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Difference between revisions of "EclipseLink/UserGuide/JPA/Basic JPA Development/Mapping/Locking/Optimistic Locking"

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=Optimistic Locking=
 
=Optimistic Locking=
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We recommend using EclipseLink optimistic locking. With optimistic locking, all users have read access to the data. When a user attempts to write a change, the application checks to ensure the data has not changed since the user read the data.
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Optimistic locking in a stateless environmentIn a stateless environment, take care to avoid processing out-of-date (stale) data
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A common strategy for avoiding stale data is to implement optimistic locking, and store the optimistic lock values in the object.
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This solution requires careful implementation if the stateless application serializes the objects, or sends the contents of the object to the client in an alternative format. In this case, transport the optimistic lock values to the client in the HTTP contents of an edit page. You must then use the returned values in any write transaction to ensure that the data did not change while the client was performing its work.
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You can use [[Introduction%20to%20Descriptors%20(ELUG)#Optimistic Version Locking Policies|Optimistic Version Locking]] or [[Introduction%20to%20Descriptors%20(ELUG)#Optimistic Field Locking Policies|Optimistic Field Locking]] locking policies. We recommend using version locking policies.
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==Optimistic Version Locking==
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Use the <tt>@Version</tt> annotation to enable the JPA-managed optimistic locking by specifying the version field or property of an entity class that serves as its optimistic lock value (recommended).
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When choosing a version field or property, ensure that the following is true:
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* there is only one version field or property per entity;
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* you choose a property or field persisted to the primary table (see Section 9.1.1 "Table Annotation" of the [http://jcp.org/en/jsr/detail?id=220 JPA Specification]);
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* your application does not modify the version property or field.
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{| class="Note oac_no_warn" width="80%" border="1" frame="hsides" rules="groups" cellpadding="3" frame="hsides" rules="groups"
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| align="left" |
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'''Note:''' The field or property type must either be a numeric type (such as <tt>Number</tt>, <tt>long</tt>, <tt>int</tt>, <tt>BigDecimal</tt>, and so on), or a <tt>java.sql.Timestamp</tt>. We recommend using a numeric type.
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|}
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The <tt>@Version</tt> annotation does not have attributes.
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The [[#Example 18-10|Usage of @Version Annotation]] example shows how to use this annotation to specify property <tt>getVersionNum</tt> as the optimistic lock value. In this example, the column name for this property is set to <tt>OPTLOCK</tt> (see Section 9.1.5 "Column Annotation" of the [http://jcp.org/en/jsr/detail?id=220 JPA Specification]) instead of the default column name for the property.
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<span id="Example 18-10"></span>
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''''' Usage of @Version Annotation'''''
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<source lang="java">
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@Entity
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public class Employee implements Serializable {
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    ...
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    @Version
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    @Column(name="OPTLOCK")
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    protected int getVersionNum() {
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        return versionNum;
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    }
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    ...
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}
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</source>
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The <tt>@Version</tt> annotation supports the use of EclipseLink converters (see [[Using%20EclipseLink%20JPA%20Extensions%20(ELUG)#Using EclipseLink JPA Converters|Using EclipseLink JPA Converters]]).
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Revision as of 14:02, 15 June 2010

Optimistic Locking

We recommend using EclipseLink optimistic locking. With optimistic locking, all users have read access to the data. When a user attempts to write a change, the application checks to ensure the data has not changed since the user read the data.

Optimistic locking in a stateless environmentIn a stateless environment, take care to avoid processing out-of-date (stale) data A common strategy for avoiding stale data is to implement optimistic locking, and store the optimistic lock values in the object. This solution requires careful implementation if the stateless application serializes the objects, or sends the contents of the object to the client in an alternative format. In this case, transport the optimistic lock values to the client in the HTTP contents of an edit page. You must then use the returned values in any write transaction to ensure that the data did not change while the client was performing its work.

You can use Optimistic Version Locking or Optimistic Field Locking locking policies. We recommend using version locking policies.

Optimistic Version Locking

Use the @Version annotation to enable the JPA-managed optimistic locking by specifying the version field or property of an entity class that serves as its optimistic lock value (recommended).

When choosing a version field or property, ensure that the following is true:

  • there is only one version field or property per entity;
  • you choose a property or field persisted to the primary table (see Section 9.1.1 "Table Annotation" of the JPA Specification);
  • your application does not modify the version property or field.


Note: The field or property type must either be a numeric type (such as Number, long, int, BigDecimal, and so on), or a java.sql.Timestamp. We recommend using a numeric type.


The @Version annotation does not have attributes.

The Usage of @Version Annotation example shows how to use this annotation to specify property getVersionNum as the optimistic lock value. In this example, the column name for this property is set to OPTLOCK (see Section 9.1.5 "Column Annotation" of the JPA Specification) instead of the default column name for the property.

Usage of @Version Annotation

 @Entity
 public class Employee implements Serializable {
     ...
     @Version
     @Column(name="OPTLOCK")
     protected int getVersionNum() {
         return versionNum;
     }
     ...
 }

The @Version annotation supports the use of EclipseLink converters (see Using EclipseLink JPA Converters).



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