Hibernate 一对多关联配置
以Department和Employee为例。Department类：
public class Department {private int id;private String name;// For one-to-many associationprivate Set<Employee> employees;public Department() {}public Department(String name) {this.name = name;}// Getters and setters are omitted
Employee类：
public class Employee {private int id;private String name;public Employee() {}public Employee(String name) {this.name = name;}// Getters and setters are omitted
Department.hbm.xml:
<hibernate-mappingpackage="com.john.myhibernate.domain"><class name="Department"><id name="id"><generator class="native"/></id><property name="name" length="20" not-null="true"/><set name="employees" lazy="true" fetch="select"><key column="depart_id" not-null="true" foreign-key="slender"></key><one-to-many class="Employee"/></set></class></hibernate-mapping>
如果key的column属性不指定，则employee表中没有department的外键。
key的foreign-key属性指定key的名称。
如果one-to-many的class不指定，hibernate生成表时会报错。
Employee.hbm.xml:
<hibernate-mapping package="com.john.myhibernate.domain"><class name="Employee"><id name="id"><generator class="native"/></id><property name="name" length="20" not-null="true"/></class></hibernate-mapping>
1. 测试cascade属性
public void testSave() {Session s = null;Transaction tx = null;Department depart = new Department("FDI");Employee em1 = new Employee("Jacy");Employee em2 = new Employee("Neil");Set<Employee> employees = new HashSet<Employee>();employees.add(em1);employees.add(em2);depart.setEmployees(employees);try {s = HibernateUtil.getSession();tx = s.beginTransaction();s.save(depart);tx.commit();} catch (HibernateException e) {tx.rollback();e.printStackTrace();} finally {if (s != null)s.close();}}
结果报错：org.hibernate.TransientObjectException
因为em1和em2是两个瞬时对象。
解决办法是：
a. 在代码里加上s.save(em1); s.save(em2);
s.save(em1);s.save(em2);s.save(depart);
或者
b. 给set加上cascade="save-update"或者cascade="all"。
<hibernate-mapping<set name="employees" lazy="true" fetch="select" cascade="save-update"></hibernate-mapping>
2. 测试inverse属性：
set的inverse属性默认是false的，也就是由department维护关系。如果修改为true，执行上面的testSave方法，结果报错：Field 'depart_id' doesn't have a default value
这时关系由Employee维护，但是它没有机会（因为没有调用s.save(em1)和s.save(em2)），那么employee表中的depart_id就会是null值，和not-null=true的约束产生冲突。
3. 测试lazy, fetch属性
public void testQueryLazy() {Session s = null;s = HibernateUtil.getSession();Department depart = (Department) s.get(Department.class, 2);s.close();System.out.println(depart.getName());System.out.println(depart.getEmployees());}
a. 当fetch为select时：
如果lazy为true，hibernate用SELECT语句查询出Department。当访问Employee时才用SELECT语句查询Employee，如果这时session已经关闭，会导致异常。
如果lazy为false，hibernate在get方法执行的时候用两个SELECT语句把Department和Employee都查询出来。
b. 当fetch为join时：
不管lazy的取值，hibernate会进行连表查询，把两个实体都查询出来。
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