简介:
传统上,Hibernate的配置依赖于外部 XML 文件:数据库映射被定义为一组 XML 映射文件,并且在启动时进行加载。
然而现在借助新的 Hibernate Annotation 库,即可一次性地分配所有旧映射文件——一切都会按照您的想法来定义——注释直接嵌入到您的Java类中,并提供一种强大及灵活的方法来声明持久性映射。即利用hibernate注解后,可不用定义持久化类对应的*.hbm.xml文件,直接以注解方式写入在持久化类中来实现。
配置:
(1)安装 Hibernate Annotation 第一步, 环境与jar包: 要使用 Hibernate Annotation,您至少需要具备 Hibernate 3.2和Java 5。可以从 Hibernate 站点下载 Hibernate 3.2 和 Hibernate Annotation库。除了标准的 Hibernate JAR 和依赖项之外,您还需要 Hibernate Annotations .jar 文件(hibernate-annotations.jar)、Java 持久性 API (lib/ejb3-persistence.jar)。
@Entity
@Table(name = "Dept") public class Dept {
@Id
@GeneratedValue
/*
对于oracle想使用各自的Sequence,设置如下:
@GeneratedValue(strategy =GenerationType.AUTO,generator="PROMOTION_SEQ")
@SequenceGenerator(name="PROMOTION_SEQ",sequenceName="PROMOTION_SEQ")
另外:
对于自动增长后,在数据表中的相应字段,要设置字段为auto_increment.
*/
private Integer deptno;
@Column
private String deptname;
@OneToMany(mappedBy="dept")
//mappedBy 属性主要针对外键而言,与之对应的是.xml中的inverse属性
//mappedBy="dept" 是把维护权交给多的一方
@LazyCollection(LazyCollectionOption.FALSE)
private Set<Emp> emps=new HashSet<Emp>();
public Integer getDeptno() { return deptno; }
public void setDeptno(Integer deptno) { this.deptno = deptno; }
public String getDeptname() { return deptname; }
public void setDeptname(String deptname) { this.deptname = deptname; }
public Set<Emp> getEmps() { return emps; }
public void setEmps(Set<Emp> emps) { this.emps = emps; } }
默认情况下,Hibernate 会将持久类以匹配的名称映射到表和字段中。
@Entity
@Table(name = "Emp")
public class Emp {
@Id
@GeneratedValue
private Integer empno;
@Column
private String ename;
//如果有多个cascade,可以是{CascadeType.PERSIST,CascadeType.MERGE}
@ManyToOne(cascade = {CascadeType.ALL})
@JoinColumn(name = "deptno")//dept类对应外键的属性:deptno
private Dept dept; public Integer getEmpno() {
return empno;
} public void setEmpno(Integer empno) {
this.empno = empno;
} public String getEname() {
return ename;
} public void setEname(String ename) {
this.ename = ename;
} public Dept getDept() {
return dept;
} public void setDept(Dept dept) {
this.dept = dept;
} }
在hibernate.cfg.xml中的
<session-factory>
.....
<mapping class="cn.onetomanydouble.entity.Dept"/>
<mapping class="cn.onetomanydouble.entity.Emp"/>
</session-factory>
这里遇到一个问题:如果配置mappedBy属性的同时加上@JoinColumn会抛出异常,所以不能同时使用@JoinColumn和mappedBy;因为@JoinColumn本身就是自己来维护外键,和mappedBy冲突了。--->>>希望大牛可以再详细的解说下!
抛出的异常如下:
java.lang.ExceptionInInitializerError
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:)
at org.junit.runners.BlockJUnit4ClassRunner$.runReflectiveCall(BlockJUnit4ClassRunner.java:)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:)
at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:)
at org.junit.runners.ParentRunner$.run(ParentRunner.java:)
at org.junit.runners.ParentRunner$.schedule(ParentRunner.java:)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:)
at org.junit.runners.ParentRunner.access$(ParentRunner.java:)
at org.junit.runners.ParentRunner$.evaluate(ParentRunner.java:)
at org.junit.runners.ParentRunner.run(ParentRunner.java:)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory' defined in class path resource [applicationContext.xml]: Invocation of init method failed; nested exception is org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn: com.lizhou.entity.test.Department.employeeList
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:)
at org.springframework.beans.factory.support.AbstractBeanFactory$.getObject(AbstractBeanFactory.java:)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:)
at com.lizhou.action.test.TestAction.<clinit>(TestAction.java:)
... more
Caused by: org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn: com.lizhou.entity.test.Department.employeeList
at org.hibernate.cfg.annotations.CollectionBinder.bind(CollectionBinder.java:)
at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:)
at org.hibernate.cfg.AnnotationBinder.processIdPropertiesIfNotAlready(AnnotationBinder.java:)
at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:)
at org.springframework.orm.hibernate4.LocalSessionFactoryBuilder.buildSessionFactory(LocalSessionFactoryBuilder.java:)
at org.springframework.orm.hibernate4.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:)
at org.springframework.orm.hibernate4.LocalSessionFactoryBean.afterPropertiesSet(LocalSessionFactoryBean.java:)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:)
... more
在这里再说一下,如果先保存员工后保存部门,会多出四条update语句。
Hibernate: insert into emp (deptno, ename, empno) values (?, ?, ?)
Hibernate: insert into emp (deptno, ename, empno) values (?, ?, ?)
Hibernate: insert into dept(dname) values (?)
Hibernate: update emp set deptno=?, ename=? ,empno=? where id=?
Hibernate: update emp set deptno=?, ename=? ,empno=? where id=?
Hibernate: update emp set deptno=? ,empno=? where id=?
Hibernate: update emp set deptno=? ,empno=? where id=?
总结:mappedBy属性跟xml配置文件里的inverse一样。在一对多或一对一的关系映射中,如果不表明mappedBy属性,默认是由本方维护外键。但如果两方都由本方来维护的话,会多出一些update语句,性能有一定的损耗。
解决的办法就是在一的一方配置上mappedBy属性,将维护权交给多的一方来维护,就不会有update语句了。
注意,配了mappedBy属性后,不要再有@JoinColumn,会冲突!