Question

我遇到了hibernate load（）和get（）

两个问题

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

但是如果我单独用load（）调用同一个对象，它的工作正常并给出输出。

正如它所说的那样，只有当我们调用它的属性时，load（）才会命中数据库，但是在调用load（）方法之后立即点击数据库，检查这个源代码..

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

你能告诉我哪里出错吗？谢谢。

Answer 1

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

因此，您需要将此返回的持久性实例引用到Student类变量，以便您可以获取数据。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());

关于您的第二个疑问，您正在将非持久性新对象s2传递给您的load方法，并且因为您的持久化上下文中没有对象，因此它正在查询数据库以使您的s2更新具有持久性状态。

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));

Answer 2

您的第一个示例的输出与示例代码不匹配，但我可以看到以下问题：

您正在调用session.get（），它将返回该对象，但您忽略了结果。试试这个：

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

与load（）示例类似。第一个参数应该是您要读取的对象的类。该调用将返回该对象。

在这两种情况下都不需要创建new Student()，Hibernate会为你做这件事。

有关get（）和load（）的更多信息，请参阅Hibernate: Difference between session.get and session.load

Hibernate get（）总是返回NULL

2 个答案: