源码解析springbatch的job运行机制

batch_job_instance
batch_job_execution
batch_job_execution_context
batch_job_execution_params
batch_step_execution
batch_step_execution_context
batch_job_seq
batch_step_execution_seq
batch_job_execution_seq

@Autowired
    @Qualifier("billJob")
    private Job job;
    
    @Test
    public void billJob() throws Exception {
        JobParameters jobParameters = new JobParametersBuilder()
                .addLong("currentTimeMillis", System.currentTimeMillis())
                .addString("batchNo","2022080402")
                .toJobParameters();
        JobExecution result = jobLauncher.run(job, jobParameters);
        System.out.println(result.toString());
 
        Thread.sleep(6000);
    }

 <!-- 账单作业 -->
    <batch:job id="billJob">
        <batch:step id="billStep">
            <batch:tasklet transaction-manager="transactionManager">
                <batch:chunk reader="csvItemReader" writer="csvItemWriter" processor="creditBillProcessor" commit-interval="3">
                </batch:chunk>
            </batch:tasklet>
        </batch:step>
    </batch:job>

// 简化部分代码（参数检查、log日志）
@Override
public JobExecution run(final Job job, final JobParameters jobParameters){
	final JobExecution jobExecution;
	JobExecution lastExecution = jobRepository.getLastJobExecution(job.getName(), jobParameters);
       // 上次执行存在，说明本次请求是重启job，先做检查
	if (lastExecution != null) {
		if (!job.isRestartable()) {
			throw new JobRestartException("JobInstance already exists and is not restartable");
		}
		
		for (StepExecution execution : lastExecution.getStepExecutions()) {
			BatchStatus status = execution.getStatus();
			if (status.isRunning() || status == BatchStatus.STOPPING) {
				throw new JobExecutionAlreadyRunningException("A job execution for this job is already running: "
						+ lastExecution);
			} else if (status == BatchStatus.UNKNOWN) {
				throw new JobRestartException(
						"Cannot restart step [" + execution.getStepName() + "] from UNKNOWN status. ");
			}
		}
	}
	// Check jobParameters
	job.getJobParametersValidator().validate(jobParameters);
       // 创建JobExecution 同一个job+参数，只能有一个Execution执行器
	jobExecution = jobRepository.createJobExecution(job.getName(), jobParameters);
	try {
           // SyncTaskExecutor 看似是异步，实际是同步执行（可扩展）
		taskExecutor.execute(new Runnable() {
			@Override
			public void run() {
				try {
                       // 关键入口，请看[org.springframework.batch.core.job.AbstractJob#execute]
					job.execute(jobExecution);
					if (logger.isInfoEnabled()) {
						Duration jobExecutionDuration = BatchMetrics.calculateDuration(jobExecution.getStartTime(), jobExecution.getEndTime());
					}
				}
				catch (Throwable t) {
					rethrow(t);
				}
			}
			private void rethrow(Throwable t) {
                   // 省略各类抛异常
				throw new IllegalStateException(t);
			}
		});
	}
	catch (TaskRejectedException e) {
        // 更新job_execution的运行状态
		jobExecution.upgradeStatus(BatchStatus.FaiLED);
		if (jobExecution.getExitStatus().equals(ExitStatus.UNKNOWN)) {
			jobExecution.setExitStatus(ExitStatus.FAILED.addExitDescription(e));
		}
		jobRepository.update(jobExecution);
	}
	return jobExecution;
}

@Ovrride
public final void execute(JobExecution execution) {
 
    // 同步控制器，防并发执行
    JobSynchronizationManager.reGISter(execution);
    // 计时器，记录耗时
    LongTaskTimer longTaskTimer = BatchMetrics.createLongTaskTimer("job.active", "Active jobs",
            Tag.of("name", execution.getJobInstance().getJobName()));
    LongTaskTimer.Sample longTaskTimerSample = longTaskTimer.start();
    Timer.Sample timerSample = BatchMetrics.createTimerSample();
 
    try {
        // 参数再次进行校验
        jobParametersValidator.validate(execution.getJobParameters());
 
        if (execution.getStatus() != BatchStatus.STOPPING) {
 
            // 更新db中任务状态
            execution.setStartTime(new Date());
            updateStatus(execution, BatchStatus.STARTED);
            // 回调所有listener的beforeJob方法
            listener.beforeJob(execution);
 
            try {
                doExecute(execution);
            } catch (RepeatException e) {
                throw e.getCause(); // 搞不懂这里包一个RepeatException 有啥用
            }
        } else {
            // 任务状态时BatchStatus.STOPPING，说明任务已经停止，直接改成STOPPED
            // The job was already stopped before we even Got this far. Deal
            // with it in the same way as any other interruption.
            execution.setStatus(BatchStatus.STOPPED);
            execution.setExitStatus(ExitStatus.COMPLETED);
        }
 
    } catch (JobInterruptedException e) {
        // 任务被打断 STOPPED
        execution.setExitStatus(getDefaultExitStatusForFailure(e, execution));
        execution.setStatus(BatchStatus.max(BatchStatus.STOPPED, e.getStatus()));
        execution.addFailureException(e);
    } catch (Throwable t) {
        // 其他原因失败 FAILED
        logger.error("Encountered fatal error executing job", t);
        execution.setExitStatus(getDefaultExitStatusForFailure(t, execution));
        execution.setStatus(BatchStatus.FAILED);
        execution.addFailureException(t);
    } finally {
        try {
            if (execution.getStatus().isLessThanOrEqualTo(BatchStatus.STOPPED)
                    && execution.getStepExecutions().isEmpty()) {
                ExitStatus exitStatus = execution.getExitStatus();
                ExitStatus newExitStatus =
                        ExitStatus.NOOP.addExitDescription("All steps already completed or no steps configured for this job.");
                execution.setExitStatus(exitStatus.and(newExitStatus));
            }
 
            // 计时器 计算总耗时
            timerSample.stop(BatchMetrics.createTimer("job", "Job duration",
                    Tag.of("name", execution.getJobInstance().getJobName()),
                    Tag.of("status", execution.getExitStatus().getExitCode())
            ));
            longTaskTimerSample.stop();
            execution.setEndTime(new Date());
 
            try {
                // 回调所有listener的afterJob方法  调用失败也不影响任务完成
                listener.afterJob(execution);
            } catch (Exception e) {
                logger.error("Exception encountered in afterJob callback", e);
            }
            // 写入db
            jobRepository.update(execution);
        } finally {
            // 释放控制
            JobSynchronizationManager.release();
        }
 
    }
 
}

org.springframework.batch.core.step.item.SimpleChunkProcessor#process
 
	@Nullable
	@Override
	public RepeatStatus execute(StepContribution contribution, ChunkContext chunkContext) throws Exception {
 
		@SuppressWarnings("unchecked")
		Chunk<I> inputs = (Chunk<I>) chunkContext.getAttribute(INPUTS_KEY);
		if (inputs == null) {
			inputs = chunkProvider.provide(contribution);
			if (buffering) {
				chunkContext.setAttribute(INPUTS_KEY, inputs);
			}
		}
 
		chunkProcessor.process(contribution, inputs);
		chunkProvider.postProcess(contribution, inputs);
 
		// Allow a message coming back from the processor to say that we
		// are not done yet
		if (inputs.isBusy()) {
			logger.debug("Inputs still busy");
			return RepeatStatus.CONTINUABLE;
		}
 
		chunkContext.removeAttribute(INPUTS_KEY);
		chunkContext.setComplete();
 
		if (logger.isDebugEnabled()) {
			logger.debug("Inputs not busy, ended: " + inputs.isEnd());
		}
		return RepeatStatus.continueIf(!inputs.isEnd());
 
	}

 // 忽略无关代码...
	@Override
	public final void process(StepContribution contribution, Chunk<I> inputs) throws Exception {
 
		// 输入为空，直接返回If there is no input we don't have to do anything more
		if (isComplete(inputs)) {
			return;
		}
 
		// Make the transfORMation, calling remove() on the inputs iterator if
		// any items are filtered. Might throw exception and cause rollback.
		Chunk<O> outputs = transform(contribution, inputs);
 
		// Adjust the filter count based on available data
		contribution.incrementFilterCount(getFilterCount(inputs, outputs));
 
		// Adjust the outputs if necessary for housekeeping purposes, and then
		// write them out...
		write(contribution, inputs, getAdjustedOutputs(inputs, outputs));
 
	}
 
    // 遍历items，逐个item调用processor
	protected Chunk<O> transform(StepContribution contribution, Chunk<I> inputs) throws Exception {
		Chunk<O> outputs = new Chunk<>();
		for (Chunk<I>.ChunkIterator iterator = inputs.iterator(); iterator.hasNext();) {
			final I item = iterator.next();
			O output;
			String status = BatchMetrics.STATUS_SUCCESS;
			try {
				output = doProcess(item);
			}
			catch (Exception e) {
				
				inputs.clear();
				status = BatchMetrics.STATUS_FAILURE;
				throw e;
			}
			if (output != null) {
				outputs.add(output);
			}
			else {
				iterator.remove();
			}
		}
		return outputs;
	}

// org.springframework.batch.core.step.tasklet.TaskletStep#doExecute
    result = new TransactionTemplate(transactionManager, transactionAttribute)
				    .execute(new ChunkTransactionCallback(chunkContext, semaphore));
 
    // 事务启用过程
    // org.springframework.transaction.support.TransactionTemplate#execute
	@Override
	@Nullable
	public <T> T execute(TransactionCallback<T> action) throws TransactionException {
		Assert.state(this.transactionManager != null, "No PlatformTransactionManager set");
 
		if (this.transactionManager instanceof CallbackPreferringPlatformTransactionManager) {
			return ((CallbackPreferringPlatformTransactionManager) this.transactionManager).execute(this, action);
		}
		else {
			TransactionStatus status = this.transactionManager.getTransaction(this);
			T result;
			try {
				result = action.doInTransaction(status);
			}
			catch (RuntimeException | Error ex) {
				// Transactional code threw application exception -> rollback
				rollbackOnException(status, ex);
				throw ex;
			}
			catch (Throwable ex) {
				// Transactional code threw unexpected exception -> rollback
				rollbackOnException(status, ex);
				throw new UndeclaredThrowableException(ex, "TransactionCallback threw undeclared checked exception");
			}
			this.transactionManager.commit(status);
			return result;
		}
	}

private RepeatStatus executeInternal(final RepeatCallback callback) {
	// Reset the termination policy if there is one...
       // 此处会调用completionPolicy.start方法，更新chunk的计数器
	RepeatContext context = start();
	// Make sure if we are already marked complete before we start then no processing takes place.
       // 通过running字段来判断是否继续处理next
	boolean running = !isMarkedComplete(context);
       // 省略listeners处理....
	// Return value, default is to allow continued processing.
	RepeatStatus result = RepeatStatus.CONTINUABLE;
	RepeatInternalState state = createInternalState(context);
	try {
		while (running) {
			
               // 省略listeners处理....
			if (running) {
				try {
                       // callback是实际处理方法，类似函数式编程
					result = getNextResult(context, callback, state);
					executeAfterInterceptors(context, result);
				}
				catch (Throwable throwable) {
					doHandle(throwable, context, deferred);
				}
                   // 检查当前chunk是否处理完，决策出是否继续处理下一条item
				// N.B. the order may be important here:
				if (isComplete(context, result) || isMarkedComplete(context) || !deferred.isEmpty() {
					running = false;
				}
			}
		}
		result = result.and(waitForResults(state));
           // 省略throwables处理....
		// Explicitly drop any references to internal state...
		state = null;
	}
	finally {
           // 省略代码...
	}
	return result;
}

// 节选部分代码
oldVersion = new StepExecution(stepExecution.getStepName(), stepExecution.getJobExecution());
copy(stepExecution, oldVersion);
 
private void copy(final StepExecution source, final StepExecution target) {
	target.setVersion(source.getVersion());
	target.setWriteCount(source.getWriteCount());
	target.setFilterCount(source.getFilterCount());
	target.setCommitCount(source.getCommitCount());
	target.setExecutionContext(new ExecutionContext(source.getExecutionContext()));
}

// 省略无关代码
try {
	try {
        // 执行w-p-r模型方法
		result = tasklet.execute(contribution, chunkContext);
		if (result == null) {
			result = RepeatStatus.FINISHED;
		}
	}
	catch (Exception e) {
		// 省略...
	}
}
finally {
	// If the step operations are asynchronous then we need to synchronize changes to the step execution (at a
	// minimum). Take the lock *before* changing the step execution.
	try {
        // 获取锁
		semaphore.acquire();
		locked = true;
	}
	catch (InterruptedException e) {
		logger.error("Thread interrupted while locking for repository update");
		stepExecution.setStatus(BatchStatus.STOPPED);
		stepExecution.setTerminateOnly();
		Thread.currentThread().interrupt();
	}
	stepExecution.apply(contribution);
}
stepExecutionUpdated = true;
stream.update(stepExecution.getExecutionContext());
try {
    // 更新上下文、DB中的状态
	// Going to attempt a commit. If it fails this flag will stay false and we can use that later.
	getJobRepository().updateExecutionContext(stepExecution);
	stepExecution.incrementCommitCount();
	getJobRepository().update(stepExecution);
}
catch (Exception e) {
	// If we get to here there was a problem saving the step execution and we have to fail.
	String msg = "JobRepository failure forcing rollback";
	logger.error(msg, e);
	throw new FatalStepExecutionException(msg, e);
}