ItemReaderAdapter to Read Custom DAO - spring

Question

I have a requirement to use the spring batch to read the existing logic retrieved from database and the existing target object method returns me the list of objects after querying from database.
So I have a task to read this in chunks. When I see the list size from existing code, I see it is around 15000 but on implementing the spring batch, I wanted to read in chunks of 100 and this was not happening through ItemReaderAdapter.
Below code snippets would give you an idea of the issue I am mentioning. So would this be possible from Spring Batch. I notice the Delegating Job Sample Spring Example, but the service there returns the object on every chunk and not the total list object.
Please advice
Job.xml
<step id="firststep">
<tasklet>
<chunk reader="myreader" writer="mywriter" commit-interval="100" />
</tasklet>
</step>
<job id="firstjob" incrementer="idIncrementer">
<step id="step1" parent="firststep" />
</job>
<beans:bean id="myreader" class="org.springframework.batch.item.adapter.ItemReaderAdapter">
<beans:property name="targetObject" ref="readerService" />
<beans:property name="targetMethod" value="getCustomer" />
</beans:bean>
<beans:bean id="readerService" class="com.sh.java.ReaderService">
</beans:bean>
ReaderService.java
public class ReaderService {
public List<CustomItem> getCustomer() throws Exception {
/*
* code to get database instances
*/
List<CustomItem> customList = dao.getCustomers(date);
System.out.println("Customer List Size: " + customList.size()); //Here it is 15K
return (List<CustomItem>) customList;
}
}

Answer 1

Before all: read a 15K List<> of objects might impact (in negative) performance; check if you can write a custom SQL query and use a JDBC/Hibernate cursor item reader instead.
What you are trying to do is not possible using ItemReaderAdapter (it wasn't designed to read a chunk of object) but you can achieve the same result writing a custom ItemReader extending AbstractItemCountingItemStreamItemReader to inherit ItemStream capabilities and override the abstract or no-op methods; especially in:
doOpen() call your readerService.getCustomers() and save List<> in class variables,
in doRead() read next item - from List<> read in doOpen() - using built-in index stored in ExecutionContext

Answer 2

#Bellabax,
Doing the way you suggested also reads the entire database records in doOpen, however, from the list retrieved from doOpen, the reader reads it in a chunks. Pls advise
CustomerReader.java
public class CustomerReader extends
AbstractItemCountingItemStreamItemReader<Customer>
{
List<Customer> customerList;
public CustomerReader ()
{
}
#Override
protected void doClose() throws Exception
{
customerList.clear();
setMaxItemCount(0);
setCurrentItemCount(0);
}
#Override
protected void doOpen() throws Exception
{
customList = dao.getCustomers(date);
System.out.println("Customer List Size: "+list.size()); //This still prints 15k
setMaxItemCount(list.size());
}
#Override
protected Customer doRead() throws Exception
{
//Here reading 15K in chunks!
Customer customer = customList.get(getCurrentItemCount() - 1);
return customer;
}
}