Java Reference
In-Depth Information
What went wrong? All you really did was swap out the CompositeItemWriter you used in the previous
section with the new ClassifierCompositeItemWriter . The issue centers around the ItemStream
The ItemStream Interface
The ItemStream interface serves as the contract to be able to periodically store and restore state.
Consisting of three methods, open , update , and close , the ItemStream interface is implemented by any
stateful ItemReader or ItemWriter. In cases, for example, where a file is involved in the input or output,
the open method opens the required file, and the close method closes the required file. The update
method records the current state (number of records written, and so on) as each chunk is completed.
The reason for the difference between CompositeItemWriter and ClassifierCompositeItemWriter is
that CompositeItemWriter implements the org.springframework.batch.item.ItemStream interface. In
CompositeItemWriter , the open method loops through the delegate ItemWriters and calls the open
method on each of them as required. The close and update methods work the same way. However,
ClassifierCompositeItemWriter doesn't implement the ItemStream method. Because of this, the XML
file is never opened or XMLEventFactory (or the underlying XML writing) created, throwing the exception
shown in Listing 9-79.
How do you fix this error? Spring Batch provides the ability to register ItemStream s to be handled in
a step manually. If an ItemReader or ItemWriter implements ItemStream , the methods are handled for
you. If they don't (as in the case of ClassifierCompositeItemWriter ), you're required to register the
ItemReader or ItemWriter as a stream to be able to work with it if it maintains state. Listing 9-80 shows
the updated configuration for the job, registering the xmlOutputWriter as an ItemStream. 6
Listing 9-80. Updated Configuration Registering the Appropriate ItemStream for Processing
<step id="formatFileStep">
<chunk reader="customerFileReader" writer="classifierWriter"
<stream ref="xmlOutputWriter"/>
<job id="formatJob">
<step id="step1" parent="formatFileStep"/>
If you rebuild and rerun the job with the updated configuration, you see that all the records are
processed as expected.
6 You only need to register the xmlOutputWriter as a stream. JdbcBatchItemWriter doesn't implement the
ItemStream interface because it doesn't maintain any state.
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