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This article focuses on "how to instantiate collections and agents in Hibernate". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let the editor take you to learn how to instantiate collections and agents in Hibernate.

Instantiated collections and proxies in Hibernate access uninitialized collections or proxies outside the scope of Session, and Hibernate will throw a LazyInitializationException exception. That is, in a detached state, this exception is thrown when you access a collection owned by an entity, or when you access its properties that point to a proxy.

Sometimes we need to make sure that an agent or collection is initialized before Session shuts down. Of course, we can ensure this by forcibly calling methods such as cat.getSex () or cat.getKittens (). Size (). But such a program will cause confusion to the reader and does not conform to the usual code specifications.

The static method Hibernate.initialized () provides a convenient way for your application to delay loading collections or proxies. As long as its Session is in the open state, Hibernate.initialize (cat) will force the agent to be instantiated for cat. Similarly, Hibernate.initialize (cat.getKittens ()) has the same function for collections of kittens.

Another option is to keep the Session in the open state until all the required collections or agents are loaded. In some application architectures, especially for code that uses Hibernate for data access, and code that uses Hibernate in different application layers and different physical processes. When a collection is instantiated, how to keep the Session in the open state is often a problem. There are two ways to solve this problem:

In a Web-based application, the servlet filter (filter) can be used to turn off Session at the end of a user request (request) and at the end of page generation (here keeping Session mode on at the presentation layer (Open Session in View)), which, of course, depends on the fact that exceptions in the application framework need to be handled correctly.

It is important to close the Session and end the transaction correctly before returning the interface to the user, or even in the event of an exception during the generation of the interface. See the "Open Session in View" mode on Hibernate wiki for examples.

In an application with a separate business tier, the business tier must "prepare" the data set it needs for the web tier before returning. This means that the business layer should load all the data required by the presentation layer / web layer and return the instantiated data.

In general, the application should call Hibernate.initialize () for each collection required by the web layer (this call must occur before our session closes), or use a Hibernate query with a FETCH clause, or FetchMode.JOIN, to get all the data collections in advance.

If you use the Command pattern instead of Session Facade in your application, the task will be much easier.

You can also use the merge () or lock () method to bind a new Session to a previously loaded object before accessing the uninstantiated collection (or proxy).

Obviously, Hibernate will not and should not automate these tasks, as this will introduce a special transaction semantics.

Sometimes, you don't need to fully instantiate a large collection, you only need to know part of its information (such as its size), or part of the contents of the collection.

You can use a collection filter to get the size of its collection without instantiating the entire collection:

((Integer) s.createFilter (collection, "select count (*)"). List (). Get (0). IntValue ()

The createFilter () method here can also be used to effectively grab part of the collection without instantiating the entire collection:

S.createFilter (lazyCollection, ") .setFirstResult (0) .setMaxResults (10) .list (); now that you have a better understanding of how to instantiate collections and agents in Hibernate, you might as well do it! Here is the website, more related content can enter the relevant channels to inquire, follow us, continue to learn!

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