The persistence lifecycle managed instance has a primary (Web site design and hosting)
The persistence lifecycle managed instance has a primary key value set as its database identifier. (There are some variations to when this identifier is assigned to a persistent instance.) Persistent instances may be objects instantiated by the application and then made persistent by calling one of the methods on the persistence manager. They may even be objects that became persistent when a reference was created from another persistent object that is already managed. Alternatively, a persistent instance may be an instance retrieved from the database by execution of a query, by an identifier lookup, or by navigating the object graph starting from another persistent instance. Persistent instances are always associated with a persistence context. Hibernate caches them and can detect whether they have been modified by the application. There is much more to be said about this state and how an instance is managed in a persistence context. We ll get back to this later in this chapter. Removed objects You can delete an entity instance in several ways: For example, you can remove it with an explicit operation of the persistence manager. It may also become available for deletion if you remove all references to it, a feature available only in Hibernate or in Java Persistence with a Hibernate extension setting (orphan deletion for entities). An object is in the removed state if it has been scheduled for deletion at the end of a unit of work, but it s still managed by the persistence context until the unit of work completes. In other words, a removed object shouldn t be reused because it will be deleted from the database as soon as the unit of work completes. You should also discard any references you may hold to it in the application (of course, after you finish working with it for example, after you ve rendered the removal-confirmation screen your users see). Detached objects To understand detached objects, you need to consider a typical transition of an instance: First it s transient, because it just has been created in the application. Now you make it persistent by calling an operation on the persistence manager. All of this happens in a single unit of work, and the persistence context for this unit of work is synchronized with the database at some point (when an SQL INSERT occurs). The unit of work is now completed, and the persistence context is closed. But the application still has a handle: a reference to the instance that was saved. As long as the persistence context is active, the state of this instance is persistent. At
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