# The Java EE 7 Tutorial

# 42.1 Overview of Entity Locking and Concurrency

Entity data is concurrently accessed if the data in a data source is accessed at the same time by multiple applications. Ensure that the underlying data's integrity is preserved when it is accessed concurrently.

When data is updated in the database tables in a transaction, the persistence provider assumes the database management system will hold short-term read locks and long-term write locks to maintain data integrity. Most persistence providers will delay database writes until the end of the transaction, except when the application explicitly calls for a flush (that is, the application calls the `EntityManager.flush` method or executes queries with the flush mode set to `AUTO`).

By default, persistence providers use optimistic locking, where, before committing changes to the data, the persistence provider checks that no other transaction has modified or deleted the data since the data was read. This is accomplished by a version column in the database table, with a corresponding version attribute in the entity class. When a row is modified, the version value is incremented. The original transaction checks the version attribute, and if the data has been modified by another transaction, a `javax.persistence.OptimisticLockException` will be thrown, and the original transaction will be rolled back. When the application specifies optimistic lock modes, the persistence provider verifies that a particular entity has not changed since it was read from the database even if the entity data was not modified.

Pessimistic locking goes further than optimistic locking. With pessimistic locking, the persistence provider creates a transaction that obtains a long-term lock on the data until the transaction is completed, which prevents other transactions from modifying or deleting the data until the lock has ended. Pessimistic locking is a better strategy than optimistic locking when the underlying data is frequently accessed and modified by many transactions.

Note: Using pessimistic locks on entities that are not subject to frequent modification may result in decreased application performance.

## 42.1.1 Using Optimistic Locking

Use the `javax.persistence.Version` annotation to mark a persistent field or property as a version attribute of an entity. The version attribute enables the entity for optimistic concurrency control. The persistence provider reads and updates the version attribute when an entity instance is modified during a transaction. The application may read the version attribute, but must not modify the value.

Note: Although some persistence providers may support optimistic locking for entities that do not have version attributes, portable applications should always use entities with version attributes when using optimistic locking. If the application attempts to lock an entity that does not have a version attribute, and the persistence provider does not support optimistic locking for non-versioned entities, a `PersistenceException` will be thrown.

The `@Version` annotation has the following requirements.

- Only a single `@Version` attribute may be defined per entity.

- The `@Version` attribute must be in the primary table for an entity mapped to multiple tables.

- The type of the `@Version` attribute must be one of the following: `int`, `Integer`, `long`, `Long`, `short`, `Short`, or `java.sql.Timestamp`.

The following code snippet shows how to define a version attribute in an entity with persistent fields:

    @Version
    protected int version;

The following code snippet shows how to define a version attribute in an entity with persistent properties:

    @Version
    protected Short getVersion() { ... }