How to implement equals and hashCode using the JPA entity identifier (Primary Key)
Imagine having a tool that can automatically detect if you are using JPA and Hibernate properly. Hypersistence Optimizer is that tool!
Introduction
As previously explained, using the JPA entity business key for equals and hashCode is always best choice. However, not all entities feature a unique business key, so we need to use another database column that is also unique, like the primary key.
But using the entity identifier for equality is very challenging, and this post is going to show you how you can use it without issues.
Test harness
When it comes to implementing equals and hashCode, there is one and only one rule you should have in mind:
Equals and hashCode must behave consistently across all entity state transitions.
To test the effectiveness of an equals and hashCode implementation, the following test can be used:
protected <T extends Identifiable<? extends Serializable>>
void assertEqualityConstraints(Class<T> clazz, T entity) {
Set<T> tuples = new HashSet<>();
assertFalse(tuples.contains(entity));
tuples.add(entity);
assertTrue(tuples.contains(entity));
doInJPA(entityManager -> {
entityManager.persist(entity);
entityManager.flush();
assertTrue("The entity is found after it's persisted",
tuples.contains(entity));
});
//The entity is found after the entity is detached
assertTrue(tuples.contains(entity));
doInJPA(entityManager -> {
T _entity = entityManager.merge(entity);
assertTrue("The entity is found after it's merged",
tuples.contains(_entity));
});
doInJPA(entityManager -> {
entityManager.unwrap(Session.class).update(entity);
assertTrue("The entity is found after it's reattached",
tuples.contains(entity));
});
doInJPA(entityManager -> {
T _entity = entityManager.find(clazz, entity.getId());
assertTrue("The entity is found after it's loaded " +
"in an other Persistence Context",
tuples.contains(_entity));
});
executeSync(() -> {
doInJPA(entityManager -> {
T _entity = entityManager.find(clazz, entity.getId());
assertTrue("The entity is found after it's loaded " +
"in an other Persistence Context and " +
"in an other thread",
tuples.contains(_entity));
});
});
}
Natural id
The first use case to test is the natural id mapping. Considering the following entity:
@Entity
public class Book implements Identifiable<Long> {
@Id
@GeneratedValue
private Long id;
private String title;
@NaturalId
private String isbn;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Book)) return false;
Book book = (Book) o;
return Objects.equals(getIsbn(), book.getIsbn());
}
@Override
public int hashCode() {
return Objects.hash(getIsbn());
}
//Getters and setters omitted for brevity
}
The isbn property is also a @NaturalId, therefore, it should be unique and not nullable. Both equals and hashCode use the isbn property in their implementations.
For more details about the
@NaturalIdannotation, check out this article.
When running the following test case:
Book book = new Book();
book.setTitle("High-PerformanceJava Persistence");
book.setIsbn("123-456-7890");
assertEqualityConstraints(Book.class, book);
Everything works fine, as expected.
Default java.lang.Object equals and hashCode
What if our entity does not have any column that can be used as a @NaturalId? The first urge is to not define your own implementations of equals and hashCode, like in the following example:
@Entity(name = "Book")
public class Book implements Identifiable<Long> {
@Id
@GeneratedValue
private Long id;
private String title;
//Getters and setters omitted for brevity
}
However, when testing this implementation:
Book book = new Book();
book.setTitle("High-PerformanceJava Persistence");
assertEqualityConstraints(Book.class, book);
Hibernate throws the following exception:
java.lang.AssertionError: The entity is found after it's merged
The original entity is not equal with the one returned by the merge method because two distinct Object(s) do not share the same reference.
Using the entity identifier for equals and hashCode
So if the default equals and hashCode is no good either, then let’s use the entity identifier for our custom implementation. Let’s just use our IDE to generate the equals and hashCode and see how it works:
@Entity
public class Book implements Identifiable<Long> {
@Id
@GeneratedValue
private Long id;
private String title;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Book)) return false;
Book book = (Book) o;
return Objects.equals(getId(), book.getId());
}
@Override
public int hashCode() {
return Objects.hash(getId());
}
//Getters and setters omitted for brevity
}
When running the previous test case, Hibernate throws the following exception:
java.lang.AssertionError: The entity is found after it's persisted
When the entity was first stored in the Set, the identifier was null. After the entity was persisted, the identifier was assigned to a value that was automatically generated, hence the hashCode differs. For this reason, the entity cannot be found in the Set after it got persisted.
Fixing the entity identifier equals and hashCode
To address the previous issue, there is only one solution: the hashCode should always return the same value:
@Entity
public class Book implements Identifiable<Long> {
@Id
@GeneratedValue
private Long id;
private String title;
@Override
public boolean equals(Object o) {
if (this == o) return true;
if (!(o instanceof Book)) return false;
Book book = (Book) o;
return id != null && id.equals(book.id);
}
@Override
public int hashCode() {
return 31;
}
//Getters and setters omitted for brevity
}
Also, when the entity identifier is null, we can guarantee equality only for the same object references. Otherwise, no transient object is equal to any other transient or persisted object. That’s why the identifier equality check is done only if the current Object identifier is not null.
With this implementation, the equals and hashCode test runs fine for all entity state transitions. The reason why it works is because the hashCode value does not change, hence, we can rely on the java.lang.Object reference equality as long as the identifier is null.
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Conclusion
The entity identifier can be used for equals and hashCode, but only if the hashCode returns the same value all the time. This might sound like a terrible thing to do since it defeats the purpose of using multiple buckets in a HashSet or HashMap.
However, for performance reasons, you should always limit the number of entities that are stored in a collection. You should never fetch thousands of entities in a @OneToMany Set because the performance penalty on the database side is multiple orders of magnitude higher than using a single hashed bucket.
All tests are available on GitHub.
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Hi, Vlad!
Thank you for this nice post.
But I have a question about this line:
return id != null && id.equals(book.id);
What if argument of equals will be an uninitialized proxy? Shouldn’t you get Id of comparable object using getter?
return id != null && id.equals(book.getId());
That would be required only if you are using property-based access, which I don’t recommend. For field-based access, you should be fine.
Can you provide a bit more details?
I got today same behavior as in pitfall #1 in this post using field access: https://xebia.com/blog/advanced-hibernate-proxy-pitfalls/
Thanks!
Send me a Pull Request with a replicating test case so I can see the issue too.
All the complexity with equals/hashCode goes away if the assigned identifiers are used for entities. I see no drawbacks of this approach, and so I don’t understand why many (maybe even the majority) of people keep using other ID generation strategies.
Vlad, do you see drawbacks of assigned identifiers (the necessity to have a high-performance ID generator which may work similarly to Hibernate’s hi-lo/pooling generators and to explicitly supplying IDs via the constructor of an entity are not drawbacks 🙂 ?
And how do generate those unique values in the application?
The same way Hibernate does this: by generating random UUID values; or if that’s not applicable, by using DB sequence with pooling implemented on top of it in the application; or by using a different approach suitable to the applications.
That’s a bad idea. It takes 128 bits, and the values are random, hence they don’t work nice with clustered indexes. And the bloat is amplified by every Foreign Key.
That’s a bad idea
That was only one of the approaches I mentioned. And it is not always a bad idea. But if you don’t like it, I also mentioned (twice already:) ) DB sequence + pooling.
The only time you’d want to use it is for multi-master replication. Otherwise, a numeric id is a much better approach.
Somehow we deviated from the original question. But since you did not mention the drawbacks of using assigned identifiers, I am guessing you also don’t see any? (the possibility of picking a non-optimal way of generating the identifiers is the same for both assigned IDs and other Hibernate ID generation strategies).
I just told 2 reasons: bloat and randomness.
But your mentions of bloat and randomness were about UUID identifiers. The application is free to choose any way of generating IDs for the assigned identifier approach. Which means that the app can easily use the DB sequence with pooling on top of it (or with hi-lo if the DB sequence does not allow advancing the sequence value for more than 1). It’s done in Hibernate, and can be done anywhere (it’s not hard to do at all).
Yes, that ca be done, but it sounds more work than just providing the right equals and hashCode.
Yes, there is this thing “implement your ID generator VS implement only hashCode/equals in an unexpected way” when choosing the assigned ID strategy. But the additional benefit we have with it is that you know that the ID of an entity is never null. And this is a noticeable simplification for a real-life applications when you can’t always easily say whether the entity you have is new or not.
Goid point. I’ll keep that in mind for my new Hypersistence Optimizer tool. I will check the equals/hashCode validity too.
“Hypersistence Optimizer tool” – I was not able to google it or locate among https://github.com/vladmihalcea?utf8=%E2%9C%93&tab=repositories&q=&type=source&language=
Is it unpublished at this stage?
I’m building it right now. Stay tuned for more information. https://twitter.com/vlad_mihalcea/status/1100336912202690561?s=19
Thanks for the explanations Vlad!
Would you say it’s a requirement to (almost) always implement the equals/hashCode methods for every @Entity?
I’m asking this because I’ve seen that many, if not most of the Entities in the project high-performance-java-persistence don’t have custom equals/hashCode methods. And I think people learn best practices by reading good, consistent code (even if it’s longer).
If you don’t have to store the entities in a collection or compare them, then you won’t need equals or hashCode. There is no universal rule that applies to any use case.
I do not see how your implementation would work when merging a new entity. The new entity would not have an id, while the object returned by the merge operation would. So your implementation would consider them different.
In fact, your test harness only works, because you first persist the entity, providing it with an id, an then you detach and merge it later on. At that time, id generation will no longer be a issue and your implementation will consider them equal.
In the case of a direct merge of new entites, this will be no better than object-references.
There is no such thing as a direct merge. Merge is for integrating changes o detached entities, which have been persisted previously because that’s how JPA works. You can find all the tests in my High-Performance Java Persistence GitHub repository, so feel free to provide a test case that proves this implementation does not work.
What do you think of the default implementation provided by Spring Data Jaa in org.springframework.data.jpa.domain.AbstractPersistable?
They use a slightly optimized version of hashCode(), if id is not null:
public int hashCode() {
int hashCode = 17;
hashCode += null == getId() ? 0 : getId().hashCode() * 31;
return hashCode;
}
https://github.com/spring-projects/spring-data-jpa/blob/master/src/main/java/org/springframework/data/jpa/domain/AbstractPersistable.java
It’s not a good implementation. Consistency will break when the entity is persisted and an I’d is auto-generated.
I copied your code into my spring data spring boot app and I only get nulls in all the audit fields.
I copied the @Embeddable Audit object as is, and I added this line to my entity:
@Embedded
private Audit audit = new Audit();
Here is the code I took it from: https://github.com/vladmihalcea/high-performance-java-persistence/blob/04c5e1920adbc9f6c0affeda42781284f4abe42d/core/src/test/java/com/vladmihalcea/book/hpjp/hibernate/mapping/embeddable/EmbeddableEntityListenerTest.java
Have you or anyone else tried this code in Spring?
Try to debug it and see why the code is not called when using Spring and why the callbacks wirk fine in the test.
What about the approach to not use a constant hash code like 31, but instead to use the hash code of the class, sanitized by any proxy generated suffixes (e.g. _$$….) to achieve a better bucket distribution?
Wouldn’t that render a constant value as well? How would that lead to multiple hash values?
I’m using this as a default implementation in an inheritance. The question is if I might have missed any case in a proxy generation tool that might change the whole class and not only append a generation suffix?
Send me a Pull Request in my high-performance-java-persistence GitHub repository so I can check out your approach. Thanks.
I withdraw my question. I’ve performed some performance tests on an application where we work with collections that have up to 2500 (small) entries. Using a constant hash code here makes the calculation about 5-10 times slower than using a hash code based on the database identifier, accepting the drawback that mixed transition state collections won’t work.
5-10 slower does not tell how much time it takes in absolute figures. If the constant hashCode makes the calculations take 10 microseconds while the “broken” hashCode takes only 2 milliceconds, chances are that this is not the next bottleneck in your application as long as the fastest queries take in the millisecond range (2 or 3 orders of magnitude more time).
Of course. In our context, building the sample database is 3 minutes vs 20-30 minutes.
I really doubt that the hashcode alone will ever make such a difference.
In this particular application, it indeed is. Just deleting the hashCode implementation without changing anything else and rerunning the sample setup again causes the big difference. In the application, everything is recalculated from scratch on a change for consistency reasons and due to the high complexity of the calculation, so it happens that several 100s of entities are recreated on one transaction and put on a set. I’m quite sure that this could be optimized to reduce the hash code impact, but this is out of scope.
Hi, beautifully explained article!
I’m anyway puzzled somehow by what seems to me a contradictory conclusion comparing this article with the previous one (https://vladmihalcea.com/hibernate-facts-equals-and-hashcode/).
The previous one concludes this:
“We can’t use an auto-incrementing database id for comparing objects since the transient and the attached object versions won’t be equal to each other.”
but in this article you are using the id inside equals so after persist the transient and the attached object versions won’t be equal to each other which is bad, right?
Could you please clarify this?
Thank you
Good point, I rephrased it like this:
Using the hashCode() method to always return a constant value isn’t much of a solution, since Sets and Map key’s will perform poorly.
Can you talk about using a GUID generator in the java code to assign a PK before making the entity persistent? I’ve seen that done, and it seems to work well. The downside is it will only work with GUIDs as PKs.
Actually, they will not. Entities are not not in-memory objects. They have a high cost to be retrieved from the DB. By the time the constant hashCode becomes your bottleneck, you had to fetch millions of records which already took a lot of time.
My video course covers the GUID topic in great detail. Enjoy watching the videos.
Anyways, if I use UUID as an Id and if I generate UUID before persisting an entity, using equals and hashCode of the Primary Key is a good idea, right?
You can find the answer in my Types and Identifiers video course.
You are going to learn the pros and cons of using UUIDs for identifiers, and how it can influence performance based on the underlyign DB indexing strategies.