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

spring-projects / spring-batch

Spring Batch is a framework for writing batch applications using Java and Spring

http://projects.spring.io/spring-batch/

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Improve JPA repository readers to be more typesafe [BATCH-2697] #910

Open spring-projects-issues opened 6 years ago

spring-projects-issues commented 6 years ago

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\
As well as needing to know that the method
- MUST return Page\ 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

spring-projects-issues commented 6 years ago

Michael Minella commented

Why not just use the JpaPagingItemReader? The RepositoryItemReader is intended to go against the Repository implementation. What you're proposing is really a new FunctionItemReader that would call a function, but what does it return? What's the contract of the Function?

spring-projects-issues commented 6 years ago

Niels Bech Nielsen commented

I know of the JpaPagingItemReader, but if I facilitate Spring Data repositories in my project, I would prefer to be able to use them as well with any batch parts. I was merely highlighting the technical debt I get should I use the RepositoryItemReader. If the method name changes I get an error at runtime. If the set of arguments change I get an error at runtime. If the return value of an innocent repository method changes from Page\ to List\ I get an error at runtime. That is just very fragile. For my own projects I have written an implementation of the above suggestion to facilitate compile-time check and thought you would welcome the feedback. If not, by all means, your choice. Given the Function interface and lambda style, you could actually pass in a repository method directly. The function would merely take the pageable as argument and return a Page\. And yes, you lose control over the repository, but I would gain from static type check. Anyway, Spring usually facilitates more than one way of doing things, and this is a suggestion of one.
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