Some information about Hibernate (continued)

In-depth Session

Session Overview

When Session interfaces are used, Hibernate provides the application with the primary interface for database operations. It provides basic methods for saving, updating, deleting, and loading Java objects.

Understanding Session caching,

The implementation of the session excuse contains Java collections of some columns, which constitute the Session cache, as long as the Session strength does not end the lifecycle, objects stored in the cache do not end the lifecycle. When the save () method of the session persists an object, the object is loaded into the cache, even if the object is not referenced in the program, as long as the cache is not cleared, the object is still in the lifecycle. When a graph load () object is created, it determines whether the object exists in the cache. If yes, it is returned. Database not being queried

Clear Session cache

Session has a cache, Objects in the cache are called persistent objects.It corresponds to the relevant records in the database. sessions can execute related SQL statements according to the changes of objects in the cache at certain time points to synchronously update the database, this process is called flush. By default, the Session clears the cache at the following time points: when the application calls Transaction commit ()This method first clears the cache (session. flush (), and submit the transaction (tx. commit () When an application performs some query operations, if the attributes of the persistence object in the cache have changed, the cache is cleared first, to ensure that the query results can reflect the latest status of the Persistent object, the difference between the flush () method of the Session is explicitly called: flush: to clear the cache (the data cached at this time is not lost, let the cache and database execute a series of SQL statements synchronously, but do not commit the transaction; commit: First call the flush () method, and then commit the transaction, this means that the operations committed to the database will be permanently saved in reresh: refresh, synchronize sessions with the database, execute queries, and display the latest database information, update the object status of the ontology cache clear: clear the cache, equivalent to list. removeAll ();

Use Session cache to read data of persistent objects

Cache function:

Reduce the frequency of accessing the database. Ensure that the objects in the cache are synchronized with related records in the database.

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Fetch (Default value: select)	Lazy (The default value is true)	Policy
Join	False	Use urgent left Outer Join search.
Join	True	Use urgent left Outer Join search.
Join	Extra	Use urgent left Outer Join search.
Select	False	Use Instant Search
Select	True	Latency Retrieval
Select	Extra	Latency Retrieval C. getOrders (). size () execute select count (id) from orders where customer_id =? For (Order o: set) {o. getOrderNumber: Select customer_id, id, order_number, price from orders where customer_id =?
Subselect	False/true/extra It is also divided into three situations	Nested subquery (when multiple customer objects are retrieved) the Lazy attribute determines the search policy) Select customer_id, order_number, price from orders where customer_id In (select id from MERs mers)

Search Policy	Advantages	Disadvantages	Applicable scenarios with priority
Search now	Completely transparent to the application. no matter whether the object is in a persistent or free state, the application can navigate from an object to the associated object.	(1) Multiple select statements (2) It may load objects that the application does not need to access, wasting a lot of memory space.	(1) class level (2) objects that the application needs to access immediately (3) The second-level cache is used.
Delayed search	It is up to the application to determine which objects need to be loaded, so as to avoid executing redundant select statements and loading objects that the application does not need to access. Therefore, it can improve retrieval performance and save memory space.	If an application wants to access a free proxy instance, it must ensure that it has been initialized during persistence.	(1) one-to-many or many-to-many Association (2) objects that the application does not need to access immediately or are not accessible at all
Urgent left Outer Join search	(1) It is completely transparent to the application. no matter whether the object is in a persistent or free state, you can navigate from one object to another. (2) When an external connection is used, fewer select statements are used.	(1) It may load objects that the application does not need to access, wasting memory. (2) Complex Database Table connections also affect retrieval performance.	(1) many-to-one (2) objects to be accessed immediately (3) The database has good table connection performance.

Hibernate Retrieval Method

HQL (Hibernate Query Language)

HQL (Hibernate Query Language) is an Object-Oriented Query Language, which is similar to the SQL Query Language. Among the various retrieval methods provided by Hibernate, HQL is the most widely used retrieval method, it has the following functions: You can set various query conditions in a query statement to support projection query. That is, only partial attributes of the retrieved object support paging query, and connection query supports grouping query, the keyword having and group by can be used to provide cohesion clustering functions, such as sum (), min (), max (); ability to call User-Defined SQL functions or standard SQL functions. Support for subqueries. Support for dynamic parameter binding.

OID Retrieval Method

Retrieve Objects Based on their OID

QBC Retrieval Method

Use the QBC (Query By Criteria) API to retrieve objects. This API encapsulates string-Based Query statements and provides a more object-oriented Query interface.

Simple Query

Use alias

Sort query results

Paging Query

SetFirstResult (int firstResult); specifies the object from which to start retrieval. The firstResult parameter indicates the index position of the object in the query result. The starting value of the index position is 0. By default, query starts from the first object in the Query result.

SetMaxResult (int maxResults); sets the maximum number of objects retrieved at a time. By default, the Query and Criteria interfaces retrieve all objects in the Query results.

Hibernate Retrieval Method

Phrase	Description
Expression. eq	Equal to =
Expression. allEq	Use Map and key/value for multiple equal judgments
Expression. gt	Greater than>
Expression. ge	Greater than or equal to> =
Expression. lt	Less than <
Expression. le	Less than or equal to <=
Expression.	Corresponding SQL between clause
Expression. like	Corresponding to the like clause of SQL
Expression. in	Corresponding SQL in Clause
Expression. and	And relationship
Expression. or	Or relationship
Expression. sqlRestriction	SQL limit Query
Expression. asc ()	Sort by input fields in ascending order
Expression. desc ()	Sort by input fields in descending order

Various operations supported by HQL and QBC

Operation Type	HQL Operator	QBC Calculation Method	Description
Comparison	=	Express. eq ()
	<>	Express. not (Express. eq ())
	> =	Express. ge ()
	<	Express. lt ()
	<=	Express. le ()
	Is null	Express. isNull ()
	Is not null	Express. isNotNull ()
Range Operators	In	Express. in ()
	Not in	Express. not (Express. in ())
	Between	Express. ()
	Not	Express. not (Express. ())

Comparison	=	Express. eq ()
	<>	Express. not (Express. eq ())
	> =	Express. ge ()
	<	Express. lt ()
	<=	Express. le ()
	Is null	Express. isNull ()
	Is not null	Express. isNotNull ()
Range Operators	In	Express. in ()
	Not in	Express. not (Express. in ())
	Between	Express. ()
	Not	Express. not (Express. ())

Logic	And	Expression. and () | Expression. conjunction ()
	Or	Expression. or () | Expression. disjunction ()
	Not	Expression. not ()

Level 2 Cache

Concept of Cache

A very general concept in the computer field. It is between an application and a permanent data storage source (such as a file or database on a hard disk). It is used to reduce the frequency of applications directly reading and writing permanent data inventory storage sources, this improves the Running Performance of applications. The cached data is a copy of the data in the data storage source. Normally, the cache memory is used for physical media.

Hibernate provides two levels of Cache

The Session-level cache is a cache within the transaction scope. This level of cache is managed by hibernate. In general, it is necessary to intervene in the SessionFactory level cache, which is a process-specific cache.

Enable secondary cache Conditions

Rarely modified. Very few system modules use data that is not private. It is a shared second-level cache vendor.