Jump to: navigation, search

Difference between revisions of "EclipseLink/Examples/JPA/Caching"

(How to use EclispeLink Caching)
Line 6: Line 6:
  
 
By default EclipseLink uses a shared object cache, that caches a subset of all objects read and persisted for the persistence unit.
 
By default EclipseLink uses a shared object cache, that caches a subset of all objects read and persisted for the persistence unit.
The EclipseLink shared cache differs from the local EntityManager cache.  The shared cache exists for the duration of the persistence unit (EntityManagerFactory, or server) and is shared by all EntityManagers and users of the persistence unit.  The local EntityManager cache only is not shared, and only exists for the duration of the EntityManager or transaction.
+
The EclipseLink shared cache differs from the local EntityManager cache.  The shared cache exists for the duration of the persistence unit (EntityManagerFactory, or server) and is shared by all EntityManagers and users of the persistence unit.  The local EntityManager cache is not shared, and only exists for the duration of the EntityManager or transaction.
  
 
The benefit of the shared cache, is that once an object has been read, if it is read again by <code>Id</code>, the database does not need to be accessed.
 
The benefit of the shared cache, is that once an object has been read, if it is read again by <code>Id</code>, the database does not need to be accessed.
Line 222: Line 222:
 
| headers="r10c1-t19 r1c5-t19" align="left" | <br>
 
| headers="r10c1-t19 r1c5-t19" align="left" | <br>
 
|}
 
|}
 
  
 
== Caching in Clustered Environments ==
 
== Caching in Clustered Environments ==
  
 
==Relevant API==
 
==Relevant API==

Revision as of 16:46, 24 November 2009


In progress

How to use EclispeLink Caching

By default EclipseLink uses a shared object cache, that caches a subset of all objects read and persisted for the persistence unit. The EclipseLink shared cache differs from the local EntityManager cache. The shared cache exists for the duration of the persistence unit (EntityManagerFactory, or server) and is shared by all EntityManagers and users of the persistence unit. The local EntityManager cache is not shared, and only exists for the duration of the EntityManager or transaction.

The benefit of the shared cache, is that once an object has been read, if it is read again by Id, the database does not need to be accessed.

The limitation of the shared cache, is that if the database is changed directly through JDBC, or by another application or server, the objects in the shared cache will be stale.

EclipseLink offers several mechanism to deal with stale data including:

  • Refreshing
  • Invalidation
  • Optimistic locking
  • Cache coordination

The shared cache can also be disabled. This can be done using the persistence unit property:

 "eclipselink.cache.shared.default"="false"

Or can be selectively enabled/disabled using the @Cache annotation.

EclipseLink also offers several different caching strategies, the configure how many objects are cached, and how much memory is used.

Note, the CacheType NONE should not be used to disable the cache, instead shared should be set to false.

Option (Identity Map) Caching Guaranteed Identity Memory Use

Full Identity Map

Yes

Yes

Very High

Weak Identity Map

Yes

Yes

Low

Soft Identity Map

Yes

Yes

High

Soft Cache Weak Identity Map
Hard Cache Weak Identity Map

Yes

Yes

Medium-high

No Identity Map

No

No

None

See the documentation for details on these types.


Using the @Cache Annotation

You may define the @Cache annotation on the following:

  • @Entity
  • @MappedSuperclass;
  • the root of the inheritance hierarchy (if applicable).

If you define the @Cache annotation on an inheritance subclass, the annotation will be ignored. If you define the @Cache annotation on @Embeddable EclipseLink will throw an exception.

Example of @Cache Annotation

 @Target({TYPE})
 @Retention(RUNTIME)
 public @interface Cache {
   CacheType type() default SOFT_WEAK;
   int size() default 100;
   boolean shared() default true;
   int expiry() default -1;
   TimeOfDay expiryTimeOfDay() default @TimeOfDay(specified=false);
   boolean alwaysRefresh() default false;
   boolean refreshOnlyIfNewer() default false;
   boolean disableHits() default false; 
   CacheCoordinationType coordinationType() default SEND_OBJECT_CHANGES;
 }

Attributes of the @Cache Annotation

Attribute Description Default Required or Optional Override with Persistence Unit Property

type

Set this attribute to the type (org.eclipse.persistence.annotations.CacheType enumerated type) of the cache that you will be using:

  • FULL
  • WEAK
  • SOFT
  • SOFT_WEAK
  • HARD_WEAK
  • CACHE
  • NONE

CacheType.SOFT_WEAK

optional

  • eclipselink.cache.type.<ENTITY>
  • eclipselink.cache.type.default

size

Set this attribute to an int value to define the size of cache to use (number of objects).

100



shared

Indicate whether cached instances should be in the shared cache or in a client isolated cache (see Isolated Client Session Cache).

  • true - use shared cache for cached instances;
  • false - use client isolated cache for cached instances.

true

optional


expiry

The int value to enable the expiration of the cached instance after a fixed period of time (milliseconds). Queries executed against the cache after this will be forced back to the database for a refreshed copy.

-1 (no expiry)

optional


expiryTimeOfDay

Specific time of day (org.eclipse.persistence.annotations.TimeOfDay) when the cached instance will expire. Queries executed against the cache after this will be forced back to the database for a refreshed copy.

@TimeOfDay(specified=false)

optional


alwaysRefresh

Set to a boolean value of true to force all queries that go to the database to always refresh the cache.

false

optional


refreshOnlyIfNewer

Set to a boolean value of true to force all queries that go to the database to refresh the cache only if the data received from the database by a query is newer than the data in the cache (as determined by the optimistic locking field).

Note: This option only applies if one of the other refreshing options, such as alwaysRefresh, is already enabled.

Note: A version field is necessary to apply this feature.

false

optional


disableHits

Set to a boolean value of true to force all queries to bypass the cache for hits, but still resolve against the cache for identity. This forces all queries to hit the database.

false

optional


coordinationType

Set this attribute to the cache coordination mode (org.eclipse.persistence.annotations.CacheCoordinationType enumerated type).

CacheCoordinationType.SEND_OBJECT_CHANGES

optional


Caching in Clustered Environments

Relevant API