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A core interface in Hibernate.

There are five core interfaces that are used in almost every Hibernate application and are the foundation of the Hibernate foundation. Using these interfaces, you can store and retrieve persistent objects or control transactions.

1 Configuration interface

The Configuration object is used to configure and boot Hibernate. The application uses a configuration instance to specify the location of the mapping file and the specific properties of the Hibernate, and then creates a session factory. Even though the configuration interface is only a relatively small part of the entire Hibernate application, it is the * * objects you encounter when you start using Hibernate.

2 SessionFactory interface

The application obtains the session instance from the SessionFactory (session factory). Compared with the conference interface, this object is not exciting enough. Conversation factories are certainly not lightweight! It intends to share among multiple application threads. Typically, there is only one session factory for the entire application-for example, it is created during application initialization. However, if your application uses Hibernate to access multiple databases, you need to use a session factory for each database. The session factory caches the generated SQL statements and the mapping metadata that Hibernate uses at run time. It also holds data read in a unit of work and may be reused in future units of work (only if class and collection mappings specify that this secondary cache is desired).

3 Session interface

Session (session) interface is the main interface used by Hibernate applications. Instances of session interfaces are lightweight and inexpensive to create and destroy. This is important because your application may be creating and destroying sessions all the time, probably with every request. Hibernate sessions are not thread-safe and should be designed to be used only in one thread at a time.

Hibernate session is a concept between connection and transaction. You can simply think of a session as a cache or collection of loaded objects for a single unit of work. Hibernate can detect changes in objects in this unit of work. We sometimes call a session a persistence manager because it is also an operation related to persistence. For example, the interface for storing and retrieving objects. Note that the Hibernate session has nothing to do with the HttpSession of the Web layer. When we use conversations in this book, we refer to Hibernate conversations. To make a difference, we sometimes refer to the HttpSession object as a user session.

4 interface between Query and Criteria

The Query interface allows you to execute queries on the database and control how queries are executed. The query is written in HQL or the SQL dialect of the local database. The query instance is used to bind query parameters, limit the number of results returned by the query, and finally execute the query.

The Criteria (standard) interface is very small and allows you to create and execute object-oriented standard queries. To help reduce redundancy in the application code, Hibernate provides some shortcut methods on the session interface that allow you to call a query in one line of code.

5 Transaction interface

The Transaction (transaction) interface is an optional API. Hibernate applications can choose not to use this interface and instead manage transactions in their own underlying code. Transactions abstract application code from the underlying transaction implementation-- this could be a JDBC transaction, a JTA user transaction, or even a common object request broker structure (CORBA)-- allowing applications to control transaction boundaries through a consistent set of API. This helps maintain the portability of Hibernate applications in different types of execution environments or containers.

Two Hibernate data types

A basic and very powerful architectural element is the concept of Hibernate types. The type object of Hibernate maps a Java type to the type of a database field (in fact, the type may span multiple fields). All persistence properties of the persistence class, including associations, have a corresponding Hibernate type. This design makes Hibernate extremely flexible and easy to extend.

Built-in types are broad in scope, covering all Java base types and many JDK classes, including java.util.Currency,java.util.Calendar,byte [] and va.io.Serializable. Even better, Hibernate supports user-defined types. It provides UserType and serType interfaces that allow you to add your own types. Using this feature, common classes used by applications, such as Address,Name or MonetaryAmount, can be handled easily and elegantly. Custom types are considered important features of Hibernate and encourage new or creative use of them.

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