CHAPTER 10 Transactions and concurrency Usually, you also
CHAPTER 10 Transactions and concurrency Usually, you also have to exit the application after you close the Session following an exception, although there are some exceptions (for example, Stale- ObjectStateException) that naturally lead to a new attempt (possibly after interacting with the application user again) in a new Session. Because these are closely related to conversations and concurrency control, we ll cover them later. FAQ Can I use exceptions for validation? Some developers get excited once they see how many fine-grained exception types Hibernate can throw. This can lead you down the wrong path. For example, you may be tempted to catch the ConstraintViolationException for validation purposes. If a particular operation throws this exception, why not display a (customized depending on the error code and text) failure message to application users and let them correct the mistake? This strategy has two significant disadvantages. First, throwing unchecked values against the database to see what sticks isn t the right strategy for a scalable application. You want to implement at least some data-integrity validation in the application layer. Second, all exceptions are fatal for your current unit of work. However, this isn t how application users will interpret a validation error they expect to still be inside a unit of work. Coding around this mismatch is awkward and difficult. Our recommendation is that you use the fine-grained exception types to display better looking (fatal) error messages. Doing so helps you during development (no fatal exceptions should occur in production, ideally) and also helps any customer-support engineer who has to decide quickly if it s an application error (constraint violated, wrong SQL executed) or if the database system is under load (locks couldn t be acquired). Programmatic transaction demarcation in Java SE with the Hibernate Transaction interface keeps your code portable. It can also run inside a managed environment, when a transaction manager handles the database resources. Programmatic transactions with JTA A managed runtime environment compatible with Java EE can manage resources for you. In most cases, the resources that are managed are database connections, but any resource that has an adaptor can integrate with a Java EE system (messaging or legacy systems, for example). Programmatic transaction demarcation on those resources, if they re transactional, is unified and exposed to the developer with JTA; javax.transaction.UserTransaction is the primary interface to begin and end transactions. The common managed runtime environment is a Java EE application server. Of course, application servers provide many more services, not only management of resources. Many Java EE services are modular installing an application server
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