Windows 2003 server web - Object identity and equality 9.2.2 The scope of
Object identity and equality 9.2.2 The scope of object identity As application developers, we identify an object using Java object identity (a==b). If an object changes state, is the Java identity guaranteed to be the same in the new state? In a layered application, that may not be the case. In order to explore this, it s extremely important to understand the relationship between Java identity, a==b, and database identity, x.getId().equals( y.getId() ). Sometimes they re equivalent; sometimes they aren t. We refer to the conditions under which Java identity is equivalent to database identity as the scope of object identity. For this scope, there are three common choices: A primitive persistence layer with no identity scope makes no guarantees that if a row is accessed twice the same Java object instance will be returned to the application. This becomes problematic if the application modifies two different instances that both represent the same row in a single unit of work. (How should we decide which state should be propagated to the database?) A persistence layer using persistence context-scoped identity guarantees that, in the scope of a single persistence context, only one object instance represents a particular database row. This avoids the previous problem and also allows for some caching at the context level. Process-scoped identity goes one step further and guarantees that only one object instance represents the row in the whole process (JVM). For a typical web or enterprise application, persistence context-scoped identity is preferred. Process-scoped identity does offer some potential advantages in terms of cache utilization and the programming model for reuse of instances across multiple units of work. However, in a pervasively multithreaded application, the cost of always synchronizing shared access to persistent objects in the global identity map is too high a price to pay. It s simpler, and more scalable, to have each thread work with a distinct set of persistent instances in each persistence context. We would say that Hibernate implements persistence context-scoped identity. So, by nature, Hibernate is best suited for highly concurrent data access in multiuser applications. However, we already mentioned some issues you ll face when objects aren t associated with a persistence context. Let s discuss this with an example. The Hibernate identity scope is the scope of a persistence context. Let s see how this works in code with Hibernate APIs the Java Persistence code is the
If you are searching for cheap webhost for your web application, please visit MySQL5 Web Hosting services.