在Spring Data JPA存储库中使用泛型 [英] Using generics in Spring Data JPA repositories
问题描述
我有许多需要持久保存到数据库的简单对象类型。我正在使用Spring JPA来管理这种持久性。对于每个对象类型，我需要构建以下内容：
I have a number of simple object types that need to be persisted to a database. I am using Spring JPA to manage this persistence. For each object type I need to build the following:
import org.springframework.data.jpa.repository.JpaRepository;
public interface FacilityRepository extends JpaRepository<Facility, Long> {
}
public interface FacilityService {
public Facility create(Facility facility);
}
@Service
public class FacilityServiceImpl implements FacilityService {
@Resource
private FacilityRepository countryRepository;
@Transactional
public Facility create(Facility facility) {
Facility created = facility;
return facilityRepository.save(created);
}
}
我想到有可能更换每个对象类型的多个类，具有三个基于泛型的类，从而节省了大量的样板编码。我不确定如何去做，事实上如果这是一个好主意？
It occurred to me that it may be possible to replace the multiple classes for each object type with three generics based classes, thus saving a lot of boilerplate coding. I am not exactly sure how to go about it and in fact if it is a good idea?
推荐答案
首先，我知道我们在这里提高了相当多的标准但是这已经比你在没有Spring Data JPA的帮助下编写的代码要少得多。
First of all, I know we're raising the bar here quite a bit but this is already tremendously less code than you had to write without the help of Spring Data JPA.
其次，我认为您首先不需要服务类，如果您所做的就是转发对存储库的调用。如果您的业务逻辑需要在事务中编写不同的存储库，或者要封装其他业务逻辑，我们建议在存储库前使用服务。
Second, I think you don't need the service class in the first place, if all you do is forward a call to the repository. We recommend using services in front of the repositories if you have business logic that needs orchestration of different repositories within a transaction or has other business logic to encapsulate.
一般来说，您当然可以做这样的事情：
Generally speaking, you can of course do something like this:
interface ProductRepository<T extends Product> extends CrudRepository<T, Long> {
@Query("select p from #{#entityName} p where ?1 member of p.categories")
Iterable<T> findByCategory(String category);
Iterable<T> findByName(String name);
}
这将允许您在客户端使用存储库，如下所示：
This will allow you to use the repository on the client side like this:
class MyClient {
@Autowired
public MyClient(ProductRepository<Car> carRepository,
ProductRepository<Wine> wineRepository) { … }
}
它会起作用正如所料。但是有一些事情需要注意：
and it will work as expected. However there are a few things to notice:
仅当域类使用单表继承时才有效。有关域类的唯一信息我们可以在bootstrap时获得它将是 Product 对象。因此，对于像 findAll（）甚至 findByName（...）这样的方法，相关查询将以 Wine 或 Car 除非为它创建一个专用的存储库接口来捕获具体的类型信息。
This only works if the domain classes use single table inheritance. The only information about the domain class we can get at bootstrap time is that it will be Product objects. So for methods like findAll() and even findByName(…) the relevant queries will start with select p from Product p where…. This is due to the fact that the reflection lookup will never ever be able to produce Wine or Car unless you create a dedicated repository interface for it to capture the concrete type information.
一般来说，我们建议每聚合根。这意味着您没有每个域类的repo本身。更重要的是，在存储库上以1：1的方式抽象服务也完全忽略了这一点。如果你构建服务，你不会为每个存储库构建一个（猴子可以做到这一点，我们不是猴子，是吗？）。服务暴露更高级别的API，更多用例驱动器并且通常协调对多个存储库的调用。
Generally speaking, we recommend creating repository interfaces per aggregate root. This means you don't have a repo for every domain class per se. Even more important, a 1:1 abstraction of a service over a repository is completely missing the point as well. If you build services, you don't build one for every repository (a monkey could do that, and we're no monkeys, are we? ;). A service is exposing a higher level API, is much more use-case drive and usually orchestrates calls to multiple repositories.
此外，如果您在存储库之上构建服务，您通常希望强制客户端使用服务而不是存储库（这里的一个典型示例是用户管理服务也会触发密码生成和加密，因此绝不让开发人员使用存储库，因为他们有效地解决加密问题）。所以你通常想要选择谁可以坚持哪些域对象不能在整个地方创建依赖关系。
Also, if you build services on top of repositories, you usually want to enforce the clients to use the service instead of the repository (a classical example here is that a service for user management also triggers password generation and encryption, so that by no means it would be a good idea to let developers use the repository directly as they'd effectively work around the encryption). So you usually want to be selective about who can persist which domain objects to not create dependencies all over the place.
是的，您可以构建通用存储库并将其与多种域类型一起使用，但存在相当严格的技术限制。不过，从架构的角度来看，上面描述的场景甚至应该弹出，因为这意味着你无论如何都要面对设计气味。
Yes, you can build generic repositories and use them with multiple domain types but there are quite strict technical limitations. Still, from an architectural point of view, the scenario you describe above should even pop up as this means you're facing a design smell anyway.
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