Pessimistic Locking in JPA

Introduction

Locking is an essential way to avoid data inconsistancy when two sources try to update same data simultaneously. In the previous blog, we covered the optimistic locking, in which a lock is applied to avoid dirty reading.
In this blog, we'll cover the Pessimistic locking. Pessimistic Locking is useful when we need to lock the data before a transaction, so that it cannot be modified by any other transaction. This lock retains until the transaction commits or roll back.

There are three types of pessimistic lock mode:
PESSIMISTIC_READ: for shared lock, it prevents data deletion or updation
PESSIMISTIC_WRITE: for exclusive lock, it prevents read, delete or update the data
PESSIMISTIC_FORCE_INCREMENT: similar to PESSIMISTIC_WRITE including a version attribute

Setting a Pessimistic Lock

There are several ways to set up a pessimistic lock:

1. Find method of entityManager
Pass a parameter for LockModeType in find function.

entityManager.find(User.class, userId, LockModeType.PESSIMISTIC_READ);

2. setLockMode of query
you can use setLockMode function of Query to enable the lock. 

Query query = entityManager.createQuery("from User where userId = :userId");
query.setParameter("userId", userId);
query.setLockMode(LockModeType.PESSIMISTIC_WRITE);
query.getResultList()

3. Refresh
Pass parameter of LockModeType in refresh, which a method of enitityManager. 

User result = entityManager.find(User.class, userId);
entityManager.refresh(result, LockModeType.PESSIMISTIC_FORCE_INCREMENT);

4. Explicit Locking
Another method, lock of enitityManager is available to directly apply lock on a particular enitiy. 

User result = entityManager.find(User.class, userId);
entityManager.lock(result, LockModeType.PESSIMISTIC_WRITE);

5. NamedQuery
You can also use Named query. 

@NamedQuery(name="userLock",
  query="SELECT s FROM User s WHERE s.id LIKE :userId",
  lockMode = PESSIMISTIC_READ)