A- Managing transactions:

To manage transactions in Hibernate, you can use the Hibernate Transaction API. A transaction is a sequence of operations that must either complete in its entirety or be rolled back. To start a transaction, you can call the beginTransaction() method on the Session object. Once the transaction is complete, you can either commit it or rollback it. Here is an example:

Session session = sessionFactory.openSession();
Transaction tx = null;
try {
    tx = session.beginTransaction();
    // Perform database operations
    session.saveOrUpdate(person);
    session.saveOrUpdate(address);
    tx.commit();
} catch (Exception e) {
    if (tx != null) {
        tx.rollback();
    }
    throw e;
} finally {
    session.close();
}

In this example, we start a transaction using the beginTransaction() method, perform some database operations, and then commit the transaction using the commit() method. If an exception is thrown, we rollback the transaction using the rollback() method. Finally, we close the session.

B- Handling concurrency:

Concurrency refers to situations where multiple users or threads access the same data at the same time. This can cause data inconsistencies and errors if not handled properly. Hibernate provides several mechanisms to handle concurrency, including optimistic locking and pessimistic locking.

Optimistic locking is a technique where a version number is associated with each record in the database. When a user retrieves a record, the version number is also retrieved. When the user updates the record, the version number is checked to ensure that no other user has updated the record since it was retrieved. If the version number matches, the record is updated. Otherwise, an exception is thrown.

Person person = session.load(Person.class, id);
person.setName(newName);
session.update(person);

Pessimistic locking is a technique where a lock is placed on the record when it is retrieved, preventing other users from accessing the record until the lock is released.

Person person = session.get(Person.class, id, LockMode.PESSIMISTIC_WRITE);
person.setName(newName);
session.update(person);

C- Using Hibernate’s built-in support for transactions and locking:

Hibernate provides built-in support for transactions and locking using the Transaction and Lock classes. Here is an example:

Session session = sessionFactory.openSession();
Transaction tx = session.beginTransaction();

try {
    Person person = session.get(Person.class, id, LockMode.UPGRADE);
    person.setName(newName);
    session.update(person);
    tx.commit();
} catch (Exception e) {
    if (tx != null) {
        tx.rollback();
    }
    throw e;
} finally {
    session.close();
}

In this example, we start a transaction using the Transaction class, retrieve a Person object using the LockMode.UPGRADE lock, update the object, and then commit the transaction using the commit() method. If an exception is thrown, we rollback the transaction using the rollback() method. Finally, we close the session.