Many-to-many and deferred loading of hibernate associations

Association relationships Many-to-many

Chestnuts: An employee can write multiple projects, and a project is done by multiple programmers, which is a typical many-to-many instance

The following gives the two entity class employee (Employee entity), Project (Project entity), based on an instance.

One. One-way, many-to-many: (Add a project and add two employees to the project.) ）

Implant project properties in the employee table

To configure the Employee.hbm.xml file:

Start one side:

@Test    //bidirectional Many-to-many association relationship 2. Add multiple employees to a department under public    Void Manytomanyt1 () {        Session session = Hibernateutil.getsession ( );        Transaction tx=session.begintransaction ();        Employee Emp=new employee ();        Emp.setempname ("pheasant");        Project Pro1=new project ();        Pro1.setproname ("Security Department");        Project Pro2=new project ();        Pro2.setproname ("Finance Department");        Emp.getprojects (). Add (Pro1);        Emp.getprojects (). Add (Pro2);        Session.save (EMP);        Session.save (Pro1);        Session.save (PRO2);        Tx.commit ();    }

Results:

Two. Many-to-many two-way (query project under a few employees/query each employee has several projects)

To populate an employee's Employee attribute in the project table:

Configuration Project.hbm.xml file (IBID.):

Test:

@Test    //bidirectional Many-to-many association relationship  1. Retrieve the employee name and find out the project name public    void T () {        Session session = Hibernateutil.getsession ();        Employee employee = Session.get (Employee.class, +);        SYSTEM.OUT.PRINTLN ("Employee Name:" +employee.getempname ());        SYSTEM.OUT.PRINTLN ("Employee's Project:");        For (Project project:employee.getProjects ()) {            System.out.println (Project.getproname ());        }    }

　　

Results:

Hibernate:     Select        employee0_.empid as empid1_2_0_,        Employee0_.empname as empname2_2_0_ from        ANNAN. Employee employee0_     where        employee0_.empid=? Employee Name: Pheasant Staff Project: Hibernate:     select        Projects0_.eid as eid1_5_0_,        projects0_.pid as pid2_5_0_,        project1_.proid as proid1_3_1_, project1_.proname as        proname2_ 3_1_     from        empandpro projects0_     inner join        ANNAN. Project project1_ on             projects0_.pid=project1_.proid     where        projects0_.eid=? Security Department Finance Department

　　

Lazy Loading:

Lazy Loading is when the data is really needed , The SQL statement is executed to query, avoiding the unnecessary performance overhead.

Deferred load classification:

query policy at the class level

the query strategy for one-to-many and many-to-many associations

A. a multiple-to-one correlation query strategy

A. Class-level query policy : immediate Retrieval and deferred retrieval , default to deferred retrieval

Class-Level lazy loading    @Test public    void T3 () {
        The default lazy is true        session session = Hibernateutil.getsession ();        Employee employee = Session.load (Employee.class, +);        System.out.println ();    }

Effect:

　

When the class's lazy is false:

Effect:

One -to-many and many-to-many correlated query strategies:

::::::: One -to-many or many-to-many retrieval strategies are determined by lazy and fetch : ::::::

in the mapping file, the <set> element is used to configure a one-to-many association and many-to-many association relationships. <set> elements have lazy and fetch attributes

When the value of 1.lazy is true: Lazy loading

Results:

When the value of 2.lazy is false: Load immediately

Fetch FETCH value

Join: Urgent left OUTER join

Select: Multiple simple SQL(default)

Subselect: Sub-query

Fetch and lazy combinations

Parse:fetch= "join" lazy will be ignored, urgent left outer join immediate retrieval

Fetch= "Select" Lazy= "false" multiple simple SQL retrieve now

Fetch= "Select" Lazy= "true" multiple statements deferred retrieval

Fetch= "Select" lazy= "Extra" multiple statements and their lazy search

Fetch= "Subselect" lazy= "false" subquery immediate Retrieval

Fetch= "Subselect" lazy= "true" subquery deferred retrieval

Fetch= "Subselect" lazy= "Extra" subquery and its lazy search

Join:

<set name= "Projects" table= "Empandpro" fetch= "join" >

Hibernate:     Select        employee0_.empid as empid1_2_0_,        employee0_.empname as empname2_2_0_,        projects1_ . Eid as Eid1_5_1_,        project2_.proid as pid2_5_1_,        project2_.proid as proid1_3_2_,        Project2_.proname as Proname2_3_2_     from        ANNAN. Employee employee0_ left     outer join        Empandpro projects1_ on             employee0_.empid=projects1_.eid     Left outer join        ANNAN. Project project2_ on             projects1_.pid=project2_.proid     where        employee0_.empid=? Employee Name: Pheasant Staff Project: Finance Department Security Department

Default Select:

<set name= "Projects" table= "Empandpro" fetch= "select" >

Hibernate:     Select        employee0_.empid as empid1_2_0_,        Employee0_.empname as empname2_2_0_ from        ANNAN. Employee employee0_     where        employee0_.empid=? Employee Name: Pheasant Staff Project: Hibernate:     select        Projects0_.eid as eid1_5_0_,        projects0_.pid as pid2_5_0_,        project1_.proid as proid1_3_1_, project1_.proname as        proname2_ 3_1_     from        empandpro projects0_     inner join        ANNAN. Project project1_ on             projects0_.pid=project1_.proid     where        projects0_.eid=? Security Department Finance Department

When the value of 3.lazy is extra: Enhanced lazy loading: (

only the properties of the collection object are loaded, when the properties of the collection itself are accessed (for example, the collection size, build count), are not loaded immediately.

)

Results:

System.out.println (Projects.size ());

Hibernate:     Select        employee0_.empid as empid1_2_0_,        Employee0_.empname as empname2_2_0_ from        ANNAN. Employee employee0_     where        employee0_.empid=? Hibernate:     Select        count (PID)     from        Empandpro     where        Eid =?2

　　

Many-to-many and deferred loading of hibernate associations