Spring Entity HashCode Eager Issue Example

This project demonstrates a potential issue with Spring Data JPA or hibernate when implementing equals and hashCode on a field of an associated entity that is fetched with fetch type EAGER when the associated entity is fetched by a repository method.

Project Structure

The project uses the following entities:

• Library: Represents a library, which can have many books and subsidiaries.
• Book: Represents a book, with a title and genre, and belongs to a library. hashcode and equals are overridden and based on the title field. The title field is not nullable and should always be present and is not changeable.
• Subsidiary: Represents a subsidiary of the library. I needed another one-to-many relationship and that's the only reason this entity exists.

The relationships are:

• Library --1:N--> Books
• Library --1:N--> Subsidiaries

Key Points

• Entities use UUID as primary keys.
• Collections are managed with Set.
• The Book entity overrides equals and hashCode based on the title.
• The test demonstrates that removing an entity is sometimes not possible

Problem description - TLDR

When implementing equals and hashCode it is sometimes not possible to remove participants via Set#remove method.

Problem description - Details

When we get a library entity by calling the LibraryRepository#findById method we get the object and the books set is already initialized. We can successfully remove an object from the set:

Library foundLibrary = libraryRepository.findById(libraryId).orElseThrow();
Book bookToRemove = foundLibrary.getBooks().stream()
        .filter(book -> "Book Two".equals(book.getTitle()))
        .findFirst()
        .orElseThrow();
boolean removed = foundLibrary.getBooks().remove(bookToRemove);
        Assertions.assertThat(removed).isTrue();

But removing a child is no longer possible when we start at the BookRepository and call a query method like findByTitleAndGenre:

Book bookOne = bookRepository.findByTitleAndGenre("Book One", BIOGRAPHY).get(0);
boolean removedBookOne = bookOne.getLibrary().getBooks().remove(bookOne);
Assertions.assertThat(removedBookOne).isTrue(); // This test fails but it should not

This is actually independent of Spring as the test canSaveAndFindLibraryWithBooksAndSubsidiariesUsingEntityManager shows where we use the entity manager directly instead of calling the repository method.

Analysis already conducted

When the findByTitleAndGenre method is called, Hibernate/JPA/Spring data runs a LEFT JOIN query joining all three tables.

While creating the objects the Book objects are created using their default constructor. This means that all fields in the JVM Object are actually null if they are not initialized by default, which the title field is not.

Before initializing the fields the objects seem to be put in the Set of the books field in the library object. When a book is put into the set it's hashCode is called. Because title is null at that point the hashCode would be 0.

It seems as if the fields of the book objects are set after the books have been added to the set.

So when we call Set#remove later with an Book object as parameter, the hashCode method is called on the parameter. Because the title field is set now the result will be the hashCode of the value of the title attribute. That value won't be 0 and therefore the Set#remove call won't find the object and return false.

Possible workarounds

FetchType.LAZY

switching the fetch type to LAZY seems to resolve the issue (see branch "fetchtype.lazy") but it may not be the intended use.

Technologies Used

• Java
• Spring Boot
• Spring Data JPA
• H2 Database (in-memory)
• AssertJ (for assertions in tests)

How to Run

1. Build the project using Gradle.
2. Run the tests to see the entity relationship and collection manipulation in action.