Improve documentation of scoped beans definition

[spring-projects-issues]

Simon Wong opened BATCH-2097 and commented

SO related questions:

• https://stackoverflow.com/questions/43043554/spring-batch-writernotopenexception
• https://stackoverflow.com/questions/23847661/spring-batch-org-springframework-batch-item-readernotopenexception-reader-must/24927005#24927005

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Affects: 2.2.1

[spring-projects-issues]

Simon Wong commented

If JavaConfig being used (no such issue for using xml config), adding @StepScope to ItemWriter for FlatFileItemWriter will throw the Exception "org.springframework.batch.item.WriterNotOpenException: Writer must be open before it can be written to" during job execution.

    @Bean
    @StepScope
    public ItemWriter<String> writer() {
        FlatFileItemWriter<String> itemWriter = new FlatFileItemWriter<String>();

        itemWriter.setResource(new FileSystemResource("c:/some_output.csv"));
        itemWriter.setLineAggregator(new PassThroughLineAggregator<String>());

        return itemWriter;
    }

[spring-projects-issues]

Tobias Flohre commented

It will work when the method writer returns the FlatFileItemWriter as type. JavaConfig does not recognize that the writer is an ItemStream as well if the return type is ItemWriter. But anyway, it would be nice if it would work without that workaround.

[spring-projects-issues]

Michael Minella commented

Per Tobias Flohre, ItemWriter doesn't have an open method...ItemStream does so there is nothing to open. If you return an ItemStream, or an ItemStreamWriter, it would should fine.

[spring-projects-issues]

Tobias Flohre commented

One more comment to make it clearer: when a bean is step-scoped, it does not get instantiated on startup, instead a cglib proxy based on the return type of the @Bean method is created. If the return type is ItemWriter, the proxy does not implement ItemStream and therefore the writer doesn't get registered as one. That's why the open method never gets called.
There's no way around it: the return type has to implement ItemStream.

[spring-projects-issues]

Simon Wong commented

Thanks both of you.
It would be nice to add a caveat in the reference document.