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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 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 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.
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.
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.
The version number mechanism for optimistic locking is provided through a @Version
annotation.
Example 5.1. The @Version annotation
@Entity
public class Flight implements Serializable {
...
@Version
@Column(name="OPTLOCK")
public Integer getVersion() { ... }
}
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 appropriate
UserVersionType.
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 or LockModeType.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).
Example 5.2. Declaring a version property in hbm.xml
<version
column="version_column"
name="propertyName"
type="typename"
access="field|property|ClassName"
unsaved-value="null|negative|undefined"
generated="never|always"
insert="true|false"
node="element-name|@attribute-name|element/@attribute|."
/>
| column | The name of the column holding the version number. Optional, defaults to the property name. |
| name | The name of a property of the persistent class. |
| type | The type of the version number. Optional, defaults to
|
| access | Hibernate's strategy for accessing the property value. Optional, defaults to
|
| unsaved-value | Indicates that an instance is newly instantiated and thus unsaved. This distinguishes it
from detached instances that were saved or loaded in a previous session. The default value,
|
| generated | Indicates that the version property value is generated by the database. Optional, defaults
to |
| insert | Whether or not to include the |
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.DB or
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.
Example 5.4. The timestamp element in hbm.xml
<timestamp
column="timestamp_column"
name="propertyName"
access="field|property|ClassName"
unsaved-value="null|undefined"
source="vm|db"
generated="never|always"
node="element-name|@attribute-name|element/@attribute|."
/>
| column | The name of the column which holds the timestamp. Optional, defaults to the property namel |
| name | The name of a JavaBeans style property of Java type Date or Timestamp of the persistent class. |
| access | The strategy Hibernate uses to access the property value. Optional, defaults to
|
| unsaved-value | A version property which indicates than instance is newly
instantiated, and unsaved. This distinguishes it from detached instances that were saved or loaded in a
previous session. The default value of |
| source | Whether Hibernate retrieves the timestamp from the database or the current JVM. Database-based timestamps incur an overhead because Hibernate needs to query the database each time to determine the incremental next value. However, database-derived timestamps are safer to use in a clustered environment. Not all database dialects are known to support the retrieval of the database's current timestamp. Others may also be unsafe for locking, because of lack of precision. |
| generated | Whether the timestamp property value is generated by the database. Optional, defaults to
|
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.
Hibernate always uses the locking mechanism of the database, and never lock objects in memory.
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 |
| LockMode.UPGRADE_NOWAIT | acquired upon explicit user request using a |
| LockMode.UPGRADE_SKIPLOCKED | acquired upon explicit user request using a |
| LockMode.READ | acquired automatically when Hibernate reads data under Repeatable Read or Serializable 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 |
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,
UPGRADE_NOWAIT or UPGRADE_SKIPLOCKED, 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, UPGRADE_NOWAIT or
UPGRADE_SKIPLOCKED. In the case of UPGRADE,
UPGRADE_NOWAIT or UPGRADE_SKIPLOCKED, 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.