import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;
import java.util.Base64;

public class HmacExample {
    public static void main(String[] args) throws Exception {
        String message = "Hello, Java Crypto API!";
        String secretKey = "my_secret_key";

        // Создание HMAC SHA-256
        Mac mac = Mac.getInstance("HmacSHA256");
        SecretKeySpec keySpec = new SecretKeySpec(secretKey.getBytes(), "HmacSHA256");
        mac.init(keySpec);

        // Вычисление HMAC
        byte[] hmacBytes = mac.doFinal(message.getBytes());
        String hmacBase64 = Base64.getEncoder().encodeToString(hmacBytes);

        System.out.println("HMAC (Base64): " + hmacBase64);
    }
}

HMAC (Base64): q8phcxg9DpPpMfKkczjRkkv6OiOBv2YoTbYJ46qZaYc=

public static boolean verifyHmac(String message, String receivedHmac, String secretKey) throws Exception {
    Mac mac = Mac.getInstance("HmacSHA256");
    SecretKeySpec keySpec = new SecretKeySpec(secretKey.getBytes(), "HmacSHA256");
    mac.init(keySpec);
    
    // Вычисляем HMAC заново
    byte[] hmacBytes = mac.doFinal(message.getBytes());
    String computedHmac = Base64.getEncoder().encodeToString(hmacBytes);

    // Сравниваем с полученным HMAC
    return computedHmac.equals(receivedHmac);
}

Редактировать
@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "username", nullable = false)
    private String username;

    @Column(name = "email", unique = true)
    private String email;
}

User user = new User();
user.setUsername("john_doe");
user.setEmail("john@example.com");
userRepository.save(user);

INSERT INTO users (username, email) VALUES ('john_doe', 'john@example.com');

user.setEmail("new_email@example.com");
userRepository.save(user);

UPDATE users SET email = 'new_email@example.com' WHERE id = 1;
Разработчик не пишет SQL-код вручную — ORM делает это автоматически.

User user = new User(); // Transient
user.setUsername("Alice");

entityManager.persist(user); // Managed
entityManager.detach(user); // Detached
entityManager.remove(user); // Removed

User user1 = entityManager.find(User.class, 1); // SQL выполняется
User user2 = entityManager.find(User.class, 1); // Берётся из кеша
Первый запрос загрузит данные из БД, второй возьмёт их из памяти.

@Entity
public class User {
    @OneToMany(mappedBy = "user", fetch = FetchType.LAZY)
    private List<Order> orders;
}

User user = userRepository.findById(1L).get();
List<Order> orders = user.getOrders(); // SQL-запрос будет только здесь, если Lazy

public class Task270225 {
    public static void main(String[] args) {
        List<Integer> list = new LinkedList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        
        (list).removeFirst();
        list.remove(1);

        System.out.println(list);
    }
}

@Bean
public DataSource dataSource() {
    return new DriverManagerDataSource(url, username, password);
}

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactory(DataSource dataSource) {
    LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
    em.setDataSource(dataSource);
    em.setPackagesToScan("com.example.domain");
    em.setJpaVendorAdapter(new HibernateJpaVendorAdapter());
    return em;
}

@Bean
public PlatformTransactionManager transactionManager(EntityManagerFactory emf) {
    return new JpaTransactionManager(emf);
}

@Bean
public LocalContainerEntityManagerFactoryBean entityManagerFactory(DataSource dataSource) {
    LocalContainerEntityManagerFactoryBean em = new LocalContainerEntityManagerFactoryBean();
    em.setDataSource(dataSource);
    em.setPackagesToScan("com.example.domain");
    em.setJpaVendorAdapter(new HibernateJpaVendorAdapter());
    return em;
}

spring.jpa.properties.hibernate.cache.use_second_level_cache=true
spring.jpa.properties.hibernate.cache.region.factory_class=org.hibernate.cache.ehcache.EhCacheRegionFactory

import javax.persistence.Entity;

import jakarta.persistence.Entity;

User user = new User();

entityManager.persist(user); // Сущность становится управляемой

entityManager.detach(user); // Сущность становится отсоединенной

entityManager.remove(user); // Сущность помечена на удаление

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=secret
spring.datasource.driver-class-name=com.mysql.cj.jdbc.Driver

spring.jpa.show-sql=true
spring.jpa.hibernate.ddl-auto=update
spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL8Dialect

spring.jpa.entity-scan.packages=com.example.entities

spring.jpa.properties.hibernate.cache.use_second_level_cache=true
spring.jpa.properties.hibernate.cache.region.factory_class=org.hibernate.cache.ehcache.EhCacheRegionFactory

spring.datasource.hikari.maximum-pool-size=10
spring.datasource.hikari.minimum-idle=2

public interface UserRepository extends JpaRepository<User, Long> { ... }

@Entity
@Table(name = "users")
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "username", nullable = false, length = 50)
    private String username;

    @Column(name = "email", unique = true)
    private String email;

    // Геттеры и сеттеры
}

public interface UserRepository extends JpaRepository<User, Long> { ... }

spring.datasource.url=jdbc:mysql://localhost:3306/mydb
spring.datasource.username=root
spring.datasource.password=secret
spring.jpa.hibernate.ddl-auto=update
spring.jpa.show-sql=true

import java.util.HashSet;
import java.util.Set;

record Person28(
        String name,
        int age
) {}

public class Task280225 {
    public static void main(String[] args) {
        Set<Person28> set = new HashSet<>();
        set.add(new Person28("Alice", 25));
        set.add(new Person28("Bob", 30));
        set.add(new Person28("Alice", 25));
        set.remove(set.iterator().next());

        System.out.println(set);
    }
}

import javax.persistence.Table;

import jakarta.persistence.Table;

@Table(name = "users")

@Table(name = "users", schema = "public")

@Table(name = "users", catalog = "my_catalog")

@Table(name = "users", uniqueConstraints = {
    @UniqueConstraint(columnNames = {"username"}),
    @UniqueConstraint(columnNames = {"email"})
})

@Table(name = "users", indexes = {
    @Index(name = "idx_username", columnList = "username"),
    @Index(name = "idx_email", columnList = "email")
})

spring.jpa.hibernate.ddl-auto=update

@Entity
@Table(
    name = "users",
    schema = "public",
    catalog = "my_catalog",
    uniqueConstraints = {
        @UniqueConstraint(columnNames = {"username"}),
        @UniqueConstraint(columnNames = {"email"})
    },
    indexes = {
        @Index(name = "idx_username", columnList = "username"),
        @Index(name = "idx_email", columnList = "email")
    }
)
public class User {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "username", nullable = false, length = 50)
    private String username;

    @Column(name = "email", unique = true)
    private String email;

    // Геттеры и сеттеры
}

spring.jpa.show-sql=true

spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL8Dialect