How to implement reader for multiple queries but same output item?

Question

For a Spring batch job, we have 2 different queries on the same table. The requirement is to have a reader that execute two queries to read data from the same table.

One way could be :

<batch:step id="firstStep" next="secondStep">
       <batch:tasklet>
          <batch:chunk reader="firstReader" writer="firstWriter" commit-        interval="2">
          </batch:chunk>
       </batch:tasklet>
    </batch:step>
    <batch:step id="secondStep" next="thirdStep">
       <batch:tasklet>
          <batch:chunk reader="secondReader" writer="secondWriter"
           commit-interval="2">
          </batch:chunk>
       </batch:tasklet>
    </batch:step>

But this demands totally another step to be defined which is a copy of the first. Is there any other way to achieve the same ? I am looking for something like MultiResourceItemReader for DB based readers that aggregates the data together.

Answer 1

You can create one view in database for different queries and call it as you call in a JdbcPagingItemReader .If thats not an option then there are different ways , but one way i have worked is as given below.Spring has other option as well, but as per developer stand point following is definitely an option.

Create two item reader ...first one is below

<!--use org.springframework.batch.item.database.JdbcCursorItemReader for  simple queries-->
<bean id="itemReader1"
    class="org.springframework.batch.item.database.JdbcPagingItemReader"
 <property name="sql"
    value=" FROM   table1" />
    .......
    <property name="rowMapper">
        <bean class="com.sjena.AccountApplicationMapper" />
    </property>
</bean>

then another reader from table 2

<bean id="itemReader2"
    class="org.springframework.batch.item.database.JdbcCursorItemReader"
<property name="sql"
    value="FROM   table2" />
    .......
    <property name="rowMapper">
        <bean class="com.sjena.AccountApplicationMapper" />
    </property>
</bean>

then delegate to your custom reader

<bean id="customItemReader" class="com.sjena.spring.reader.MyCustomReader"
    scope="step">
    <property name="itemReader1" ref="itemReader1" />
    <property name="itemReader2" ref="itemReader2" />
    <property name="pageSize" value="5" />

</bean>

And eventually use this custom reader

<job id="testJob" xmlns="http://www.springframework.org/schema/batch">
    <step id="step1">
        <tasklet>
            <chunk reader="itemReader" writer="itemWriter"
                commit-interval="1" />
        </tasklet>
    </step>
</job>

Then your class is as given below

public class MyCustomReader implements ItemReader<AccountApplicationSummary> {

int pagesize;// you may have diff pagesize for diff item readers
ItemReader<AccountApplication>  itemReader1;
ItemReader<AccountApplication>  itemReader2;


@Override
public AccountApplicationSummary read()
        throws Exception, UnexpectedInputException, ParseException, NonTransientResourceException {

    // itemReader1.setPageSize(pageSize),Be sure, itemReader is   JdbcPagingItemReader type and better to do these initiatlization in a init method (implement InitializingBean and use afterpropertyset to set them..).. 
    //Like pageSize, you can set anyproperty that you may need

    AccountApplication application1 = itemReader1.read();
    AccountApplication application2 = itemReader2.read();
    //And you have results from both tables and now you can play with it 

    AccountApplicationSummary summary = new AccountApplicationSummary();

    return summary;
}

}