Springbatch failing | Why the transaction was completed automatically

I have meet some question which i can not understand; Why the transaction was completed automatically without any rollback code or commit code

This is a sample demo

@Component("sampleDemo")
@Scope("step")
public class sampleDemo implements Tasklet {
    @Autowired
    private PlatformTransactionManager transactionManager;

    @Override
    public RepeatStatus execute(StepContribution stepContribution, ChunkContext chunkContext) throws Exception {
        TransactionStatus status = null;
        try {
            while((testDto = dbReader01.read()) != null) {
                if (status == null || status.isCompleted()) {
                    status = transactionManager.getTransaction(definition);
                }
                ....
                if(someCheck) {
                    return RepeatStatus.FINISHED;
                }
                if (status != null && !status.isCompleted()) {
                    transactionManager.commit(status);
                }
            }
        } catch {
            ...
        } finally {
            dbReader01.close();
        }
        return RepeatStatus.FINISHED;;
    }
}

When this someCheck was true, this jobs was return with an unfinished transaction.so that,it had some errcode like these

org.springframework.transaction.TransactionSystemException: Could not roll back JDBC transaction; nested exception is java.sql.SQLException: Connection is null.
    at org.springframework.jdbc.datasource.DataSourceTransactionManager.doRollback(DataSourceTransactionManager.java:347)
    at org.springframework.transaction.support.AbstractPlatformTransactionManager.doRollbackOnCommitException(AbstractPlatformTransactionManager.java:895)
    at org.springframework.transaction.support.AbstractPlatformTransactionManager.processCommit(AbstractPlatformTransactionManager.java:767)
    at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:714)
    at org.springframework.transaction.support.TransactionTemplate.execute(TransactionTemplate.java:152)
    at org.springframework.batch.core.step.tasklet.TaskletStep$2.doInChunkContext(TaskletStep.java:273)
    at org.springframework.batch.core.scope.context.StepContextRepeatCallback.doInIteration(StepContextRepeatCallback.java:82)
    at org.springframework.batch.repeat.support.RepeatTemplate.getNextResult(RepeatTemplate.java:375)
    at org.springframework.batch.repeat.support.RepeatTemplate.executeInternal(RepeatTemplate.java:215)
    at org.springframework.batch.repeat.support.RepeatTemplate.iterate(RepeatTemplate.java:145)
    at org.springframework.batch.core.step.tasklet.TaskletStep.doExecute(TaskletStep.java:258)
    at org.springframework.batch.core.step.AbstractStep.execute(AbstractStep.java:203)
    at org.springframework.batch.core.job.SimpleStepHandler.handleStep(SimpleStepHandler.java:148)
    at org.springframework.batch.core.job.flow.JobFlowExecutor.executeStep(JobFlowExecutor.java:68)
    at org.springframework.batch.core.job.flow.support.state.StepState.handle(StepState.java:67)
    at org.springframework.batch.core.job.flow.support.SimpleFlow.resume(SimpleFlow.java:169)
    at org.springframework.batch.core.job.flow.support.SimpleFlow.start(SimpleFlow.java:144)
    at org.springframework.batch.core.job.flow.FlowJob.doExecute(FlowJob.java:136)
    at org.springframework.batch.core.job.AbstractJob.execute(AbstractJob.java:313)
    at org.springframework.batch.core.launch.support.SimpleJobLauncher$1.run(SimpleJobLauncher.java:144)
    at org.springframework.core.task.SyncTaskExecutor.execute(SyncTaskExecutor.java:50)
    at org.springframework.batch.core.launch.support.SimpleJobLauncher.run(SimpleJobLauncher.java:137)
    at org.springframework.batch.test.JobLauncherTestUtils.launchJob(JobLauncherTestUtils.java:152)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:50)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
    at mockit.integration.junit4.internal.JUnit4TestRunnerDecorator.executeTestMethod(JUnit4TestRunnerDecorator.java:156)
    at mockit.integration.junit4.internal.JUnit4TestRunnerDecorator.invokeExplosively(JUnit4TestRunnerDecorator.java:65)
    at mockit.integration.junit4.internal.MockFrameworkMethod.invokeExplosively(MockFrameworkMethod.java:37)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.springframework.test.context.junit4.statements.RunBeforeTestExecutionCallbacks.evaluate(RunBeforeTestExecutionCallbacks.java:73)
    at org.springframework.test.context.junit4.statements.RunAfterTestExecutionCallbacks.evaluate(RunAfterTestExecutionCallbacks.java:83)
    at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:26)
    at org.springframework.test.context.junit4.statements.RunBeforeTestMethodCallbacks.evaluate(RunBeforeTestMethodCallbacks.java:75)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
    at org.springframework.test.context.junit4.statements.RunAfterTestMethodCallbacks.evaluate(RunAfterTestMethodCallbacks.java:86)
    at org.springframework.test.context.junit4.statements.SpringRepeat.evaluate(SpringRepeat.java:84)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:251)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:97)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.junit.internal.runners.statements.RunAfters.evaluate(RunAfters.java:27)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:70)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:190)
    at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:89)
    at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:41)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:541)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:763)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:463)
    at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:209)
Caused by: java.sql.SQLException: Connection is null.
    at org.apache.commons.dbcp2.DelegatingConnection.checkOpen(DelegatingConnection.java:607)
    at org.apache.commons.dbcp2.DelegatingConnection.rollback(DelegatingConnection.java:482)
    at org.springframework.jdbc.datasource.DataSourceTransactionManager.doRollback(DataSourceTransactionManager.java:344)
    ... 61 common frames omitted

BUT, if i change the position where the transaction is initialized,such like this demo

@Component("sampleDemo")
@Scope("step")
public class sampleDemo implements Tasklet {
    @Autowired
    private PlatformTransactionManager transactionManager;

    @Override
    public RepeatStatus execute(StepContribution stepContribution, ChunkContext chunkContext) throws Exception {
        TransactionStatus status = null;
        try {
            if (status == null || status.isCompleted()) {
                status = transactionManager.getTransaction(definition);
            }
            while((testDto = dbReader01.read()) != null) {
                ....
                if(someCheck) {
                    return RepeatStatus.FINISHED;
                }
                if (status != null && !status.isCompleted()) {
                    transactionManager.commit(status);
                }
            }
        } catch {
            ...
        } finally {
            dbReader01.close();
        }
        return RepeatStatus.FINISHED;;
    }
}

It will be completed without any errorcodes.I don't understand.why this transaction was closed automatically.

Tasklet#execute is already executed in a transaction driven by Spring Batch. This is explained in the documentation here. So you don't need to create another transaction inside that method.