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

我通过使用本教程学习Spring,Hibernate,Maven:Chad Lung:一个使用Netbeans

7,JUnit,Maven,HSQLDB,Spring和Hibernate的项目。可以,但是我需要建立一对多关系(一个雇员有很多任务)。我已经尝试了许多示例,但仍然不知道如何使代码正常工作:

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;

}

}

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;}

}

<?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 />

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;

}

回答:

出问题的是以下内容:

@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使用名为的连接列(即外键)进行映射idEmployees。但是联接列未命名为idEmployees。idEmployees是Employee表的主键。连接列的名称为TasksIdEmployees。输入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,这导致了第二个原因…

  2. 在双向关联中,没有必要(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。另一边必须是反面,只需说一下:去看另一边如何映射此关联。这是使用mapledBy属性完成的,该属性告诉Hibernate关联的另一端的字段或属性的名称:

        @OneToMany(mappedBy = "employee")

public List<Task> getTasks() {

return tasks;

}

请注意,LAZY是toMany关联的默认设置,因此没有必要指定它。

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