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This article mainly explains "what is the relevance of Hibernate operation delay loading". The explanation in this article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "what is the relevance of Hibernate operation delay loading".

By default, Hibernate 3 uses deferred select fetching for collections and deferred proxy fetching for associations that return a single value. For almost all applications, the vast majority of their associations, this strategy is effective.

Note: if you set hibernate.default_batch_fetch_size,Hibernate, batch crawl optimization will be taken for delayed loading (this optimization may also be turned on at a more refined level).

However, you must understand a problem with delayed fetching. Calling a deferred collection outside of an open Hibernate session context can cause an accident. For example:

S = sessions.openSession (); Transaction tx = s.beginTransaction (); User u = (User) s.createQuery ("from User u where u.name=:userName") .setString ("userName", userName). UniqueResult (); Map permissions = u.getPermissions (); tx.commit (); s.close (); Integer accessLevel = (Integer) permissions.get ("accounts"); / / Error!

After Session is closed, the permessions collection will be uninstantiated and no longer available, so its state cannot be loaded properly. Hibernate does not support delayed instantiation of unmanaged objects. The modification here is to move the code that permissions reads the data to before tx.commit ().

In addition, we can also use non-delayed collections or associations by specifying lazy= "false" to the association mapping.

However, for most collections, it is more recommended to crawl data in a deferred manner. If you define too many non-deferred associations in your object model, Hibernate will eventually need to load the entire database into memory in almost every transaction!

However, on the other hand, in some special transactions, we often need to use join fetching (which itself is non-delayed) instead of query fetching.

Thank you for your reading, the above is the content of "what is the relevance of Hibernate operation delay loading". After the study of this article, I believe you have a deeper understanding of what the relevance of Hibernate operation delay loading is, and the specific use needs to be verified in practice. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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