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。
@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…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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