Why should you be careful using @EqualsAndHashCode on Hibernate entities with lazy-loaded proxies?

Hibernate sometimes returns a lazy-loading proxy object standing in for the real entity, rather than the actual entity instance itself — and a proxy's runtime class differs from the real entity's class (it's a dynamically generated subclass). A naive equals() comparing this.getClass() == other.getClass(), which is what @EqualsAndHashCode can generate by default in some configurations, can incorrectly report a real entity and its own proxy as unequal, purely because their runtime classes differ.

@EqualsAndHashCode(onlyExplicitlyIncluded = true)
@Entity
public class Product {
    @Id
    @EqualsAndHashCode.Include
    private Long id;   //  compare by id, ignoring runtime class differences from proxying
}

The safer pattern is to base equality on the entity's stable identifier field alone, using onlyExplicitlyIncluded = true combined with @EqualsAndHashCode.Include on just the id, and to compare using instanceof rather than exact class equality, avoiding the proxy/class mismatch entirely.

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