An entity manager factory is typically create at application initialization time and closed at application end.
It's creation is an expensive process. For those who are familiar with Hibernate, an entity manager factory is very much like a session factory. Actually, an entity manager factory is a wrapper on top of a session factory. Calls to the entityManagerFactory are thread safe. Thanks to the EntityManagerFactory , you can retrieve an extended entity manager.
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The extended entity manager keep the same persistence context for the lifetime of the entity manager: in other words, the entities are still managed between two transactions unless you call entityManager. You can see an entity manager as a small wrapper on top of an Hibernate session. Hibernate Entity Manager comes with Hibernate Validator configured out of the box. You don't have to override any event yourself. If you do not use Hibernate Validator annotations in your domain model, there will be no performance cost.
For more information on Hibernate Validator, please refer to the Hibernate Annotations reference guide. Like in Hibernate comparable terms in parentheses , an entity instance is in one of the following states:. New transient : an entity is new if it has just been instantiated using the new operator, and it is not associated with a persistence context.
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It has no persistent representation in the database and no identifier value has been assigned. Managed persistent : a managed entity instance is an instance with a persistent identity that is currently associated with a persistence context.
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Detached: the entity instance is an instance with a persistent identity that is no longer associated with a persistence context, usually because the persistence context was closed or the instance was evicted from the context. Removed: a removed entity instance is an instance with a persistent identity, associated with a persistence context, but scheduled for removal from the database. The EntityManager API allows you to change the state of an entity, or in other words, to load and store objects. You will find persistence with JPA easier to understand if you think about object state management, not managing of SQL statements.
Once you've created a new entity instance using the common new operator it is in new state. You can make it persistent by associating it to an entity manager:. If the DomesticCat entity type has a generated identifier, the value is associated to the instance when persist is called.
If the identifier is not automatically generated, the application-assigned usually natural key value has to be set on the instance before persist is called. Load an entity instance by its identifier value with the entity manager's find method:. In some cases, you don't really want to load the object state, but just having a reference to it ie a proxy.
You can get this reference using the getReference method.
This is especially useful to link a child to its parent without having to load the parent. You can reload an entity instance and it's collections at any time using the em. This is useful when database triggers are used to initialize some of the properties of the entity. Note that only the entity instance and its collections are refreshed unless you specify REFRESH as a cascade style of any associations:.
If you don't know the identifier values of the objects you are looking for, you need a query. Both query languages are portable across databases, the use entity and property names as identifiers instead of table and column names. You may also express your query in the native SQL of your database, with optional support from JPA for result set conversion into Java business objects.
This interface offers methods for parameter binding, result set handling, and for execution of the query. Queries are always created using the current entity manager:. A query is usually executed by invoking getResultList. This method loads the resulting instances of the query completely into memory. Entity instances retrieved by a query are in persistent state. The getSingleResult method offers a shortcut if you know your query will only return a single object. JPA 2 provides more type-safe approaches to queries.
But you can benefit form some type-safe convenience even when using JP-QL note that it's not as type-safe as the compiler has to trust you with the return type. We highly recommend the Criteria API approach. While more verbose, it provides compiler-enforced safety including down to property names which will pay off when the application will move to maintenance mode. JPA queries can return tuples of objects if projection is used. Each result tuple is returned as an object array:. The criteria API provides a type-safe approach to projection results.
Queries may specify a particular property of an entity in the select clause, instead of an entity alias. You may call SQL aggregate functions as well. Returned non-transactional objects or aggregation results are considered "scalar" results and are not entities in persistent state in other words, they are considered "read only" :. Both named and positional query parameters are supported, the Query API offers several methods to bind arguments.
The JPA specification numbers positional parameters from one. Named parameters are identifiers of the form :paramname in the query string.
Named parameters should be preferred, they are more robust and easier to read and understand:. Note that the actual program code is independent of the query language that is used, you may also define native SQL queries in metadata, or use Hibernate's native facilities by placing them in XML mapping files. Use the SqlResultSetMapping please see the Hibernate Annotations reference documentation on how to map a SQL resultset mapping or the entity mapping if the column names of the query result are the same as the names declared in the entity mapping; remember that all entity columns have to be returned for this mechanism to work :.
For more information about scalar support in named queries, please refers to the Hibernate Annotations documentation.
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You can adjust the flush mode used when executing the query as well as define the lock mode used to load the entities. Adjusting the flush mode is interesting when one must guaranty that a query execution will not trigger a flush operation. Most of the time you don't need to care about this. Adjusting the lock mode is useful if you need to lock the objects returns by the query to a certain level.
If you want to use FlushMode. See below. Query hints for performance optimization, usually are implementation specific. Hints are declared using the query. The Hibernate EJB3 implementation offers the following query hints:. The value object accept both the native type or its string equivalent eg. Please refer to the Hibernate reference documentation for more information. Transactional managed instances ie. There is no need to call a particular method to make your modifications persistent. A straightforward way to update the state of an entity instance is to find it, and then manipulate it directly, while the persistence context is open:.
Therefore Hibernate offers an alternate approach, using detached instances.
http://kinun-mobile.com/wp-content/2020-05-08/tiqon-mobile-location.php An object when loaded in the persistence context is managed by Hibernate. You can force an object to be detached ie. Many applications need to retrieve an object in one transaction, send it to the presentation layer for manipulation, and later save the changes in a new transaction. There can be significant user think and waiting time between both transactions. Applications that use this kind of approach in a high-concurrency environment usually use versioned data to ensure isolation for the "long" unit of work.
The JPA specifications supports this development model by providing for persistence of modifications made to detached instances using the EntityManager.
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