如何使用 Hibernate 注释 @ManyToOne 和 @OneToMany 进行关联

Question

我正在使用本教程学习 Spring、Hibernate、Maven：Chad Lung: A project using Netbeans 7, JUnit, Maven, HSQLDB, Spring and Hibernate。它工作正常，但我需要建立一对多的关系（一个员工有很多任务）。我尝试了很多示例，但仍然不知道如何使我的代码正常工作：

Employee.java：

package com.giantflyingsaucer.simplespringhibernate.entity;

import javax.persistence.*;
import java.io.Serializable;
import java.util.List;

@Entity
@Table(name = "Employees")
public class Employee implements Serializable {

    private Integer employeeId;
    private List<Task> tasks;

    @Id
    @Column(name = "idEmployees", nullable=false)
    public Integer getEmployeeId() {
        return this.employeeId;
    }

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

    @OneToMany(fetch = FetchType.LAZY)
    @JoinColumn(name="idEmployees")
    public List<Task> getTasks() {
        return tasks;
    }
}

Task.java：

package com.giantflyingsaucer.simplespringhibernate.entity;

import javax.persistence.*;
import java.io.Serializable;

@Entity
@Table(name = "Tasks")
public class Task implements Serializable {

    private Integer taskId;
    private Employee employee;


    @Id
    @Column(name = "idTasks", nullable=false)
    public Integer getTaskId() {
        return this.taskId;
    }

    public void setTaskId(Integer taskId) {
        this.taskId = taskId;
    }

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "TasksIdEmployees")
    public Employee getEmployee() {return employee;}

}

db-config.xml：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
xmlns:context="http://www.springframework.org/schema/context" xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.0.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
">
<bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource"
    destroy-method="close">

    <property name="driverClass">
        <value>${jdbc.driver.className}</value>
    </property>
    <property name="jdbcUrl">
        <value>${jdbc.url}</value>
    </property>
    <property name="user">
        <value>${jdbc.username}</value>
    </property>
    <property name="password">
        <value>${jdbc.password}</value>
    </property>
</bean>
<bean id="sessionFactory"
    class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean">
    <property name="dataSource">
        <ref bean="dataSource" />
    </property>
    <property name="packagesToScan" value="com.giantflyingsaucer.simplespringhibernate.entity" />
    <property name="hibernateProperties">
        <props>
            <prop key="hibernate.dialect">${jdbc.hibernate.dialect}</prop>
            <prop key="hibernate.hbm2ddl.auto">update</prop>
            <prop key="hibernate.show_sql">false</prop>
        </props>
    </property>
</bean>
<bean id="transactionManager"
    class="org.springframework.orm.hibernate3.HibernateTransactionManager">
    <property name="sessionFactory">
        <ref bean="sessionFactory" />
    </property>
</bean>
<tx:annotation-driven />

MySQL 表：

CREATE TABLE employees (
`idEmployees` int(11) NOT NULL,
PRIMARY KEY (`idEmployees`)
);

CREATE TABLE tasks (
`idTasks` int(11) NOT NULL,
`TasksIdEmployees` int(11) DEFAULT NULL,
PRIMARY KEY (`idTasks`),
KEY `FkTasksEmployees_idx` (`TasksIdEmployees`),
CONSTRAINT `FkTasksEmployees` FOREIGN KEY (`TasksIdEmployees`) REFERENCES `employees`   (`idEmployees`) ON DELETE NO ACTION ON UPDATE NO ACTION
);

非常感谢！

我通过在 NetBeans 中自动生成映射文件和 POJO 找到了答案：

// Employee.java:
    @OneToMany(fetch = FetchType.LAZY, mappedBy = "employees")
    public List<Task> getTasks() {
        return this.tasks;
    }

    public void setTasks(List<Task> tasks) {
        this.tasks = tasks;
    }

// Task.java:
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name = "TasksIdEmployees")
public Employee getEmployees() {
    return this.employee;
}

public void setEmployees(Employee employee) {
    this.employee = employee;
}

Answer 1

问题如下：

@OneToMany(fetch = FetchType.LAZY)
@JoinColumn(name="idEmployees")
public List<Task> getTasks() {
    return tasks;
}

这是错误的，原因有两个。

1. @JoinColumn(name="idEmployees") 表示：此 OneToMany 使用名为 idEmployees 的连接列（即外键）映射。但连接列未命名为 idEmployees。 idEmployees 是 Employee 表的主键。连接列名称为TasksIdEmployees。放置正确的名称将使映射对于单向 OneToMany 关联正确。但是关联是双向的，这就引出了第二个原因……

2. 在双向关联中，不需要（这是一个错误）在关联的两侧重复映射信息。一方（多方）必须是关联的所有者并定义映射。对方一定是反面，简单地说：去看看对方是如何映射这个关联的。这是使用 mappedBy 属性完成的，该属性告诉 Hibernate 另一侧的字段或属性的名称，即关联的所有者：

   @OneToMany(mappedBy = "employee")
   public List<Task> getTasks() {
       return tasks;
   }
   

注意 LAZY 是 toMany 关联的默认值，因此没有必要指定它。