当使用 JPA 和 Hibernate 时，equals 和 hashcode 应该如何实现

Yeah, it's hard. In my project equals and hashCode both rely on the id of the object. The problem of this solution is that neither of them works if the object has not been persisted yet, as the id is generated by database. In my case that's tolerable since in almost all cases objects are persisted right away. Other than that, it works great and is easy to implement.

Hibernate has a nice and long description of when / how to override equals() / hashCode() in documentation

The gist of it is you only need to worry about it if your entity will be part of a Set or if you're going to be detaching / attaching its instances. The latter is not that common. The former is usually best handled via:

1. Basing equals() / hashCode() on a business key - e.g. a unique combination of attributes that is not going to change during object (or, at least, session) lifetime.
2. If the above is impossible, base equals() / hashCode() on primary key IF it's set and object identity / System.identityHashCode() otherwise. The important part here is that you need to reload your Set after new entity has been added to it and persisted; otherwise you may end up with strange behavior (ultimately resulting in errors and / or data corruption) because your entity may be allocated to a bucket not matching its current hashCode().

I don't think that the accepted answer is accurate.

To answer the original question:

Is the default implementation good enough for most cases?

The answer is yes, in most cases it is.

You only need to override equals() and hashcode() if the entity will be used in a Set (which is very common) AND the entity will be detached from, and subsequently re-attached to, hibernate sessions (which is an uncommon usage of hibernate).

The accepted answer indicates that the methods need to be overriden if either condition is true.

When an entity is loaded through lazy loading, it's not an instance of the base type, but is a dynamically generated subtype generated by javassist, thus a check on the same class type will fail, so don't use:

if (getClass() != that.getClass()) return false;

instead use:

if (!(otherObject instanceof Unit)) return false;

which is also a good practice, as explained on Implementing equals in Java Practices.

for the same reason, accessing directly fields, may not work and return null, instead of the underlying value, so don't use comparison on the properties, but use the getters, since they might trigger to load the underlying values.

The best equals and hashCode implementation is when you use a unique business key or natural identifier, like this:

@Entity
public class Company {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;
 

@Column(unique = true, updatable = false)
private String name;
 

@Override
public int hashCode() {
HashCodeBuilder hcb = new HashCodeBuilder();
hcb.append(name);
return hcb.toHashCode();
}
 

@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Company)) {
return false;
}
Company that = (Company) obj;
EqualsBuilder eb = new EqualsBuilder();
eb.append(name, that.name);
return eb.isEquals();
}
}

The business key should be consistent across all entity state transitions (transient, attached, detached, removed), that's why you can't rely on id for equality.

Another option is to switch to using UUID identifiers, assigned by the application logic. This way, you can use the UUID for the equals/hashCode because the id is assigned before the entity gets flushed.

You can even use the entity identifier for equals and hashCode, but that requires you to always return the same [hashCode value so that you make sure that the entity hashCode value is consistent across all entity state transitions, like this:

@Entity(name = "Post")
@Table(name = "post")
public class Post implements Identifiable<Long> {
 

@Id
@GeneratedValue
private Long id;
 

private String title;
 

public Post() {}
 

@Override
public boolean equals(Object o) {
if (this == o) return true;
 

if (!(o instanceof Post))
return false;
 

Post other = (Post) o;
 

return id != null &&
id.equals(other.getId());
}
 

@Override
public int hashCode() {
return getClass().hashCode();
}
  

//Getters and setters omitted for brevity
}

There is very nice article here: https://docs.jboss.org/hibernate/stable/core.old/reference/en/html/persistent-classes-equalshashcode.html

Quoting an important line from the article:

We recommend implementing equals() and hashCode() using Business key equality. Business key equality means that the equals() method compares only the properties that form the business key, a key that would identify our instance in the real world (a natural candidate key):

In simple terms

public class Cat {


...
public boolean equals(Object other) {
//Basic test / class cast
return this.catId==other.catId;
}


public int hashCode() {
int result;


return 3*this.catId; //any primenumber
}


}

If you happened to override equals, make sure you fulfill its contracts:-

• SYMMETRY
• REFLECTIVE
• TRANSITIVE
• CONSISTENT
• NON NULL

And override hashCode, as its contract rely on equals implementation.

Joshua Bloch(designer of Collection framework) strongly urged these rules to be followed.

• item 9: Always override hashCode when you override equals

There are serious unintended effect when you don't follow these contracts. For example List#contains(Object o) might return wrong boolean value as the general contract not fulfilled.

In the documentation of Hibernate 5.2 it says you might not want to implement hashCode and equals at all - depending on your situation.

https://docs.jboss.org/hibernate/orm/5.2/userguide/html_single/Hibernate_User_Guide.html#mapping-model-pojo-equalshashcode

Generally, two objects loaded from the same session will be equal if they are equal in the database (without implementing hashCode and equals).

It gets complicated if you're using two or more sessions. In this case, the equality of two objects depends on your equals-method implementation.

Further, you'll get into trouble if your equals-method is comparing IDs that are only generated while persisting an object for the first time. They might not be there yet when equals is called.