validate step listeners

[hosuaby]

The problem: StepBuilderFactory is very permissive concerning listeners.

Method AbstractTaskletStepBuilder#listener(Object listener) accepts an Object as parameter, and this can lead to wrong step configurations as following:

@Configuration
public class BadJobConfig {
    @Autowired private JobBuilderFactory jobs;
    @Autowired private StepBuilderFactory steps;

    @Bean
    public Job someJob(Step someStep) {
        return jobs
                .get("some-job")
                .start(someStep)
                .build();
    }

    @Bean
    public Step someStep() {
        return steps
                .get("some-step")
                .tasklet((contribution, chunkContext) -> null)
                .listener(new DummyJobExecutionListener())  // mistake! It's job listener, not step listener
                .build();
    }

    static class DummyJobExecutionListener implements JobExecutionListener {
        @Override
        public void beforeJob(JobExecution jobExecution) {
        }

        @Override
        public void afterJob(JobExecution jobExecution) {
        }
    }
}

As you see DummyJobExecutionListener is a job listener! Not step listener. Code compiles and executes without a single warning. This mis-configuration can lead to long painful hours of debugging, before user will notice that he passed wrong listener.

Solution: add runtime assert that will check that Object listener is a valid step execution listener. We can simply use method StepListenerFactoryBean#isListener.

With this feature, the precedent code will throw an exception during application start-up:

Caused by: java.lang.IllegalArgumentException: Object of type [com.adelean.a2.batch.admin.config.BadJobConfig$DummyJobExecutionListener] is not a valid step listener. It must ether implement StepListener interface or have methods annotated with any of:
	- @BeforeStep
	- @AfterStep
	- @BeforeChunk
	- @AfterChunk
	- @AfterChunkError
	- @BeforeRead
	- @AfterRead
	- @OnReadError
	- @BeforeProcess
	- @AfterProcess
	- @OnProcessError
	- @BeforeWrite
	- @AfterWrite
	- @OnWriteError
	- @OnSkipInRead
	- @OnSkipInProcess
	- @OnSkipInWrite

[hosuaby]

Works also with XML config:

<?xml version="1.0" encoding="UTF-8"?>
<beans:beans
        xmlns="http://www.springframework.org/schema/batch"
        xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
        xmlns:beans="http://www.springframework.org/schema/beans"
        xsi:schemaLocation="http://www.springframework.org/schema/batch
    http://www.springframework.org/schema/batch/spring-batch.xsd
    http://www.springframework.org/schema/beans
    http://www.springframework.org/schema/beans/spring-beans.xsd">

    <job id="someJob">
        <step id="someStep">
            <tasklet ref="someTasklet"/>
            <listeners>
                <listener ref="dummyJobExecutionListener" />
            </listeners>
        </step>
    </job>

</beans:beans>