Spring Data Repositories 的比较

1. Overview

In this quick article, we’ll focus on different kinds of Spring Data repository interfaces and their functionality. We’ll touch on:

CrudRepository
PagingAndSortingRepository
JpaRepository

Simply put, every repository in Spring Data extends the generic Repository interface, but beyond that, they each have different functionality.

2. Spring Data Repositories

Let’s start with the JpaRepository – which extends PagingAndSortingRepository and, in turn, the CrudRepository.

Each of these defines its functionality:

CrudRepository provides CRUD functions
PagingAndSortingRepository provides methods to do pagination and sorting of records
JpaRepository provides JPA-related methods such as flushing the persistence context and deleting records in a batch

And so, because of this inheritance relationship, the JpaRepository contains the full API of CrudRepository and PagingAndSortingRepository.

When we don’t need the full functionality provided by JpaRepository and PagingAndSortingRepository, we can use the CrudRepository.

Let’s now look at a quick example to understand these APIs better.

We’ll start with a simple Product entity:

@Entity
public class Product {

    @Id
    private long id;
    private String name;

    // getters and setters
}

And let’s implement a simple operation – find a Product based on its name:

@Repository
public interface ProductRepository extends JpaRepository<Product, Long> {
    Product findByName(String productName);
}

That’s all. The Spring Data Repository will auto-generate the implementation based on the name we provided it.

This was a very simple example, of course; you can go deeper into Spring Data JPA here.

3. CrudRepository

Let’s now have a look at the code for the CrudRepository interface:

public interface CrudRepository<T, ID extends Serializable>
  extends Repository<T, ID> {

    <S extends T> S save(S entity);

    T findOne(ID primaryKey);

    Iterable<T> findAll();

    Long count();

    void delete(T entity);

    boolean exists(ID primaryKey);
}

Notice the typical CRUD functionality:

save(…) – save an Iterable of entities. Here, we can pass multiple objects to save them in a batch
findOne(…) – get a single entity based on passed primary key value
findAll() – get an Iterable of all available entities in the database
count() – return the count of total entities in a table
delete(…) – delete an entity based on the passed object
exists(…) – verify if an entity exists based on the passed primary key value

This interface looks quite generic and simple, but actually, it provides all the basic query abstractions needed in an application.

4. PagingAndSortingRepository

Now, let’s have a look at another repository interface, which extends CrudRepository:

public interface PagingAndSortingRepository<T, ID extends Serializable> 
  extends CrudRepository<T, ID> {

    Iterable<T> findAll(Sort sort);

    Page<T> findAll(Pageable pageable);
}

This interface provides a method findAll(Pageable pageable), which is the key to implementing Pagination.

When using Pageable, we create a Pageable object with certain properties, and we’ve to specify at least the following:

Page size
Current page number
Sorting

So, let’s assume that we want to show the first page of a result set sorted by lastName, ascending, having no more than five records each. This is how we can achieve this using a PageRequest and a Sort definition:

Sort sort = new Sort(new Sort.Order(Direction.ASC, "lastName"));
Pageable pageable = new PageRequest(0, 5, sort);

Passing the pageable object to the Spring data query will return the results in question (the first parameter of PageRequest is zero-based).

5. JpaRepository

Finally, we’ll have a look at the JpaRepository interface:

public interface JpaRepository<T, ID extends Serializable> extends
  PagingAndSortingRepository<T, ID> {

    List<T> findAll();

    List<T> findAll(Sort sort);

    List<T> save(Iterable<? extends T> entities);

    void flush();

    T saveAndFlush(T entity);

    void deleteInBatch(Iterable<T> entities);
}

Again, let’s look at each of these methods in brief:

findAll() – get a List of all available entities in the database
findAll(…) – get a List of all available entities and sort them using the provided condition
save(…) – save an Iterable of entities. Here, we can pass multiple objects to save them in a batch
flush() – flush all pending tasks to the database
saveAndFlush(…) – save the entity and flush changes immediately
deleteInBatch(…) – delete an Iterable of entities. Here, we can pass multiple objects to delete them in a batch

Clearly, the above interface extends PagingAndSortingRepository, which means it also has all methods present in the CrudRepository.

6. Spring Data Repositories in Spring Data 3

In the new version of Spring Data, the internals of some Repository classes have changed slightly, adding new functionalities and providing a simpler development experience.

We now have access to the advantageous List-based CRUD repository interface. Also, the class hierarchy of some spring-data Repository classes is based on a different structure.

All details are available in our New CRUD Repository Interfaces in Spring Data 3 article.

7. Downsides of Spring Data Repositories

Beyond all the very useful advantages of these repositories, there are some basic downsides of directly depending on these as well:

We couple our code to the library and to its specific abstractions, such as `Page` or `Pageable`; that’s, of course, not unique to this library – but we do have to be careful not to expose these internal implementation details
By extending, e.g. CrudRepository, we expose a complete set of persistence methods at once. This is probably fine in most circumstances as well, but we might run into situations where we’d like to gain more fine-grained control over the methods exposed, e.g. to create a ReadOnlyRepository that doesn’t include the save(…) and delete(…) methods of CrudRepository

8. Conclusion

This article covered some brief but important differences and features of Spring Data JPA repository interfaces. We can find examples using these APIs over on GitHub.

For more information, have a look at the series on Spring Persistence

Persistence

REST

Security