Spring Data JPA 删除关系

1. Overview

In this tutorial, we’ll have a look at how deleting is done in Spring Data JPA.

2. Sample Entity

As we know from the Spring Data JPA reference documentation, repository interfaces provide us some basic support for entities.

Let’s say we have an entity, such as a Book:

@Entity
public class Book {

    @Id
    @GeneratedValue
    private Long id;
    private String title;

    // standard constructors

    // standard getters and setters
}

Then we can extend Spring Data JPA’s CrudRepository to give us access to CRUD operations on Book:

@Repository
public interface BookRepository extends CrudRepository<Book, Long> {}

3. Delete from Repository

Among others, CrudRepository contains two methods: deleteById and deleteAll.

Let’s test these methods directly from our BookRepository:

@RunWith(SpringRunner.class)
@SpringBootTest(classes = {Application.class})
public class DeleteFromRepositoryUnitTest {

    @Autowired
    private BookRepository repository;

    Book book1;
    Book book2;
    List<Book> books;

    // data initialization

    @Test
    public void whenDeleteByIdFromRepository_thenDeletingShouldBeSuccessful() {
        repository.deleteById(book1.getId());
        assertThat(repository.count()).isEqualTo(1);
    }

    @Test
    public void whenDeleteAllFromRepository_thenRepositoryShouldBeEmpty() {
        repository.deleteAll();
        assertThat(repository.count()).isEqualTo(0);
    }
}

And even though we are using CrudRepository, note that these same methods exist for other Spring Data JPA interfaces such as JpaRepository or PagingAndSortingRepository.

4. Derived Delete Query

We can also derive query methods for deleting entities. There is a set of rules for writing them, but let’s just focus on the simplest example.

A derived delete query must start with deleteBy, followed by the name of the selection criteria. These criteria must be provided in the method call.

Let’s say that we want to delete Books by title. Using the naming convention, we’d start with deleteBy and list title as our criteria:

@Repository
public interface BookRepository extends CrudRepository<Book, Long> {
    long deleteByTitle(String title);
}

The return value, of type long, indicates how many records the method deleted.

Let’s write a test and make sure that is correct:

@Test
@Transactional
public void whenDeleteFromDerivedQuery_thenDeletingShouldBeSuccessful() {
    long deletedRecords = repository.deleteByTitle("The Hobbit");
    assertThat(deletedRecords).isEqualTo(1);
}

Persisting and deleting objects in JPA requires a transaction. That’s why we should use a @Transactional annotation when using these derived delete queries, to make sure a transaction is running. This is explained in detail in the ORM with Spring documentation.

5. Custom Delete Query

The method names for derived queries can get quite long, and they are limited to just a single table.

When we need something more complex, we can write a custom query using @Query and @Modifying together.

Let’s check the equivalent code for our derived method from earlier:

@Modifying
@Query("delete from Book b where b.title=:title")
void deleteBooks(@Param("title") String title);

Again, we can verify it works with a simple test:

@Test
@Transactional
public void whenDeleteFromCustomQuery_thenDeletingShouldBeSuccessful() {
    repository.deleteBooks("The Hobbit");
    assertThat(repository.count()).isEqualTo(1);
}

Both solutions presented above are similar and achieve the same result. However, they take a slightly different approach.

The @Query method creates a single JPQL query against the database. By comparison, the deleteBy methods execute a read query and then delete each of the items one by one.

6. Delete in Relationships

Now let’s see what happens when we have relationships with other entities.

Assume we have a Category entity that has a OneToMany association with the Book entity:

@Entity
public class Category {

    @Id
    @GeneratedValue
    private Long id;
    private String name;

    @OneToMany(mappedBy = "category", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Book> books;

    // standard constructors

    // standard getters and setters
}

The CategoryRepository can just be an empty interface that extends CrudRepository:

@Repository
public interface CategoryRepository extends CrudRepository<Category, Long> {}

We should also modify the Book entity to reflect this association:

@ManyToOne
private Category category;

Let’s now add two categories and associate them with the books we currently have.

Now if we try to delete the categories, the books will also be deleted:

@Test
public void whenDeletingCategories_thenBooksShouldAlsoBeDeleted() {
    categoryRepository.deleteAll();
    assertThat(bookRepository.count()).isEqualTo(0);
    assertThat(categoryRepository.count()).isEqualTo(0);
}

This is not bi-directional, though, meaning that if we delete the books, the categories are still there:

@Test
public void whenDeletingBooks_thenCategoriesShouldAlsoBeDeleted() {
    bookRepository.deleteAll();
    assertThat(bookRepository.count()).isEqualTo(0);
    assertThat(categoryRepository.count()).isEqualTo(2);
}

We can change this behavior by changing the properties of the relationship, such as the CascadeType.

7. Conclusion

In this article, we saw different ways to delete entities in Spring Data JPA.

We looked at the provided delete methods from CrudRepository as well as our derived queries or custom ones using @Query annotation.

We also saw how deleting is done in relationships.

As always, all of the code snippets mentioned in this article can be found on our GitHub repository.