hibernate 中多对多关系对象集合的保存

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<Project> projects = new HashSet<Project>();
    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<Project> getProjects() {
        return projects;
    }
    public void setProjects(Set<Project> projects) {
        this.projects = projects;
    }
}

Project类

public class Project {
    private int id;
    private String pname;
    private Date startDate = new Date();
    private Set<Employee> employees = new HashSet<Employee>();
    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<Employee> getEmployees() {
        return employees;
    }
    public void setEmployees(Set<Employee> employees) {
        this.employees = employees;
    }
}

3 映射配置

Employee.hbm.xml配置

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping 
    package="com.baidu.entity">
    
    <class name="Employee" table="employee">
        <id name="id" column="id">
            <generator class="native"/>
        </id>
        
        <property name="ename" column="ename"></property>
        
        <set name="projects" table="emp_pro_relation">
            <key column="eid"></key>
            <many-to-many column="pid" class="Project"></many-to-many>
        </set>

    </class>


</hibernate-mapping>

Project.hbm.xml配置

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC 
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping 
    package="com.baidu.entity">
    
    <class name="Project" table="project">
        
        <id name="id" column="id">
            <generator class="native"/>
        </id>
        
        <property name="pname" column="pname"></property>
        
        <set name="employees" table="emp_pro_relation" cascade="save-update">
            <key column="pid"></key>
            <many-to-many column="eid" class="Employee"></many-to-many>
        </set>

    </class>


</hibernate-mapping>

4 全局配置hibernate.cfg.xml

<hibernate-configuration>
    <session-factory>
       <!-- 1 数据库连接配置 -->
       <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
       <property name="hibernate.connection.url">jdbc:mysql:///day26?useSSL=true</property>
       <property name="hibernate.connection.username">root</property>
       <property name="hibernate.connection.password">root</property>
       
       <!-- 2 其他设置 -->
       <property name="hibernate.dialect">org.hibernate.dialect.MySQL5Dialect</property>
       <property name="show_sql">true</property>
       <property name="format_sql">true</property>
       
       <!-- 3 映射文件加载 -->
       <mapping resource="com/huitong/entity/Project.hbm.xml"/>
       <mapping resource="com/huitong/entity/Employee.hbm.xml"/>

    </session-factory>

</hibernate-configuration>

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。