Spring Scheduling Annotations

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In this quick article, Spring provides annotation support for both task scheduling and asynchronous method execution so we’re going to explore the Spring Scheduling Annotations.

This article is part of a series:

• Spring Core Annotations
• Spring Web Annotations 
• Spring Boot Annotations 
• Spring Scheduling Annotations (current article)

Let's explore task scheduling and asynchronous annotations from the org.springframework.scheduling.annotation package.

1. @Scheduled Annotation

The @Scheduled annotation is added to a method along with some information about when to execute it, and Spring takes care of the rest.

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
}

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.

@Scheduled(initialDelay=1000, 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
}

2. @EnableScheduling Annotation

@EnableScheduling annotation is used to enable scheduling in the application. We also have to use it in conjunction with @Configuration:

@Configuration
@EnableScheduling
public class Appconfig{}

As a result, we can now run methods periodically with @Scheduled.

3. @Async Annotation

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 cannot 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 SampleBeanInitializer {

    private final SampleBean bean;

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

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

4. @EnableAsync Annotation

With this annotation, we can enable asynchronous functionality in Spring. We must use it with @Configuration:

@Configuration
@EnableAsync
public class AppConfig{}

Now, that we enabled asynchronous calls, we can use @Async to define the methods supporting it.

5. @Schedules Annotation

We can use this annotation to specify multiple @Scheduled rules:

@Schedules({ 
  @Scheduled(fixedRate = 10000), 
  @Scheduled(cron = "0 * * * * MON-FRI")
})
void checkVehicle() {
    // ...
}

Check out a complete Spring application example using Scheduling annotations on Spring Boot 2 - Scheduling Tasks