parallel sub flow list is not actually executed in spring batch

Question

I wrote a spring batch program to create subflow list. The number of sub flows will be dynamically decided during the runtime. The sub flows will be run in parallel. The sample code is pasted below:

@Configuration
public class JobConfiguration {

@Bean
public Job myJob() {

    return jobBuilderFactory.get("myJob")
            .preventRestart()
            .incrementer(new RunIdIncrementer())
            .start(step1())
            .next(splitStep())
            .build();
}

@Bean
public Step splitStep() {

    return stepBuilderFactory.get("splitStep")
            .flow(createSubFlow())
            .build();
}

@Bean
@StepScope
public Flow createSubFlow() {
    List<Flow> streamSubFlowList = new ArrayList<>();

    List<Stream> streamList;

    try {
        streamList = dataSourceConfig.streamMapper().getStreamList();
    } catch (Exception e) {
        logger.error("getStreamList failed for", e);
    }
    streamList.forEach(stream -> {
        long streamId = stream.getStreamId();
        String streamFlowName = "stream" + streamId + "_flow";
        Flow streamSubFlow = new FlowBuilder<Flow>(streamFlowName)
                .start(step2(streamId))
                .next(step3(streamId))
                .end();

        logger.info("streamSubFlow is created for streamId: {}", streamId);

        streamSubFlowList.add(streamSubFlow);
    });

    return new FlowBuilder<Flow>("splitStreamFlow").split(new SimpleAsyncTaskExecutor())
            .add(streamSubFlowList.toArray(new Flow[0])).build();
}



public Step step2(long streamId) {
    return stepBuilderFactory.get("step2")
            .<Slot, Slot>chunk(1)
            .reader(reader2(streamId))
            .processor(processor2)
            .writer(writer2())
            .build();
}


public Step step3(long streamId) {
    return stepBuilderFactory.get("step3")
            .<Group, Group>chunk(1)
            .reader(reader3(streamId))
            .processor(processor3)
            .writer(writer3())
            .build();
}


public JdbcCursorItemReader<Slot> reader2(Long streamId) {
        return new JdbcCursorItemReaderBuilder<Slot>()
                        .dataSource(dataSourceConfig.dataSource())
                        .name("reader2")
                        .sql(Constants.QUERY2_SQL)
                        .preparedStatementSetter(preparedStatementSetter(streamId))
                        .rowMapper(new reader2RowMapper())
                        .build();
}

public JdbcCursorItemReader<Slot> reader3(Long streamId) {
        return new JdbcCursorItemReaderBuilder<Group>()
                        .dataSource(dataSourceConfig.dataSource())
                        .name("reader3")
                        .sql(Constants.QUERY3_SQL)
                        .preparedStatementSetter(preparedStatementSetter(streamId))
                        .rowMapper(new reader3RowMapper())
                        .build();
}


@Bean
public ItemWriter<Slot> writer2 () {

    JdbcBatchItemWriter<ForecastSlot> writer2 = new JdbcBatchItemWriter<>();
    writer2.setDataSource(dataSourceConfig.dataSource());
    writer2.setSql(Constants.INSERT_SLOT_SQL);
    writer2.setItemSqlParameterSourceProvider(new BeanPropertyItemSqlParameterSourceProvider<Slot>());

    return writer2;
}

@Bean
public ItemWriter<Group> writer3 () {

    JdbcBatchItemWriter<ForecastSkillGroup> writer3 = new JdbcBatchItemWriter<>();
    writer3.setDataSource(dataSourceConfig.dataSource());
    writer3.setSql(Constants.INSERT_GROUP_SQL);
    writer3.setItemSqlParameterSourceProvider(new BeanPropertyItemSqlParameterSourceProvider<Group>());

    return writer3;
}

}

After I run the batch, the batch completes successfully. It seems 3 sub flows have been created and run in parallel. However, the database is not updated. I set the break points in rowmapper, but it does not trigger. I verified there is data in database, so rowmapper should be invoked.

2018-Oct-26 20:52:27.985 INFO  [main] c.t.w.p.b.c. - streamSubFlow is created for streamId: 3

2018-Oct-26 20:52:28.209 INFO  [main] c.t.w.p.b.c. - streamSubFlow is created for streamId: 4

2018-Oct-26 20:52:28.434 INFO  [main] c.t.w.p.b.c. - streamSubFlow is created for streamId: 5

2018-Oct-26 20:52:42.835 INFO  [SimpleAsyncTaskExecutor-4] o.s.b.c.j.SimpleStepHandler - Executing step: [step2]

2018-Oct-26 20:52:42.951 INFO  [SimpleAsyncTaskExecutor-17] o.s.b.c.j.SimpleStepHandler - Executing step: [step2]

2018-Oct-26 20:52:43.097 INFO  [SimpleAsyncTaskExecutor-7] o.s.b.c.j.SimpleStepHandler - Executing step: [step2]

2018-Oct-26 20:52:58.989 INFO  [SimpleAsyncTaskExecutor-17] o.s.b.c.j.SimpleStepHandler - Executing step: [step3]

2018-Oct-26 20:52:59.010 INFO  [SimpleAsyncTaskExecutor-4] o.s.b.c.j.SimpleStepHandler - Executing step: [step3]

2018-Oct-26 20:52:59.142 INFO  [SimpleAsyncTaskExecutor-38] o.s.b.c.j.SimpleStepHandler - Executing step: [step3]

Any idea why the subflows did not update the database. Thanks in advance!