问题描述
我有一个方法persistData()持久化一个实体对象。我有另一个方法findData()执行find()操作在同一个实体类的主键值被持久化。当我调用实体类的@PostPersist中的findData()时,我得到一个空指针异常。这在我脑海中提出了几个问题:
I have a method persistData() which persists an entity object. I have another method findData() which performs find() operation on the same entity class for the primary key value which was persisted. When I call the findData() in the @PostPersist of the entity class, I get a null pointer exception. This has raised a few questions in my mind:
- 为什么会产生空指针错误?
- @PostPersist在现实中的用途是什么?
- @Postpersist实际上是什么时候调用的?提交后,提交期间或提交之前?
任何进一步的见解也将不胜感激。请在下面找到相关代码和stacktrace:
Any further insights would also be appreciated. Please find the relevant code and stacktrace below:
public void persistData(){
EntityManagerFactory fac= Persistence.createEntityManagerFactory("test");
EntityManager man = fac.createEntityManager();
Employee e = new Employee();
e.setEmpId(500);
e.setEmpName("Emp5");
e.setSalary(5000);
man.getTransaction().begin();
man.persist(e);
man.getTransaction().commit();
man.close();
}
public void findData(){
EntityManagerFactory fac= Persistence.createEntityManagerFactory("test");
EntityManager man = fac.createEntityManager();
Employee e=man.find(Employee.class, 500);
System.out.println(e.getEmpName());
man.close();
}
@PostPersist
public void getData(){
new Service().findData();
}
堆栈跟踪(部分):
Exception in thread "main" javax.persistence.RollbackException: java.lang.NullPointerException
at oracle.toplink.essentials.internal.ejb.cmp3.transaction.base.EntityTransactionImpl.commit(EntityTransactionImpl.java:120)
at oracle.toplink.essentials.internal.ejb.cmp3.transaction.EntityTransactionImpl.commit(EntityTransactionImpl.java:60)
at Service.persistData(Service.java:18)
at Service.main(Service.java:34)
Caused by: java.lang.NullPointerException
at Service.findData(Service.java:28)
at Employee.getData(Employee.java:33)
/ strong>:我使用JPA 1.0
推荐答案
要回答您的问题1:
(需要代码和stacktrace)
(need the code and the stacktrace)
回答你的问题2:
@PostPersist表示JPA回调方法。它允许您通过实体生命周期事件触发一些代码。
The @PostPersist indicate a JPA callback method. It allows you to trigger some code through the entity life-cycle events.
现实生活中的例子
假设您有一个User表格,并且您希望在每次新用户持久化时生成确认电子邮件:您可以使用 PostPersist 方法。
Assume you have a User table and you want to generate a confirmation email every time a new User is persisted: you can do it in a PostPersist method.
要回答您的问题3:
相关部分的规格是在blod。
The relevant part of specs are in blod.
-2.0 specs:
From JPA-2.0 specs:
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