Hibernate Interview Questions And Answers - DoAnswers.com

Hibernate's primary feature is mapping from Java classes to database tables (and from Java data types to SQL data types). Hibernate also provides data query and retrieval facilities. Hibernate generates the SQL calls and relieves the developer from manual result set handling and object conversion, keeping the application portable to all SQL databases, with database portability delivered at very little performance overhead.

Hibernate provides transparent persistence for Plain Old Java Objects (POJOs). The only strict requirement for a persistent class is a no-argument constructor, not compulsorily public. (Proper behavior in some applications also requires special attention to the equals() and hashCode() methods.)

Hibernate provides a dirty checking feature that avoids unnecessary database write actions by performing SQL updates only on the modified fields of persistent objects.

Hibernate can be used both in standalone Java applications and in Java EE applications using servlets or EJB session beans.

Preserving the data inside a database is database persistence. Persistence is once of the fundamental concepts of application development.

There are many alternatives to implement persistence. Object/relational mapping (ORM) is the technique that Hibernate tool uses to persist the data. By using ORM tool like Hibernate, you get advantages like low SQL coding (improved productivity), easy to maintain the system and also leads to better performance.

Proxies are created dynamically by sub classing your object at runtime. The subclass has all the methods of the parent, and when any of the methods are accessed, the proxy loads up the real object from the DB and calls the method for you

There are two types of Loading in application. Eager loading and Lazy
loading. In eager loading we will fetch all the values from the Persistent storage and cache it. It will make serious performance issues. We use lazy loading to avoid that scenario.
Lazy setting decides whether to load child objects while loading the Parent Object. You need to do this setting respective hibernate mapping file of the parent class.Lazy = true (means not to load child) ( In the terms of database its primary key-foreign key relationship).
By default the lazy loading of the child objects is true. This make sure that the child objects are not loaded unless they are explicitly invoked in the application by calling getChild() method on parent. In this case hibernate issues a fresh database call to load the child when getChild() is actually called on the Parent object. But in some cases you do need to load the child objects when parent is loaded. Just make the lazy=false and hibernate will load the child when parent is loaded from the database
But Sometimes it doesn't work properly, it doesn't load the child objects for the parent .and second problem is that if you have condition like ....
Employee Table -1-----n-> Emp_Dept Table<-n------1- Department Table
And you have many to many relationship between them, then by specifying lazy="false" for both the parent object(Employee And Department) hibernate try to get all the child for this parent and for this child. Now this child becomes the parent and hibernate will try to get the entire child for this parent. This process may continue. This results in performance issue.
So U has to take care while writing the mapping files in case of many-to-many relationships.