Object identity and equality Later in this chapter, (Affordable web hosting)
Sunday, March 23rd, 2008Object identity and equality Later in this chapter, we ll show you how objects are added to this cache (basically, whenever they become persistent) and how you can manage this cache (by detaching objects manually from the persistence context, or by clearing the persistence context). The last two items on our list of benefits of a persistence context, the guaranteed scope of identity and the possibility to extend the persistence context to span a conversation, are closely related concepts. To understand them, you need to take a step back and consider objects in detached state from a different perspective. 9.2 Object identity and equality A basic Hibernate client/server application may be designed with server-side units of work that span a single client request. When a request from the application user requires data access, a new unit of work is started. The unit of work ends when processing is complete and the response for the user is ready. This is also called the session-per-request strategy (you can replace the word session with persistence context whenever you read something like this, but it doesn t roll off the tongue as well). We already mentioned that Hibernate can support an implementation of a possibly long-running unit of work, called a conversation. We introduce the concept of conversations in the following sections as well as the fundamentals of object identity and when objects are considered equal which can impact how you think about and design conversations. Why is the concept of a conversation useful? 9.2.1 Introducing conversations For example, in web applications, you don t usually maintain a database transaction across a user interaction. Users take a long time to think about modifications, but, for scalability reasons, you must keep database transactions short and release database resources as soon as possible. You ll likely face this issue whenever you need to guide the user through several screens to complete a unit of work (from the user s perspective) for example, to fill an online form. In this common scenario, it s extremely useful to have the support of the persistence service, so you can implement such a conversation with a minimum of coding and best scalability. Two strategies are available to implement a conversation in a Hibernate or Java Persistence application: with detached objects or by extending a persistence con text. Both have strength and weaknesses.
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