JobParametersIncrementer

JobParametersIncrementer

Most of the methods on JobOperator are self-explanatory, and you can find more detailed explanations in the Javadoc of the interface. However, the startNextInstance method is worth noting. This method always starts a new instance of a Job. This can be extremely useful if there are serious issues in a JobExecution and the Job needs to be started over again from the beginning. Unlike JobLauncher (which requires a new JobParameters object that triggers a new JobInstance), if the parameters are different from any previous set of parameters, the startNextInstance method uses the JobParametersIncrementer tied to the Job to force the Job to a new instance:

 

public interface JobParametersIncrementer {

    JobParameters getNext(JobParameters parameters);

}

 

 

The contract of JobParametersIncrementer is that, given a JobParameters object, it returns the “next” JobParameters object by incrementing any necessary values it may contain. This strategy is useful because the framework has no way of knowing what changes to the JobParameters make it the “next” instance. For example, if the only value in JobParameters is a date and the next instance should be created, should that value be incremented by one day or one week (if the job is weekly, for instance)? The same can be said for any numerical values that help to identify the Job, as the following example shows:

 

public class SampleIncrementer implements JobParametersIncrementer {

    public JobParameters getNext(JobParameters parameters) {
        if (parameters==null || parameters.isEmpty()) {
            return new JobParametersBuilder().addLong("run.id", 1L).toJobParameters();
        }
        long id = parameters.getLong("run.id",1L) + 1;
        return new JobParametersBuilder().addLong("run.id", id).toJobParameters();
    }
}

 

In this example, the value with a key of run.id is used to discriminate between JobInstances. If the JobParameters passed in is null, it can be assumed that the Job has never been run before and, thus, its initial state can be returned. However, if not, the old value is obtained, incremented by one, and returned.

 

For jobs defined in Java, you can associate an incrementer with a Job through the incrementer method provided in the builders, as follows:

 

@Bean
public Job footballJob(JobRepository jobRepository) {
    return new JobBuilder("footballJob", jobRepository)
    				 .incrementer(sampleIncrementer())
    				 ...
                     .build();
}

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