(ix) Many-to-many association relationships in hibernate

I. Overview

Many-to-many association relationships can be implemented in Java objects by defining collection types. In the relational data model, it is not possible to directly express many-to-many association relationships between tables and tables, but rather to create an intermediate table that contains both sides of the primary key to express the many-to-many association relationships between the two tables.

Example: We use an example of student and course (students and courses) to demonstrate a many-to-many association relationship.

(1) Creating student and course classes

   public class Student {            private Integer ID;            private String name;             Use a collection that contains the course object selected by the student            private set<course> courses=new hashset<course> ();            Public Integer getId () {                return ID;            }            public void SetId (Integer id) {                this.id = ID;            }            Public String GetName () {                return name;            }            public void SetName (String name) {                this.name = name;            }            Public set<course> getcourses () {                return courses;            }            public void setcourses (set<course> courses) {                this.courses = courses;            }        }

　　

public class Course {private Integer ID;                     private String name;                    Use a collection to include all students who choose the course private set<student> students=new hashset<student> ();                    Public Integer GetId () {return id;                    } public void SetId (Integer id) {this.id = ID;                    } public String GetName () {return name;                    } public void SetName (String name) {this.name = name;                    } public set<student> getstudents () {return students; } public void Setstudents (set<student> students) {this.students = Studen                    Ts }            } 

(2) Write our mapping file

        
　　
            
Test:
            public class Demo {@Test public void fun () {//Read config file                    Configuration Conf=new configuration (). Configure ();                    Create Factory Sessionfactory Sessionfactory=conf.buildsessionfactory () based on configuration;                      Session session = Sessionfactory.opensession ();                     Transaction ts=session.begintransaction ();                    Creation of two Student Student s1=new Student ();                    S1.setname ("Tom");                    Student s2=new Student ();                    S2.setname ("Jack");                    Creation of three Course Course c1=new Course ();                    C1.setname ("language");                    Course c2=new Course ();                    C2.setname ("mathematics");                    Course c3=new Course ();                    C3.setname ("English"); Because the association relationship is maintained by student, no courses are required to correlate student s1.getcourses (). Add(C1);                    S1.getcourses (). Add (C2);                     S1.getcourses (). Add (C3);                    S2.getcourses (). Add (C1);                    S2.getcourses (). Add (C2);                    S2.getcourses (). Add (C3);                    Because the Cascade save is set, only save student can Session.save (S1);                    Session.save (S2);                    Ts.commit ();                    Session.close ();                Sessionfactory.close ();    }} results, three tables were generated in the database. Student and course, and the data in the intermediate table Student_course, correspond to each other.
　　
(ix) Many-to-many association relationships in hibernate