近日工作中发现对Hibernate一些概念模糊.故望此文可以帮助我理顺思路.
写了一个简单的Demo做测试.
两个实体
public class Type {
private int id;
private String name;
private Set books;
//省略getter,setter等方法,下同
}
public class Book implements Serializable {
private int id;
private String name;
private String desc;
private String price;
private Date createTime;
private Date updateTime;
private Type type;
private Set orders;
}
public class Order {
private int id;
private Date createTime;
private Set books;
}
关系映射文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="hadix.demo.hibernate.model.Book" table="book">
<id name="id">
<column name="id" sql-type="integer" length="2000000000" precision="10"
not-null="true"/>
<generator class="increment"/>
</id>
<property name="name">
<column name="name" sql-type="varchar(500)" length="2000000000" precision="10"/>
</property>
<property name="desc">
<column name="desc" sql-type="text" length="2000000000" precision="10"/>
</property>
<property name="price">
<column name="price" sql-type="numeric" length="2000000000" precision="10"/>
</property>
<property name="createTime">
<column name="create_time" sql-type="datetime" length="2000000000" precision="10"/>
</property>
<property name="updateTime">
<column name="update_time" sql-type="datetime" length="2000000000" precision="10"/>
</property>
<many-to-one name="type" column="type_id" class="hadix.demo.hibernate.model.Type"/>
<set name="orders" lazy="true" table="book_order_link" inverse="true">
<key column="book_id"/>
<many-to-many column="order_id" class="hadix.demo.hibernate.model.Order"/>
</set>
</class>
<class name="hadix.demo.hibernate.model.Order" table="order">
<id name="id">
<column name="id" sql-type="integer" length="2000000000" precision="10"
not-null="true"/>
</id>
<property name="createTime">
<column name="create_time" sql-type="datetime" length="2000000000" precision="10"/>
</property>
<set name="books" table="book_order_link" lazy="false">
<key column="order_id"/>
<many-to-many column="book_id" class="hadix.demo.hibernate.model.Book"/>
</set>
</class>
<class name="hadix.demo.hibernate.model.Type" table="type">
<id name="id">
<column name="id" sql-type="integer" length="2000000000" precision="10"
not-null="true"/>
</id>
<property name="name">
<column name="name" sql-type="varchar(500)" length="2000000000" precision="10"
not-null="true" unique="true"/>
</property>
<set name="books" lazy="true" inverse="true">
<key column="type_id"/>
<one-to-many class="hadix.demo.hibernate.model.Book"/>
</set>
</class>
</hibernate-mapping>
关系图
Book-Order(many-to-many)
Book-Type(many-to-one)
均为双向关系
运行测试用例
@Test
public void testManyToOne() {
Session session = Sessions.getSession();
session.beginTransaction();
//持久态 books
List<Book> books = getBooks(session);
for (Book book : books) {
//数据库中原本已经存放了书籍数据,但未指定类型,所以测试可以通过该断言
assertNull(book.getType());
}
//瞬态 type
Type type = new Type("Book_Type_1");
type.setBooks(new HashSet<Book>(books));
session.saveOrUpdate(type);//type 变为持久态
session.getTransaction().commit();
session.clear();//清除缓存避免之后取到旧数据
//重新读取书籍数据,以确保获得正确关系更新
for (Book book : getBooks(session)) {
assertEquals(type, book.getType());//期望类型保存后会更新数据库中书跟类型之间的关系
}
session.close();
}
private List<Book> getBooks(Session session) {
return session.createQuery("from Book").list();
}
结果测试没有通过第二个断言.测试结果是:java.lang.AssertionError:
expected:<Type{id=1, name='Book_Type_1'}> but was:<null>
因为在Type一端使用了inverse="true",所以在保存Type对象时并没有保存跟Book间的关系.去掉inverse="true",清除数据库中的type数据,重新执行改测试,则测试通过.
接下来的测试验证book和order的多对多关系
@Test
public void testManyToMany() {
Session session = Sessions.getSession();
session.beginTransaction();
List<Book> books = getBooks(session);
HashSet<Book> bookSet = new HashSet<Book>(books);
//把同一批书添加到两个订单中
Order order1 = newOrder(session, bookSet);
Order order2 = newOrder(session, bookSet);
session.getTransaction().commit();
//验证建立了正确的多对多关系
assertEquals(books.size(),order1.getBooks().size());
assertEquals(books.size(),order2.getBooks().size());
for (Book book : books) {
assertEquals(2, book.getOrders().size());
}
session.close();
}
private Order newOrder(Session session, HashSet<Book> bookSet) {
Order order = new Order();
order.setBooks(bookSet);
session.save(order);
//order对象在保存后变为"持久态"
return order;
}
前面的测试,中注释中标示了一些对象状态转换,下图描述了Hibernate对象状态转换
总结
问题:1.Hibernate对象状态转换是隐式的,没经验的用户会比较困惑,在大规模的系统中也很容易出现错误.
2.Hibernate的异常信息是很令人困惑的,写这个demo的过程中,曾出现过一些异常.经检查是由于缺少javassit.jar,但是异常信息却完全与之无关,十分诡异.系统调试经常需要花费很多时间,也未必能找到根本问题.
3.Hibernate的文档描述不明确,inverse和cascade的语义本应十分明确,但是文档描述却不够明确.
我不喜欢这样的工具.作为工具它太复杂了,这种复杂度使学习曲线陡峭,系统调试困难.不适用于我目前的工作.
(第一次写,基本没什么养分..好在让自己重新温习了一下hibernate开发环境配置,明确一下inverse的作用)
