Spring Scheduling Annotations

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