Hibernate Interview QA

What is Hibernate?

Hibernate is a powerful, high performance object/relational persistence and query service.It is an open-source technology which fits well both with Java and .NET technologies.Hibernate lets developers write persistence classes with hibernate query features of HQL within principles of Object Oriented paradigm.It means one can include association,inheritance,polymorphism,composition and collection of these persisting objects to build applications.

Hibernate Architecture

The main objective of Hibernate is to relieve the developers from manual handling of SQLs,JDBC APIs for resultsets handling and it helps in keeping your data portable to various SQL databases,just by switching the delegate and driver details in hibernate.cfg.xml file.
Hibernate offers sophisticated query options, you can write plain SQL, object-oriented HQL (Hibernate Query Language), or create programmatic criteria and example queries. Hibernate can optimize object loading all the time, with various fetching and caching options.
Some snapshots of Hibernate:
-Free open source
-OO Concepts can be implemented.
-A rich variety of mappings for collections and dependent objects
-No extra code generation or bytecode processing steps in your build procedure
-Great performance, has a dual-layer cache architecture, and may be used in a cluster
-Its own query language support
-Efficient transaction handling
-The Java Persistence API is the standard object/relational mapping and persistence management interface of the Java EE 5.0 platform which are implemented with the Hibernate Annotations and Hibernate EntityManager modules, on top of the Hibernate Core.(As part of EJB3.0 spec)

Why Hibernate?

The reasons are plenty,weighing in favor of Hibernate clearly.

-Cost effective.Just imagine when you are using EJBs instead of Hibernate.One has to invest in Application Server(Websphere,Weblogic etc.),learning curve for EJB is slow and requires special training if your developers are not equipped with the EJB know-how.

-The developers get rid of writing complex SQLs and no more need of JDBC APIs for resultset handling.Even less code than JDBC.In fact the OO developers work well when they have to deal with object then writing lousy queries.

-High performance then EJBs(if we go by their industry reputation),which itself a container managed,heavyweight solution.

-Switching to other SQL database requires few changes in Hibernate configuration file and requires least clutter than EJBs.

-EJB itself has modeled itself on Hibernate principle in its latest version i.e. EJB3 because of apparent reasons.

What is ORM ?

Object Relational Mapping(ORM) is a technique/solution that provides an object-based view of data to applications which it can manipulate.The basic purpose of ORM is to allow an application written in an object oriented language to deal with the information it manipulates in terms of objects, rather than in terms of database-specific concepts such as rows, columns and tables.

In the Java world, ORM's first appearance was under the form of entity beans. But entity beans have limited scope in Java EE domain,they can not be exploited for Java SE based applications.The mapping of class lever attributes is done to table columns.For example a String variabe of a class will directly map onto a VARCHAR column. A relationship mapping is the one that you use when you have an attribute of a class that holds a reference to an instance of some other class in your domain model. The most common types of relationship mappings are "one to one", "one to many" or "many to many".

What are core interfaces for Hibernate framework?

Most Hibernate-related application code primarily interacts with four interfaces provided by Hibernate Core:

org.hibernate.Session
org.hibernate.SessionFactory
org.hibernate.Criteria
org.hibernate.Query

The Session is a persistence manager that manages operation like storing and retrieving objects. Instances of Session are inexpensive to create and destroy. They are not thread safe.

The application obtains Session instances from a SessionFactory. SessionFactory instances are not lightweight and typically one instance is created for the whole application. If the application accesses multiple databases, it needs one per database.

The Criteria provides a provision for conditional search over the resultset.One can retrieve entities by composing Criterion objects. The Session is a factory for Criteria.Criterion instances are usually obtained via the factory methods on Restrictions.

Query represents object oriented representation of a Hibernate query. A Query instance is obtained by calling Session.createQuery().

What is dirty checking in Hibernate?

Hibernate automatically detects object state changes in order to synchronize the updated state with the database, this is called dirty checking. An important note here is, Hibernate will compare objects by value, except for Collections, which are compared by identity. For this reason you should return exactly the same collection instance as Hibernate passed to the setter method to prevent unnecessary database updates.

What are different fetch strategies Hibernate have?

A fetching strategy in Hibernate is used for retrieving associated objects if the application needs to navigate the association. They may be declared in the O/R mapping metadata, or over-ridden by a particular HQL or Criteria query.

Hibernate3 defines the following fetching strategies:

Join fetching - Hibernate retrieves the associated instance or collection in the same SELECT, using an OUTER JOIN.

Select fetching - a second SELECT is used to retrieve the associated entity or collection. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you actually access the association.

Subselect fetching - a second SELECT is used to retrieve the associated collections for all entities retrieved in a previous query or fetch. Unless you explicitly disable lazy fetching by specifying lazy="false", this second select will only be executed when you actually access the association.

Batch fetching - an optimization strategy for select fetching - Hibernate retrieves a batch of entity instances or collections in a single SELECT, by specifying a list of primary keys or foreign keys.

Hibernate also distinguishes between:
Immediate fetching - an association, collection or attribute is fetched immediately, when the owner is loaded.

Lazy collection fetching - a collection is fetched when the application invokes an operation upon that collection. (This is the default for collections.)

"Extra-lazy" collection fetching - individual elements of the collection are accessed from the database as needed. Hibernate tries not to fetch the whole collection into memory unless absolutely needed (suitable for very large collections)

Proxy fetching - a single-valued association is fetched when a method other than the identifier getter is invoked upon the associated object.

"No-proxy" fetching - a single-valued association is fetched when the instance variable is accessed. Compared to proxy fetching, this approach is less lazy (the association is fetched even when only the identifier is accessed) but more transparent, since no proxy is visible to the application. This approach requires buildtime bytecode instrumentation and is rarely necessary.

Lazy attribute fetching - an attribute or single valued association is fetched when the instance variable is accessed. This approach requires buildtime bytecode instrumentation and is rarely necessary.

We use fetch to tune performance. We may use lazy to define a contract for what data is always available in any detached instance of a particular class.

Can you compare JDBC/DAO with Hibernate?

Hibernate and straight SQL through JDBC are different approaches.They both have their specific significance in different scenarios.If your application is not to big and complex,not too many tables and queries involved then it will be better to use JDBC. While Hibernate is a POJO based ORM tool,using JDBC underneath to connect to database, which lets one to get rid of writing SQLs and associated JDBC code to fetch resultset,meaning less LOC but more of configuration work.It will suit better when you have large application involving large volume of data and queries.Moreover lazy loading,caching of data helps in having better performance and you need not call the database every time rather data stays in object form which can be reused.

Explain different inheritance mapping models in Hibernate

There can be three kinds of inheritance mapping in hibernate

1. Table per concrete class with unions
2. Table per class hierarchy
3. Table per subclass

Example:
We can take an example of three Java classes like Vehicle, which is an abstract class and two subclasses of Vehicle as Car and UtilityVan.

1. Table per concrete class with unions
In this scenario there will be 2 tables
Tables: Car, UtilityVan, here in this case all common attributes will be duplicated.

2. Table per class hierarchy
Single Table can be mapped to a class hierarchy
There will be only one table in database named 'Vehicle' which will represent all attributes required for all three classes.
Here it is be taken care of that discriminating columns to differentiate between Car and UtilityVan

3. Table per subclass
Simply there will be three tables representing Vehicle, Car and UtilityVan

Question: What are common mechanisms of configuring Hibernate?
Answer: 1. By placing hibernate.properties file in the classpath.
2. Including elements in hibernate.cfg.xml in the classpath.

Question:How can you create a primary key using Hibernate?
Answer: The 'id' tag in .hbm file corresponds to primary key of the table:

Here Id ="empid", that will act as primary key of the table "EMPLOYEE".

Question: In how many ways one can map files to be configured in Hibernate?
Answer: 1. Either mapping files are added to configuration in the application code or,
2.hibernate.cfg.xml can be used for configuring in .

Question: How to set Hibernate to log all generated SQL to the console?
Answer: By setting the hibernate.show_sql property to true.

Question: What happens when both hibernate.properties and hibernate.cfg.xml are in the classpath?
Answer: The settings of the XML configuration file will override the settings used in the properties.

Question: What methods must the persistent classes implement in Hibernate?
Answer: Since Hibernate instantiates persistent classes using Constructor.newInstance(), it requires a constructor with no arguments for every persistent class. And getter and setter methods for all the instance variables.

Question: How can Hibernate be configured to access an instance variable directly and not through a setter method?
Answer: By mapping the property with access="field" in Hibernate metadata. This forces hibernate to bypass the setter method and access the instance variable directly while initializing a newly loaded object.

Question: How to declare mappings for multiple classes in one mapping file?
Answer:Use multiple elements. But, the recommended practice is to use one mapping file per persistent class.

Question: How are the individual properties mapped to different table columns?
Answer: By using multiple elements inside the element.

Question: What are derived properties?
Answer: The properties that are not mapped to a column, but calculated at runtime by evaluation of an expression are called derived properties. The expression can be defined using the formula attribute of the element.

Question: How can you make a property be read from the database but not modified in anyway (make it immutable)?
Answer: Use insert="false" and update="false" attributes.

Question: How can a whole class be mapped as immutable?
Answer: By using the mutable="false" attribute in the class mapping.