Hibernate Locking

Locking refers to actions taken to prevent data in a relational database from changing between the time it is read and the time that it is used.

Your locking strategy can be either optimistic or pessimistic.

Locking strategies

Optimistic

Optimistic locking assumes that multiple transactions can complete without affecting each other, and that therefore transactions can proceed without locking the data resources that they affect. Before committing, each transaction verifies that no other transaction has modified its data. If the check reveals conflicting modifications, the committing transaction rolls back^[1].

Pessimistic

Pessimistic locking assumes that concurrent transactions will conflict with each other, and requires resources to be locked after they are read and only unlocked after the application has finished using the data.

Hibernate provides mechanisms for implementing both types of locking in your applications.

Optimistic

When your application uses long transactions or conversations that span several database transactions, you can store versioning data, so that if the same entity is updated by two conversations, the last to commit changes is informed of the conflict, and does not override the other conversation's work. This approach guarantees some isolation, but scales well and works particularly well in Read-Often Write-Sometimes situations.
Hibernate provides two different mechanisms for storing versioning information, a dedicated version number or a timestamp.

Version number

Timestamp

A version or timestamp property can never be null for a detached instance. Hibernate detects any instance with a null version or timestamp as transient, regardless of other unsaved-value strategies that you specify. Declaring a nullable version or timestamp property is an easy way to avoid problems with transitive reattachment in Hibernate, especially useful if you use assigned identifiers or composite keys.

Here, the version property is mapped to the OPTLOCK column, and the entity manager uses it to detect conflicting updates, and prevent the loss of updates that would be overwritten by a last-commit-wins strategy.

The version column can be any kind of type, as long as you define and implement the appropriateUserVersionType.

Your application is forbidden from altering the version number set by Hibernate. To artificially increase the version number, see the documentation for properties LockModeType.OPTIMISTIC_FORCE_INCREMENT orLockModeType.PESSIMISTIC_FORCE_INCREMENTcheck in the Hibernate Entity Manager reference documentation.

If the version number is generated by the database, such as a trigger, use the annotation@org.hibernate.annotations.Generated(GenerationTime.ALWAYS).

5.1.2. Timestamp

Timestamps are a less reliable way of optimistic locking than version numbers, but can be used by applications for other purposes as well. Timestamping is automatically used if you the @Version annotation on a Date or Calendar.

Example 5.3. Using timestamps for optimistic locking

@Entity
public class Flight implements Serializable {
...
    @Version
    public Date getLastUpdate() { ... }
} 

Hibernate can retrieve the timestamp value from the database or the JVM, by reading the value you specify for the @org.hibernate.annotations.Source annotation. The value can be either org.hibernate.annotations.SourceType.DBor org.hibernate.annotations.SourceType.VM. The default behavior is to use the database, and is also used if you don't specify the annotation at all.

The timestamp can also be generated by the database instead of Hibernate, if you use the@org.hibernate.annotations.Generated(GenerationTime.ALWAYS) annotation.

5.2. Pessimistic

Typically, you only need to specify an isolation level for the JDBC connections and let the database handle locking issues. If you do need to obtain exclusive pessimistic locks or re-obtain locks at the start of a new transaction, Hibernate gives you the tools you need.

5.2.1. The LockMode class

The LockMode class defines the different lock levels that Hibernate can acquire.

LockMode.WRITE	acquired automatically when Hibernate updates or inserts a row.
LockMode.UPGRADE	acquired upon explicit user request using SELECT ... FOR UPDATE on databases which support that syntax.
LockMode.UPGRADE_NOWAIT	acquired upon explicit user request using a SELECT ... FOR UPDATE NOWAIT in Oracle.
LockMode.READ	acquired automatically when Hibernate reads data under Repeatable Read orSerializable isolation level. It can be re-acquired by explicit user request.
LockMode.NONE	The absence of a lock. All objects switch to this lock mode at the end of a Transaction. Objects associated with the session via a call to update() orsaveOrUpdate() also start out in this lock mode.

The explicit user request mentioned above occurs as a consequence of any of the following actions:

• A call to Session.load(), specifying a LockMode.
• A call to Session.lock().
• A call to Query.setLockMode().

If you call Session.load() with option UPGRADE or UPGRADE_NOWAIT, and the requested object is not already loaded by the session, the object is loaded using SELECT ... FOR UPDATE. If you call load() for an object that is already loaded with a less restrictive lock than the one you request, Hibernate calls lock() for that object.

Session.lock() performs a version number check if the specified lock mode is READ, UPGRADE, or UPGRADE_NOWAIT. In the case of UPGRADE or UPGRADE_NOWAIT, SELECT ... FOR UPDATE syntax is used.

If the requested lock mode is not supported by the database, Hibernate uses an appropriate alternate mode instead of throwing an exception. This ensures that applications are portable.