixedRate(Runnable task, Instant startTime, Duration period);

		ScheduledFuture scheduleAtFixedRate(Runnable task, Duration period);

		ScheduledFuture scheduleWithFixedDelay(Runnable task, Instant startTime, Duration delay);

		ScheduledFuture scheduleWithFixedDelay(Runnable task, Duration delay);

----

The simplest method is the one named `schedule` that takes only a `Runnable` and an `Instant`.
That causes 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.


[[scheduling-trigger-interface]]
=== `Trigger` Interface

The `Trigger` interface is essentially inspired by JSR-236. 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 take into account the outcome of the
preceding execution, that information is available within a `TriggerContext`. The
`Trigger` interface itself is quite simple, as the following listing shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	public interface Trigger {

		Instant nextExecution(TriggerContext triggerContext);
	}
----

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). The following listing shows the available methods for `Trigger` implementations.

[source,java,indent=0,subs="verbatim,quotes"]
----
	public interface TriggerContext {

		Clock getClock();

		Instant lastScheduledExecution();

		Instant lastActualExecution();

		Instant lastCompletion();
	}
----


[[scheduling-trigger-implementations]]
=== `Trigger` Implementations

Spring provides two implementations of the `Trigger` interface. The most interesting one
is the `CronTrigger`. It enables the scheduling of tasks based on
xref:integration/scheduling.adoc#scheduling-cron-expression[cron expressions].
For example, the following task is scheduled to run 15 minutes past each hour but only
during the 9-to-5 "business hours" on weekdays:

[source,java,indent=0]
[subs="verbatim"]
----
	scheduler.schedule(task, new CronTrigger("0 15 9-17 * * MON-FRI"));
----

The other 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 you can use it 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 you can configure
such `Triggers` externally and, therefore, easily modify or extend them.


[[scheduling-task-scheduler-implementations]]
=== `TaskScheduler` implementations

As with Spring's `TaskExecutor` abstraction, the primary benefit of the `TaskScheduler`
arrangement is that an application's scheduling needs are decoupled from the deployment
environment. This abstraction level is particularly relevant when deploying to an
application server environment where threads should not be created directly by the
application itself. For such scenarios, Spring provides a `DefaultManagedTaskScheduler`
that delegates to a JSR-236 `ManagedScheduledExecutorService` in a Jakarta EE environment.

Whenever external thread management is not a requirement, a simpler alternative is
a local `ScheduledExecutorService` setup within the application, which can be adapted
through Spring's `ConcurrentTaskScheduler`. As a convenience, Spring also provides a
`ThreadPoolTaskScheduler`, which internally delegates to a `ScheduledExecutorService`
to provide common bean-style configuration along the lines of `ThreadPoolTaskExecutor`.
These variants work perfectly fine for locally embedded thread pool setups in lenient
application server environments, as well -- in particular on Tomcat and Jetty.

As of 6.1, `ThreadPoolTaskScheduler` provides a pause/resume capability and graceful
shutdown through Spring's lifecycle management. There is also a new option called
`SimpleAsyncTaskScheduler` which is aligned with JDK 21's Virtual Threads, using a
single scheduler thread but firing up a new thread for every scheduled task execution
(except for fixed-delay tasks which all operate on a single scheduler thread, so for
this virtual-thread-aligned option, fixed rates and cron triggers are recommended).



[[scheduling-annotation-support]]
== Annotation Support for Scheduling and Asynchronous Execution

Spring provides annotation support for both task scheduling and asynchronous method
execution.


[[scheduling-enable-annotation-support]]
=== Enable Scheduling Annotations

To enable support for `@Scheduled` and `@Async` annotations, you can add `@EnableScheduling`
and `@EnableAsync` to one of your `@Configuration` classes, or `<task:annotation-driven>` element,
as the following example shows:

include-code::./SchedulingConfiguration[tag=snippet,indent=0]

You can pick and choose the relevant annotations for your application. For example,
if you need only support for `@Scheduled`, you can omit `@EnableAsync`. For more
fine-grained control, you can additionally implement the `SchedulingConfigurer`
interface, the `AsyncConfigurer` interface, or both. See the
{spring-framework-api}/scheduling/annotation/SchedulingConfigurer.html[`SchedulingConfigurer`]
and {spring-framework-api}/scheduling/annotation/AsyncConfigurer.html[`AsyncConfigurer`]
javadoc for full details.

Note that, with the preceding XML, 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`.

NOTE: The default advice mode for processing `@Async` annotations is `proxy` which allows
for interception of calls through the proxy only. Local calls within the same class
cannot get intercepted that way. For a more advanced mode of interception, consider
switching to `aspectj` mode in combination with compile-time or load-time weaving.


[[scheduling-annotation-support-scheduled]]
=== The `@Scheduled` annotation

You can add the `@Scheduled` annotation to a method, along with trigger metadata. For
example, the following method is invoked every five seconds (5000 milliseconds) with a
fixed delay, meaning that the period is measured from the completion time of each
preceding invocation.

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedDelay = 5000)
	public void doSomething() {
		// something that should run periodically
	}
----

[NOTE]
====
By default, milliseconds will be used as the time unit for fixed delay, fixed rate, and
initial delay values. If you would like to use a different time unit such as seconds or
minutes, you can configure this via the `timeUnit` attribute in `@Scheduled`.

For example, the previous example can also be written as follows.

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedDelay = 5, timeUnit = TimeUnit.SECONDS)
	public void doSomething() {
		// something that should run periodically
	}
----
====

If you need a fixed-rate execution, you can use the `fixedRate` attribute within the
annotation. The following method is invoked every five seconds (measured between the
successive start times of each invocation):

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedRate = 5, timeUnit = TimeUnit.SECONDS)
	public void doSomething() {
		// something that should run periodically
	}
----

For fixed-delay and fixed-rate tasks, you can specify an initial delay by indicating
the amount of time to wait before the first execution of the method, as the following
`fixedRate` example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(initialDelay = 1000, fixedRate = 5000)
	public void doSomething() {
		// something that should run periodically
	}
----

For one-time tasks, you can just specify an initial delay by indicating the amount
of time to wait before the intended execution of the method:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(initialDelay = 1000)
	public void doSomething() {
		// something that should run only once
	}
----

If simple periodic scheduling is not expressive enough, you can provide a
xref:integration/scheduling.adoc#scheduling-cron-expression[cron expression].
The following example runs only on weekdays:

[source,java,indent=0]
[subs="verbatim"]
----
	@Scheduled(cron="*/5 * * * * MON-FRI")
	public void doSomething() {
		// something that should run on weekdays only
	}
----

TIP: You can also use the `zone` attribute to specify the time zone in which the cron
expression is resolved.

Notice that the methods to be scheduled must have void returns and must not accept any
arguments. If the method needs to interact with other objects from the application
context, those would typically have been provided through dependency injection.

`@Scheduled` can be used as a repeatable annotation. If several scheduled declarations
are found on the same method, each of them will be processed independently, with a
separate trigger firing for each of them. As a consequence, such co-located schedules
may overlap and execute multiple times in parallel or in immediate succession.
Please make sure that your specified cron expressions etc do not accidentally overlap.

[NOTE]
====
As of Spring Framework 4.3, `@Scheduled` methods are supported on beans of any scope.

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 that are annotated with `@Scheduled` and registered as regular Spring beans
with the container. Otherwise, you would get double initialization (once through the
container and once through the `@Configurable` aspect), with the consequence of each
`@Scheduled` method being invoked twice.
====

[[scheduling-annotation-support-scheduled-reactive]]
=== The `@Scheduled` annotation on Reactive methods or Kotlin suspending functions

As of Spring Framework 6.1, `@Scheduled` methods are also supported on several types
of reactive methods:

 - methods with a `Publisher` return type (or any concrete implementation of `Publisher`)
like in the following example:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedDelay = 500)
	public Publisher<Void> reactiveSomething() {
		// return an instance of Publisher
	}
----

 - methods with a return type that can be adapted to `Publisher` via the shared instance
of the `ReactiveAdapterRegistry`, provided the type supports _deferred subscription_ like
in the following example:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedDelay = 500)
	public Single<String> rxjavaNonPublisher() {
		return Single.just("example");
	}
----

[NOTE]
====
The `CompletableFuture` class is an example of a type that can typically be adapted
to `Publisher` but doesn't support deferred subscription. Its `ReactiveAdapter` in the
registry denotes that by having the `getDescriptor().isDeferred()` method return `false`.
====

 - Kotlin suspending functions, like in the following example:

[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedDelay = 500)
	suspend fun something() {
		// do something asynchronous
	}
----

 - methods that return a Kotlin `Flow` or `Deferred` instance, like in the following example:

[source,kotlin,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(fixedDelay = 500)
	fun something(): Flow<Void> {
		flow {
			// do something asynchronous
		}
	}
----

All these types of methods must be declared without any arguments. In the case of Kotlin
suspending functions, the `kotlinx.coroutines.reactor` bridge must also be present to allow
the framework to invoke a suspending function as a `Publisher`.

The Spring Framework will obtain a `Publisher` for the annotated method once and will
schedule a `Runnable` in which it subscribes to said `Publisher`. These inner regular
subscriptions occur according to the corresponding `cron`/`fixedDelay`/`fixedRate` configuration.

If the `Publisher` emits `onNext` signal(s), these are ignored and discarded (the same way
return values from synchronous `@Scheduled` methods are ignored).

In the following example, the `Flux` emits `onNext("Hello")`, `onNext("World")` every 5
seconds, but these values are unused:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(initialDelay = 5000, fixedRate = 5000)
	public Flux<String> reactiveSomething() {
		return Flux.just("Hello", "World");
	}
----

If the `Publisher` emits an `onError` signal, it is logged at `WARN` level and recovered.
Because of the asynchronous and lazy nature of `Publisher` instances, exceptions are
not thrown from the `Runnable` task: this means that the `ErrorHandler` contract is not
involved for reactive methods.

As a result, further scheduled subscription occurs despite the error.

In the following example, the `Mono` subscription fails twice in the first five seconds.
Then subscriptions start succeeding, printing a message to the standard output every five
seconds:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Scheduled(initialDelay = 0, fixedRate = 5000)
	public Mono<Void> reactiveSomething() {
		AtomicInteger countdown = new AtomicInteger(2);

		return Mono.defer(() -> {
			if (countDown.get() == 0 || countDown.decrementAndGet() == 0) {
				return Mono.fromRunnable(() -> System.out.println("Message"));
			}
			return Mono.error(new IllegalStateException("Cannot deliver message"));
		})
	}
----

[NOTE]
====
When destroying the annotated bean or closing the application context, Spring Framework cancels
scheduled tasks, which includes the next scheduled subscription to the `Publisher` as well
as any past subscription that is still currently active (for example, for long-running publishers
or even infinite publishers).
====


[[scheduling-annotation-support-async]]
=== The `@Async` annotation

You can provide the `@Async` annotation on a method so that invocation of that method
occurs asynchronously. In other words, the caller returns immediately upon
invocation, while the actual execution of the method occurs in a task that has been
submitted to a Spring `TaskExecutor`. In the simplest case, you can apply the annotation
to a method that returns `void`, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Async
	void doSomething() {
		// this will be run asynchronously
	}
----

Unlike the methods annotated with the `@Scheduled` annotation, these methods can expect
arguments, because they are invoked in the "`normal`" way by callers at runtime rather
than from a scheduled task being managed by the container. For example, the following
code is a legitimate application of the `@Async` annotation:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Async
	void doSomething(String s) {
		// this will be run 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`. The following example shows how to use `@Async` on a method
that returns a value:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Async
	Future<String> returnSomething(int i) {
		// this will be run asynchronously
	}
----

TIP: `@Async` methods may not only declare a regular `java.util.concurrent.Future` return
type but also Spring's `org.springframework.util.concurrent.ListenableFuture` or, as of
Spring 4.2, JDK 8's `java.util.concurrent.CompletableFuture`, for richer interaction with
the asynchronous task and for immediate composition with further processing steps.

You can not use `@Async` in conjunction with lifecycle callbacks such as `@PostConstruct`.
To asynchronously initialize Spring beans, you currently have to use a separate
initializing Spring bean that then invokes the `@Async` annotated method on the target,
as the following example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	public class SampleBeanImpl implements SampleBean {

		@Async
		void doSomething() {
			// ...
		}

	}

	public class SampleBeanInitializer {

		private final SampleBean bean;

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

		@PostConstruct
		public void initialize() {
			bean.doSomething();
		}

	}
----

NOTE: There is no direct XML equivalent for `@Async`, since such methods should be designed
for asynchronous execution in the first place, not externally re-declared to be asynchronous.
However, you can manually set up Spring's `AsyncExecutionInterceptor` with Spring AOP,
in combination with a custom pointcut.


[[scheduling-annotation-support-qualification]]
=== Executor Qualification with `@Async`

By default, when specifying `@Async` on a method, the executor that is used is the
one xref:integration/scheduling.adoc#scheduling-enable-annotation-support[configured when enabling async support],
i.e. the "`annotation-driven`" element if you are using XML or your `AsyncConfigurer`
implementation, if any. However, you can use the `value` attribute of the `@Async`
annotation when you need to indicate that an executor other than the default should be
used when executing a given method. The following example shows how to do so:

[source,java,indent=0,subs="verbatim,quotes"]
----
	@Async("otherExecutor")
	void doSomething(String s) {
		// this will be run asynchronously by "otherExecutor"
	}
----

In this case, `"otherExecutor"` can be the name of any `Executor` bean in the Spring
container, or it may be the name of a qualifier associated with any `Executor` (for example,
as specified with the `<qualifier>` element or Spring's `@Qualifier` annotation).


[[scheduling-annotation-support-exception]]
=== Exception Management with `@Async`

When an `@Async` method has a `Future`-typed return value, it is easy to manage
an exception that was thrown during the method execution, as this exception is
thrown when calling `get` on the `Future` result. With a `void` return type,
however, the exception is uncaught and cannot be transmitted. You can provide an
`AsyncUncaughtExceptionHandler` to handle such exceptions. The following example shows
how to do so:

[source,java,indent=0,subs="verbatim,quotes"]
----
	public class MyAsyncUncaughtExceptionHandler implements AsyncUncaughtExceptionHandler {

		@Override
		public void handleUncaughtException(Throwable ex, Method method, Object... params) {
			// handle exception
		}
	}
----

By default, the exception is merely logged. You can define a custom `AsyncUncaughtExceptionHandler`
by using `AsyncConfigurer` or the `<task:annotation-driven/>` XML element.



[[scheduling-task-namespace]]
== The `task` Namespace

As of version 3.0, Spring includes an XML namespace for configuring `TaskExecutor` and
`TaskScheduler` instances. It also provides a convenient way to configure tasks to be
scheduled with a trigger.


[[scheduling-task-namespace-scheduler]]
=== The `scheduler` Element

The following element creates a `ThreadPoolTaskScheduler` instance with the
specified thread pool size:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<task:scheduler id="scheduler" pool-size="10"/>
----

The value provided for the `id` attribute is 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 has only a single thread.
There are no other configuration options for the scheduler.


[[scheduling-task-namespace-executor]]
=== The `executor` Element

The following creates a `ThreadPoolTaskExecutor` instance:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<task:executor id="executor" pool-size="10"/>
----

As with the scheduler shown in the xref:integration/scheduling.adoc#scheduling-task-namespace-scheduler[previous section],
the value provided for the `id` attribute is 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 only a single size,
an executor's thread pool can have different values for the core and the max size.
If you provide a single value, the executor has 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`. The following example sets a minimum value of
`5` and a maximum value of `25`:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<task:executor
			id="executorWithPoolSizeRange"
			pool-size="5-25"
			queue-capacity="100"/>
----

In the preceding 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, see the documentation for
{java-api}/java.base/java/util/concurrent/ThreadPoolExecutor.html[`ThreadPoolExecutor`].
The main idea is that, when a task is submitted, the executor first tries 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, the task is added to the queue, as long as its
capacity has not yet been reached. Only then, if the queue's capacity has been
reached, does the executor create a new thread beyond the core size. If the max size
has also been reached, then the executor rejects the task.

By default, the queue is unbounded, but this is rarely the desired configuration,
because it can lead to `OutOfMemoryError` if enough tasks are added to that queue while
all pool threads are busy. Furthermore, if the queue is unbounded, the max size has
no effect at all. Since the executor always tries 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).

Consider the case, as mentioned above, when a task is rejected. By default, when a
task is rejected, a thread pool executor throws 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, you can instead
configure either `DiscardPolicy` or `DiscardOldestPolicy`. 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 forces the thread that is calling the submit method to run the task itself.
The idea is that such a caller is 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. You can choose any of these options from an
enumeration of values available for the `rejection-policy` attribute on the `executor`
element.

The following example shows an `executor` element with a number of attributes to specify
various behaviors:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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 stopped. 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 get stopped. A time value of zero causes excess threads to stop
immediately after executing a task without remaining follow-up work in the task queue.
The following example sets the `keep-alive` value to two minutes:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<task:executor
			id="executorWithKeepAlive"
			pool-size="5-25"
			keep-alive="120"/>
----


[[scheduling-task-namespace-scheduled-tasks]]
=== 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. The following listing shows a simple example:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<task:scheduled-tasks scheduler="myScheduler">
		<task:scheduled ref="beanA" method="methodA" fixed-delay="5000"/>
	</task:scheduled-tasks>

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

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 run regardless of how long
any previous execution takes. Additionally, for both `fixed-delay` and `fixed-rate`
tasks, you can specify an 'initial-delay' parameter, indicating the number of
milliseconds to wait before the first execution of the method. For more control,
you can instead provide a `cron` attribute to provide a
xref:integration/scheduling.adoc#scheduling-cron-expression[cron expression].
The following example shows these other options:

[source,xml,indent=0]
[subs="verbatim"]
----
	<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="10"/>
----



[[scheduling-cron-expression]]
== Cron Expressions

All Spring cron expressions have to conform to the same format, whether you are using them in
xref:integration/scheduling.adoc#scheduling-annotation-support-scheduled[`@Scheduled` annotations],
xref:integration/scheduling.adoc#scheduling-task-namespace-scheduled-tasks[`task:scheduled-tasks` elements],
or someplace else. A well-formed cron expression, such as `* * * * * *`, consists of six
space-separated time and date fields, each with its own range of valid values:


....
 ┌───────────── second (0-59)
 │ ┌───────────── minute (0 - 59)
 │ │ ┌───────────── hour (0 - 23)
 │ │ │ ┌───────────── day of the month (1 - 31)
 │ │ │ │ ┌───────────── month (1 - 12) (or JAN-DEC)
 │ │ │ │ │ ┌───────────── day of the week (0 - 7)
 │ │ │ │ │ │          (0 or 7 is Sunday, or MON-SUN)
 │ │ │ │ │ │
 * * * * * *
....

There are some rules that apply:

* A field may be an asterisk (`*`), which always stands for "`first-last`".
For the day-of-the-month or day-of-the-week fields, a question mark (`?`) may be used instead of an
asterisk.
* Commas (`,`) are used to separate items of a list.
* Two numbers separated with a hyphen (`-`) express a range of numbers.
The specified range is inclusive.
* Following a range (or `*`) with `/` specifies the interval of the number's value through the range.
* English names can also be used for the month and day-of-week fields.
Use the first three letters of the particular day or month (case does not matter).
* The day-of-month and day-of-week fields can contain an `L` character, which has a different meaning.
** In the day-of-month field, `L` stands for _the last day of the month_.
If followed by a negative offset (that is, `L-n`), it means _``n``th-to-last day of the month_.
** In the day-of-week field, `L` stands for _the last day of the week_.
If prefixed by a number or three-letter name (`dL` or `DDDL`), it means _the last day of week (`d`
or `DDD`) in the month_.
* The day-of-month field can be `nW`, which stands for _the nearest weekday to day of the month ``n``_.
If `n` falls on Saturday, this yields the Friday before it.
If `n` falls on Sunday, this yields the Monday after, which also happens if `n` is `1` and falls on
a Saturday (that is: `1W` stands for _the first weekday of the month_).
* If the day-of-month field is `LW`, it means _the last weekday of the month_.
* The day-of-week field can be `d#n` (or `DDD#n`), which stands for _the ``n``th day of week `d`
(or ``DDD``) in the month_.

Here are some examples:

|===
| Cron Expression       | Meaning

|`0 0 * * * *`          | top of every hour of every day
|`*/10 * * * * *`       | every ten seconds
| `0 0 8-10 * * *`      | 8, 9 and 10 o'clock of every day
| `0 0 6,19 * * *`      | 6:00 AM and 7:00 PM every day
| `0 0/30 8-10 * * *`   | 8:00, 8:30, 9:00, 9:30, 10:00 and 10:30 every day
| `0 0 9-17 * * MON-FRI`| on the hour nine-to-five weekdays
| `0 0 0 25 DEC ?`       | every Christmas Day at midnight
| `0 0 0 L * *`         | last day of the month at midnight
| `0 0 0 L-3 * *`       | third-to-last day of the month at midnight
| `0 0 0 * * 5L`        | last Friday of the month at midnight
| `0 0 0 * * THUL`      | last Thursday of the month at midnight
| `0 0 0 1W * *`        | first weekday of the month at midnight
| `0 0 0 LW * *`        | last weekday of the month at midnight
| `0 0 0 ? * 5#2`       | the second Friday in the month at midnight
| `0 0 0 ? * MON#1`     | the first Monday in the month at midnight
|===

[[macros]]
=== Macros

Expressions such as `0 0 * * * *` are hard for humans to parse and are, therefore,
hard to fix in case of bugs. To improve readability, Spring supports the following
macros, which represent commonly used sequences. You can use these macros instead
of the six-digit value, thus: `@Scheduled(cron = "@hourly")`.

|===
|Macro                          | Meaning

| `@yearly` (or `@annually`)    | once a year (`0 0 0 1 1 *`)
| `@monthly`                    | once a month (`0 0 0 1 * *`)
| `@weekly`                     | once a week (`0 0 0 * * 0`)
| `@daily` (or `@midnight`)     | once a day (`0 0 0 * * *`), or
| `@hourly`                     | once an hour, (`0 0 * * * *`)
|===



[[scheduling-quartz]]
== Using the Quartz Scheduler

Quartz uses `Trigger`, `Job`, and `JobDetail` objects to realize scheduling of all
kinds of jobs. For the basic concepts behind Quartz, see the
https://www.quartz-scheduler.org/[Quartz Web site]. For convenience purposes, Spring
offers a couple of classes that simplify using Quartz within Spring-based applications.


[[scheduling-quartz-jobdetail]]
=== Using the `JobDetailFactoryBean`

Quartz `JobDetail` objects contain all the information needed to run a job. Spring
provides a `JobDetailFactoryBean`, which provides bean-style properties for XML
configuration purposes. Consider the following example:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean name="exampleJob" class="org.springframework.scheduling.quartz.JobDetailFactoryBean">
		<property name="jobClass" value="example.ExampleJob"/>
		<property name="jobDataAsMap">
			<map>
				<entry key="timeout" value="5"/>
			</map>
		</property>
	</bean>
----

The job detail configuration has all the 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 `JobDetail` also gets
its properties from the job data mapped to properties of the job instance. So, in the
following example, the `ExampleJob` contains a bean property named `timeout`, and the
`JobDetail` has it applied automatically:

[source,java,indent=0,subs="verbatim,quotes",chomp="-packages"]
----
	package example;

	public class ExampleJob extends QuartzJobBean {

		private int timeout;

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

		protected void executeInternal(JobExecutionContext ctx) throws JobExecutionException {
			// do the actual work
		}
	}
----

All additional properties from the job data map are available to you as well.

NOTE: By 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 `JobDetailFactoryBean` (`exampleJob` in the preceding example above).


[[scheduling-quartz-method-invoking-job]]
=== Using the `MethodInvokingJobDetailFactoryBean`

Often you merely need to invoke a method on a specific object. By using the
`MethodInvokingJobDetailFactoryBean`, you can do exactly this, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean id="jobDetail" class="org.springframework.scheduling.quartz.MethodInvokingJobDetailFactoryBean">
		<property name="targetObject" ref="exampleBusinessObject"/>
		<property name="targetMethod" value="doIt"/>
	</bean>
----

The preceding example results in the `doIt` method being called on the
`exampleBusinessObject` method, as the following example shows:

[source,java,indent=0,subs="verbatim,quotes"]
----
	public class ExampleBusinessObject {

		// properties and collaborators

		public void doIt() {
			// do the actual work
		}
	}
----

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<bean id="exampleBusinessObject" class="examples.ExampleBusinessObject"/>
----

By using the `MethodInvokingJobDetailFactoryBean`, you need not create one-line jobs
that merely invoke a method. You need only 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 is possible
that the second one starts before the first job has finished. If `JobDetail` classes
implement the `Stateful` interface, this does not happen: the second job does not start
before the first one has finished.

To make jobs resulting from the `MethodInvokingJobDetailFactoryBean` be non-concurrent,
set the `concurrent` flag to `false`, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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.


[[scheduling-quartz-cron]]
=== Wiring up Jobs by Using Triggers and `SchedulerFactoryBean`

We have created job details and jobs. We have also reviewed the convenience bean that
lets you invoke a method on a specific object. Of course, we still need to schedule the
jobs themselves. This is done by using triggers and a `SchedulerFactoryBean`. Several
triggers are available within Quartz, and Spring offers two Quartz `FactoryBean`
implementations with convenient defaults: `CronTriggerFactoryBean` and
`SimpleTriggerFactoryBean`.

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.

The following listing uses both a `SimpleTriggerFactoryBean` and a `CronTriggerFactoryBean`:

[source,xml,indent=0]
[subs="verbatim"]
----
	<bean id="simpleTrigger" class="org.springframework.scheduling.quartz.SimpleTriggerFactoryBean">
		<!-- 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.CronTriggerFactoryBean">
		<property name="jobDetail" ref="exampleJob"/>
		<!-- run every morning at 6 AM -->
		<property name="cronExpression" value="0 0 6 * * ?"/>
	</bean>
----

The preceding example sets up two triggers, one running every 50 seconds with a starting
delay of 10 seconds and one running every morning at 6 AM. To finalize everything,
we need to set up the `SchedulerFactoryBean`, as the following example shows:

[source,xml,indent=0,subs="verbatim,quotes"]
----
	<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`, such as the calendars used by the
job details, properties to customize Quartz with, and a Spring-provided JDBC DataSource. See
the {spring-framework-api}/scheduling/quartz/SchedulerFactoryBean.html[`SchedulerFactoryBean`]
javadoc for more information.

NOTE: `SchedulerFactoryBean` also recognizes a `quartz.properties` file in the classpath,
based on Quartz property keys, as with regular Quartz configuration. Please note that many
`SchedulerFactoryBean` settings interact with common Quartz settings in the properties file;
it is therefore not recommended to specify values at both levels. For example, do not set
an "org.quartz.jobStore.class" property if you mean to rely on a Spring-provided DataSource,
or specify an `org.springframework.scheduling.quartz.LocalDataSourceJobStore` variant which
is a full-fledged replacement for the standard `org.quartz.impl.jdbcjobstore.JobStoreTX`.