Dynamically choose a spring-batch reader at runtime

Question

I have a spring-batch job that converts various bank statements into my app. There is a different reader for each bank statement type and only one writer for all of them. The job is very simple - read, process and write:

<batch:job id="importer" restartable="true">
    <batch:step id="import">
        <batch:tasklet>
            <batch:chunk reader="reader" writer="writer" processor="processor" commit-interval="10" />
        </batch:tasklet>
    </batch:step>
</batch:job>

Now, I would like the end user to provide a statement file as well as bank information and then choose a corresponding spring-batch reader at runtime for this job to run. Ideally it would be great if I could read-ahead in a first step and tried to "guess" the bank form statement format first. But I digress.

Is it possible to inject a reader into a job prior to running it? Or is there a better way to achieve my goal?

Answer 1

You can set the reader from an input parameter/or a propeties file:

<batch:chunk reader="#{jobParameters['reader.beanId']}" writer="writer" processor="processor" commit-interval="10" />

Also you could consider late binding of job and step attributes, here is the link from spring-batch step-scope

Answer 2

the problem is, that (File) Itemreaders are constructed before the step starts, meaning the file will be opened before the step starts too

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did you try to move the reader transformation and mapping logic to a processor ? you can use a PassThroughLineMapper for this

<bean id="dumbItemReader" class="org.springframework.batch.item.file.FlatFileItemReader" scope="step">
    <property name="resource" value="#{jobParameters['input.file']}" />
    <property name="lineMapper">
        <bean class="org.springframework.batch.item.file.mapping.PassThroughLineMapper" />
    </property>
    <property name="strict" value="true" /><!-- optional -->
</bean>

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another very clean solution might work with an extra pre-step to find out the needed reader type:

• create one step who checks the type (either read first line with a classic step configuration, or use a tasklet step)
• use programmatic flow decision and
• a step configuration for each reader type
• each reader step scoped