CRUD Operation

1. Question and Concepts

1.1 @Transactional with Hibernate

• opens a transaction before method execution and commits/rolls back after.
• Executes Hibernate operations (INSERT/UPDATE/DELETE).
• Without @Transactional : insert may not execute (no commit), or may hit LazyInitializationException.

1.2 save()

save() does two things: insert or update

1. If the object is new : it will insert it into the database.
2. If the object already exists : it will update the existing record.

How does it know if it’s new or existing?

It looks at the ID field (@Id):

1. ID is null or not in DB → new → insert
2. ID exists in DB → update

1.3 Why Hibernate runs SELECT before INSERT?

• Manual ID / merge() → check if entity exists or not.
• ID generation Strategy(AUTO, TABLE, SEQUENCE).
• Relationship checks (@ManyToOne, @OneToOne).

  sql: insert into user (id, name,email) values (1, 'a','a@gmail.com');

  Hibernate: select u1_0.uid,u1_0.email,u1_0.name from user u1_0 where u1_0.uid=?
  Hibernate: insert into user (email,name,uid) values (?,?,?)

1.4 Custom / Dervied findBy method

Rules

• Define abstract methods in the repository.
• Format: findBy<EntityClassPropertyName>(DataType propertyArgumentName).
• Return type: allow zero or more records.
• Property name: first letter uppercase after findBy.

Entity

@Entity
public class User{
  private String name;

  @Column(name="email")
  private String emailAddress;
}

Repository

public interface UserRepository extends JpaRepository<User, Integer> {

  List<User> findByName(String n);
  List<User> findByEmailAddress(String e);
  List<User> findByNameAndEmailAddress(String e,String n);
  List<User> findByNameOrEmailAddress(String e,String n);
}

2. Insert

save()

• SQL:

  INSERT INTO user VALUES (?, ?, ?);

• Inserts a single record.

Example:

User u = new User(1, "a", "a@gmail.com");
userRepository.save(u);

saveAll()

• SQL:

INSERT INTO user VALUES (?, ?, ?);
INSERT INTO user VALUES (?, ?, ?);

• Inserts multiple records at once.

Example:

List<User> users = new ArrayList<>();
users.add(new User(2, "b", "b@gmail.com"));
users.add(new User(3, "c", "c@gmail.com"));
userRepository.saveAll(users);

3. Update

    // load and save
public void updateUserById(int id, String email) {

    Optional<User> optional = userRepository.findById(id);
    if (optional.isPresent()) {
      User user = optional.get();
      user.setEmail(email);
      userRepository.save(user);
      } 
    else System.out.println("No record Found!");
}

4. Read / Select

findAll()

• SQL:

  SELECT * FROM user;

• Returns all records from the table.

Example:

List<User> users = userRepository.findAll();
users.forEach(u -> {
  System.out.println(u.getId()+" "+u.getName()+" "+u.getEmail());
});

findById()

• SQL:

  SELECT * FROM user WHERE id = ?;

• Always points to the identifier column of the entity.
• Returns single record wrapped in Optional<T> no record may be present.

Example:

Optional<User> userOptional = userRepository.findById(1L);
if (userOptional.isPresent()) {
    User u = userOptional.get();
    System.out.println(u.getId() + " " + u.getName() + " " + u.getEmail());
} else {
    System.out.println("User not found!");
}

5. Delete

deleteById

• The method does not return anything.
• should check record existence ( with existsById) before calling it to avoid exceptions.

public boolean deleteUserById(int id) {
    if (userRepository.existsById(id)) {
        userRepository.deleteById(id);
        return true; // deleted
    }
    return false; // not found
}

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Github Code : CRUD Operation