When TaskExecutor concurrencyLimit is less than the number of flow steps, job will be blocked - spring-batch

Question

I'm using Spring Batch 4.1.2.RELEASE. I have a problem about Parallel Steps
(using Split Flow). When SplitFlow's concurrencyLimit (or ThreadPoolTaskExecutor.corePoolSize) is less than the number of A split flow's steps. the job never stops, and no Exception thrown.
I know the solution is to increase the concurrencyLimit or decrease the number of steps in each flow. But I want to make sure whether there is a problem with job's TaskExecutor and task's TaskExecutor or my code is wrong.
Without consideration of SplitFlow, I found that if if the number of Jobs (as simple as possible) submitted to jobLauncher is more than its TaskExecutor.corePoolSize(assume 1), the job will be executed one by one. This is the expected result.
#Bean
public TaskExecutor taskExecutor() {
SimpleAsyncTaskExecutor executor = new SimpleAsyncTaskExecutor("tsk-Exec-");
executor.setConcurrencyLimit(2);
return executor;
}
#SpringBootApplication
#EnableBatchProcessing
#EnableWebMvc
public class BatchJobApplication {
public static void main(String[] args) {
SpringApplication.run(BatchJobApplication.class, args);
}
}
The code below create a single job, contains a split flow with 4 tasklet step.
#Autowired
private TaskExecutor taskExecutor;
public JobExecution experiment(Integer flowId) {
String dateFormat = LocalDate.now(ZoneId.of("+8")).format(DateTimeFormatter.BASIC_ISO_DATE);
JobBuilder job1 = this.jobBuilderFactory.get("Job_" + flowId + "_" + dateFormat);
List<TaskletStep> taskletSteps = Lists.newArrayList();
for (int i = 0; i < 4; i++) {
taskletSteps.add(this.stepBuilderFactory.get("step:" + i).tasklet(
(contribution, chunkContext) -> {
Thread.sleep(3000);
return RepeatStatus.FINISHED;
}).build());
}
JobExecution run = null;
FlowBuilder.SplitBuilder<SimpleFlow> splitFlow = new FlowBuilder<SimpleFlow>("splitFlow").split(taskExecutor);
FlowBuilder<SimpleFlow> lastFlowNode = null;
for (TaskletStep taskletStep : taskletSteps) {
SimpleFlow singleNode = new FlowBuilder<SimpleFlow>("async-fw-" + taskletStep.getName()).start(taskletStep).build();
lastFlowNode = splitFlow.add(singleNode);
}
Job build = job1.start(lastFlowNode.end()).build().build();
JobParametersBuilder jobParametersBuilder = new JobParametersBuilder();
jobParametersBuilder.addDate("parameterGenerated", new Date());
try {
run = jobLauncher.run(build, jobParametersBuilder.toJobParameters());
} catch (JobExecutionAlreadyRunningException e) {
e.printStackTrace();
} catch (JobRestartException e) {
e.printStackTrace();
} catch (JobInstanceAlreadyCompleteException e) {
e.printStackTrace();
} catch (JobParametersInvalidException e) {
e.printStackTrace();
}
return run;
}
Now It's blocked.
2019-07-29 18:08:10.321 INFO 24416 --- [ job-Exec-1] o.s.b.c.l.support.SimpleJobLauncher : Job: [FlowJob: [name=Job_2124_20190729]] launched with the following parameters: [{parameterGenerated=1564394890193}]
2019-07-29 18:08:13.392 DEBUG 24416 --- [ job-Exec-1] cTaskExecutor$ConcurrencyThrottleAdapter : Entering throttle at concurrency count 0
2019-07-29 18:08:13.393 DEBUG 24416 --- [ job-Exec-1] cTaskExecutor$ConcurrencyThrottleAdapter : Entering throttle at concurrency count 1
2019-07-29 18:08:13.393 DEBUG 24416 --- [ tsk-Exec-2] cTaskExecutor$ConcurrencyThrottleAdapter : Concurrency count 2 has reached limit 2 - blocking
2019-07-29 18:08:13.425 INFO 24416 --- [ tsk-Exec-1] o.s.batch.core.job.SimpleStepHandler : Executing step: [step:3]
2019-07-29 18:08:16.466 DEBUG 24416 --- [ tsk-Exec-1] cTaskExecutor$ConcurrencyThrottleAdapter : Returning from throttle at concurrency count 1
2019-07-29 18:08:16.466 DEBUG 24416 --- [ tsk-Exec-2] cTaskExecutor$ConcurrencyThrottleAdapter : Entering throttle at concurrency count 1
2019-07-29 18:08:16.466 DEBUG 24416 --- [ tsk-Exec-2] cTaskExecutor$ConcurrencyThrottleAdapter : Concurrency count 2 has reached limit 2 - blocking
2019-07-29 18:08:16.484 INFO 24416 --- [ tsk-Exec-3] o.s.batch.core.job.SimpleStepHandler : Executing step: [step:2]
2019-07-29 18:08:19.505 DEBUG 24416 --- [ tsk-Exec-3] cTaskExecutor$ConcurrencyThrottleAdapter : Returning from throttle at concurrency count 1
2019-07-29 18:08:19.505 DEBUG 24416 --- [ tsk-Exec-2] cTaskExecutor$ConcurrencyThrottleAdapter : Entering throttle at concurrency count 1
2019-07-29 18:08:19.506 DEBUG 24416 --- [ tsk-Exec-4] cTaskExecutor$ConcurrencyThrottleAdapter : Concurrency count 2 has reached limit 2 - blocking

Answer 1

I think I have found the answer yesterday evening.
When The TaskExecutor in SplitFlow with a few concurrencyLimit or ThreadPoolTaskExecutor.corePoolSize. According to the code, It's very likely to happen that All the Thread is blocked by future.get(), but no Thread available has chance to run taskletStep.
//SplitState.java:114
results.add(task.get());
In addition, the Threads created by TaskExecutor in JobLauncher doesn't have to wait results of future. So TaskExcutor always have enough free Thread to accept jobs, no one need to wait any condition.