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Inheriting from a Parent Step

If a group of Steps share similar configurations, then it may be helpful to define a “parent” Step from which the concrete Steps may inherit properties. Similar to class inheritance in Java, the “child” Step combines its elements and attributes with the parent’s. The child also overrides any of the parent’s Steps.

In the following example, the Step, concreteStep1, inherits from parentStep. It is instantiated with itemReader, itemProcessor, itemWriter, startLimit=5, and allowStartIfComplete=true. Additionally, the commitInterval is 5, since it is overridden by the concreteStep1 Step, as the following example shows:

<step id="parentStep">
    <tasklet allow-start-if-complete="true">
        <chunk reader="itemReader" writer="itemWriter" commit-interval="10"/>
    </tasklet>
</step>

<step id="concreteStep1" parent="parentStep">
    <tasklet start-limit="5">
        <chunk processor="itemProcessor" commit-interval="5"/>
    </tasklet>
</step>

The id attribute is still required on the step within the job element. This is for two reasons:

  • The id is used as the step name when persisting the StepExecution. If the same standalone step is referenced in more than one step in the job, an error occurs.

  • When creating job flows, as described later in this chapter, the next attribute should refer to the step in the flow, not the standalone step.

Abstract Step

Sometimes, it may be necessary to define a parent Step that is not a complete Step configuration. If, for instance, the reader, writer, and tasklet attributes are left off of a Step configuration, then initialization fails. If a parent must be defined without one or more of these properties, the abstract attribute should be used. An abstract Step is only extended, never instantiated.

In the following example, the Step (abstractParentStep) would not be instantiated if it were not declared to be abstract. The Step, (concreteStep2) has itemReader, itemWriter, and commit-interval=10.

<step id="abstractParentStep" abstract="true">
    <tasklet>
        <chunk commit-interval="10"/>
    </tasklet>
</step>

<step id="concreteStep2" parent="abstractParentStep">
    <tasklet>
        <chunk reader="itemReader" writer="itemWriter"/>
    </tasklet>
</step>

Merging Lists

Some of the configurable elements on Steps are lists, such as the <listeners/> element. If both the parent and child Steps declare a <listeners/> element, the child’s list overrides the parent’s. To allow a child to add additional listeners to the list defined by the parent, every list element has a merge attribute. If the element specifies that merge="true", then the child’s list is combined with the parent’s instead of overriding it.

In the following example, the Step "concreteStep3", is created with two listeners: listenerOne and listenerTwo:

<step id="listenersParentStep" abstract="true">
    <listeners>
        <listener ref="listenerOne"/>
    <listeners>
</step>

<step id="concreteStep3" parent="listenersParentStep">
    <tasklet>
        <chunk reader="itemReader" writer="itemWriter" commit-interval="5"/>
    </tasklet>
    <listeners merge="true">
        <listener ref="listenerTwo"/>
    <listeners>
</step>