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1. What is SessionFactory? What is Session? What's the difference between httpsession and hibernate's session?

The SessionFactory interface is responsible for initializing Hibernate. It acts as a proxy for the data storage source and is responsible for creating Session objects. Factory mode is used here. It is important to note that SessionFactory is not lightweight, because in general, a project usually requires only one SessionFactory, and when you need to operate on multiple databases, you can specify a SessionFactory for each database. -Baidu encyclopedia

The session in hibernate is not the session in http, and the HttpSession object is generally called the user session.

What about Session in hibernate? Is used to represent an interaction (session) between the application and the database. In this Session, the general persistence method (CRUD) is included. Moreover, Session is a lightweight object (thread unsafe). Usually, each Session instance is bound to a database transaction, that is, every database transaction is executed, a new Session instance should be created first, and Session needs to be closed after using Session.

2. Nasty 1 problem?

Generally speaking, nroom1 means that after querying n pieces of data, each piece of data will be associated with querying its associated object once. This is called nroom1.

Here are 3 solutions:

Set the value of the fetch attribute of @ ManyToOne to fetchType.LAZY. After this is resolved, the next n sql statements are issued on demand. However, the drawback is that if you need a cascading query, you cannot get the cascading object.

Set @ BatchSize (size=5) (this annotation is to be added to the class in the same place as @ Entity) so that fewer sql statements are issued. This setting improves efficiency to some extent.

Join fetch, if you use "from Student s left join fetch s.group g" to make a table join query, a SQL statement is issued.

Using QBC queries, the default effect is the same as 3.

1) fetch= "select" will issue another statement to query the collection

2) set the lazy failure of fetch= "join" with external join sets

3) this fetch= "subselect" issues another select statement to grab the association set of all the entity objects queried previously. Fetch only affects the HQL query and others will not.

3. What's the difference between get and load loading in Hibernate?

When we use the session.load () method to load an object, there is no sql statement issued at this time, the current object is actually a proxy object, this proxy object only holds the id value of the entity object, only when we want to use this object to get other properties, then we will issue a sql statement to query our objects from the database.

Get is much more straightforward than the deferred loading mode of load. When we use the session.get () method to get an object, regardless of whether we use the object or not, we will issue a sql statement to query it from the database.

4. Dirty reading and illusory reading?

Dirty read: when a transaction is accessing data and has made changes to the data, but has not yet committed to the database. Another transaction queries this data, this data is dirty data, and the operation done by relying on this data is incorrect.

Illusion: for example, if the first transaction modifies all the rows of the database, while the second transaction inserts a piece of data into the database, the first transaction will find that a record in the database has not been modified. It's like an illusion.

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