Spring Batch: pass data between reader and writer - spring

Question

I would like to get data in the Writer that I've set in the Reader of my step. I know about ExecutionContexts (step and job) and about ExecutionContextPromotionListener via http://docs.spring.io/spring-batch/trunk/reference/html/patterns.html#passingDataToFutureSteps
The problem is that in Writer I'm retrieving a null value of 'npag'.
Line on ItemWriter:
LOG.info("INSIDE WRITE, NPAG: " + nPag);
I've being doing some workarounds without luck, looking answer for other similar questions... Any help? thanks!
Here's my code:
READER
#Component
public class LCItemReader implements ItemReader<String> {
private StepExecution stepExecution;
private int nPag = 1;
#Override
public String read() throws CustomItemReaderException {
ExecutionContext stepContext = this.stepExecution.getExecutionContext();
stepContext.put("npag", nPag);
nPag++;
return "content";
}
#BeforeStep
public void saveStepExecution(StepExecution stepExecution) {
this.stepExecution = stepExecution;
}
}
WRITER
#Component
#StepScope
public class LCItemWriter implements ItemWriter<String> {
private String nPag;
#Override
public void write(List<? extends String> continguts) throws Exception {
try {
LOG.info("INSIDE WRITE, NPAG: " + nPag);
} catch (Throwable ex) {
LOG.error("Error: " + ex.getMessage());
}
}
#BeforeStep
public void retrieveInterstepData(StepExecution stepExecution) {
JobExecution jobExecution = stepExecution.getJobExecution();
ExecutionContext jobContext = jobExecution.getExecutionContext();
this.nPag = jobContext.get("npag").toString();
}
}
JOB/STEP BATCH CONFIG
#Bean
public Job lCJob() {
return jobs.get("lCJob")
.listener(jobListener)
.start(lCStep())
.build();
}
#Bean
public Step lCStep() {
return steps.get("lCStep")
.<String, String>chunk(1)
.reader(lCItemReader)
.processor(lCProcessor)
.writer(lCItemWriter)
.listener(promotionListener())
.build();
}
LISTENER
#Bean
public ExecutionContextPromotionListener promotionListener() {
ExecutionContextPromotionListener executionContextPromotionListener = new ExecutionContextPromotionListener();
executionContextPromotionListener.setKeys(new String[]{"npag"});
return executionContextPromotionListener;
}

Answer 1

The ExecutionContextPromotionListener specifically states that it works at the end of a step so that would be after the writer executes. So the promotion I think you are counting on does not occur when you think it does.
If i were you I would set it in the step context and get it from the step if you need the value with in a single step. Otherwise I would set it to the job context.
The other aspect is the #BeforeStep. That marks a method for executing before the step context exists. The way you are setting the nPag value in the reader would be after the step had started executing.

Answer 2

You are trying to read the value for nPag even before it is set in the reader, ending up with a default value which is null. You need to read the value on nPag at the time of logging from the execution context directly. You can keep a reference to the jobContext. Try this
#Component
#StepScope
public class LCItemWriter implements ItemWriter<String> {
private String nPag;
private ExecutionContext jobContext;
#Override
public void write(List<? extends String> continguts) throws Exception {
try {
this.nPag = jobContext.get("npag").toString();
LOG.info("INSIDE WRITE, NPAG: " + nPag);
} catch (Throwable ex) {
LOG.error("Error: " + ex.getMessage());
}
}
#BeforeStep
public void retrieveInterstepData(StepExecution stepExecution) {
JobExecution jobExecution = stepExecution.getJobExecution();
jobContext = jobExecution.getExecutionContext();
}
}

Answer 3

In your Reader and Writer you need to implement ItemStream interface and use ExecutionContext as member variable.Here i have given example with Processor instead of Writer but same is applicable for Writer as well .Its working fine for me and i am able to take values from reader to processor.
I have set the value in context in reader and getting the value in processor.
public class EmployeeItemReader implements ItemReader<Employee>, ItemStream {
ExecutionContext context;
#Override
public Employee read() throws Exception, UnexpectedInputException, ParseException, NonTransientResourceException {
context.put("ajay", "i am going well");
Employee emp=new Employee();
emp.setEmpId(1);
emp.setFirstName("ajay");
emp.setLastName("goswami");
return emp;
}
#Override
public void close() throws ItemStreamException {
// TODO Auto-generated method stub
}
#Override
public void open(ExecutionContext arg0) throws ItemStreamException {
context = arg0;
}
#Override
public void update(ExecutionContext arg0) throws ItemStreamException {
// TODO Auto-generated method stub
context = arg0;
}
}
My processor
public class CustomItemProcessor implements ItemProcessor<Employee,ActiveEmployee>,ItemStream{
ExecutionContext context;
#Override
public ActiveEmployee process(Employee emp) throws Exception {
//See this line
System.out.println(context.get("ajay"));
ActiveEmployee actEmp=new ActiveEmployee();
actEmp.setEmpId(emp.getEmpId());
actEmp.setFirstName(emp.getFirstName());
actEmp.setLastName(emp.getLastName());
actEmp.setAdditionalInfo("Employee is processed");
return actEmp;
}
#Override
public void close() throws ItemStreamException {
// TODO Auto-generated method stub
}
#Override
public void open(ExecutionContext arg0) throws ItemStreamException {
// TODO Auto-generated method stub
}
#Override
public void update(ExecutionContext arg0) throws ItemStreamException {
context = arg0;
}
}
Hope this helps.