Add parameter supplier support to spring batch readers

[happysca]

In my use case of MyBatisCursorItemReader like below, I have to specify query parameters before the Spring Container is initialized, which works well at first.

@Configuration
public class SomeStepConfig {
    ...

    @Bean
    public Step someStep() {
        return stepBuilderFactory.get("someStep")
                .<SomeIn, SomeOut>chunk(1000)
                .reader(reader())
                .processor(processor())
                .writer(writer())
                .build();
    }
    MyBatisCursorItemReader reader() {
        return new MyBatisCursorItemReaderBuilder<SomeIn>()
                .sqlSessionFactory(this.sqlSessionFactory)
                .queryId("selectSelectId")
                .parameterValues(Collections.singletonMap("key", "value"))
                .build();
    }

    ...
}

After a few iterations, the query parameters needs to be determined by a preceding step, which is after Spring Container initialized, and I didn't come up with any elegant solution. So I made a copy of source code of MyBatisCursorItemReaderBuilder & MyBatisCursorItemReader in my project, and then made some changes to them as shown in this PR.

The client code becomes

@Configuration
public class SomeStepConfig {
    ...

    MyBatisCursorItemReader reader() {
        return new MyBatisCursorItemReaderBuilder<SomeIn>()
                .sqlSessionFactory(this.sqlSessionFactory)
                .queryId("selectSelectId")
                .parameterSupplier(() -> parameterMapFromLocalOrRemote())
                .build();
    }
    private Map<String, Object> parameterMapFromLocalOrRemote() {
        ...
    }

    ...
}

It works great so far, except that copying whole source of MyBatisCursorItemReaderBuilder & MyBatisCursorItemReader is not elegant enough. I considered subclassing, but the private fields makes it rather difficult.

Considering this is a very useful feature, at least for me, the changes are very small and simple, I was wondering if I could make a contribute. So I added the same feature to MyBatisPagingItemReaderBuilder & MyBatisPagingItemReader and made some changes to unit tests for the feature. Then there is this PR.

Thanks for you time for anyone review or read this PR.

[happysca]

A shared Map instance might just work in my case (occurred to me just now hahahaha). Parameter supplier is not that important to me, but still more powerful than shared instances.

[kazuki43zoo]

Thank you for your contribution!! I think your suggestion is reasonable and useful. Could you confirm my trivial comments?

[coveralls]

Coverage increased (+0.1%) to 89.345% when pulling 24b2b75 on happysca:batch_reader_query_parameter_supplier into 26d492c on mybatis:master.