举例讲解Java的Hibernate框架中的多对一和一对多映射
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2024-03-07 20:10:57
多对一（many-to-one）映射
多对一（many-to-one）关联是最常见的关联关系，其中一个对象可以与多个对象相关联。例如，一个相同的地址对象可以与多个雇员的对...
多对一（many-to-one）映射
多对一（many-to-one）关联是最常见的关联关系，其中一个对象可以与多个对象相关联。例如，一个相同的地址对象可以与多个雇员的对象相关联。
定义rdbms表：
考虑一个情况，我们需要员工记录存储在employee表，将有以下结构：
create table employee ( id int not null auto_increment, first_name varchar(20) default null, last_name varchar(20) default null, salary int default null, address int not null, primary key (id) );
此外，许多员工都可以有相同的地址，所以这种关联可以使用许多一对一的关联呈现。我们将存储地址相关的信息在一个单独的表，该表具有以下结构：
create table address ( id int not null auto_increment, street_name varchar(40) default null, city_name varchar(40) default null, state_name varchar(40) default null, zipcode varchar(10) default null, primary key (id) );
同时创建rbdms表，并让他们准备下一个实现。
定义pojo类：
让我们实现一个pojo类员工将被用于保存与employee表的对象和其地址类型的变量。
import java.util.*;
public class employee{
private int id;
private string firstname;
private string lastname;
private int salary;
private address address;
public employee() {}
public employee(string fname, string lname,
int salary, address address ) {
this.firstname = fname;
this.lastname = lname;
this.salary = salary;
this.address = address;
}
public int getid() {
return id;
}
public void setid( int id ) {
this.id = id;
}
public string getfirstname() {
return firstname;
}
public void setfirstname( string first_name ) {
this.firstname = first_name;
}
public string getlastname() {
return lastname;
}
public void setlastname( string last_name ) {
this.lastname = last_name;
}
public int getsalary() {
return salary;
}
public void setsalary( int salary ) {
this.salary = salary;
}
public address getaddress() {
return address;
}
public void setaddress( address address ) {
this.address = address;
}
}
我们需要定义相应的地址表，这样地址对象可以存储和检索到地址表中的另一个pojo类。
import java.util.*;
public class address{
private int id;
private string street;
private string city;
private string state;
private string zipcode;
public address() {}
public address(string street, string city,
string state, string zipcode) {
this.street = street;
this.city = city;
this.state = state;
this.zipcode = zipcode;
}
public int getid() {
return id;
}
public void setid( int id ) {
this.id = id;
}
public string getstreet() {
return street;
}
public void setstreet( string street ) {
this.street = street;
}
public string getcity() {
return city;
}
public void setcity( string city ) {
this.city = city;
}
public string getstate() {
return state;
}
public void setstate( string state ) {
this.state = state;
}
public string getzipcode() {
return zipcode;
}
public void setzipcode( string zipcode ) {
this.zipcode = zipcode;
}
}
定义hibernate映射文件：
开发我们的映射文件，可指示hibernate如何定义的类映射到数据库表。<many-to-one>进行元素将被用来定义规则建立employee和address实体之间的多对一关系。
<?xml version="1.0" encoding="utf-8"?>
<!doctype hibernate-mapping public
"-//hibernate/hibernate mapping dtd//en"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="employee" table="employee">
<meta attribute="class-description">
this class contains the employee detail.
</meta>
<id name="id" type="int" column="id">
<generator class="native"/>
</id>
<property name="firstname" column="first_name" type="string"/>
<property name="lastname" column="last_name" type="string"/>
<property name="salary" column="salary" type="int"/>
<many-to-one name="address" column="address"
class="address" not-null="true"/>
</class>
<class name="address" table="address">
<meta attribute="class-description">
this class contains the address detail.
</meta>
<id name="id" type="int" column="id">
<generator class="native"/>
</id>
<property name="street" column="street_name" type="string"/>
<property name="city" column="city_name" type="string"/>
<property name="state" column="state_name" type="string"/>
<property name="zipcode" column="zipcode" type="string"/>
</class>
</hibernate-mapping>
应该保存的映射文件中的格式<classname>.hbm.xml。保存映射文件中的文件employee.hbm.xml。已经熟悉了大部分的映射细节，但让我们再次看看映射文件中的所有元素：
映射文档是具有<hibernate-mapping>为对应于每一个类包含2个<class>元素的根元素的xml文档。
<class>元素被用于定义数据库表从一个java类特定的映射。 java类名指定使用class元素的name属性和使用表属性数据库表名指定。
<meta>元素是可选元素，可以用来创建类的描述。
<id>元素映射在类中的唯一id属性到数据库表的主键。 id元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了hibernate映射类型，这种类型的映射将会从java转换为sql数据类型。
id元素内<generator>元素被用来自动生成的主键值。将生成元素class属性设置为原生让hibernate拾取无论是identity，sequence或者hilo的算法来创建主键根据底层数据库的支持能力。
<property>元素用于一个java类的属性映射到数据库表中的列。元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了hibernate映射类型，这种类型的映射将会从java转换为sql数据类型。
<many-to-one>进行元素是用来设置employee和地址的实体之间的关系。name属性被设置为在父类中定义的变量，在我们的情况下，它是地址。列属性用于在父表employee集的列名。
最后，我们将创建应用程序类的main（）方法来运行应用程序。我们将使用这个应用程序，以节省一些employee连同的记录他们的地址，然后我们将申请crud操作上的记录。
import java.util.*;
import org.hibernate.hibernateexception;
import org.hibernate.session;
import org.hibernate.transaction;
import org.hibernate.sessionfactory;
import org.hibernate.cfg.configuration;
public class manageemployee {
private static sessionfactory factory;
public static void main(string[] args) {
try{
factory = new configuration().configure().buildsessionfactory();
}catch (throwable ex) {
system.err.println("failed to create sessionfactory object." + ex);
throw new exceptionininitializererror(ex);
}
manageemployee me = new manageemployee();
/* let us have one address object */
address address = me.addaddress("kondapur","hyderabad","ap","532");
/* add employee records in the database */
integer empid1 = me.addemployee("manoj", "kumar", 4000, address);
/* add another employee record in the database */
integer empid2 = me.addemployee("dilip", "kumar", 3000, address);
/* list down all the employees */
me.listemployees();
/* update employee's salary records */
me.updateemployee(empid1, 5000);
/* delete an employee from the database */
me.deleteemployee(empid2);
/* list down all the employees */
me.listemployees();
}
/* method to add an address record in the database */
public address addaddress(string street, string city,
string state, string zipcode) {
session session = factory.opensession();
transaction tx = null;
integer addressid = null;
address address = null;
try{
tx = session.begintransaction();
address = new address(street, city, state, zipcode);
addressid = (integer) session.save(address);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
return address;
}
/* method to add an employee record in the database */
public integer addemployee(string fname, string lname,
int salary, address address){
session session = factory.opensession();
transaction tx = null;
integer employeeid = null;
try{
tx = session.begintransaction();
employee employee = new employee(fname, lname, salary, address);
employeeid = (integer) session.save(employee);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
return employeeid;
}
/* method to list all the employees detail */
public void listemployees( ){
session session = factory.opensession();
transaction tx = null;
try{
tx = session.begintransaction();
list employees = session.createquery("from employee").list();
for (iterator iterator =
employees.iterator(); iterator.hasnext();){
employee employee = (employee) iterator.next();
system.out.print("first name: " + employee.getfirstname());
system.out.print(" last name: " + employee.getlastname());
system.out.println(" salary: " + employee.getsalary());
address add = employee.getaddress();
system.out.println("address ");
system.out.println(" street: " + add.getstreet());
system.out.println(" city: " + add.getcity());
system.out.println(" state: " + add.getstate());
system.out.println(" zipcode: " + add.getzipcode());
}
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
}
/* method to update salary for an employee */
public void updateemployee(integer employeeid, int salary ){
session session = factory.opensession();
transaction tx = null;
try{
tx = session.begintransaction();
employee employee =
(employee)session.get(employee.class, employeeid);
employee.setsalary( salary );
session.update(employee);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
}
/* method to delete an employee from the records */
public void deleteemployee(integer employeeid){
session session = factory.opensession();
transaction tx = null;
try{
tx = session.begintransaction();
employee employee =
(employee)session.get(employee.class, employeeid);
session.delete(employee);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
}
}
编译和执行：
下面是步骤来编译并运行上述应用程序。请确保已在进行的编译和执行之前，适当地设置path和classpath。
- 创建hibernate.cfg.xml配置文件中配置章节解释。
- 创建employee.hbm.xml映射文件，如上图所示。
- 创建employee.java源文件，如上图所示，并编译它。
- 创建address.java源文件，如上图所示，并编译它。
- 创建manageemployee.java源文件，如上图所示，并编译它。
- 执行manageemployee二进制文件来运行程序。
在屏幕上获得以下结果，并同时记录会在员工和地址表创建。
$java manageemployee
.......various log messages will display here........
first name: manoj last name: kumar salary: 4000
address
street: kondapur
city: hyderabad
state: ap
zipcode: 532
first name: dilip last name: kumar salary: 3000
address
street: kondapur
city: hyderabad
state: ap
zipcode: 532
first name: manoj last name: kumar salary: 5000
address
street: kondapur
city: hyderabad
state: ap
zipcode: 532
如果检查员工和地址表，就应该记录下了：
mysql> select * from employee;
+----+------------+-----------+--------+---------+ | id | first_name | last_name | salary | address | +----+------------+-----------+--------+---------+ | 1 | manoj | kumar | 5000 | 5 | +----+------------+-----------+--------+---------+ 1 row in set (0.00 sec)
mysql> select * from address;
+----+-------------+-----------+------------+---------+ | id | street_name | city_name | state_name | zipcode | +----+-------------+-----------+------------+---------+ | 1 | kondapur | hyderabad | ap | 532 | +----+-------------+-----------+------------+---------+ 1 row in set (0.00 sec)
一对多（one-to-many）映射
一对多的映射可以使用一组java集合不包含任何重复的元素来实现。我们已经看到了如何设置映射集合在hibernate中，所以如果你已经学会了集合（set）映射，那么所有设置可用于一对多的映射。
集合被映射到与映射表中<set>元素，并java.util.hashset中初始化。您可以使用set集合在类中，有一个集合中不需要重复的元素。
rdbms表与pojo类我们依然采用上面例子中定义好的，
定义hibernate映射文件：
让我们指示hibernate如何定义的类映射到数据库表的映射文件。
<?xml version="1.0" encoding="utf-8"?>
<!doctype hibernate-mapping public
"-//hibernate/hibernate mapping dtd//en"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="employee" table="employee">
<meta attribute="class-description">
this class contains the employee detail.
</meta>
<id name="id" type="int" column="id">
<generator class="native"/>
</id>
<set name="certificates" cascade="all">
<key column="employee_id"/>
<one-to-many class="certificate"/>
</set>
<property name="firstname" column="first_name" type="string"/>
<property name="lastname" column="last_name" type="string"/>
<property name="salary" column="salary" type="int"/>
</class>
<class name="certificate" table="certificate">
<meta attribute="class-description">
this class contains the certificate records.
</meta>
<id name="id" type="int" column="id">
<generator class="native"/>
</id>
<property name="name" column="certificate_name" type="string"/>
</class>
</hibernate-mapping>
应该保存的映射文件中的格式<classname>.hbm.xml。我们保存映射文件中的文件employee.hbm.xml。你已经熟悉了大部分的映射细节，但让我们再次看看映射文件中的所有元素：
映射文档是具有<hibernate-mapping>为对应于每一个类包含2个<class>元素的根元素的xml文档。
<class>元素被用于定义数据库表从一个java类特定的映射。 java类名指定使用class元素的name属性和使用表属性数据库表名指定。
<meta>元素是可选元素，可以用来创建类的描述。
<id>元素映射在类中的唯一id属性到数据库表的主键。 id元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了hibernate映射类型，这种类型的映射将会从java转换为sql数据类型。
id元素内的<generator>元素被用来自动生成的主键值。将生成元素的class属性设置为原生让hibernate拾取identity，sequence或者hilo中的算法来创建主键根据底层数据库的支持能力。
<property>元素用于一个java类的属性映射到数据库表中的列。元素的name属性是指属性的类和column属性是指在数据库表中的列。 type属性保存了hibernate映射类型，这种类型的映射将会从java转换为sql数据类型。
<set>元素设置证书和employee类之间的关系。我们使用cascade属性中<set>元素来告诉hibernate来保存证书的对象，同时为employee对象。 name属性被设置为在父类中定义的变量集，在我们的例子是证书。对于每一组变量，我们需要定义在映射文件中单独的一组元素。
<key>元素是包含外键的父对象，即在证书表中的列。表employee。
<one-to-many>元素表示一个employee对象涉及到很多证书的对象。
创建应用程序类：
最后，我们将创建应用程序类的main（）方法来运行应用程序。我们将使用这个应用程序，以节省一些员工连同记录证书，然后我们将应用上crud操作记录。
import java.util.*;
import org.hibernate.hibernateexception;
import org.hibernate.session;
import org.hibernate.transaction;
import org.hibernate.sessionfactory;
import org.hibernate.cfg.configuration;
public class manageemployee {
private static sessionfactory factory;
public static void main(string[] args) {
try{
factory = new configuration().configure().buildsessionfactory();
}catch (throwable ex) {
system.err.println("failed to create sessionfactory object." + ex);
throw new exceptionininitializererror(ex);
}
manageemployee me = new manageemployee();
/* let us have a set of certificates for the first employee */
hashset set1 = new hashset();
set1.add(new certificate("mca"));
set1.add(new certificate("mba"));
set1.add(new certificate("pmp"));
/* add employee records in the database */
integer empid1 = me.addemployee("manoj", "kumar", 4000, set1);
/* another set of certificates for the second employee */
hashset set2 = new hashset();
set2.add(new certificate("bca"));
set2.add(new certificate("ba"));
/* add another employee record in the database */
integer empid2 = me.addemployee("dilip", "kumar", 3000, set2);
/* list down all the employees */
me.listemployees();
/* update employee's salary records */
me.updateemployee(empid1, 5000);
/* delete an employee from the database */
me.deleteemployee(empid2);
/* list down all the employees */
me.listemployees();
}
/* method to add an employee record in the database */
public integer addemployee(string fname, string lname,
int salary, set cert){
session session = factory.opensession();
transaction tx = null;
integer employeeid = null;
try{
tx = session.begintransaction();
employee employee = new employee(fname, lname, salary);
employee.setcertificates(cert);
employeeid = (integer) session.save(employee);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
return employeeid;
}
/* method to list all the employees detail */
public void listemployees( ){
session session = factory.opensession();
transaction tx = null;
try{
tx = session.begintransaction();
list employees = session.createquery("from employee").list();
for (iterator iterator1 =
employees.iterator(); iterator1.hasnext();){
employee employee = (employee) iterator1.next();
system.out.print("first name: " + employee.getfirstname());
system.out.print(" last name: " + employee.getlastname());
system.out.println(" salary: " + employee.getsalary());
set certificates = employee.getcertificates();
for (iterator iterator2 =
certificates.iterator(); iterator2.hasnext();){
certificate certname = (certificate) iterator2.next();
system.out.println("certificate: " + certname.getname());
}
}
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
}
/* method to update salary for an employee */
public void updateemployee(integer employeeid, int salary ){
session session = factory.opensession();
transaction tx = null;
try{
tx = session.begintransaction();
employee employee =
(employee)session.get(employee.class, employeeid);
employee.setsalary( salary );
session.update(employee);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
}
/* method to delete an employee from the records */
public void deleteemployee(integer employeeid){
session session = factory.opensession();
transaction tx = null;
try{
tx = session.begintransaction();
employee employee =
(employee)session.get(employee.class, employeeid);
session.delete(employee);
tx.commit();
}catch (hibernateexception e) {
if (tx!=null) tx.rollback();
e.printstacktrace();
}finally {
session.close();
}
}
}
编译和执行：
$java manageemployee
.......various log messages will display here........ first name: manoj last name: kumar salary: 4000 certificate: mba certificate: pmp certificate: mca first name: dilip last name: kumar salary: 3000 certificate: bca certificate: ba first name: manoj last name: kumar salary: 5000 certificate: mba certificate: pmp certificate: mca
如果检查员工和证书表，就应该记录下了：
mysql> select * from employee;
+----+------------+-----------+--------+ | id | first_name | last_name | salary | +----+------------+-----------+--------+ | 1 | manoj | kumar | 5000 | +----+------------+-----------+--------+ 1 row in set (0.00 sec)
mysql> select * from certificate;
+----+------------------+-------------+ | id | certificate_name | employee_id | +----+------------------+-------------+ | 1 | mba | 1 | | 2 | pmp | 1 | | 3 | mca | 1 | +----+------------------+-------------+ 3 rows in set (0.00 sec)
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