单向N:1无中间表映射

员工与部门的关系：假如员工只能属于一个部门，那么员工与部门之间是N:1的关系，如果只能从员工来找到员工所属的部门，而不是从部门来找到对应的员工，那么这就成了单向的N：1关联

Hibernate工具类（简单测试）

public class HibernateUtil {
    
    private static final SessionFactory sessionFactory = buildSessionFactory();
    private static SessionFactory buildSessionFactory() {
    try {
        // Create the SessionFactory from hibernate.cfg.xml
        return new Configuration().configure().buildSessionFactory();
    }
    catch (Exception ex) {
        // Make sure you log the exception, as it might be swallowed
        System.out.println("Initial SessionFactory creation failed." + ex);
        throw new ExceptionInInitializerError(ex);
    }
    }
    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
    
    public static Session getSession(){
        SessionFactory sf=getSessionFactory();
        return sf.openSession();
    }
}

员工

public class Employee {
	
	private Long id;
	private String name;
	private Department department;
	
	public Long getId() {
		return id;
	}
	public void setId(Long id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public Department getDepartment() {
		return department;
	}
	public void setDepartment(Department department) {
		this.department = department;
	}
}

员工配置文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.longtech.hibernate.domain.Employee"
        table="Employee"> 
        <id name="id" column="id" type="java.lang.Long" >
            <meta attribute="field-description">唯一标识</meta>
            <generator class="native"/>
        </id>
        <property name="name" type="java.lang.String" column="name" length="20">
            <meta attribute="field-description">员工名称</meta>
        </property>
        <many-to-one name="department" column="dept_id" cascade="all"></many-to-one>
    </class>
</hibernate-mapping>

部门

public class Department {
    private Long id;
    private String name;
    
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

}

 

部门配置文件

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
    "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
    "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
    <class name="com.longtech.hibernate.domain.Department"
        table="Department"> 
        <id name="id" column="id" type="java.lang.Long" >
            <meta attribute="field-description">唯一标识</meta>
            <generator class="native"/>
        </id>
        <property name="name" type="java.lang.String" column="name" length="20">
            <meta attribute="field-description">部门名称</meta>
        </property>
    </class>
</hibernate-mapping>

持久化类

            Department dept=new Department();
            dept.setName("department");
            
            Employee emp1=new Employee();
            emp1.setName("emp1");
            emp1.setDepartment(dept); 
            session.save(emp1);

在持久化类中，当执行session.save(emp1)的时候，如果没有指定cascade="all",那么执行的时候就会抛出org.hibernate.TransientObjectException的异常。

通过以上操作，就可以发现当执行了程序后，数据库里会生成两条记录。

而控制台会输入两条记录：

Hibernate: insert into Department (name) values (?)
Hibernate: insert into Employee (name, dept_id) values (?, ?)

这个说明，如果使用了cassade="all",那么数据库会先插入Department记录，然后再插入员工，否则它就会报异常。其实这个逻辑是和jdbc操作数据库的时候是一样的。如果不先插入department,则会出来外键引用不到的情况。