"SSH Three frames" Hibernate Foundation seventh: operation of a one-to-many association relationship

In contrast to the many-to-one relationship above, this illustrates the next-to-many correlation.

In the above, we describe a many-to-one relationship, which is a many-to-one relationship in a relational database, and it is a pair-to-many relationship. However, this is not enough, Hibernate is an object-oriented structure, and in hibernate it is still a many-to-one relationship, but there is not a pair, so we need to add a one-to-many relationship.

Department entity class: Department.java

Package Cn.itcast.hibernate.domain;import Java.util.set;public class Department {      private int id;      private String name;      Private set<employee> Emps;        Public set<employee> Getemps () {return emps;} public void Setemps (set<employee> emps) {this.emps = Emps;} public int getId () {          return ID;      }      public void setId (int id) {          this.id = ID;      }      Public String GetName () {          return name;      }      public void SetName (String name) {          this.name = name;      }  }  

Department Mapping relationship: Department.hbm.xml

<?xml version= "1.0"?>  <! DOCTYPE hibernate-mapping public       "-//hibernate/hibernate mapping DTD 3.0//en"      "http// Hibernate.sourceforge.net/hibernate-mapping-3.0.dtd ">  
We need to note that in the Department entity class, we add a set set property, and in the mapping file we also add a set property, which is to map one-to-many relationships.
In the set tag in the mapping file, we define a key tag, which is a foreign key and must be the same as the foreign key of the employee class.

Employee Entity class: Employee.java
Package Cn.itcast.hibernate.domain;public class Employee {      private int id;      private String name;      Private Department depart;            public int getId () {          return ID;      }      public void setId (int id) {          this.id = ID;      }      Public String GetName () {          return name;      }      public void SetName (String name) {          this.name = name;      }      Public Department Getdepart () {          return depart;      }      public void Setdepart (Department depart) {          This.depart = depart;      }  }  

Employee Mapping Relationship: Employee.hbm.xml
<?xml version= "1.0"?>  <! DOCTYPE hibernate-mapping public       "-//hibernate/hibernate mapping DTD 3.0//en"      "http// Hibernate.sourceforge.net/hibernate-mapping-3.0.dtd ">  

Next, we write a test class: Many2one.java
public class Many2one {public static void main (string[] arg) {Department depart = Add ();     Query (Depart.getid ());          } static Department Add () {Session s = null;          Transaction tx = NULL;              try{Department depart = new Department ();                            Depart.setname ("Depart name");              Employee EMP1 = new Employee (); Emp1.setdepart (depart);                          Establish an association relationship of two objects emp1.setname ("emp name");              Employee EMP2 = new Employee (); Emp2.setdepart (depart);                            Establish an association relationship of two objects emp2.setname ("emp name");              s = hibernateutil.getsession ();              tx = S.begintransaction ();            S.save (depart);   S.save (EMP1); The EMP object and depart object are saved in different order and there will be some difference s.save (EMP2);              If an employee (EMP) is inserted after the department (depart), it generates three database statements//Because the EMP's depart_id field is not inserted after both insertions, so there is a last statement to insert Tx.commit ();              return depart;              }finally{if (s!=null) {s.close ();          }}} static Department query (int departid) {Session s = null;          Transaction tx = NULL;              try{s = hibernateutil.getsession ();              tx = S.begintransaction (); Department depart = (Department) s.get (Department.class, Departid);            Query System.out.println by ID ("EMP size:" +depart.getemps (). Size ());            Hibernate.initialize (Depart.getemps ());              Tx.commit ();          return depart;              }finally{if (s!=null) {s.close ();   }          }      }  }
In the main function, we add two data and then query the department ID to get two results.




"SSH Three frames" Hibernate Foundation seventh: operation of a one-to-many association relationship