How do I load a Set of Strings from a JoinTable?

Question

I’m using MySQL 5.5, JPA 2.0, and Hibernate 4.1.0.Final. I have this field in my entity …

@Entity
@Table(name = "teacher",
    uniqueConstraints = {…}
)
public class TrainingSessionTeacher implements Serializable 
{
    …
    @ManyToMany
    @JoinTable(name = "teacher_subj", joinColumns = { @JoinColumn(name = "EACHER_ID") }, inverseJoinColumns = { @JoinColumn(name = "SUBJECT") })
    private Set<String> subjects;

The join table in question has the following structure:

| Field                       | Type        | Null | Key | Default | Extra |
+-----------------------------+-------------+------+-----+---------+-------+
| ID                          | varchar(32) | NO   | PRI | NULL    |       |
| TEACHER_ID                  | varchar(32) | NO   | MUL | NULL    |       |
| SUBJECT                     | varchar(32) | NO   |     | NULL    |       |

However when loading my application context, I get the exception, “org.hibernate.AnnotationException: Use of @OneToMany or @ManyToMany targeting an unmapped class: org.mainco.subco.pd.domain.TrainingSessionTeacher.subjects[java.lang.String]”. Is there a way I can map a Set of Strings in my entity? I’m hoping I don’t have to create a separate entity for the “teacher_subj” class.

The full stack trace is as follows:

Caused by: org.hibernate.AnnotationException: Use of @OneToMany or @ManyToMany targeting an unmapped class: org.mainco.subco.pd.domain.TrainingSessionTeacher.subjects[java.lang.String]
    at org.hibernate.cfg.annotations.CollectionBinder.bindManyToManySecondPass(CollectionBinder.java:1160)
    at org.hibernate.cfg.annotations.CollectionBinder.bindStarToManySecondPass(CollectionBinder.java:691)
    at org.hibernate.cfg.annotations.CollectionBinder$1.secondPass(CollectionBinder.java:626)
    at org.hibernate.cfg.CollectionSecondPass.doSecondPass(CollectionSecondPass.java:66)
    at org.hibernate.cfg.Configuration.originalSecondPassCompile(Configuration.java:1587)
    at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1362)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1727)
    at org.hibernate.ejb.EntityManagerFactoryImpl.<init>(EntityManagerFactoryImpl.java:88)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:904)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:889)
    at org.hibernate.ejb.HibernatePersistence.createContainerEntityManagerFactory(HibernatePersistence.java:73)
    at org.springframework.orm.jpa.LocalContainerEntityManagerFactoryBean.createNativeEntityManagerFactory(LocalContainerEntityManagerFactoryBean.java:287)
    at org.springframework.orm.jpa.AbstractEntityManagerFactoryBean.afterPropertiesSet(AbstractEntityManagerFactoryBean.java:310)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1514)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1452)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:519)
    at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
    at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:294)
    at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:225)
    at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:291)
    at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
    at org.springframework.context.support.AbstractApplicationContext.getBean(AbstractApplicationContext.java:1105)
    at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:915)
    at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:472)
    at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:103)
    at org.springframework.test.context.support.AbstractGenericContextLoader.loadContext(AbstractGenericContextLoader.java:1)
    at org.springframework.test.context.support.DelegatingSmartContextLoader.loadContext(DelegatingSmartContextLoader.java:228)
    at org.springframework.test.context.TestContext.loadApplicationContext(TestContext.java:124)
    at org.springframework.test.context.TestContext.getApplicationContext(TestContext.java:148)
    at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.injectDependencies(DependencyInjectionTestExecutionListener.java:109)
    at org.springframework.test.context.support.DependencyInjectionTestExecutionListener.prepareTestInstance(DependencyInjectionTestExecutionListener.java:75)
    at org.springframework.test.context.TestContextManager.prepareTestInstance(TestContextManager.java:321)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.createTest(SpringJUnit4ClassRunner.java:211)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner$1.runReflectiveCall(SpringJUnit4ClassRunner.java:288)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.methodBlock(SpringJUnit4ClassRunner.java:290)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.runChild(SpringJUnit4ClassRunner.java:231)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:49)
    at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
    at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
    at org.springframework.test.context.junit4.statements.RunBeforeTestClassCallbacks.evaluate(RunBeforeTestClassCallbacks.java:61)
    at org.springframework.test.context.junit4.statements.RunAfterTestClassCallbacks.evaluate(RunAfterTestClassCallbacks.java:71)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
    at org.springframework.test.context.junit4.SpringJUnit4ClassRunner.run(SpringJUnit4ClassRunner.java:174)
    at org.apache.maven.surefire.junit4.JUnit4Provider.execute(JUnit4Provider.java:264)
    at org.apache.maven.surefire.junit4.JUnit4Provider.executeTestSet(JUnit4Provider.java:153)
    at org.apache.maven.surefire.junit4.JUnit4Provider.invoke(JUnit4Provider.java:124)
    at org.apache.maven.surefire.booter.ForkedBooter.invokeProviderInSameClassLoader(ForkedBooter.java:200)
    at org.apache.maven.surefire.booter.ForkedBooter.runSuitesInProcess(ForkedBooter.java:153)
    at org.apache.maven.surefire.booter.ForkedBooter.main(ForkedBooter.java:103)

Thanks for your help, - Dave

Edit: Using element collection, I changed my field to be this

@ElementCollection(fetch = FetchType.EAGER)
@CollectionTable(name = "teacher_subj", joinColumns = {@JoinColumn(name="TEACHER_ID")})
private Set<String> subject;

but when I set this value using the entity class' "setSubject" method, nothing gets persisted to the database. Is there some other annotation I need to add to get Hibernate to generate SQL for my teacher_subj to save to the database properly?

Answer 1

Use the @ElementCollection instead of the @ManyToMany, because @ManyToMany is between entities, and a String is not an Entity. Additionally replace @JoinTable with @CollectionTable(name="teacher_subj", joinColumns=@JoinColumn(name="TEACHER_ID"))