@Entity
public class User {
    @Id
    private Long id;

    @NaturalId(mutable = false) // Неизменяемый (оптимизирует кеширование)
    private String email;

    @NaturalId(mutable = true)  // Изменяемый (редкий случай)
    private String passportNumber;
}

@Entity
@Table(uniqueConstraints = @UniqueConstraint(columnNames = "isbn"))
public class Book {
    @Id
    private Long id;

    @NaturalId // immutable=false по умолчанию
    private String isbn;
}

Book book = session.bySimpleNaturalId(Book.class)
    .load("978-3-16-148410-0");

@Entity
public class Order {
    @ManyToOne
    @NotFound(action = NotFoundAction.IGNORE) // Игнорировать отсутствие пользователя
    private User user;
}

-- order.user_id = 999, но users.id = 999 не существует
@NotFound(action = IGNORE) → order.user = null.

@EventListener(ApplicationReadyEvent.class)
public void checkBrokenReferences() {
    // Ручная проверка...
}

logging.level.org.hibernate=DEBUG

@Entity
public class Comment {
    @ManyToOne
    @NotFound(action = NotFoundAction.IGNORE)
    private Post post; // Если post удалён, comment.post = null
}

public CommentDTO getComment(Long id) {
    Comment comment = commentRepository.findById(id).orElseThrow();
    if (comment.getPost() == null) {
        log.warn("Пост для комментария {} не найден", id);
    }
    return toDto(comment);
}

import java.util.HashMap;

class Key170425 {
    int id;
    
    Key170425(int id) {
        this.id = id;
    }
    
    @Override
    public int hashCode() {
        return id % 2;
    }

    @Override
    public boolean equals(Object o) {
        return this.id % 2 == ((Key170425)o).id % 2;
    }
}

public class Task170425 {
    public static void main(String[] args) {
        HashMap<Key170425, String> map = new HashMap<>();
        map.put(new Key170425(1), "Apple");
        map.put(new Key170425(2), "Banana");
        map.put(new Key170425(3), "Cherry");
        
        System.out.println(map.size());
    }
}

@Entity  
public class Order {  
    @ManyToOne  
    @OnDelete(action = OnDeleteAction.CASCADE) // При удалении User удалятся его Order  
    private User user;  
}  

@Entity  
public class Comment {  
    @ManyToOne  
    @OnDelete(action = OnDeleteAction.CASCADE) // Удалить комментарии при удалении Post  
    private Post post;  
}  

@Entity  
public class Employee {  
    @ManyToOne  
    @OnDelete(action = OnDeleteAction.SET_NULL) // department_id = NULL при удалении Department  
    private Department department;  
}  

SELECT next_val FROM hibernate_sequences WHERE sequence_name = 'employee_seq' FOR UPDATE;
UPDATE hibernate_sequences SET next_val = next_val + 50 WHERE sequence_name = 'employee_seq';

spring.jpa.hibernate.ddl-auto=update  

@TableGenerator(  
    name = "book_seq",  
    table = "custom_sequences",  
    pkColumnName = "seq_name",  
    valueColumnName = "seq_value",  
    allocationSize = 30  
)  

@Entity  
public class Employee {  
    @Id  
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "emp_seq")  
    @TableGenerator(  
        name = "emp_seq",  
        allocationSize = 20  
    )  
    private Long id;  
}  

@TableGenerator(  
    name = "project_seq",  
    schema = "hr",  
    table = "project_sequences",  
    initialValue = 1000  
)  

import java.util.HashSet;

public class Task180425 {
    public static void main(String[] args) {
        HashSet<Short> set = new HashSet<>();
        for (short i = 0; i < 100; i++) {
            set.add(i);
            set.remove(i - 1);
        }
        System.out.println(set.size());
    }
}

@Entity  
@OptimisticLock(type = OptimisticLockType.ALL)  
public class Account {  
    @Id  
    private Long id;  

    @OptimisticLock(excluded = true) // Не проверяется при обновлении  
    private String secretCode;  
}  

UPDATE account SET ... WHERE id = ? AND (secretCode IS NULL OR secretCode = ?)  

spring.jpa.properties.hibernate.optimistic_lock.strategy=all  

@ExceptionHandler(OptimisticLockException.class)  
public void handleConflict() { /* ... */ }  

@Entity  
@OptimisticLock(type = OptimisticLockType.ALL)  
public class Product {  
    @Version  
    private Long version; // Дополнительная проверка  
}  

@Entity  
@OptimisticLock(type = OptimisticLockType.DIRTY)  
public class User {  
    @OptimisticLock(excluded = true)  
    private LocalDateTime lastLogin;  
}