Improve JPA repository readers to be more typesafe [BATCH-2697]

Niels Bech Nielsen opened BATCH-2697 and commented

The JPA implementation org.springframework.batch.item.data.RepositoryItemReader
use a loosely typed configuration to request a page of objects.

Currently one needs to:

• set the repository
• Specify the method name
• Store the arguments in a List<Object>

As well as needing to know that the method

• MUST return Page<T> and
• MUST take a Pageable as last parameter.

All of this is very fragile to changes, and there is no refactoring tools or type tools to help in this.

Given that none of this is very typesafe, I suggest introduce a function interface similar to:

public interface JPARepositoryPageReader<T> {
  Page<T> readNextPage(Pageable page);
}

such that the repository read can be specified more typesafe. One derivative of this interface could use the method invocation of the RepositoryItemReader and have methodName and args injected.

In the below example I use the name 'JPARepositoryItemReader' as the 'new' implementation, and use a Java8 lambda to simply the page reader:

@Bean
public ItemReader<Customer> createReader() {
   JPARepositoryItemReader reader = new JPARepositoryItemReader();
   reader.setPageSize(1000);
   reader.setSort(someSort);  // BTW, would be nice to be able to use Sort object directly as well
   reader.setPageReader ( (pageable) -> customerRepository.findByLastName("Jones", pageable) );
   return reader;
}

No further details from BATCH-2697

Start by reading the existing RepositoryItemReader configuration and its methodName, repository, and argument handling. Compare that entry point with the proposed JPARepositoryPageReader interface and lambda-based pageReader API. Done means the JPA repository reader has a clearly defined type-safe page-reading design and its existing behavior remains covered.