多对多关系映射和一对多关系映射开发步骤差不多，

例子如下：员工和项目之间的关系，一个员工可以参与多个项目；一个项目可以有多个开发人员参与。因此是多对多的关系。

1 分析数据表

1.1）员工表

CREATE TABLE employee(id INT PRIMARY KEY AUTO_INCREMENT,ename VARCHAR(50) NOT NULL);

 

1.2）项目表

CREATE TABLE project(id INT PRIMARY KEY AUTO_INCREMENT,pname VARCHAR(100) NOT NULL,startDate DATE);

 

1.3）员工和项目关系表

CREATE TABLE emp_pro_relation(eid INT,pid INT,FOREIGN KEY (eid) REFERENCES employee(id),FOREIGN KEY (pid) REFERENCES project(id));

 

2 编写实体类

Employee类

public class Employee {    private int id;    private String ename;    private Set
      
        projects = new HashSet
       
        ();    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getEname() {        return ename;    }    public void setEname(String ename) {        this.ename = ename;    }    public Set
        
          getProjects() {        return projects;    }    public void setProjects(Set
         
           projects) {        this.projects = projects;    }}
         
        
       
      

Project类

public class Project {    private int id;    private String pname;    private Date startDate = new Date();    private Set
      
        employees = new HashSet
       
        ();    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getPname() {        return pname;    }    public void setPname(String pname) {        this.pname = pname;    }    public Date getStartDate() {        return startDate;    }    public void setStartDate(Date startDate) {        this.startDate = startDate;    }    public Set
        
          getEmployees() {        return employees;    }    public void setEmployees(Set
         
           employees) {        this.employees = employees;    }}
         
        
       
      

 

3 映射配置

Employee.hbm.xml配置

Project.hbm.xml配置

 

4 全局配置hibernate.cfg.xml

         
             
              
       
        com.mysql.jdbc.Driver
              
       
        jdbc:mysql:///day26?useSSL=true
              
       
        root
              
       
        root
                     
              
       
        org.hibernate.dialect.MySQL5Dialect
              
       
        true
              
       
        true
                     
              
              
           
      
     

 

二、简单测试

public void fun1(){        Configuration configuration = new Configuration();        configuration.configure();        SessionFactory sessionFac = configuration.buildSessionFactory();        Session session = sessionFac.openSession();        Transaction bt = session.beginTransaction();                Employee emp1 = new Employee();        emp1.setEname("good1");                Employee emp2 = new Employee();        emp2.setEname("good2");                Employee emp3 = new Employee();        emp3.setEname("nice1");                Employee emp4 = new Employee();        emp4.setEname("nice2");                Project pro1 = new Project();        pro1.setPname("OA");                Project pro2 = new Project();        pro2.setPname("电商");                pro1.getEmployees().add(emp1);        pro1.getEmployees().add(emp2);        pro1.getEmployees().add(emp3);                pro2.getEmployees().add(emp3);        pro2.getEmployees().add(emp4);                session.save(pro1);        session.save(pro2);                bt.commit();        session.close();        sessionFac.close();    }

需要注意的是：在使用一方保存另一方时，可能会有错误。此时需要在保存方配置cascade属性。