Hibernate

Page 1 of 26 http://www.developersbook.com 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. 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. 2) 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) 3) What is a pure relational ORM? The entire application, including the user interface, is designed around the relational model and SQL-based relational operations. 4) What are the benefits of ORM and Hibernate? There are many benefits from these. Out of which the following are the most important one. i. Productivity – Hibernate reduces the burden of developer by providing much of the functionality and let the developer to concentrate on business logic. ii. 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. iii. 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. iv. Vendor independence – Irrespective of the different types of databases that are there, hibernate provides a much easier way to develop a cross platform application. 5) What does ORM consists of? Or What does an ORM solution comprises of?  API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes  Should have a language or a API for specifying queries that refer to the classes and the properties of classes  An ability for specifying mapping metadata  It should have a Optimization technique for ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions 6) What is Hibernate? Hibernate is a pure Java object-relational mapping (ORM) and persistence framework and query service that allows you to map plain old Java objects to relational database tables using (XML) configuration files. Hibernate is a powerful, high performance object/relational persistence and query service. This lets the users to develop persistent classes following object-oriented principles such as association, inheritance, polymorphism, composition, and collections. 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 productivity o High-level object-oriented API o Less Java code to write o No SQL to write  Improved performance o Sophisticated caching o Lazy loading o Eager loading  Improved maintainability o A lot less code to write

Page 2 of 26 

Improved portability o ORM framework generates database-specific SQL for you

6.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 7.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:

8.What are the most common methods of Hibernate configuration? The most common methods of Hibernate configuration are:  Programmatic configuration  XML configuration (hibernate.cfg.xml) 8) Explain about hibernate.cfg.xml? Hibernate can be configured with two types of files out of which hibernate.cfg.xml is widely used and popular feature. Hibernate consults hibernate.cfg.xml file for its operating properties such as database dialect, connection string and mapping files. These files are searched on class path. 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. 9.What are the important tags of hibernate.cfg.xml? Following are the important tags of hibernate.cfg.xml:

Page 3 of 26

10) What are the Core interfaces 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. i. ii. iii. iv. v.

Session Interface – This is the primary interface used by hibernate applications. The instances of this interface are lightweight and are inexpensive to create and destroy. Hibernate sessions are not thread safe. SessionFactory Interface – This is a factory that delivers the session objects to hibernate application. Generally there will be a single SessionFactory for the whole application and it will be shared among all theapplication threads. Configuration Interface – This interface is used to configure and bootstrap hibernate. The instance of this interface is used by the application in order to specify the location of hibernate specific mapping documents. Transaction Interface – This is an optional interface but the above three interfaces are mandatory in each and every application. This interface abstracts the code from any kind of transaction implementations such as JDBC transaction, JTA transaction. Query and Criteria Interface – This interface allows the user to perform queries and also control the flow of the query execution.

11.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 connection  Factory for Transaction  Holds 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 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 work SessionFactory sessionFactory = configuration.buildSessionFactory();

Page 4 of 26 19) How do you create a SessionFactory? Configuration cfg = new Configuration(); cfg.addResource("myinstance/MyConfig.hbm.xml"); cfg.setProperties( System.getProperties() ); SessionFactory sessions = cfg.buildSessionFactory(); First, we need to create an instance of Configuration and use that instance to refer to the location of the configuration file. After configuring this instance is used to create the SessionFactory by calling the method 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 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 14.What is Hibernate Query Language (HQL)? This language, the Hibernate query Language (HQL), is an object-oriented extension to SQL. Allows the user to express queries in its own portable SQL extension and this is called as HQL. It also allows the user to express in native 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() vs. 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, the theget() methods.

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

get() method will return null if the unique id 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.

The load() method is older; get() was added to Hibernate’s API due to user request. The difference is trivial: The following Hibernate code snippet retrieves a User object from the database: User user = (User) session.get(User.class, userID); The get() method is special because the identifier uniquely identifies a single instance of a class. Hence it’s common for applications to use the identifier as a convenient handle to a persistent object. Retrieval by identifier can use the cache when retrieving an object, avoiding a database hit if the object is already cached. Hibernate also provides a load() method: User user = (User) session.load(User.class, userID); If load() can’t find the object in the cache or database, an exception is thrown. The load() method never returns null. The get() method returns null if the object can’t be found. The load() method may return a proxy instead of a real persistent instance. A

Page 5 of 26 proxy is a placeholder instance of a runtime-generated subclass (through cglib or Javassist) of a mapped persistent class, it can initialize itself if any method is called that is not the mapped database identifier getter-method. On the other hand, get() never returns a proxy. Choosing between get() and load() is easy: If you’re certain the persistent object exists, and nonexistence would be considered exceptional, load() is a good option. If you aren’t certain there is a persistent instance with the given identifier, use get() and test the return value to see if it’s null. Using load() has a further implication: The application may retrieve a valid reference (a proxy) to a persistent instance without hitting the database to retrieve its persistent state. So load() might not throw an exception when it doesn’t find the persistent object in the cache or database; the exception would be thrown later, when the proxy is accessed. 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 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? 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?

Page 6 of 26 { ? = call selectAllEmployees() }

22.Explain 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. 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 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

Page 7 of 26 Example:

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 specifying the order-by 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 efficient way to sort it .

31.What is the advantage of Hibernate over jdbc? Hibernate 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 to map Java classes to database tables. Hibernate itself takes care of this mapping using XML files so developer does not need to write code for this.

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 developer does not need to write code explicitly to map database tables tuples to application objects during interaction with RDBMS.

JDBC supports only native Structured Query Language (SQL). 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.

Hibernate provides a powerful query language Hibernate Query Language (independent from type of database) that is expressed in a familiar SQL like syntax and includes full support for polymorphic queries. Hibernate also supports native SQL statements. It also selects an

Page 8 of 26 effective way to perform a database manipulation task for 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 tableto-object/object-to-table.

Hibernate provides this mapping itself. The actual mapping between tables and application objects is done in XML files. If there is change in Database or in any table then the only need to change XML file 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 objecttable mapping itself and returns result to application in form of Java objects. It relieves programmer from manual handling of persistent data, hence reducing the development time and maintenance cost.

With JDBC, caching is maintained by hand-coding.

Hibernate, with Transparent Persistence, cache is set to application work space. Relational tuples are moved to this cache as a result of query. It improves performance if client application reads same data many times for same write. Automatic Transparent Persistence allows the developer to concentrate more on business logic rather 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 type field to application, due to this defined field Hibernate updates version field of database table every time relational tuple is updated in form of Java class object to that table. So if two users retrieve same tuple and then modify it and one user save this modified tuple to database, version is automatically updated for this tuple by Hibernate. When other user tries to save updated tuple to database then it does not allow saving it because this user does not have updated data.

32.What are the Collection types in Hibernate ?  Bag  Set  List  Array  Map 33.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. 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?

Page 9 of 26 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. Proxies are created dynamically by subclassing 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. Very nice in simple cases with no object hierarchy. Typecasting and instanceof work perfectly on the proxy in this case since it is a direct subclass. By default Hibernate creates a proxy for each of the class you map in mapping file. This class contain the code to invoke JDBC. This class is created by hibernate using CGLIB. Mapping of classes can be made into a proxy instead of a table. A proxy is returned when actually a load is called on a session. The proxy contains actual method to load the data. The proxy is created by default by Hibernate, for mapping a class to a file. The code to invoke Jdbc is contained in this class. 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? 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 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. 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 writebehind. 42.What are Callback interfaces? (2) 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 context  Persistent -The instance is associated with a persistence context.  Detached -The instance was associated with a persistence context which has been closed – currently not associated 44.What are the differences between EJB 3.0 & Hibernate Hibernate Vs EJB 3.0 :Hibernate

EJB 3.0

Page 10 of 26 Session–Cache or collection of loaded objects relating to a single unit of work

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

XDoclet Annotations used to support Attribute Oriented Programming

Java 5.0 Annotations used to support Attribute Oriented 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 annotations

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

Provides and Entity Manager Interface for managing CRUD operations for an Entity

Provides callback support through lifecycle, interceptor, and validatable interfaces

Provides callback support through Entity Listener and Callback methods

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

Entity Relationships are bidirectional or unidirectional

45.What are the types of inheritance models in Hibernate? There are three types of inheritance models in Hibernate:  Table per class hierarchy  Table per subclass  Table per concrete class http://www.javabeat.net/articles/104-hibernate-interview-questions-1.html 6) 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. 7) 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. 8) 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) How does hibernate code looks like? Session session = getSessionFactory().openSession(); Transaction tx = session.beginTransaction(); MyPersistanceClass mpc = new MyPersistanceClass ("Sample App"); session.save(mpc); tx.commit(); session.close(); The Session and Transaction are the interfaces provided by hibernate. There are many other interfaces besides this. 11) What is a hibernate xml mapping document and how does it look like? In order to make most of the things work in hibernate, usually the information is provided in an xml document. This document is called as xml mapping document. The document defines, among other things, how properties of the user defined persistence classes’ map to the columns of the relative tables in database.

Page 11 of 26 Everything should be included under tag. This is the main tag for an xml mapping document. 14) What are Callback interfaces? These interfaces are used in the application to receive a notification when some object events occur. Like when an object is loaded, saved or deleted. There is no need to implement callbacks in hibernate applications, but they’re useful for implementing certain kinds of generic functionality. 15) What are Extension interfaces? When the built-in functionalities provided by hibernate is not sufficient enough, it provides a way so that user can include other interfaces and implement those interfaces for user desire functionality. These interfaces are called as Extension interfaces. 16) What are the Extension interfaces that are there in hibernate? There are many extension interfaces provided by hibernate.  ProxyFactory interface - used to create proxies  ConnectionProvider interface – used for JDBC connection management  TransactionFactory interface – Used for transaction management  Transaction interface – Used for transaction management  TransactionManagementLookup interface – Used in transaction management.  Cahce interface – provides caching techniques and strategies  CacheProvider interface – same as Cache interface  ClassPersister interface – provides ORM strategies  IdentifierGenerator interface – used for primary key generation  Dialect abstract class – provides SQL support 17) What are different environments to configure hibernate? There are mainly two types of environments in which the configuration of hibernate application differs. i. Managed environment – In this kind of environment everything from database connections, transaction boundaries, security levels and all are defined. An example of this kind of environment is environment provided by application servers such as JBoss, Weblogic and WebSphere. ii. Non-managed environment – This kind of environment provides a basic configuration template. Tomcat is one of the best examples that provide this kind of environment. 18) What is the file extension you use for hibernate mapping file? The name of the file should be like this : filenam.hbm.xml The filename varies here. The extension of these files should be ―.hbm.xml‖. This is just a convention and it’s not mandatory. But this is the best practice to follow this extension. 20) What is meant by Method chaining? Method chaining is a programming technique that is supported by many hibernate interfaces. This is less readable when compared to actual java code. And it is not mandatory to use this format. Look how a SessionFactory is created when we use method chaining. SessionFactory sessions = new Configuration() .addResource("myinstance/MyConfig.hbm.xml") .setProperties( System.getProperties() ) .buildSessionFactory();

Page 12 of 26 21) What does hibernate.properties file consist of? This is a property file that should be placed in application class path. So when the Configuration object is created, hibernate is first initialized. At this moment the application will automatically detect and read this hibernate.properties file. hibernate.connection.datasource = java:/comp/env/jdbc/AuctionDB hibernate.transaction.factory_class = net.sf.hibernate.transaction.JTATransactionFactory hibernate.transaction.manager_lookup_class = net.sf.hibernate.transaction.JBossTransactionManagerLookup hibernate.dialect = net.sf.hibernate.dialect.PostgreSQLDialect 22) What should SessionFactory be placed so that it can be easily accessed? As far as it is compared to J2EE environment, if the SessionFactory is placed in JNDI then it can be easily accessed and shared between different threads and various components that are hibernate aware. You can set the SessionFactory to a JNDI by configuring a property hibernate.session_factory_name in the hibernate.properties file. 23) What are POJOs? POJO stands for plain old java objects. These are just basic JavaBeans that have defined setter and getter methods for all the properties that are there in that bean. Besides they can also have some business logic related to that property. Hibernate applications works efficiently with POJOs rather then simple java classes. 24) What is object/relational mapping metadata? ORM tools require a metadata format for the application to specify the mapping between classes and tables, properties and columns, associations and foreign keys, Java types and SQL types. This information is called the object/relational mapping metadata. It defines the transformation between the different data type systems and relationship representations. 26) What are the different types of property and class mappings?  Typical and most common property mapping



Or Derived properties



Typical and most common property mapping



Controlling inserts and updates

27) What is Attribute Oriented Programming? XDoclet has brought the concept of attribute-oriented programming to Java. Until JDK 1.5, the Java language had no support for annotations; now XDoclet uses the Javadoc tag format (@attribute) to specify class-, field-, or method-level metadata attributes. These attributes are used to generate hibernate mapping file automatically when theapplication is built. This kind of programming that works on attributes is called as Attribute Oriented Programming. 28) What are the different methods of identifying an object? There are three methods by which an object can be identified. i. Object identity – Objects are identical if they reside in the same memory location in the JVM. This can be checked by using the = = operator. ii. Object equality – Objects are equal if they have the same value, as defined by the equals( ) method. Classes that don’t explicitly override this method inherit the implementation defined by java.lang.Object, which compares object identity.

Page 13 of 26 iii.

Database identity – Objects stored in a relational database are identical if they represent the same row or, equivalently, share the same table and primary key value.

29) What are the different approaches to represent an inheritance hierarchy? i. Table per concrete class. ii. Table per class hierarchy. iii. Table per subclass. 30) What are managed associations and hibernate associations? Associations that are related to container management persistence are called managed associations. These are bidirectional associations. Coming to hibernate associations, these are unidirectional. http://www.geekinterview.com/question_details/80518 What will happened if level 2 cache is enable and query cache is diabled in Hibernate? You cannot disable Query Cache because it is compulsory. Q. How do you configure 2nd level cach in hibernate? A. To activate second-level caching, you need to define the hibernate.cache.provider_class property in the hibernate.cfg.xml file as follows: org.hibernate.cache.EHCacheProvider By default, the second-level cache is activated and uses the EHCache provider. To use the query cache you must first enable it by setting the property hibernate.cache.use_query_cache to true in hibernate.properties. What is different between Hibernate and iBatis? What is really difference between two and really more importantly when should I use one over the other. Its pretty interesting question because there are major differences between iBatis and Hibernate. Within in the java persistence there is no one size, fits all solution. So, in this case Hibernate which is a de facto standard is used in lot of places. Let us consider a scenario where Hibernate work great for initial model. Now Suddenly if you are using stored procedures, well we can do it in Hibernate but its little difficult; ok we map those, all of sudden we got some reporting type of queries, those don’t have keys have group bys; with some difficulty here we can use name queries and stuff like that, but now starts getting more complicated, we have complex joins, yes you can do in hibernate, but we can’t do with average developer. We have SQL that just doesn’t work. So these are some of the complexities. One of the other things I find is, if am looking at an application that does not work very well with an ORM, aside from these considerations of using stored procedures, already using SQL, complex joins. In other words, Hibernate works very well if your data model is well in sync with object model, because ORM solutions like Hibernate map object to tables. However, let’s suppose data model is not in sync with object model, in this case you have do lot of additional coding and complexities are entering into your application, start coming the beyond the benefits of ORM. So, again all of sudden you are noticing that the flow is gone; our application is becoming very very complex and developers can’t maintain the code. This is where the model starts breaking down. One size does not fit all. So this is where I like to use iBatis; as the alternative solution for these type of situations, iBatis maps results sets to objects, so no need to care about table structures. This works very well for stored procedures, works very well for reporting applications, etc,. Now the question is , does it work well for simple CRUD applications? Well, it works because what we have to write is sql. Then why not use Hibernate for that? You can start see Some of the decision criteria that comes into play. So one of the other follow onquestions that typically get is , can I use both? That’s really interesting question! because the answer is sure. But, such a thing will never ever exists is java persistence world. However we can kind of use both to create this little hybrid. So think of this kind scenario, we have very large application where Hibernate is working very well for it, but we have a reporting piece that just is a real nag , its query only , so we can do is, we can use iBatis to pull up the queries for reporting piece and still use Hibernate for all the operational stuff and updates. This model actually works well, it doesn’t break the transactional model, and it doesn’t affect any of the primary & secondary caches with a Hibernate. It’s a good solution. Use iBatis if 

You want to create your own SQL's and are willing to maintain them



your environment is driven by relational data model

Page 14 of 26  you have to work existing and complex schema's Use Hibernate if  your environment is driven by object model and wants generates SQL automatically The message is, 

One size does not fit all the java persistence and the important to know there are other solutions besides the traditional ORMs, and that would be iBatis.



Both the solutions work well, given their specific domain.



Look for the opportunity where you can use both.

How to call stored procedure in mysql through hibernate? In order to call a stored procedure using the hibernate , define a named query for a persistent class mapping document. Then call the named query from your java application .Example is given in the Hibernate Referencedocumentation as follows:First create a stored procedure in your database as, CREATE OR REPLACE FUNCTION selectAllEmployments RETURN SYS_REFCURSOR AS st_cursor SYS_REFCURSOR; BEGIN OPEN st_cursor FOR SELECT EMPLOYEE, EMPLOYER, STARTDATE, ENDDATE, REGIONCODE, EID, VALUE, CURRENCY FROM EMPLOYMENT; RETURN st_cursor; END; Then in the mapping document create a named query as follows: { ? = call selectAllEmployments() } Then, in your java code just call the named query as follows:List people = sess.getNamedQuery("selectAllEmployees_SP") .setString("namePattern", namePattern) .setMaxResults(50) .list(); -------------------- OR ---------------------------As per sql--> create a procedure in your sql by using the following code. create or replace procedure first-procedure(x in number y out number) begin y: x*x; end; write your sql connection code in hibernate configuration file. In your hibernate client application get the session obj. On the session object call the connection() method and get the connection. ex: Connection con session.connection(); CallableStatement cst con.prepareCall("{call first-procedure(? ?)}");

Page 15 of 26 cst.registerOutParameter(2 Types.Integer); cst.setInt(1 20); cst.execute(); int res cst.getInt(2); s.o.p(res); ses.close(); What are different fetching strategies in hibernate? Hibernate defines following fetching strategies: 1. Join Fetching : retrieval of assosiated obects or collections are done by OUTER JOIN in the same SELECT. 2. Select Fetching : A second select is used to retrieve the associated instance or collection. 3. Subselect Fetching : It is almost same as select fetching. 4. Batch Fetching : Hibernate retrieves a batch of entity instance or collections ina single SELECT. Hibernate also distinguishes between: 1. Immediate fetching. 2. Lazy collection fetching. 3. "Extra-lazy collection fetching. 4. Proxy fetching. 5."No-proxy" fetching. 6. Lazy attribute fetching. what is lazy initialisation in hibernate? Hibernate supports the feature of lazy initilasation for both entities and collections. What this actually means is, the Hibernate engine loads only those objects that we are querying for and doesnt try to fetch other entities(that are associated with the entity we are querying) or collections. An attribute 'lazy' can be used to let Hibernate know if the associated entity or collection has to be lazily loaded or prefetched. Ex: This causes the collection to be eagerly fetched rather than doing a lazy fetch. If on the other hand , the attribute value of lazy is set to true, then hibernate will not make an attempt to fire the query for fetching the collection object until the request is made by the user. 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)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 actully 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.Exampleslazy true (default)Address child of User class can be made lazy if it is not required frequently.lazy falseBut you may need to load the Author object for Book parent whenever you deal with the book for online bookshop. Lazy setting decides whether to load child objects while loading the Parent Object. You need to specify parent class.Lazy = true in hibernate mapping file. 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 actully 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. Examples: Address child of User class can be made lazy if it is not required frequently. But you may need to load the Author object for Book parent whenever you deal with the book for online bookshop.

Page 16 of 26 Hibernate does not support lazy initialization for detached objects. Access to a lazy association outside of the context of an open Hibernate session will result in an exception. what is the use of cascade in hbm file? 1) cascade "none" the default tells Hibernate to ignore the association. 2) cascade "save-update" tells Hibernate to navigate the association when the transaction is committed and when an object is passed to save() or update() and save newly instantiated transient instances and persist changes to detached instances. 3) cascade "delete" tells Hibernate to navigate the association and delete persistent instances when an object is passed to delete(). 4) cascade "all" means to cascade both save-update and delete as well as calls to evict and lock. 5) cascade "all-delete-orphan" means the same as cascade "all" but in addition Hibernate deletes any persistent entity instance that has been removed (dereferenced) from the association (for example from a collection). 6) cascade "delete-orphan" Hibernate will delete any persistent entity instance that has been removed (dereferenced) from the association (for example from a collection). http://www.bestjavainterviewquestions.com/hibernate-interview-questions-2/ Q) What role does the Session interface play in Hibernate? A) 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 connection * Factory for Transaction * Holds a mandatory (first-level) cache of persistent objects, used when navigating the object graph or looking up objects by identifier Q) What role does the SessionFactory interface play in Hibernate? A) 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 work SessionFactory sessionFactory = configuration.buildSessionFactory(); Q) What is the general flow of Hibernate communication with RDBMS? A) 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 Q) How do you map Java Objects with Database tables? A) * First we need to write Java domain objects (beans with setter and getter). The variables should be same as database columns. * Write hbm.xml, where we map java class to table and database columns to Java class variables. Example :

Page 17 of 26 Q) What Does Hibernate Simplify? A) 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 Q) What’s the difference between load() and get()? A) load() vs. 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. 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.

load() just returns a proxy

Q) What is the difference between and merge and update ? A)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. Q) How do you define sequence generated primary key in hibernate? A) Using tag. Example: SEQUENCE_NAME Q) Define cascade and inverse option in one-many mapping? A) 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? Q) What does it mean to be inverse? A) It informs hibernate to ignore that end of the relationship. If the one–to–many was marked as inverse, hibernate would create a child–>parent relationship (child.getParent). If the one–to–many was marked as non–inverse then a child–>parent relationship would be created. Q) What do you mean by Named – SQL query? A) 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

Page 18 of 26 Invoke Named Query : List people = session.getNamedQuery(―empdetails‖) .setString(―TomBrady‖, name) .setMaxResults(50) .list(); Q) How do you invoke Stored Procedures? A) { ? = call selectAllEmployees() } Q) Explain Criteria API A) 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. Example : List employees = session.createCriteria(Employee.class) .add(Restrictions.like(―name‖, ―a%‖) ) .add(Restrictions.like(―address‖, ―Boston‖)) .addOrder(Order.asc(―name‖) ) .list(); Q) Define HibernateTemplate? A) 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. Q) What are the benefits does HibernateTemplate provide? A) 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. Q) How do you switch between relational databases without code changes? A) Using Hibernate SQL Dialects , we can switch databases. Hibernate will generate appropriate hql queries based on the dialect defined. Q) If you want to see the Hibernate generated SQL statements on console, what should we do? A) In Hibernate configuration file set as follows: true Q) What are derived properties? A) 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. People who read this also read: Core Java Questions Spring Questions SCJP 6.0 Certification EJB Interview Questions Servlets Questions Q) What is component mapping in Hibernate? A) * 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

Page 19 of 26 * Required to define an empty constructor * Shared references not supported Q) What is the difference between sorted and ordered collection in hibernate? A) sorted collection vs. order collection 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. 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 . http://www.javalobby.org/java/forums/t104442.html Q. What is a SessionFactory? Is it a thread-safe object? Answer: SessionFactory is Hibernate s 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; } } 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? Answer: Detached objects can be passed across layers all the way up to the presentation layer without having to use any DTOs (Data Transfer Objects). You can later on re-attach the detached objects to another session. Q. What are the pros and cons of detached objects? Answer: 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.

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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? Answer : " Hibernate uses the version property, 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 own strategy with Interceptor.isUnsaved(). Q. What is the difference between 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? 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. Q. What are the key benifits of Hibernate? A: These are the key benifits of Hibernate:  Transparent persistence based on POJOs without byte code processing  Powerful object-oriented hibernate query language  Descriptive O/R Mapping through mapping file.

Page 21 of 26   

Automatic primary key generation Hibernate cache : Session Level, Query and Second level cache. Performance: Lazy initialization, Outer join fetching, Batch fetching

Q. What is hibernate session and session factory? How do you configure sessionfactory in spring configuration file? A. Hibernate Session is the main runtime interface between a Java application and Hibernate. SessionFactory allows applications to create hibernate session by reading hibernate configurations file hibernate.cfg.xml. // Initialize the Hibernate environment Configuration cfg = new Configuration().configure(); // Create the session factory SessionFactory factory = cfg.buildSessionFactory(); // Obtain the new session object Session session = factory.openSession(); Q. How do you define hibernate mapping file in spring? A. Add the hibernate mapping file entry in mapping resource inside Spring’s applicationContext.xml file in the web/WEB-INF directory. org/appfuse/model/User.hbm.xml Q. What type of transaction management is supported in hibernate? A. Hibernate communicates with the database via a JDBC Connection; hence it must support both managed and non-managed transactions. non-managed in web containers: managed in application server using JTA: Q. What are the different fetching strategy in Hibernate? A. 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. Q. What are different types of cache hibernate supports ? A. Caching is widely used for optimizing database applications. Hibernate uses two different caches for

Page 22 of 26 objects: first-level cache and second-level cache. First-level cache is associated with the Session object, while second-level cache is associated with the Session Factory object. By default, Hibernate uses first-level cache on a per-transaction basis. Hibernate uses this cache mainly to reduce the number of SQL queries it needs to generate within a given transaction. For example, if an object is modified several times within the same transaction, Hibernate will generate only one SQL UPDATE statement at the end of the transaction, containing all the modifications. To reduce database traffic, second-level cache keeps loaded objects at the Session Factory level between transactions. These objects are available to the whole application, not just to the user running the query. This way, each time a query returns an object that is already loaded in the cache, one or more database transactions potentially are avoided. In addition, you can use a query-level cache if you need to cache actual query results, rather than just persistent objects. The query cache should always be used in conjunction with the secondlevel cache. Hibernate supports the following open-source cache implementations out-of-the-box:  

  

EHCache is a fast, lightweight, and easy-to-use in-process cache. It supports read-only and read/write caching, and memory- and disk-based caching. However, it does not support clustering. OSCache is another open-source caching solution. It is part of a larger package, which also provides caching functionalities for JSP pages or arbitrary objects. It is a powerful and flexible package, which, like EHCache, supports read-only and read/write caching, and memory- and disk-based caching. It also provides basic support for clustering via either JavaGroups or JMS. SwarmCache is a simple cluster-based caching solution based on JavaGroups. It supports read-only or nonstrict read/write caching (the next section explains this term). This type of cache is appropriate for applications that typically have many more read operations than write operations. JBoss TreeCache is a powerful replicated (synchronous or asynchronous) and transactional cache. Use this solution if you really need a true transaction-capable caching architecture. Commercial Tangosol Coherence cache.

Q. What are the different caching strategies? A. The following four caching strategies are available:  Read-only: This strategy is useful for data that is read frequently but never updated. This is by far the simplest and best-performing cache strategy.  Read/write: Read/write caches may be appropriate if your data needs to be updated. They carry more overhead than read-only caches. In non-JTA environments, each transaction should be completed when Session.close() or Session.disconnect() is called.  Nonstrict read/write: This strategy does not guarantee that two transactions won't simultaneously modify the same data. Therefore, it may be most appropriate for data that is read often but only occasionally modified.  Transactional: This is a fully transactional cache that may be used only in a JTA environment. Q. What are the types of inheritence models and describe how they work like vertical inheritence and horizontal? A. There are three types of inheritance mapping in hibernate : Example: Let us take the simple example of 3 java classes. Class Manager and Worker are inherited from Employee Abstract class. 1. Table per concrete class with unions : In this case there will be 2 tables. Tables: Manager, Worker [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 called 'Employee' that will represent all the attributes required for all 3 classes. But it needs some discriminating column to differentiate between Manager and worker; 3. Table per subclass: In this case there will be 3 tables represent Employee, Manager and Worker 1) Explain about Hibernate? Hibernate solves problems such as Object Relational impedance mismatch, etc. It is commonly used for object and query service. It helps data base developers develop classes which include inheritance, association, composition and polymorphism. A developer or user can express queries either in HQL or SQL. 2) Explain about the primary feature of Hibernate? Primary feature of hibernate is to java classes to database tables. Data query and retrieval is also possible with Hibernate. Application portability is a key feature in Hibernate it allows developers to port applications to almost all SQL databases. 3) Explain about transparent persistence of Hibernate? Transparent persistence is provided for Plain old Java objects or POJOs. For proper functioning of the applications

Page 23 of 26 importance should be given to the methods equals () and hash Code methods (). It has a requirement which should be strictly followed in the applications which is a no-argument constructor. 4) Explain about the dirty checking feature of Hibernate? Dirty checking feature of the Hibernate allows users or developers to avoid time consuming data base write actions. This feature makes necessary updations and changes to the fields which require a change, remaining fields are left unchanged or untouched. 5) Brief about the Session factory interface? It creates new hibernate sessions by referencing immutable and thread safe objects. Application using hibernate are usually allowed and desgined to implement single instance of the class using this interface. Only single instance of a class can be used which is using this interface. 6) Explain about session interface? This represents hibernate session which perform the manipulation on the database entities. Some of the activities performed by session interface are as follows they are managing the persistence state, fetching persisted ones and management of the transaction demarcation. 7) Explain the steps involved in creating database applications with Java using Hibernate? Creating Database applications with Java is made simpler with Hibernate. First Plain old java object needs to be written, XML mapping file should be created which shows relationship between database and class attributes. Hibernate APIs can be used to store persistent objects. 9) Explain about mapping description file? Mapping description file is the second file which Hibernate uses to configure its functions. This mapping file has an extension *.hbm which instructs mapping between Java class and database tables. The usage of mapping description file rests entirely upon the business entity. 10) Explain about transaction file? Transactions denote a work file which can save changes made or revert back the changes. A transaction can be started by session.beginTransaction() and it uses JDBC connection, CORBA or JTA. When this session starts several transactions may occur. 11) Explain about mapping files in Hibernate? Mapping files forms the core of any database mapping tools. These files contain field to field mapping, usually this mapping occurs between classes and attributes. After mapping files they can be persist to the database. Tags can be used to indicate the presence of a primary key. 12) What is the effect when a transient mapped object is passed onto a Sessions save? When a Sessions save () is passed to a transient mapped object it makes the method to become more persistent. Garbage collection and termination of the Java virtual machine stays as long as it is deleted explicitly. It may head back to its transient state. 13) Explain about version field? Application level data integrity constants are important if you are making changes to offline information which is again backed by database. Higher level locking or versioning protocol is required to support them. Version field usage comes at this stage but the design and implementation process is left to the developer. 14) State some advantages of hibernate? Some of the advantages which a developer can get from Hibernate are as follows: Mapping of one POJO table to one table is not required in hibernate. It supports inheritance relationships and is generally a fast tool. Portability is necessary the greater benefit from hibernate. POJOs can be used in other applications where they are applicable. 15) Explain about addClass function? This function translates a Java class name into file name. This translated file name is then loaded as an input stream from the Java class loader. This addclass function is important if you want efficient usage of classes in your code. 16) Explain about addjar() and addDirectory() methods? These methods are the most convenient to use in hibernate. These methods allow you to load all your Hibernate

Page 24 of 26 documents at a time. These methods simplify code configuration, refactoring, layout, etc. These functions help you to add your hibernate mapping to Hibernate initialization files. 17) Explain about the id field? This id field corresponds to the surrogate key which is generated by the database. These fields are handled by the id field. Name attribute is used to specify the names of the field and it should correspond to the method name of getid. This also should correspond to long type and the values should be stored I the database in the long column. Explain the difference between transient (i.e. newly instantiated) and detached objects in hibernate. Transient objects do not have association with the databases and session objects. They are simple objects and not persisted to the database. Once the last reference is lost, that means the object itself is lost. And of course , garbage collected. The commits and rollbacks will have no effects on these objects. They can become into persistent objects through the save method calls of Session object. The detached object 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. Since then, the change may be done in the database which makes this object stale. The detached object can be reattached after certain time to another object in order to become persistent again. 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 Explain about the dirty checking feature of Hibernate? Dirty checking feature of the Hibernate allows users or developers to avoid time consuming data base write actions. This feature makes necessary updations and changes to the fields which require a change, remaining fields are left unchanged or untouched. If you want to see the Hibernate generated SQL statements on console, what should we do? In Hibernate configuration file set as follows: true 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 writebehind. Brief about the Session factory interface? It creates new hibernate sessions by referencing immutable and thread safe objects. Application using hibernate are usually allowed and desgined to implement single instance of the class using this interface. Only single instance of a class can be used which is using this interface. 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. 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)

Page 25 of 26 .setMaxResults(50) .list(); 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. 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. What are the types of inheritance models in Hibernate? There are three types of inheritance models in Hibernate: * Table per class hierarchy * Table per subclass * Table per concrete class How do you invoke Stored Procedures? { ? = call selectAllEmployees() } 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. Explain about session interface? This represents hibernate session which perform the manipulation on the database entities. Some of the activities performed by session interface are as follows they are managing the persistence state, fetching persisted ones and management of the transaction demarcation. What are the types of Hibernate instance states ? Three types of instance states: * Transient -The instance is not associated with any persistence context * Persistent -The instance is associated with a persistence context * Detached -The instance was associated with a persistence context which has been closed ? currently not associated 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. How do you define sequence generated primary key in hibernate? Using tag. Example: SEQUENCE_NAME

Page 26 of 26 What is SessionFactory ? Is it thread safe? SessionFactory is Hibernate's 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();