Question

我之前问了一个问题，但是我觉得还不够清楚，我会详细说明代码。
我有一个教师实体和部门实体
许多教师属于一个部门和一个部门有一名教师领导该部门。
以下是我实施它的方法。

@Entity
public class Teacher extends Model {
@Required
public String surname;

@Required
public String othernames;

    ... 

@OneToOne(mappedBy = "deptHead")
public Department headedBy = new Department();

@ManyToOne(cascade=CascadeType.ALL)
public Department dept = new Department();

... 
}

和部门实体

@Entity
public class Department extends Model {
@Required
public String deptName; 

@OneToMany(mappedBy="dept")
public List<Teacher> teachers = new ArrayList<Teacher>();

@OneToOne
public Teacher deptHead = new Teacher();

    ...

}

我收到以下错误

    @6c4nj8nmg
    Internal Server Error (500) for request GET /

    JPA error
    A JPA error occurred (Unable to build EntityManagerFactory): could not instantiate   test objectmodels.Department

    play.exceptions.JPAException: Unable to build EntityManagerFactory
    at play.db.jpa.JPAPlugin.onApplicationStart(JPAPlugin.java:269)
    at play.plugins.PluginCollection.onApplicationStart(PluginCollection.java:525)
    at play.Play.start(Play.java:526)
    at play.Play.detectChanges(Play.java:630)
    at play.Invoker$Invocation.init(Invoker.java:198)
    at Invocation.HTTP Request(Play!)
    Caused by: org.hibernate.InstantiationException: could not instantiate test objectmodels.Department
    at org.hibernate.engine.UnsavedValueFactory.instantiate(UnsavedValueFactory.java:48)
    at org.hibernate.engine.UnsavedValueFactory.getUnsavedIdentifierValue(UnsavedValueFactory.java:67)
    at org.hibernate.tuple.PropertyFactory.buildIdentifierProperty(PropertyFactory.java:67)
    at org.hibernate.tuple.entity.EntityMetamodel.<init>(EntityMetamodel.java:135)
    at org.hibernate.persister.entity.AbstractEntityPersister.<init>(AbstractEntityPersister.java:485)
    at org.hibernate.persister.entity.SingleTableEntityPersister.<init>(SingleTableEntityPersister.java:133)
    at org.hibernate.persister.PersisterFactory.createClassPersister(PersisterFactory.java:84)
    at org.hibernate.impl.SessionFactoryImpl.<init>(SessionFactoryImpl.java:286)
    at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1872)
    at org.hibernate.ejb.Ejb3Configuration.buildEntityManagerFactory(Ejb3Configuration.java:906)
    at play.db.jpa.JPAPlugin.onApplicationStart(JPAPlugin.java:267)
    ... 5 more
    Caused by: java.lang.reflect.InvocationTargetException
    at  org.hibernate.engine.UnsavedValueFactory.instantiate(UnsavedValueFactory.java:45)
    ... 15 more
     Caused by: java.lang.StackOverflowError
    at java.lang.Class.searchMethods(Unknown Source)
    at java.lang.Class.getMethod0(Unknown Source)
    at java.lang.Class.getMethod(Unknown Source)
    at      play.classloading.enhancers.PropertiesEnhancer$FieldAccessor.invokeWriteProperty(PropertiesEnhancer.java:268)
    at models.Department.<init>(Department.java:23)
    at models.Teacher.<init>(Teacher.java:47)
    at models.Department.<init>(Department.java:26)
    ...

需要这些帮助

Answer 1

这不是JPA问题，而是由Teacher / Department的递归实例化引起的。

当您创建或要求JPA创建Teacher的实例时，Teacher会尝试实例化Department，实例化Teacher ...，无限。

因此，您在堆栈跟踪结束时看到StackOverflowError错误。

从类定义中删除= new Teacher()和= new Department()表达式;在创建它们时依赖并使用适当的setter方法。

Answer 2

我删除了教师和部门之间的@oneToOne关系并创建了一个名为DepartmentHeads的新实体，现在一切正常。
这是结果。

@Entity
public class Teacher extends Model {
    @Required
    public String surname;

    @Required
    public String othernames;

    ... 

    @ManyToOne(cascade=CascadeType.ALL)
    public Department dept = new Department();

    ... 
}

@Entity
public class Department extends Model {
    @Required
    public String deptName; 

    @OneToMany(mappedBy="dept")
    public List<Teacher> teachers = new ArrayList<Teacher>();

    ...

}

@Entity
public class DepartmentHead extends Model{
    @OneToOne
    public Teacher teacher = new Teacher();

    @OneToOne
    public Department dept = new Department();  
}

现在一切正常。

两个实体之间的JPA OneToOne和ManyToMany

2 个答案: