Running a Spring batch at a schedule

Last time, I wrote a Spring batch application to index local markdown files into Apache Solr. While the default configuration of Spring batch is great, I don’t want to re-run the application to re-index all documents. In this tutorial I’ll show you how you can run a batch job at a certain schedule.

Disabling default behaviour

The default behaviour, as explained, is that every Spring batch job will run at the start of the application. To disable this, you need to configure the spring.batch.job.enabled property in your application configuration, for example:

spring:
  batch:
    job:
      enabled: false

Adding the @EnableScheduling annotation

To allow using scheduled tasks, you also have to add the @EnableScheduling annotation to either a configuration class (a class annotated with @Configuration) or to the main class, for example:

@SpringBootApplication
@EnableScheduling
public class SpringBootSolrBatchApplication {

    public static void main(String[] args) {
        SpringApplication.run(SpringBootSolrBatchApplication.class, args);
    }
}

Creating a scheduled task

Now that we have disabled Spring batch from running at the start of the application, it’s time to re-enable it in a different way, by scheduling it.

To do this, you have to create a new component, so that we can autowire:

• A Spring batch Job, like the one I made in my previous tutorial to index my Markdown files.
• The JobLauncher, which is created by Spring boot if you’re using the Spring batch starter and if you’ve defined a job.

@Component
@AllArgsConstructor
public class MarkdownSolrBatchScheduler {
    private JobLauncher jobLauncher;
    private Job indexMarkdownDocumentsJob;

}

After that, we can use Spring’s @Scheduled annotation to invoke a method at certain times. For example:

@Scheduled(cron = "0 * * * * *")
public void schedule() {

}

In this example, I’m using a cron job to run the schedule() method every minute at zero seconds specifically. This means that the job will run at 05:00:00, 05:01:00, 05:02:00 and so on.

Alternatively, you can use the fixedRate property of @Scheduled to run the job every x milliseconds, for example:

@Scheduled(fixedRate = 60000)
public void schedule() {

}

In this example, the method will run every minute since the start of the application. If you don’t want to run your job every x milliseconds, but you want to run the method x milliseconds after it has been run, you can use the fixedDelay property:

@Scheduled(fixedDelay = 60000)
public void schedule() {

}

In this example, the method will run a minute after it stopped last time. So basically this allows you to run the batch job again, a minute after it has been stopped.

Using the JobLauncher

Launching a job isn’t that difficult with the JobLauncher. All you need to do is to call the run() method and passing the job and the parameters it needs, for example:

jobLauncher.run(indexMarkdownDocumentsJob, new JobParametersBuilder().toJobParameters());

If we don’t want to send any parameters, we can’t just leave null but we have to use the JobParametersBuilder to pass a valid object.

However, Spring batch won’t run the same job with the same parameters twice. This will be a problem if we want to run the same job at different times. A possible solution is to pass the date of when the scheduler started as a parameter:

@Scheduled(cron = "0 * * * * *")
public void schedule() throws JobParametersInvalidException, JobExecutionAlreadyRunningException, JobRestartException, JobInstanceAlreadyCompleteException {
    jobLauncher.run(indexMarkdownDocumentsJob, new JobParametersBuilder()
        .addDate("date", new Date())
        .toJobParameters());
}

By doing this, the parameters will always be different, which means the scheduler will always properly run the Spring batch job.

Externalizing your cron expression

One thing you might want to do, is to externalize the cron expression, in our case 0 * * * * *. To do this, you can create a separate property within your application.yml or application.properties file, for example:

batch:
  cron: 0 * * * * *

Now you can reference it like this:

@Scheduled(cron = "${batch.cron}")
public void schedule() {
    // ...
}

Sadly, at this moment support is limited to properties, so you can’t use the Spring expression language or SpEL to its fullest extend.

Both the fixedDelay and fixedRate properties have string-based variants as well, such as fixedDelayString and fixedRateString. These allow you to use external configuration for those properties as well:

@Scheduled(fixedDelayString = "${batch.delay}")
public void schedule() {
    // ...
}

@Scheduled(fixedRateString = "${batch.rate}")
public void schedule2() {
    // ...
}