Problem
When using getResultList() to retrieve the Rows of the DB table there may be times where you would end up getting the same rows multiple times.

Why this Happens
Let’s have a table tblGroup the one below

ParentGrp	GrpName
Finance	Accounts
Finance	Sales

1. You have 2 records in database.Both the records has Finance as value in parentGrp column
2. Now when you run a select query to select parentGrp = ‘Finance’ you will get 2 records
3. Lets query them WHERE parentGrp = ‘Finance’
4. SQL Query returns 2 rows
5. Hibernate loads first one, and puts into session, with parentGrp as a key.Now this will happen if you have specified it as @ID in entity class or you havent specified it while joining two tables in entity class.The object is placed into the result list.
6. Hibernate loads second one, notices that an object with the same @Id is already in the session, and just places the reference into the result List. Row data are ignored.
7. Now we have two copies of the same record

Solution:
We can solve this by introducing a primary key column something like tblGroup_pk_id.Now this helps to uniquely identify the records in the table so it won’t get overridden when the next rows retrieved.

Now the GrpId should be entitled with @ID annotation in the entity class.

GrpId	ParentGrp	GrpName
101	Finance	Accounts
102	Finance	Sales

org.hibernate.AnnotationException: Use of @OneToMany or @ManyToMany targeting an unmapped class
Sol

1. @Entity Should be added in the Model class
2. If you have missed to add the Model class in the xml file
3. Make sure the annotation is javax.persistence.Entity, and not org.hibernate.annotations.Entity. The former makes the entity detectable.

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javax.naming.NoInitialContextException: Need to specify class name in environment or system property
Sol
“I want to find the telephone number for John Smith, but I have no phonebook to look in”.This exception is thrown when no initial context implementation can be created.JNDI (javax.naming) is all about looking up objects or resources from some directory or provider. To look something up, you need somewhere to look (this is the InitialContext).

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hibernate exception: org.hibernate.AnnotationException: No identifier specified for entity
Sol

1. You are missing a field annotated with @Id. Each @Entity needs an @Id – this is the primary key in the database.
2. If you don’t want your entity to be persisted in a separate table, but rather be a part of other entities, you can use @Embeddable instead of @Entity.
3. If you want simply a data transfer object to hold some data from the hibernate entity, use no annotations on it whatsoever – leave it a simple pojo.

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org.hibernate.hql.internal.ast.QuerySyntaxException: table is not mapped
Sol
In the HQL , you should use the java class name and property name of the mapped @Entity instead of the actual table name and column name

For example if your bean class name is UserDetails then the Hibernate code should be as below.Not Tbl_UserDetails instead of UserDetails

 Query query = entityManager. createQuery("Select UserName from UserDetails"); 

The problem can also be because of wrong import

import javax.persistence.Entity;

instead of

import org.hibernate.annotations.Entity;

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