java - Hibernate get() 总是返回 NULL

Question

我在使用 hibernate load() 和 get() 时面临两个问题

1. get() 始终返回 NULL，即使该对象存在于数据库中。

   Student s1=new Student();
   System.out.println("Student is calling with get()");        
   session.get(Student.class,new Integer(107));    
   System.out.println("Student called with get()");
   System.out.println("Printing Student Name___"+s1.getMarks());       
   
   System.out.println("--------------------------------");
   
   Student s2=new Student();
   System.out.println("Student is calling with load()");       
   session.load(s2,new Integer(107));  
   System.out.println("Student called with load()");
   System.out.println("Printing Student Name___"+s2.getStdName());     
   

输出:

Student is calling with load()
Hibernate: select student0_.stdtId as stdtId1_0_, student0_.stdName as stdName1_0_,     student0_.marks as marks1_0_ from Student student0_ where student0_.stdtId=?
Student called with load()
Printing Student Name___0.0

--------------------------------

Student is calling with get()
Student called with get()
Printing Student Name___null

但是如果我单独使用 load() 调用同一个对象，它工作正常并给出输出。

正如上面所说，只有当我们调用 load() 的属性时，load() 才会访问数据库，但这里它在调用 load() 方法后立即访问数据库，请检查源代码..

Student s2=new Student();
System.out.println("Student is calling with get()");        
session.load(s2,new Integer(107));  
System.out.println("Student called with get()");
System.out.println("Printing Student Name___"+s2.getStdName()); 

输出:

Student is calling with get()
Hibernate: select student0_.stdtId as stdtId1_0_, student0_.stdName as stdName1_0_, student0_.marks as marks1_0_ from Student student0_ where student0_.stdtId=?
Student called with get()
Printing Student Name___Jaswanth

您能建议一下我在哪里犯了错误吗？谢谢。

Answer 1

Hibernate Session.get(..) 返回给定类的持久实体。引用 API Hibernate Session

因此，您需要将返回的持久性实例引用到 Student 类变量，以便获取数据。因此您的代码需要进行以下修改:-

Student s1= null;
System.out.println("Student is calling with get()");        
s1 = (Student)session.get(Student.class,new Integer(107));    
System.out.println("Student called with get()");
System.out.println("Printing Student Name___"+s1.getMarks()); 

关于你的第二个疑问，你正在将一个非持久的新对象 s2 传递给你的加载方法，并且由于你的持久上下文中没有对象，所以它正在查询数据库以使你的 s2 更新为持久状态。

执行以下操作不会立即发出数据库读取:-

Student s2 = session.load(Student.class, new Integer(107));