Hibernate Questions

Hibernate 1 Q. How will you configure Hibernate? Answer: 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. Q. What is a SessionFactory? Is it a thread-safe object? Answer: SessionFactory is Hibernates concept of a single datastore and is threadsafe so that many threads can access it concurrently and request for sessions and immutable cache of compiled mappings for a single database. A SessionFactory is usually only built once at startup. SessionFactory should be wrapped in some kind of singleton so that it can be easily accessed in an application code. SessionFactory sessionFactory = new Configuration().configure().buildSessionfactory(); Q. What is a Session? Can you share a session object between different theads? Answer: Session is a light weight and a non-threadsafe object (No, you cannot share it between threads) that represents a single unit-of-work with the database. Sessions are opened by a SessionFactory and then are closed when all work is complete. Session is the primary interface for the persistence service. A session obtains a database connection lazily (i.e. only when required). To avoid creating too many sessions ThreadLocal class can be used as shown below to get the current session no matter how many times you make call to the currentSession() method. & public class HibernateUtil { & public static final ThreadLocal local = new ThreadLocal(); public static Session currentSession() throws HibernateException { Session session = (Session) local.get(); //open a new session if this thread has no session if(session == null) { session = sessionFactory.openSession(); local.set(session); } return session; }

Hibernate 2 } It is also vital that you close your session after your unit of work completes. Note: Keep your Hibernate Session API handy. Q. What are the benefits of detached objects? Detached objects can be passed across layers all the way up to the presentation layer without having to use any DTOs (DataTransfer Objects). You can later on re-attach the detached objects to another session. Q. What are the pros and cons of detached objects? Pros: " When long transactions are required due to user think-time, it is the best practice to break the long transaction up into two or more transactions. You can use detached objects from the first transaction to carry data all the way up to the presentation layer. These detached objects get modified outside a transaction and later on re-attached to a new transaction via another session. Cons " In general, working with detached objects is quite cumbersome, and better to not clutter up the session with them if possible. It is better to discard them and re-fetch them on subsequent requests. This approach is not only more portable but also more efficient because - the objects hang around in Hibernate's cache anyway. " Also from pure rich domain driven design perspective it is recommended to use DTOs (DataTransferObjects) and DOs (DomainObjects) to maintain the separation between Service and UI tiers. Q. How does Hibernate distinguish between transient (i.e. newly instantiated) and detached objects? " Hibernate uses the versionproperty, if there is one. " If not uses the identifier value. No identifier value means a new object. This does work only for Hibernate managed surrogate keys. Does not work for natural keys and assigned (i.e. not managed by Hibernate) surrogate keys. " Write your

the session.get() method and the session.load() method? Both the session.get(..) and session.load() methods create a persistent object by loading the required object from the database. But if there was not such object in the database then the method session.load(..) throws an exception whereas session.get(&) returns null. Q. What is the difference between the session.update() method and the session.lock() method? Both of these methods and saveOrUpdate() method are intended for reattaching a detached object. The session.lock() method simply reattaches the object to the session without checking or updating the database on the assumption that the database in sync with the detached object. It is the best practice to use either session.update(..) or session.saveOrUpdate(). Use session.lock() only if you are absolutely sure that the detached object is in sync with your detached object or if it does not matter because you will be overwriting all the columns that would have changed later on within the same transaction. Note: When you reattach detached objects you need to make sure that the dependent objects are reatched as well. Q. How would you reatach detached objects to a session when the same object has already been loaded into the session? You can use the session.merge() method call. Q. What are the general considerations or best practices for defining your Hibernate persistent classes?

Hibernate 3 1.You must have a default no-argument constructor for your persistent classes and there should be getXXX() (i.e accessor/getter) and setXXX( i.e. mutator/setter) methods for all your persistable instance variables. 2.You should implement the equals() and hashCode() methods based on your business key and it is important not to use the id field in your equals() and hashCode() definition if the id field is a surrogate key (i.e. Hibernate managed identifier). This is because the Hibernate only generates and sets the field when saving the object. 3. It is recommended to implement the Serializable interface. This is potentially useful if you want to migrate around a multi-processor cluster. 4.The persistent class should not be final because if it is final then lazy loading cannot be used by creating proxy objects. 5.Use XDoclet tags for generating your *.hbm.xml files or Annotations (JDK 1.5 onwards), which are less verbose than *.hbm.xml files. 1.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.2.What does ORM consists of ?An ORM solution consists of the followig four pieces:API for performing basic CRUD operationsAPI to express queries refering to classesFacilities to specify metadataOptimization facilities : dirty checking,lazy associations fetching3.What are the ORM levels ?The ORM levels are:Pure relational (stored procedure.)Light objects mapping (JDBC)Medium object mappingFull object Mapping (composition,inheritance, polymorphism, persistence by reachability)4.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. 5.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 productivityHigh-level object-oriented APILess Java code to writeNo SQL to writeImproved performanceSophisticated cachingLazy loadingEager loadingImproved maintainabilityA lot less code to writeImproved portabilityORM framework generates database-specific SQL for you6.What Does Hibernate Simplify?Hibernate simplifies:Saving and retrieving your domain objectsMaking database column and table name changesCentralizing pre save and post retrieve logicComplex joins for retrieving related itemsSchema creation from object model7.What is the need for Hibernate xml mapping file?Hibernate mapping file tells Hibernate which tables and columns to use to load and store objects. Typical mapping file look as follows:

Hibernate 4

8.What are the most common methods of Hibernate configuration?The most common methods of Hibernate configuration are:Programmatic configurationXML configuration ( hibernate.cfg.xml) 9.What are the important tags of hibernate.cfg.xml?Following are the important tags of hibernate.cfg.xml:

10.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 interfaceSessionFactory interfaceConfiguration interfaceTransaction interfaceQuery and Criteria interfaces11.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

Hibernate 5 objects. Session session = sessionFactory.openSession();Session interface role:Wraps a JDBC

connectionFactory for TransactionHolds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier 12.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 applicationå¹¼reated 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 SessionFactory sessionFactory = configuration.buildSessionFactory();

13.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 filesCreate session factory from configuration objectGet one session from this session factoryCreate HQL QueryExecute query to get list containing Java objects 14.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. 15.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 :

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

get()

Only use the load() method if you are sure that the object exists.

If you are not sure that the object exists, theget() methods.

load() method will throw an exception if the unique id is not found in the

get() method will return null if the uniqu

database.

the database.

load() just returns a proxy by default and database won’t be hit until the

proxy is first invoked.

get() will hit the database immediately.

17.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. 18.How do you define sequence generated primary key in hibernate?Using tag. Example:-

SEQUENCE_NAME 19.Define cascade and inverse option in onemany mapping?cascade - enable operations to cascade to child entities. cascade="all|none|save-update|delete|all-delete-orphan"

Hibernate 6 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? 20.What do you mean by Named – SQL query?Named SQL queries are defined in the mapping xml document and called wherever required. Example:

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 Invoke Named Query :List people = session.getNamedQuery("empdetails") .setString("TomBrady", name) .setMaxResults(50) .list(); 21.How do you invoke Stored Procedures?

{ ? = call selectAllEmployees() }

22.Explain Criteria APICriteria 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. Example :List employees = session.createCriteria(Employee.class)

.add(Restrictions.like("name", "a%") ) .add(Restrictions.like("address", "Boston")) .addOrder(Order.asc("name") ) .list(); 23.Define 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. 24.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.25.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. 26.If you want to see the Hibernate generated SQL statements on console, what should we do?In Hibernate configuration file set as follows: true

27.What are derived properties?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.28.What is component mapping in Hibernate?A component is an object saved as a value, not as a referenceA component can be saved directly without needing to declare interfaces or identifier propertiesRequired to define an empty constructorShared

Hibernate 7 references not supportedExample:

29.What is the difference between sorted and ordered collection in hibernate?sorted collection vs. order collection :sorted collection

order 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.

Order collection is sorting a collection by specify clause for sorting this collection when retrieval.

If your collection is not large, it will be more efficient way to sort it.

If your collection is very large, it will be more ef it .

31.What is the advantage of Hibernate over jdbc003FHibernate Vs. JDBC :JDBC

Hibernate

With JDBC, developer has to write code to map an object model's data representation to a relational data model and its corresponding database schema.

Hibernate is flexible and powerful ORM solution t to database tables. Hibernate itself takes care of XML files so developer does not need to write cod

With JDBC, the automatic mapping of Java objects with database tables and vice versa conversion is to be taken care of by the developer manually with lines of code.

Hibernate provides transparent persistence and d need to write code explicitly to map database ta toapplication objects during interaction with RDB

JDBC supports only native Structured Query Language (SQL).

Hibernate provides a powerful query language Hi

Hibernate 8 Developer has to find out the efficient way to access database, i.e. to select effective query from a number of queries to perform same task.

Language (independent from type of database) th familiar SQL like syntax and includes full support queries. Hibernate also supports native SQL state selects an effective way to perform a database m an application.

Application using JDBC to handle persistent data (database tables) having database specific code in large amount. The code written to map table data to application objects and vice versa is actually to map table fields to object properties. As table changed or database changed then it’s essential to change object structure as well as to change code written to map table-to-object/object-to-table.

Hibernate provides this mapping itself. The actua tables and application objects is done in XML file in Database or in any table then the only need to properties.

With JDBC, it is developer’s responsibility to handle JDBC result set and convert it to Java objects through code to use this persistent data in application. So with JDBC, mapping between Java objects and database tables is done manually.

Hibernate reduces lines of code by maintaining o mapping itself and returns result to application in objects. It relieves programmer from manual han data, hence reducing the development time and

With JDBC, caching is maintained by hand-coding.

Hibernate, with Transparent Persistence, cache i work space. Relational tuples are moved to this c query. It improves performance if client applicat many times for same write. Automatic Transpare allows the developer to concentrate more on bus than this application code.

In JDBC there is no check that always every user has updated data. This check has to be added by the developer.

Hibernate enables developer to define version ty application, due to this defined field Hibernate u of database table every time relational tuple is u ofJava class object to that table. So if two users and then modify it and one user save this modifie database, version is automatically updated for th Hibernate. When other user tries to save updated then it does not allow saving it because this user updated data.

32.What are the Collection types in Hibernate ?BagSetListArrayMap 33.What are the ways to express joins in HQL?HQL provides four ways of expressing (inner and outer) joins:-An implicit association joinAn ordinary join in the FROM clauseA fetch join in the FROM clause.A theta-style join in the WHERE clause. 34.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? 35.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.36.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. 37.How can a whole class be mapped as immutable?

Hibernate 9 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. 38.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 changeddynamic-insert (defaults to false): Specifies that INSERT SQL should be generated at runtime and contain only the columns whose values are not null. 39.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.40.What is automatic dirty checking? Automatic dirty checking is a feature that saves us the effort of explicitly asking Hibernate to update the database when we modify the state of an object inside a transaction. 41.What is transactional write-behind?Hibernate uses a sophisticated algorithm to determine an efficient ordering that avoids database foreign key constraint violations but is still sufficiently predictable to the user. This feature is called transactional write-behind. 42.What are Callback interfaces?Callback interfaces allow the application to receive a notification when something interesting happens to an object—for example, when an object is loaded, saved, or deleted. Hibernate applications don't need to implement these callbacks, but they're useful for implementing certain kinds of generic functionality. 43.What are the types of Hibernate instance states ?Three types of instance states:Transient -The instance is not associated with any persistence contextPersistent -The instance is associated with a persistence contextDetached -The instance was associated with a persistence context which has been closed – currently not associated 44.What are the differences between EJB 3.0 & HibernateHibernate Vs EJB 3.0 :Hibernate

EJB 3.0

Session–Cache or collection of loaded objects relating to a single unit of work

Persistence Context-Set of entities that can be given EntityManager is defined by a persistence

XDoclet Annotations used to support Attribute Oriented Programming

Java 5.0 Annotations used to support Attribute Programming

Defines HQL for expressing queries to the database

Defines EJB QL for expressing queries

Supports Entity Relationships through mapping files and annotations in JavaDoc

Support Entity Relationships through Java 5.0

Provides a Persistence Manager API exposed via the Session, Query, Criteria, and Transaction API

Provides and Entity Manager Interface for man operations for an Entity

Provides callback support through lifecycle, interceptor, and validatable interfaces

Provides callback support through Entity Listen methods

Entity Relationships are unidirectional. Bidirectional relationships are implemented by two unidirectional relationships

Entity Relationships are bidirectional or unidi

 45.What are the types of inheritance models in Hibernate?There are three types of inheritance models in Hibernate:Table per class hierarchyTable per subclassTable per concrete class2.What does ORM consists of ?An ORM solution consists of the followig four pieces:API for performing basic CRUD operationsAPI to express ries refering to classesFacilities to specify metadataOptimization facilities : dirty checking,lazy associations fetching3.What are the ORM levels ?The ORM levels are:Pure relational (stored procedure.)Light objects mapping (JDBC)Medium object mappingFull

Hibernate 10 object Mapping (composition,inheritance, polymorphism, persistence by reachability)4.What is Hibernate?Hibernate is a pure Javaobject-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 persistencerelated programming tasks.5.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 productivityHigh-level object-oriented APILess Java code to writeNo SQL to writeImproved performanceSophisticated cachingLazy loadingEager loadingImproved maintainabilityA lot less code to writeImproved portabilityORM framework generates databasespecific SQL for you6.What Does Hibernate Simplify?Hibernate simplifies:Saving and retrieving your domain objectsMaking database column and table name changesCentralizing pre save and post retrieve logicComplex joins for retrieving related itemsSchema creation from object model10.What are the Core interfaces are of Hibernate framework? People who read this, also read:-JSP Interview QuestionsJSF LifecycleiBatis TutorialiBatis an alternative to HibernateSpring Interview QuestionsThe 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 interfaceSessionFactory interfaceConfiguration interfaceTransaction interfaceQuery and Criteria interfaces11.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();Session interface role:Wraps a JDBC connectionFactory for TransactionHolds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier12.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—created 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 workSessionFactory sessionFactory = configuration.buildSessionFactory();13.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 filesCreate session factory from configuration objectGet one session from this session factoryCreate HQL QueryExecute query to get list containing Java objects14.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.

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.Hibernate simplifies:Saving and retrieving your domain objectsMaking database column and table name changesCentralizing pre save and post retrieve logicComplex joins for retrieving related itemsSchema creation from object model. 2. Benefits of hibernate over JDBC?Hibernate is database independent while in case of JDBC, developer has to write database specific queries.In case of Hibernate developer doesn‟t need to be an expert of writing complex queries as HQL simplifies query writing process while in case of JDBC, its job of developer to write and tune queries.In Hibernate, Criteria Query takes care of Query Tuning. In case of JDBC you need to tune your queries.No need to create any connection pool in case of Hibernate. You can use c3p0. In case of JDBC you need to write your own connection pool.Automatic primary key generation.Performance benefits like Lazy initialization, Outer join fetching, Batch fetching

Hibernate 11 3. Difference between different saving methods in Hibernate? saveOrUpdate: Based on some checks it will calls either save or update. E.g. if no identifier exists, save is called. Otherwise update is called.save It will Persists an entity and will assign an identifier if one doesn‟t exist. If one does, it‟s essentially doing an update. And returns the generated ID of the entity.update:It will persist the entity using an existing identifier. If no identifier exists, I believe an exception is thrown.merge Hibernate has following three Object states Transient, Detached and Persistant.Object states, with save & update, you are dealing with persistant objects. They are linked to a Session so Hibernate knows what has changed. But when you have a transient object, there is no session involved. In these cases you need to use merge for updates and persist for saviing.persist: As mentioned above, this is used on transient objects. It does not return the generated ID. 4.What are the Core interfaces of Hibernate framework?The five core interfaces are used in just about every Hibernate application. Using these interfaces, you an store and retrieve persistent objects and control transactions.Session interfaceSessionFactory interfaceTransaction interfaceQuery and Criteria interfaces 5. What are the three Types Hibernate Object States?Transient – Newly created object so not associated with any Session and database.Persistent – Associated with a unique SessionDetached – Previously persistent, not associated with any Session. 6. What is lazy loading in Hibernate?Lazy-loading can help improve the performance significantly since often you won‟t need the children and so they will not be loaded.If you have a parent and that parent has a collection of children. Hibernate now can “lazy-load” the children, which means that it does not actually load all the children when loading the parent. Instead, it loads them when requested to do so. You can either request this explicitly or, and this is far more common, hibernate will load them automatically when you try to access a child. 7. Define HibernateTemplate? HibernateTemplate is a helper class which provides different methods for querying/retrieving data from the database. It also converts checked Hibernate Exceptions into unchecked DataAccessExceptions.It automatically opens and closes sessions and commit or rollback transactions after your code executed. 8.What are the Collection types in Hibernate? Following are collection types used in Hibernate.ArrayMapBagSetList 9. What is Session?Session is a light weight and a non-thread safe and main function of the Session is to offer create, read and delete operations for instances of mapped entity classes. You can use like this SessionFactory sessionFactory = new Configuration ().configure ().buildSessionFactory (); Session session = sessionFactory.openSession (); 10. What is a SessionFactory? SessionFactory Interface is used to create session instances in hibernate. Usually an application has a single SessionFactory instance. SessionFactory is threadsafe. 11. Difference between in merge and update? Check the Question no 2. 12. What is transient Object in Hibernate? If persistent class is just instantiated using the new operator which is not associated with a Session. No persistent representation in the database and no identifier value have been

Hibernate 12 assigned to an object. And it will be destroyed by the garbage collector if the application does not hold a reference anymore. This kind of object is called transient Object. Example: Student studentObj = new Student (); studentObj.setName(“Kumar”); studentObj- transient object. 13. What is HQL? HibernateQuery Language isa powerful query language its syntax is quite similar to database SQL language,compared with SQL, however, HQL is fully object-oriented and understands notions like inheritance, polymorphism and association.HQL Select Query ExampleQuery query = session.createQuery(“from Student where studentName = „suresh‟ “); 14. What is POJO? POJO(Plain Old Java Object)is a javabean that have defined setter and getter methods for all properties. Decoupling the application code from the infrastructure frameworks is one of the many benefits of using POJOs. They also simplify development because rather than being forced to think about everything – business logic, persistence, transactions, etc. 15.What the four ORM levels are in hibernate? Following are the four ORM levels in hibernate: a. Pure Relational b. Light Object Mapping c. Medium Object Mapping d. Full Object Mapping 16. How to reattach detached objects in hibernate? Detached objects can be reattached by calling session.merge() method of session class. 17. Difference between load() and get() method in hibernate? If you are sure that the object exists then use load() else use get() methods. load() method will throw an exception if the unique id is not found in the database but get() method will return null if the unique id is not found in the database. 18. Difference between detached state and transient state Objects? Transient objects do not have association with the databases and session objects but detached objects have corresponding entries in the database. These are persistent and not connected to the Session object. These objects have the synchronized data with the database when the session was closed.Version property is used in hibernate to know whether an object is in transient state or in detached state. 19. Difference between the session.update() method and the session.lock() method? The session.lock() method simply reattaches the object to the session without checking or updating the database on the assumption that the database in sync with the detached object. Use session.lock() only if you are absolutely sure that the detached object is in sync with your detached object otherwise use session.update(). 20. Different fetching strategy in Hibernate? Join fetching: Using an OUTER JOIN hibernate retrieves the associated instance or collection in the same SELECT.

Hibernate 13 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 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 specifyinglazy=”false”, this second select will only be executed when you access the association. Batch fetching: It is an optimization strategy for select fetching.Using primary or foreign keys hibernate retrieves a batch of entity instances or collections in a single SELECT. Find the more info about Fetching strategies here.