HQL query and caching mechanism in hibernate

HQL: Fully Object-oriented query

execution order of SQL :

1. from

2.Where to filter the underlying data

The difference between where and having : 1. different order 2.where filtering underlying data 3. filtering aggregate functions

3.Group by

4.Select

5. Having

6.Order by

Use Hibernate Query , use One of Hibernate interface query

HQL is an object-oriented query statement , so it is followed by the class name

Query query = Session.createquery ("Select Id,name,stu.cla.name from Student");

list<object[]> list = Query.list ();

For (object[] obj:list) {

........ printing, etc.

OBJ[0] = ID;  OBJ[1] = name; OBJ[2] = Stu.cla.name;

}

The data after the query exists in an Object array

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List get query

list<list> list = = Session.createquery ("Select New List (id,name,stu.cla.name) from Student"). List ();

for (List list1:list) {

System.out.print (list1.get (0) + list1.get (1));

}

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Java Object query requires a constructor with parameters

Create an Intermediate class , including the required properties, such as the new object : Package name . class name

list<mydata> list = Session.createquery ("Select New com.lovo.my.MyData(id,name,classname) from MyData" ). List ();

for (MyData data:list) {

System.out.print (data.id + "+ data.name +" "+ data.classname);

}

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Map Get Query

list<map> list = = Session.createquery ("Select New Map (name as Stuname,stu.cla.name as Claname) f Rom Student"). List ();

for (Map map:list) {

System.out.print (Map.get ("stuname") + "" + map.get ("classname");

}

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know that the structure of a query is an object that gets a result set

Student stu = (Student) session.createquery ("from Student where id=2"). Uniqueresult ();

( lazy loading ) when the session is cleared , lazy loading errors are reported

System.out.print (Stu.getname () + "" + STU.GETCLA (). GetName ());

Open Lazy Loading : 1. Settings in Configuration lazy="false" 2. in the Session before closing , Initialize Hibernate. Initialize ()

Internal connection : matching is not connected , does not match does not connect , can cause data loss

Dto: Data Persistence

Singleton mode : For example :p rivate hibernateutil () {} cannot be new

Hibernate placeholder : 1.id=: ID ( named parameter ) 2.id=?

The Group by projection has a limit of 1.group by the ability projection 2. Aggregation Functions

Round ( need to take the whole function , keep a few decimals )

Ifnull ( Judging data , 0);--- if the data is empty , use 0 instead

Subqueries can only be used in Where/select in HQL

with local SQL/HQL Statement :

Select name from T_student; Select ID from T_student;

returns a column of data : accepted with an Integer using string

Session.createsqlquery (SELECT * from T_student). Addscalar ("name"). List ();

in the there is a scalar in the SQL statement , * it has no effect

Session.createsqlquery (Select * from T_student). Addenitity (Student.class). List ();

Store all the data you have queried to in the Student object

Hibernate is the encapsulation of JDBC : A reduction in the efficiency of database access and a decline in performance ( caching is generated for this ---- in Memory ) is located in the database and database access layer

First- level cache : Session Cache

Session.clear ();----- Clear session, all data in the cache is cleared

Lazy ( lazy loading ) has lazy loading :

1. What data is required to send what SQL statement

2. Turn off lazy loading : all associated tables are concatenated with one SQL statement , too many objects are stored in memory

3. when the session is closed , lazy loading is also turned off

4. Default lazy loading on

Crawl Strategy :

Immediately after the Session is closed, fetch the data ----- Fetch all the data into the database

From Student Stu the left join fetch stu,myclass where stu.id = 1;

Level Two cache : sessionfactory level with third-party jar Packages

under Hibernate configuration :

<!-- Query cache --

<property name= "Cache.use_query_cache" >true</property>

<!-- Turn on level two cache--

<property name= "Cache.use_second_level_cache" >true</property>

<!-- configuration echche Implementation Class--

<property name= "Cache.region.factory_class" >

Org.hibernate.cache.EhCacheRegionFactory

</property>

Hibernate pessimistic and optimistic lock : Control Data is not destroyed by the outside when it is operated

pessimistic lock : The state of the lock table Setlockmode ( object , Lockmode.upgpade) depends on the database lock table mechanism , T release after submitting

optimistic lock : mainly in version form

Configuration :

1. when creating a class , Add a property , version int

2.Version must be configured at The back of the ID <version name="version" column= "Version" ></ Version>

Gson Gson = new Gson ();

String list = Gson.tojson ();

To make a jar package :

1.File--export

2. Enter the jar---jarfile

3. Select the jar package you want to hit to write the jar Package name Select Save Path

HQL query and caching mechanism in hibernate