使用注解的Hibernate one-to-many映射

One to many映射关系指的是两个实体间一个实体可以和多个实体有关联关系，但是多的这一端只能和一的这一端的一个实例有关系。它是一个1 到 n的关系。例如在任何的公司员工可以注册多个银行账户，一个银行账户只能和一个员工相关联，在这篇文章中我们将会学习怎么在Hibernate3中建立这种映射关系。

问题陈述

我们要写两个实体一个是Employee实体另一个是Account实体，这样多个银行账户就可以和一个员工关联了，但是这些账户不能被两个或以上的用户共享。

设计解决方案

这种问题可以使用两种方式解决。一种方式是在Account表中设置一个外键EMPLOYEE_ID，这一列指向Employee表的主键，这种方式没有两个账号可以和多个用户相关联，显然，为了完成这种限制，账号应该是独特的。另一种方式是建立一个连接表，比如说是叫EMPLOYEE_ACCOUNT，这个表有两列，EMP_ID作为EMPLOYEE表中主键的外键，对于ACCOUNT_ID也是这种情况。

使用外键连接

这种方式，两个实体都要负责建立关系并维护这种关系，EMPLOYEE实体应该申明的关系是one to many，Account实体应该声明的关系是many  to one。首先来看一下关系设计：

EMPLOYEE实体

package hibernate.test.oneToMany.foreignKeyAsso;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "ForeignKeyAssoEntity")
@Table(name = "Employee", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID"),
@UniqueConstraint(columnNames = "EMAIL") })
public class EmployeeEntity implements Serializable {

    private static final long serialVersionUID = -1798070786993154676L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Integer employeeId;

    @Column(name = "EMAIL", unique = true, nullable = false, length = 100)
    private String email;

    @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
    private String firstName;

    @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
    private String lastName;

    @OneToMany(cascade=CascadeType.ALL)
    @JoinColumn(name="EMPLOYEE_ID")
    private Set<AccountEntity> accounts;

    public Integer getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(Integer employeeId) {
        this.employeeId = employeeId;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Set<AccountEntity> getAccounts() {
        return accounts;
    }

    public void setAccounts(Set<AccountEntity> accounts) {
        this.accounts = accounts;
    }
}

Account实体

package hibernate.test.oneToMany.foreignKeyAsso;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "ForeignKeyAssoAccountEntity")
@Table(name = "ACCOUNT", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID")})
public class AccountEntity implements Serializable
{

    private static final long serialVersionUID = -6790693372846798580L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Integer accountId;

    @Column(name = "ACC_NUMBER", unique = true, nullable = false, length = 100)
    private String accountNumber;

    @ManyToOne
    private EmployeeEntity employee;

    public Integer getAccountId() {
        return accountId;
    }

    public void setAccountId(Integer accountId) {
        this.accountId = accountId;
    }

    public String getAccountNumber() {
        return accountNumber;
    }

    public void setAccountNumber(String accountNumber) {
        this.accountNumber = accountNumber;
    }

    public EmployeeEntity getEmployee() {
        return employee;
    }

    public void setEmployee(EmployeeEntity employee) {
        this.employee = employee;
    }
}

测试代码

package hibernate.test.oneToMany;

import hibernate.test.HibernateUtil;
import hibernate.test.oneToMany.foreignKeyAsso.AccountEntity;
import hibernate.test.oneToMany.foreignKeyAsso.EmployeeEntity;

import java.util.HashSet;
import java.util.Set;

import org.hibernate.Session;

public class TestForeignKeyAssociation
{

    public static void main(String[] args)
    {
        Session session = HibernateUtil.getSessionFactory().openSession();
        session.beginTransaction();

        AccountEntity account1 = new AccountEntity();
        account1.setAccountNumber("Account detail 1");

        AccountEntity account2 = new AccountEntity();
        account2.setAccountNumber("Account detail 2");

        AccountEntity account3 = new AccountEntity();
        account3.setAccountNumber("Account detail 3");

        //Add new Employee object
        EmployeeEntity firstEmployee = new EmployeeEntity();
        firstEmployee.setEmail("demo-user-first@mail.com");
        firstEmployee.setFirstName("demo-one");
        firstEmployee.setLastName("user-one");

        EmployeeEntity secondEmployee = new EmployeeEntity();
        secondEmployee.setEmail("demo-user-second@mail.com");
        secondEmployee.setFirstName("demo-two");
        secondEmployee.setLastName("user-two");

        Set<AccountEntity> accountsOfFirstEmployee = new HashSet<AccountEntity>();
        accountsOfFirstEmployee.add(account1);
        accountsOfFirstEmployee.add(account2);

        Set<AccountEntity> accountsOfSecondEmployee = new HashSet<AccountEntity>();
        accountsOfSecondEmployee.add(account3);

        firstEmployee.setAccounts(accountsOfFirstEmployee);
        secondEmployee.setAccounts(accountsOfSecondEmployee);
        //Save Employee
        session.save(firstEmployee);
        session.save(secondEmployee);

        session.getTransaction().commit();
        HibernateUtil.shutdown();
    }
}
Output:

Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER, employee_ID) values (?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER, employee_ID) values (?, ?)
Hibernate: insert into Employee (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into ACCOUNT (ACC_NUMBER, employee_ID) values (?, ?)
Hibernate: update ACCOUNT set EMPLOYEE_ID=? where ID=?
Hibernate: update ACCOUNT set EMPLOYEE_ID=? where ID=?
Hibernate: update ACCOUNT set EMPLOYEE_ID=? where ID=?

使用关联表

这种方式使用关联表存储两个实体间的关系@JoinTable注解是用来建立这种关系的，先来看一下数据库模式

EMPLOYEE实体

package hibernate.test.oneToMany.joinTable;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "JoinTableEmployeeEntity")
@Table(name = "Employee", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID"),
@UniqueConstraint(columnNames = "EMAIL") })
public class EmployeeEntity implements Serializable
{
    private static final long serialVersionUID = -1798070786993154676L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Integer employeeId;

    @Column(name = "EMAIL", unique = true, nullable = false, length = 100)
    private String email;

    @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
    private String firstName;

    @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
    private String lastName;

    @OneToMany(cascade=CascadeType.ALL)
    @JoinTable(name="EMPLOYEE_ACCOUNT", joinColumns={@JoinColumn(name="EMPLOYEE_ID", referencedColumnName="ID")}
    , inverseJoinColumns={@JoinColumn(name="ACCOUNT_ID", referencedColumnName="ID")})
    private Set<AccountEntity> accounts;

    public Integer getEmployeeId() {
        return employeeId;
    }

    public void setEmployeeId(Integer employeeId) {
        this.employeeId = employeeId;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Set<AccountEntity> getAccounts() {
        return accounts;
    }

    public void setAccounts(Set<AccountEntity> accounts) {
        this.accounts = accounts;
    }
}

Account实体

package hibernate.test.oneToMany.joinTable;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;
import javax.persistence.UniqueConstraint;

@Entity(name = "JoinTableAccountEntity")
@Table(name = "ACCOUNT", uniqueConstraints = {
@UniqueConstraint(columnNames = "ID")})
public class AccountEntity implements Serializable
{

    private static final long serialVersionUID = -6790693372846798580L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID", unique = true, nullable = false)
    private Integer accountId;

    @Column(name = "ACC_NUMBER", unique = true, nullable = false, length = 100)
    private String accountNumber;

    public Integer getAccountId() {
        return accountId;
    }

    public void setAccountId(Integer accountId) {
        this.accountId = accountId;
    }

    public String getAccountNumber() {
        return accountNumber;
    }

    public void setAccountNumber(String accountNumber) {
        this.accountNumber = accountNumber;
    }
}

在配置文件中配置实体，我们已经有了两个在运行时的实体，我们必须在配置文件中增加他们。请注意只有一个集合实体可以在配置文件中配置，否则会有意外的情况发生

< ?xml version="1.0" encoding="utf-8"?>
< !DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernatetest</property>
        <property name="hibernate.connection.password">XXXXXX</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create</property>
        <mapping clas="hibernate.test.oneToMany.foreignKeyAsso.AccountEntity"></mapping>
        <mapping clas="hibernate.test.oneToMany.foreignKeyAsso.EmployeeEntity"></mapping>
    </session-factory>
</hibernate-configuration>

测试代码：

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