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:
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; }
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…
@JoinColumn(name="idEmployees")
idEmployees
TasksIdEmployees
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。