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Hibernate Interview Questions

1 . What is Hibernate?

  • Hibernate is a pure Java object-relational mapping (ORM) and persistence framework that allows you to map plain old Java objects to relational database tables using (XML) configuration files.Its purpose is to relieve the developer from a significant amount of relational data persistence-related programming tasks.

2 . What is ORM ?

  • ORM stands for Object Relational Mapping. ORM is the automated persistence of objects in a Java application to the tables in a relational database.
    ORM is the fundamental concept of Hibernate framework which maps database tables with Java Objects and then provides various API’s to perform different types of operations on the data tables.

3 . What does ORM consists of ?

  • It should have an API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes Should have a language or an API for specifying queries that refer to the classes and the properties of classes An ability for specifying mapping metadata It should have a technique for ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions. It is the programmed and translucent perseverance of objects in a Java application in to the tables of a relational database using the metadata that describes the mapping between the objects and the database. It works by transforming the data from one representation to another.

4 . Why hibernate is advantageous over Entity Beans & JDBC?

  • An entity bean always works under the EJB container, which allows reusing of the object external to the container. An object can not be detached in entity beans and in hibernate detached objects are supported.
    Hibernate is not database dependent where as JDBC is database dependent. Query tuning is not needed for hibernate as JDBC is needed. Data can be placed in multiple cache which is supported by hibernate, whereas in JDBC the cache is to be implemented.

5 . What are the ORM levels ?

  • The ORM levels are:
    • Pure relational (stored procedure.)
    • Light objects mapping (JDBC)
    • Medium object mapping
    • Full object Mapping (composition,inheritance, polymorphism, persistence by reachability)

6 . What is a pure relational ORM?

  • The entire application, including the user interface, is designed around the relational model and SQL-based relational operations.

7 . What is a meant by light object mapping?

  • The entities are represented as classes that are mapped manually to the relational tables. The code is hidden from the business logic using specific design patterns. This approach is successful for applications with a less number of entities, or applications with common, metadata-driven data models. This approach is most known to all.

8 . What is a meant by medium object mapping?

  • The application is designed around an object model. The SQL code is generated at build time. And the associations between objects are supported by the persistence mechanism, and queries are specified using an object-oriented expression language. This is best suited for medium-sized applications with some complex transactions. Used when the mapping exceeds 25 different database products at a time.

9 . What is meant by full object mapping?

  • Full object mapping supports sophisticated object modeling: composition, inheritance, polymorphism and persistence. The persistence layer implements transparent persistence; persistent classes do not inherit any special base class or have to implement a special interface. Efficient fetching strategies and caching strategies are implemented transparently to the application.

10 . Why do you need ORM tools like hibernate?

  • The main advantage of ORM like hibernate is that it shields developers from messy SQL. Apart from this, ORM provides following benefits:
    • Improved productivity – Hibernate reduces the burden of developer by providing much of the functionality and let the developer to concentrate on business logic.
      • High-level object-oriented API
      • Less Java code to write
      • No SQL to write
    • Improved performance– Hand-coded persistence provided greater performance than automated one. But this is not true all the times. But in hibernate, it provides more optimization that works all the time there by increasing the performance. If it is automated persistence then it still increases the performance.
      • Sophisticated caching
      • Lazy loading
      • Eager loading
    • Improved maintainability– As hibernate provides most of the functionality, the LOC for the application will be reduced and it is easy to maintain. By automated object/relational persistence it even reduces the LOC.
      • A lot less code to write
    • Improved portability- Irrespective of the different types of databases that are there, hibernate provides a much easier way to develop a cross platform application.
      • ORM framework generates database-specific SQL for you

11 . What Does Hibernate Simplify?

  • Hibernate simplifies:
    • Saving and retrieving your domain objects
    • Making database column and table name changes
    • Centralizing pre save and post retrieve logic
    • Complex joins for retrieving related items
    • Schema creation from object model

12 . How will you configure Hibernate?

  • The configuration files hibernate.cfg.xml (or hibernate.properties) and mapping files *.hbm.xml are used by the Configuration class to create (i.e. configure and bootstrap hibernate) the SessionFactory, which in turn creates the Session instances. Session instances are the primary interface for the persistence service.
    " hibernate.cfg.xml (alternatively can use hibernate.properties): These two files are used to configure the hibernate sevice (connection driver class, connection URL, connection username, connection password, dialect etc). If both files are present in the classpath then hibernate.cfg.xml file overrides the settings found in the hibernate.properties file.
    " Mapping files (*.hbm.xml): These files are used to map persistent objects to a relational database. It is the best practice to store each object in an individual mapping file (i.e mapping file per class) because storing large number of persistent classes into one mapping file can be difficult to manage and maintain. The naming convention is to use the same name as the persistent (POJO) class name. For example Account.class will have a mapping file named Account.hbm.xml. Alternatively hibernate annotations can be used as part of your persistent class code instead of the *.hbm.xml files.

13 . What are the Core interfaces are of Hibernate framework?

  • The five core interfaces are used in just about every Hibernate application. Using these interfaces, you can store and retrieve persistent objects and control transactions.
    • Session interface
    • SessionFactory interface
    • Configuration interface
    • Transaction interface
    • Query and Criteria interfaces

14 . What role does the Session interface play in Hibernate?

  • The Session interface is the primary interface used by Hibernate applications. It is a single-threaded, short-lived object representing a conversation between the application and the persistent store. It allows you to create query objects to retrieve persistent objects.
    Session session = sessionFactory.openSession();
    It Wraps a JDBC connection Factory for Transaction and Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier

15 . What are different ways to disable hibernate second level cache?

  • Hibernate second level cache can be disabled using any of the following ways:
    • By setting use_second_level_cache as false.
    • By using CACHEMODE.IGNORE
    • Using cache provider as org.hibernate.cache.NoCacheProvider

16 . What role does the SessionFactory interface play in Hibernate?

  • The application obtains Session instances from a SessionFactory. There is typically a single SessionFactory for the whole application reated during application initialization. The SessionFactory caches generate SQL statements and other mapping metadata that Hibernate uses at runtime. It also holds cached data that has been read in one unit of work and may be reused in a future unit of work.
    Configuration config = new Configuration();
    config.addResource("myinstance/configuration.hbm.xml");
    config.setProperties( System.getProperties() );
    SessionFactory sessions = config.buildSessionFactory();

17 . What is the general flow of Hibernate communication with RDBMS?

  • The general flow of Hibernate communication with RDBMS is :
    Load the Hibernate configuration file and create configuration object. It will automatically load all hbm mapping files
    Create session factory from configuration object
    Get one session from this session factory
    Create HQL Query
    Execute query to get list containing Java objects

18 . How can we bind hibernate session factory to JNDI ?

  • Hibernate session factory can be bound to JNDI by making configuration changes in hibernate.cfg file.

19 . What is Hibernate Query Language (HQL)?

  • Hibernate offers a query language that embodies a very powerful and flexible mechanism to query, store, update, and retrieve objects from a database. This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL.

20 . What are the ways to express joins in HQL?

  • HQL provides four ways of expressing (inner and outer) joins:-
    • An implicit association join
    • An ordinary join in the FROM clause
    • A fetch join in the FROM clause.
    • A theta-style join in the WHERE clause.

21 . How do you map Java Objects with Database tables?

  • First we need to write Java domain objects (beans with setter and getter). Write hbm.xml, where we map java class to table and database columns to Java class variables. Example :
    <hibernate-mapping>
    <class name="com.test.User" table="user">
    <property column="USER_NAME" length="255" name="userName" not-null="true" type="java.lang.String"/>
    <property column="USER_PASSWORD" length="255" name="userPassword" not-null="true" type="java.lang.String"/>
    </class>
    </hibernate-mapping>

22 . Should all the mapping files of hibernate have .hbm.xml extension to work properly?

  • No, having .hbm.xml extension is a convention and not a requirement for hibernate mapping file names. We can have any extension for these mapping files.

23 . How can we reattach any detached objects in Hibernate?

  • Objects which have been detached and are no longer associated with any persistent entities can be reattached by calling session.merge() method of session class.

24 . What is the difference between and merge and update ?

  • Use update() if you are sure that the session does not contain an already persistent instance with the same identifier, and merge() if you want to merge your modifications at any time without consideration of the state of the session.

25 . What’s the difference between load() and get()?

    • load()
      • Only use the load() method if you are sure that the object exists.
      • load() method will throw an exception if the unique id is not found in the database.
      • load() just returns a proxy by default and database won’t be hit until the proxy is first invoked.
    • get()
      • If you are not sure that the object exists, then use one of the get() methods.
      • get() method will return null if the unique id is not found in the database.
      • get() will hit the database immediately.

26 . What’s transaction management in hibernate? How it works?

  • Transaction management is the process of managing a set of statements or commands. In hibernate; transaction management is done by transaction interface as shown in below code:
    Session s = null;
    Transaction tr = null;
    try {
    s = sessionFactory.openSession();
    tr = s.beginTransaction();
    doTheAction(s);
    tr.commit();
    } catch (RuntimeException exc) {
    tr.rollback();
    } finally {
    s.close();
    }

27 . How primary key is created by using hibernate?

  • Database primary key is specified in the configuration file hbm.xml. Generator can also be used to specify how primary key is being created in the database. In the below example, deptId acts as primary key:

28 . How do you define sequence generated primary key in hibernate?

  • Using tag. Example:
    <id column="USER_ID" name="id" type="java.lang.Long">
    <generator class="sequence">
    <param name="table">SEQUENCE_NAME</param>
    <generator>

29 . what is cascading in hibernate?

  • Cascading is about deletion of one object from the database causing deletion of other (dependent) objects .... though of course it is typically database rows that are actually getting deleted. Cascading can also apply to saves / updates as well.

30 . What is the difference between cascade & inverse in hibernate, what is the use of them?

    1. inverse: This is used to decide which side is the relationship owner to manage the relationship (insert or update of the foreign key column).
    2. cascade: In cascade, after one operation (save, update and delete) is done, it decide whether it need to call other operations (save, update and delete) on another entities which has relationship with each other.
    Conclusion: In short, the “inverse” is decide which side will update the foreign key, while “cascade” is decide what’s the follow by operation should execute. Both are look quite similar in relationship, but it’s totally two different things. Hibernate developers are worth to spend time to research on it, because misunderstand the concept or misuse it will bring serious performance or data integrity issue in your application.

31 . Define cascade and inverse option in one-many mapping?

    • cascade - enable operations to cascade to child entities.
      cascade="all|none|save-update|delete|all-delete-orphan"
    • inverse - mark this collection as the "inverse" end of a bidirectional association.
      inverse="true|false"
      Essentially "inverse" indicates which end of a relationship should be ignored, so when persisting a parent who has a collection of children, should you ask the parent for its list of children, or ask the children who the parents are?

32 . What do you mean by Named – SQL query?

  • Named SQL queries are defined in the mapping xml document and called wherever required. Example:
    <sql-query name = "empdetails">
    <return alias="emp" class="com.test.Employee"/>
    SELECT emp.EMP_ID AS {emp.empid},
    emp.EMP_ADDRESS AS {emp.address},
    emp.EMP_NAME AS {emp.name}
    FROM Employee EMP WHERE emp.NAME LIKE :name
    </sql-query>
    Invoke Named Query :
    List people = session.getNamedQuery("empdetails")
    .setString("TomBrady", name)
    .setMaxResults(50)
    .list();

33 . How do you invoke Stored Procedures?

  • <sql-query name="selectAllEmployees_SP" callable="true">
    <return alias="emp" class="employee">
    <return-property name="empid" column="EMP_ID"/>
    <return-property name="name" column="EMP_NAME"/>
    <return-property name="address" column="EMP_ADDRESS"/>
    { ? = call selectAllEmployees() }
    </return>
    </sql-query>

34 . What is Criteria API?

  • Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set. Ex :
    List employees = session.createCriteria(Employee.class)
    .add(Restrictions.like("name", "a%") )
    .add(Restrictions.like("address", "Boston"))
    .addOrder(Order.asc("name") )
    .list();

35 . What is HibernateTemplate?

  • org.springframework.orm.hibernate.HibernateTemplate is a helper class which provides different methods for querying/retrieving data from the database. It also converts checked HibernateExceptions into unchecked DataAccessExceptions.

36 . What are the benefits does HibernateTemplate provide?

  • The benefits of HibernateTemplate are :
    • HibernateTemplate, a Spring Template class simplifies interactions with Hibernate Session.
    • Common functions are simplified to single method calls.
    • Sessions are automatically closed.
    • Exceptions are automatically caught and converted to runtime exceptions.

37 . How do you switch between relational databases without code changes?

  • Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined.

38 . If you want to see the Hibernate generated SQL statements on console, what should we do?

  • In Hibernate configuration file set as follows:
    <property name="show_sql">true </property>

39 . What are derived properties?

  • Derived properties are those properties which are not mapped to any columns of a database table. Such properties are calculated at runtime by evaluation of any expressions.

40 . What is component mapping in Hibernate?

  • A component is an object saved as a value, not as a reference
    A component can be saved directly without needing to declare interfaces or identifier properties
    Required to define an empty constructor
    Shared references not supported

41 . What is the difference between sorted and ordered collection in hibernate?

    1. sorted collection
      • A sorted collection is sorting a collection by utilizing the sorting features provided by the Java collections framework. The sorting occurs in the memory of JVM which running Hibernate, after the data being read from database using java comparator.
      • If your collection is not large, it will be more efficient way to sort it.
    2. order collection
      • Order collection is sorting a collection by specifying the order-by clause for sorting this collection when retrieval.
      • If your collection is very large, it will be more efficient way to sort it .

42 . What’s the role of JMX in hibernate?

  • Java Applications and components are managed in hibernate by a standard API called JMX API. JMX provides tools for development of efficient and robust distributed, web based solutions.

43 . What are different Collections in Hibernate ?

    • Bag
    • Set
    • List
    • Array
    • Map

44 . What are the ways to express joins in HQL?

  • HQL provides four ways of expressing (inner and outer) joins:-
    • An implicit association join
    • An ordinary join in the FROM clause
    • A fetch join in the FROM clause.
    • A theta-style join in the WHERE clause.

45 . What is Hibernate proxy?

  • The proxy attribute enables lazy initialization of persistent instances of the class. Hibernate will initially return CGLIB proxies which implement the named interface. The actual persistent object will be loaded when a method of the proxy is invoked.

46 . How can Hibernate be configured to access an instance variable directly and not through a setter method ?

  • 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.

47 . How can a whole class be mapped as immutable?

  • Mark the class as mutable="false" (Default is true),. This specifies that instances of the class are (not) mutable. Immutable classes, may not be updated or deleted by the application.

48 . What is the use of dynamic-insert and dynamic-update attributes in a class mapping?

  • Criteria is a simplified API for retrieving entities by composing Criterion objects. This is a very convenient approach for functionality like "search" screens where there is a variable number of conditions to be placed upon the result set.
    • dynamic-update (defaults to false): Specifies that UPDATE SQL should be generated at runtime and contain only those columns whose values have changed
    • dynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null.

49 . In how many ways objects can be identified in Hibernate?

  • Object identification can be done in hibernate in following three ways:
    • Using Object Identity: Using == operator.
    • Using Object Equality: Using equals() method.
    • Using database identity: Relational database objects can be identified if they represent same row.

50 . What do you mean by fetching strategy ?

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