Expose contextToJobNames from DefaultJobLoader [BATCH-2864]

Yanming Zhou opened BATCH-2864 and commented

I'm using AutomaticJobRegistrar to register job per xml application context

<bean id="jobLoader" class="org.springframework.batch.core.configuration.support.DefaultJobLoader"/>
<bean id="automaticJobRegistrar" class="org.springframework.batch.core.configuration.support.AutomaticJobRegistrar">
     <property name="applicationContextFactories">
          <bean class="org.springframework.batch.core.configuration.support.ClasspathXmlApplicationContextsFactoryBean">
               <property name="resources" value="resources/batch/*.xml" />
          </bean>
     </property>
     <property name="jobLoader" ref="jobLoader"/>
</bean>

Currently there is no way to get bean from job context since context instances are hold by private field contextToJobNames, such feature is required by execute step via remote invocation to locate step instance, I had to use the dangerous reflection.

@Component
public class JobStepExecutor {

	@Autowired
	private ApplicationContext rootApplicationContext;

	@Autowired
	private JobLoader jobLoader;

	@Autowired
	private JobExplorer jobExplorer;

	public void execute(Long jobExecutionId, Long stepExecutionId, String stepName)
			throws JobInterruptedException, NoSuchJobException {
		String jobName = jobExplorer.getJobExecution(jobExecutionId).getJobInstance().getJobName();
		Map<ConfigurableApplicationContext, Collection<String>> contextToJobNames = ReflectionUtils
				.getFieldValue(jobLoader, "contextToJobNames");
		Step step = contextToJobNames.entrySet().stream().filter(entry -> entry.getValue().contains(jobName))
				.map(entry -> entry.getKey()).findAny().map(ctx -> ctx.getBean(stepName, Step.class))
				.orElseGet(() -> rootApplicationContext.getBean(stepName, Step.class));
		StepExecution stepExecution = jobExplorer.getStepExecution(jobExecutionId, stepExecutionId);
		step.execute(stepExecution);
	}

}

Affects: 4.2.1

Start with DefaultJobLoader and its private contextToJobNames field, then review the JobLoader interface and surrounding configuration support classes to understand the intended public access point. The issue is done when callers can locate job contexts and their job names without reflection for the remote step execution use case.