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EclipseLink/UserGuide/JPA/Basic JPA Development/Caching/Query Options

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Revision as of 12:44, 29 May 2012 by (Talk | contribs) (Query Options and In-memory Querying)

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Query Options and In-memory Querying

JPA defines standard query hints for configuring how a query interacts with the shared persistence unit cache (L2). EclipseLink also provides some additional query hints for configuring the cache usage.

Entities can be accessed through JPA using either find() or queries. find() will first check the persistence context cache (L1) for the Id, if the object is not found it will check the shared persistence unit cache (L2), if the object is still not found it will access the database. By default all queries will access the database, unless querying by Id or by cache indexed fields. Once the query retrieves the rows from the database, it will resolve each row with the cache. If the object is already in the cache, then the row will be discarded, and the object will be used. If the object is not in the shared cache, then it will be built from the row and put into the shared cache. A copy will also be put in the persistence context cache and returned as the query result.

This is the general process, but it differs if the transaction is dirty. If the transaction is diry then the shared cache will be ignored and objects will be built directly from the row data.

A transaction is considered dirty in the following circumstances:

  • A flush() has written changes to the database.
  • A pessimistic lock query has been executed.
  • An update or delete query has been executed.
  • A native SQL query has been executed.
  • This persistence unit property "eclipselink.transaction.join-existing" is used.
  • The JDBC connection has been unwrapped from the EntityManager.
  • The UnitOfWork API beginEarlyTransaction has been called.

Entities can also be configured to be isolated, or non cacheable, in which case they will never be placed in the shared cache (See Shared, Isolated, Protected, Weak and Read-only Cache.

Configuring Query Results Cache

The query results cache is configured through query hints.

Query Results Cache Query Hints
Hint Description Default Required?
eclipselink.query-results-cache Enable query results cache for the named query. no cached results Required
eclipselink.query-results-cache.size Sets the fixed size of the query results cache for the named query. The size is the number of queries with unique parameters to cache. If the query has no parameters, a value of 1 should be used. 100 Optional
eclipselink.query-results-cache.expiry Sets the expiry time in milliseconds of the cached results. no expiry Optional
eclipselink.query-results-cache.expiry-time-of-day Configures the time of day expiry. Valid values are string time format, "HH:MM:SS". no expiry Optional
eclipselink.query-results-cache.randomize-expiry Set if the expiry time should be randomized by 10% of the set expiry time. This can be used to avoid a bottleneck on a fixed expiry time. false Optional
eclipselink.query-results-cache.type Set the cache type. Valid values are defined in the CacheType enum. Normally an LRU fixed sized CACHE is used, a FULL, WEAK or SOFT cache can also be used. CACHE Optional
In-memory query example
Query query = em.createQuery("Select e from Employee e where e.firstName like :firstName and e.lastName like :lastName");
query.setParameter("firstName", "B%");
query.setParameter("lastName", "S%");
List<Employee> employees = query.getResultList();

Version: 2.4 DRAFT
Other versions...

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