Java Synchronization

Synchronization in Java:

Java Synchronization is a process of controlling the access of shared resources by the multiple threads in such a manner where only one thread can access one resource at a time. In non synchronized multithreaded application, it is possible for one thread to modify a shared object, while another thread is in the process of using or updating the object's value.

Synchronization prevents such type of data corruption. Synchronization is best use with the Multi-Threading in Java. Synchronization can be done at two levels:

• Synchronizing a function
• Synchronizing a block of code

Please find the sample code below

Synchronizing a function: 

public synchronized void  Display() 

{
   // related code
}

Synchronizing a block of code inside a function:

 public myFunction ()
{
  synchronized (this)
   {
  // Synchronized code
   }
}

Read more...

Java Spring Framework Dependency Injection Variants

Variants in Java Spring Framework Dependency Injection:

Constructor dependency Injection: Dependencies are provided through the constructors of the component.

Setter dependency injection: Dependencies are provided through the JavaBeanstyle setter methods of the component and are more popular than Constructor dependency injection.

Constructor Dependency Injection Syntax:

public class ConstructorInjection
{
private Dependency dep;
public ConstructorInjection(Dependency dep)
{
this.dep = dep;
}
}

Setter Dependency Injection Syntax

public class SetterInjection
{
private Dependency dep;
public void setMyDependency(Dependency dep)
{
this.dep = dep;
}
}

Read more...

Java Stack and Heap Memory

Java Stack and Heap Memory: 

Each time an object is created in Java it goes into the area of memory known as heap. The primitive variables like int and double are allocated in the stack, if they are local method variables and in the heap if they are member variables (i.e. fields of a class). In Java methods local variables are pushed into stack when a method is invoked and stack pointer is decremented when a method call is completed. In a multi-threaded application each thread will have its own stack but will share the same heap. This is why care should be taken in your code to avoid any concurrent access issues in the heap space. The stack is threadsafe (each thread will have its own stack) but the heap is not threadsafe unless guarded with synchronization through your code. 

A method in stack is re-entrant allowing multiple concurrent invocations that do not interfere with each other. A function is recursive if it calls itself. Given enough stack space, recursive method calls are perfectly valid in Java though it is tough to debug. Recursive functions are useful in removing iterations from many sorts of algorithms. All recursive functions are re-entrant but not all re-entrant functions are recursive. Idempotent methods are methods, which are written in such a way that repeated calls to the same method with the same arguments yield same results. For example clustered EJBs, which are written with idempotent methods, can automatically recover from a server failure as long as it can reach another server.

Read more...

STRUTS 2 Interceptors

Interceptors in STRUTS 2

Many of the features provided in the Struts2 framework are implemented using interceptors; examples include exception handling, file uploading, lifecycle callbacks and validation. Interceptors are conceptually the same as servlet filters or the JDKs Proxy class. They provide a way to supply pre-processing and post-processing around the action. Similar to servlet filters,interceptors can be layered and ordered. They have access to the action being executed, as well as all environmental variables and execution 

properties. (Reference: Ian Roughley - Starting Struts 2 Book)

Below are the implementing interceptors mentioned:

1. Spring Framework  the ActionAutowiringInterceptor interceptor.
2. Request String and Form Values  the ParametersInterceptor interceptor.
3. Servlet-based objects  the ServletConfigInterceptor interceptor.

The first two interceptors work independently, with no requirements from the action, but the last interceptor ServletConfigInterceptor is different.It works with the assistance of the following interfaces:

• SessionAware to provide access to all the session attributes via a Map
• ServletRequestAware to provide access to the HttpServletRequest object
• RequestAware to provide access to all the request attributes via a Map
• ApplicationAware to provide access to all the application attributes via a Map
• ServletResponseAware to provide access to the HttpServletResponse object
• ParameterAware to provide access to all the request string and form values attributes via a Map
• PrincipalAware to provide access to the PrincipleProxy object; this object implements the principle and role methods of the HttpServletRequest object in implementation, but by providing a proxy,allows for implementation independence in the action
• ServletContextAware to provide access to the ServletContext object

Read more...

Difference between JRE/JVM/JDK

JDK is the Java Development Kit i.e. JDK is bundle of software that you can use to develop Java based software. JRE is the Java Runtime Environment i.e. JRE is an implementation of the Java Virtual Machine which actually executes Java programs. Typically, each JDK contains one (or more) JRE’s along with the various development tools like the Java source compilers, bundling and deployment tools, debuggers,development libraries, etc.

Read more...

Spring Interview Questions Part 5

Spring faq's -Part 5

34. How the AOP used in Spring?

AOP is used in the Spring Framework: To provide declarative enterprise services, especially as a replacement for EJB declarative services. The most important such service is declarative transaction management, which builds on the Spring Framework's transaction abstraction.To allow users to implement custom aspects, complementing their use of OOP with AOP.

35. What do you mean by Aspect ?

A modularization of a concern that cuts across multiple objects. Transaction management is a good example of a crosscutting concern in J2EE applications. In Spring AOP, aspects are implemented using regular classes (the schema-based approach) or regular classes annotated with the @Aspect annotation (@AspectJ style).

36. What do you mean by JointPoint?

A point during the execution of a program, such as the execution of a method or the handling of an exception. In Spring AOP, a join point always represents a method execution.

37. What do you mean by Advice?

Action taken by an aspect at a particular join point. Different types of advice include "around," "before" and "after" advice. Many AOP frameworks, including Spring, model an advice as an interceptor, maintaining a chain of interceptors "around" the join point.

38. What are the types of Advice?

Types of advice:

Before advice: Advice that executes before a join point, but which does not have the ability to prevent execution flow proceeding to the join point (unless it throws an exception).

After returning advice: Advice to be executed after a join point completes normally: for example, if a method returns without throwing an exception.

After throwing advice: Advice to be executed if a method exits by throwing an exception.

After (finally) advice: Advice to be executed regardless of the means by which a join point exits (normal or exceptional return).

Around advice: Advice that surrounds a join point such as a method invocation. This is the most powerful kind of advice. Around advice can perform custom behavior before and after the method invocation. It is also responsible for choosing whether to proceed to the join point or to shortcut the advised method execution by returning its own return value or throwing an exception

39. What is Spring's JdbcTemplate ?

Spring's JdbcTemplate is central class to interact with a database through JDBC. JdbcTemplate provides many convenience methods for doing things such as converting database data into primitives or objects, executing prepared and callable statements, and providing custom database error handling.

JdbcTemplate template = new JdbcTemplate(myDataSource)

40. What is PreparedStatementCreator ?

PreparedStatementCreator:

Is one of the most common used interfaces for writing data to database.
Has one method – createPreparedStatement(Connection)
Responsible for creating a PreparedStatement.
Does not need to handle SQLExceptions.

41. What is SQLProvider ?

SQLProvider:

Has one method – getSql()
Typically implemented by PreparedStatementCreator implementers.
Useful for debugging.

42. What is RowCallbackHandler ?

The RowCallbackHandler interface extracts values from each row of a ResultSet.

Has one method – processRow(ResultSet)
Called for each row in ResultSet.
Typically stateful.

Read more...

Spring Interview Questions Part 4

Spring faq's -Part 4

26. What are the types of Dependency Injection Spring supports?

Setter Injection:Setter-based DI is realized by calling setter methods on your beans after invoking a no-argument constructor or no-argument static factory method to instantiate your bean.

Constructor Injection:
Constructor-based DI is realized by invoking a constructor with a number of arguments, each representing a collaborator.

27. What is Bean Factory ?

A BeanFactory is like a factory class that contains a collection of beans. The BeanFactory holds Bean Definitions of multiple beans within itself and then instantiates the bean whenever asked for by clients.

BeanFactory is able to create associations between collaborating objects as they are instantiated. This removes the burden of configuration from bean itself and the beans client.
BeanFactory also takes part in the life cycle of a bean, making calls to custom initialization and destruction methods.

28. What is Application Context?

A bean factory is fine to simple applications, but to take advantage of the full power of the Spring framework, you may want to move up to Springs more advanced container, the application context. On the surface, an application context is same as a bean factory.Both load bean definitions, wire beans together, and dispense beans upon request. But it also provides:

A means for resolving text messages, including support for internationalization.
A generic way to load file resources.
Events to beans that are registered as listeners.

29. What is the difference between Bean Factory and Application Context ?

On the surface, an application context is same as a bean factory. But application context offers much more..
Application contexts provide a means for resolving text messages, including support for i18n of those messages.
Application contexts provide a generic way to load file resources, such as images.
Application contexts can publish events to beans that are registered as listeners.
Certain operations on the container or beans in the container, which have to be handled in a programmatic fashion with a bean factory, can be handled declaratively in an application context.

ResourceLoader support: Spring’s Resource interface us a flexible generic abstraction for handling low-level resources. An application context itself is a ResourceLoader, Hence provides an application with access to deployment-specific Resource instances.

MessageSource support: The application context implements MessageSource, an interface used to obtain localized messages, with the actual implementation being pluggable.

30. What are ORM’s Spring supports ?

Spring supports the following ORM’s :

• Hibernate
• iBatis
• JPA (Java Persistence API)
• TopLink
• JDO (Java Data Objects)
• OJB
  

31. What are the ways to access Hibernate using Spring ?

There are two approaches to Spring’s Hibernate integration:

Inversion of Control with a HibernateTemplate and Callback
Extending HibernateDaoSupport and Applying an AOP Interceptor

32. How to integrate Spring and Hibernate using HibernateDaoSupport?

Spring and Hibernate can integrate using Spring’s SessionFactory called LocalSessionFactory. The integration process is of 3 steps.

• Configure the Hibernate SessionFactory
• Extend your DAO Implementation from HibernateDaoSupport
• Wire in Transaction Support with AOP

33. What is AOP?

Aspect-oriented programming, or AOP, is a programming technique that allows programmers to modularize crosscutting concerns, or behavior that cuts across the typical divisions of responsibility, such as logging and transaction management. The core construct of AOP is the aspect, which encapsulates behaviors affecting multiple classes into reusable modules.

Read more...

Spring Interview Questions Part 3

Spring faq's -Part 3

19. What is SQLExceptionTranslator ?

SQLExceptionTranslator, is an interface to be implemented by classes that can translate between SQLExceptions and Spring's own data-access-strategy-agnostic org.springframework.dao.DataAccessException.

20. What are the different types of IOC (dependency injection) ?

There are three types of dependency injection:

Constructor Injection (e.g. Pico container, Spring etc): Dependencies are provided as constructor parameters.
Setter Injection (e.g. Spring): Dependencies are assigned through JavaBeans properties (ex: setter methods).
Interface Injection (e.g. Avalon): Injection is done through an interface.

Note: Spring supports only Constructor and Setter Injection

21. What are the benefits of IOC (Dependency Injection)?

Benefits of IOC (Dependency Injection) are as follows:

Minimizes the amount of code in your application. With IOC containers you do not care about how services are created and how you get references to the ones you need. You can also easily add additional services by adding a new constructor or a setter method with little or no extra configuration.

Make your application more testable by not requiring any singletons or JNDI lookup mechanisms in your unit test cases. IOC containers make unit testing and switching implementations very easy by manually allowing you to inject your own objects into the object
under test.

Loose coupling is promoted with minimal effort and least intrusive mechanism. The factory design pattern is more intrusive because components or services need to be requested explicitly whereas in IOC the dependency is injected into requesting piece of code. Also some containers promote the design to interfaces not to implementations design concept by encouraging managed objects to implement a well-defined service interface of your own.

IOC containers support eager instantiation and lazy loading of services. Containers also provide support for instantiation of managed objects, cyclical dependencies, life cycles management, and dependency resolution between managed objects etc.

22. What is Spring ?

Spring is an open source framework created to address the complexity of enterprise application development. One of the chief advantages of the Spring framework is its layered architecture, which allows you to be selective about which of its components you use while also providing a cohesive framework for J2EE application development.

23. What are the advantages of Spring framework?

The advantages of Spring are as follows:

• Spring has layered architecture. Use what you need and leave you don't need now.
• Spring Enables POJO Programming. There is no behind the scene magic here. POJO programming enables continuous integration and testability.
• Dependency Injection and Inversion of Control Simplifies JDBC
• Open source and no vendor lock-in.

24. What are features of Spring ?

Lightweight:
Spring is lightweight when it comes to size and transparency. The basic version of spring framework is around 1MB. And the processing overhead is also very negligible.

Inversion of control (IOC):
Loose coupling is achieved in spring using the technique Inversion of Control. The objects give their dependencies instead of creating or looking for dependent objects.

Aspect oriented (AOP):
Spring supports Aspect oriented programming and enables cohesive development by separating application business logic from system services.

Container:
Spring contains and manages the life cycle and configuration of application objects.

MVC Framework:
Spring comes with MVC web application framework, built on core Spring functionality. This framework is highly configurable via strategy interfaces, and accommodates multiple view technologies like JSP, Velocity, Tiles, iText, and POI. But other frameworks can be easily used instead of Spring MVC Framework.

Transaction Management:
Spring framework provides a generic abstraction layer for transaction management. This allowing the developer to add the pluggable transaction managers, and making it easy to demarcate transactions without dealing with low-level issues. Spring's transaction support is not tied to J2EE environments and it can be also used in container less environments.

JDBC Exception Handling:
The JDBC abstraction layer of the Spring offers a meaningful exception hierarchy, which simplifies the error handling strategy. Integration with Hibernate, JDO, and iBATIS: Spring provides best Integration services with Hibernate, JDO and iBATIS

25. How many modules are there in Spring? What are they?

Spring comprises of seven modules. They are..

The core container:
The core container provides the essential functionality of the Spring framework. A primary component of the core container is the BeanFactory, an implementation of the Factory pattern. The BeanFactory applies the Inversion of Control (IOC) pattern to separate an application's configuration and dependency specification from the actual application code.

Spring context:
The Spring context is a configuration file that provides context information to the Spring framework. The Spring context includes enterprise services such as JNDI, EJB, e-mail, internalization, validation, and scheduling functionality.

Spring AOP:
The Spring AOP module integrates aspect-oriented programming functionality directly into the Spring framework, through its configuration management feature. As a result you can easily AOP-enable any object managed by the Spring framework. The Spring AOP module provides transaction management services for objects in any Spring-based application. With Spring AOP you can incorporate declarative transaction management into your applications without relying on EJB components.

Spring DAO:
The Spring JDBC DAO abstraction layer offers a meaningful exception hierarchy for managing the exception handling and error messages thrown by different database vendors. The exception hierarchy simplifies error handling and greatly reduces the amount of exception code you need to write, such as opening and closing connections. Spring DAO's JDBC-oriented exceptions comply to its generic DAO exception hierarchy.

Spring ORM:
The Spring framework plugs into several ORM frameworks to provide its Object Relational tool, including JDO, Hibernate, and iBatis SQL Maps. All of these comply to Spring's generic transaction and DAO exception hierarchies.

Spring Web module:
The Web context module builds on top of the application context module, providing contexts for Web-based applications. As a result, the Spring framework supports integration with Jakarta Struts. The Web module also eases the tasks of handling multi-part requests and binding request parameters to domain objects.

Spring MVC framework:
The Model-View-Controller (MVC) framework is a full-featured MVC implementation for building Web applications. The MVC framework is highly configurable via strategy interfaces and accommodates numerous view technologies including JSP, Velocity, Tiles, iText, and POI.

Read more...

Spring Interview Questions Part 2

Spring faq's -Part 2

9. What is Java Server Faces (JSF) - Spring integration mechanism?

Spring provides a custom JavaServer Faces VariableResolver implementation that extends the standard JavaServer Faces managed beans mechanism. When asked to resolve a variable name, the following algorithm is performed:

Does a bean with the specified name already exist in some scope (request, session, application)? If so, return it Is there a standard JavaServer Faces managed bean definition for this variable name? If so, invoke it in the usual way, and return the bean that was created. Is there configuration information for this variable name in the Spring WebApplicationContext for this application? If so, use it to create and configure an instance, and return that instance to the caller.

If there is no managed bean or Spring definition for this variable name, return null instead. BeanFactory also takes part in the life cycle of a bean, making calls to custom initialization and destruction methods. As a result of this algorithm, you can transparently use either JavaServer Faces or Spring facilities to create beans on demand.

10. What is Significance of JSF- Spring integration ?

Spring - JSF integration is useful when an event handler wishes to explicitly invoke the bean factory to create beans on demand, such as a bean that encapsulates the business logic to be performed when a submit button is pressed.

11. How to integrate your Struts application with Spring?

To integrate your Struts application with Spring, we have two options:

Configure Spring to manage your Actions as beans, using the ContextLoaderPlugin, and set their dependencies in a Spring context file.

Subclass Spring's ActionSupport classes and grab your Spring-managed beans explicitly using a getWebApplicationContext() method.

12. What are the types of the transaction management Spring supports ?

Spring Framework supports:

• Programmatic transaction management.
• Declarative transaction management.
  

13. What are the benefits of the Spring Framework transaction management ?

The Spring Framework provides a consistent abstraction for transaction management that delivers the following benefits:

• Provides a consistent programming model across different transaction APIs such as JTA, JDBC, Hibernate, JPA, and JDO.
• Supports declarative transaction management.
• Provides a simpler API for programmatic transaction management than a number of complex transaction APIs such as JTA.
• Integrates very well with Spring's various data access abstractions.
  

14. Why most users of the Spring Framework choose declarative transaction management ?

Most users of the Spring Framework choose declarative transaction management because it is the option with the least impact on application code, and hence is most consistent with the ideals of a non-invasive lightweight container.

15. Explain the similarities and differences between EJB CMT and the Spring Framework's declarative transaction management ?

The basic approach is similar: it is possible to specify transaction behavior (or lack of it) down to individual method level. It is possible to make a setRollbackOnly() call within a transaction context if necessary. The differences are:

• Spring Certification article
• Spring Basic Tutorial
• JSF Basic Tutorial
• JSF-Spring2.0 Integration
• Spring-iBatis Integration
  

Unlike EJB CMT, which is tied to JTA, the Spring Framework's declarative transaction management works in any environment. It can work with JDBC, JDO, Hibernate or other transactions under the covers, with configuration changes only.

• The Spring Framework enables declarative transaction management to be applied to any class, not merely special classes such as EJBs.
• The Spring Framework offers declarative rollback rules: this is a feature with no EJB equivalent. Both programmatic and declarative support for rollback rules is provided.
• The Spring Framework gives you an opportunity to customize transactional behavior, using AOP. With EJB CMT, you have no way to influence the container's transaction management other than setRollbackOnly().
• The Spring Framework does not support propagation of transaction contexts across remote calls, as do high-end application servers.

16. When to use programmatic and declarative transaction management ?

Programmatic transaction management is usually a good idea only if you have a small number of transactional operations.
On the other hand, if your application has numerous transactional operations, declarative transaction management is usually worthwhile. It keeps transaction management out of business logic, and is not difficult to configure.

17. Explain about the Spring DAO support ?

The Data Access Object (DAO) support in Spring is aimed at making it easy to work with data access technologies like JDBC, Hibernate or JDO in a consistent way. This allows one to switch between the persistence technologies fairly easily and it also allows one to code without worrying about catching exceptions that are specific to each technology.

18. What are the exceptions thrown by the Spring DAO classes ?

Spring DAO classes throw exceptions which are subclasses of DataAccessException(org.springframework.dao.DataAccessException).Spring provides a convenient translation from technology-specific exceptions like SQLException to its own exception class hierarchy with the DataAccessException as the root exception. These exceptions wrap the original exception.

Read more...

Spring Interview Questions Part-1

Spring faq's -Part1

1. What is IOC (or Dependency Injection)?

The basic concept of the Inversion of Control pattern (also known as dependency injection) is that you do not create your objects but describe how they should be created. You don't directly connect your components and services together in code but describe which services are needed by which components in a configuration file. A container (in the case of the Spring framework, the IOC container) is then responsible for hooking it all up.

i.e., Applying IoC, objects are given their dependencies at creation time by some external entity that coordinates each object in the system. That is, dependencies are injected into objects. So, IoC means an inversion of responsibility with regard to how an object obtains references to collaborating objects.

2. What are the common implementations of the Application Context ?

The three commonly used implementation of 'Application Context' are

ClassPathXmlApplicationContext : It Loads context definition from an XML file located in the classpath, treating context definitions as classpath resources. The application context is loaded from the application's classpath by using the code .

ApplicationContext context = new
ClassPathXmlApplicationContext("bean.xml");

FileSystemXmlApplicationContext : It loads context definition from an XML file in the filesystem. The application context is loaded from the file system by using the code .

ApplicationContext context = new
FileSystemXmlApplicationContext("bean.xml");

XmlWebApplicationContext : It loads context definition from an XML file contained within a web application.

3. How is a typical spring implementation look like ?

For a typical Spring Application we need the following files:

• An interface that defines the functions.
• An Implementation that contains properties, its setter and getter methods, functions etc.,
• Spring AOP (Aspect Oriented Programming)
• A XML file called Spring configuration file.
• Client program that uses the function.

4. What is the typical Bean life cycle in Spring Bean Factory Container ?

Bean life cycle in Spring Bean Factory Container is as follows:

• The spring container finds the bean’s definition from the XML file and instantiates the bean.
• Using the dependency injection, spring populates all of the properties as specified in the bean definition
• If the bean implements the BeanNameAware interface, the factory calls setBeanName() passing the bean’s ID.
• If the bean implements the BeanFactoryAware interface, the factory calls setBeanFactory(), passing an instance of itself.
• If there are any BeanPostProcessors associated with the bean, their post- ProcessBeforeInitialization() methods will be called.
• If an init-method is specified for the bean, it will be called.
• Finally, if there are any BeanPostProcessors associated with the bean, their postProcessAfterInitialization() methods will be called.

5. What do you mean by Bean wiring ?

The act of creating associations between application components (beans) within the Spring container is reffered to as Bean wiring.

6. What do you mean by Auto Wiring?

The Spring container is able to autowire relationships between collaborating beans. This means that it is possible to automatically let Spring resolve collaborators (other beans) for your bean by inspecting the contents of the BeanFactory. The autowiring functionality has five modes.

1. no
2. byName
3. byType
4. constructor
5. autodirect

7. What is DelegatingVariableResolver?

Spring provides a custom JavaServer Faces VariableResolver implementation that extends the standard Java Server Faces managed beans mechanism which lets you use JSF and Spring together. This variable resolver is called as DelegatingVariableResolver

8. How to integrate Java Server Faces (JSF) with Spring?

JSF and Spring do share some of the same features, most noticeably in the area of IOC services. By declaring JSF managed-beans in the faces-config.xml configuration file, you allow the FacesServlet to instantiate that bean at startup. Your JSF pages have access to these beans and all of their properties.

Read more...

Hibernate Interview Questions

Java Hibernate FAQ's & Interview Questions

1. What is Hibernate?

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.

2. What is ORM?

ORM stands for Object/Relational mapping. 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.

3. What does an ORM solution comprises of?

• It should have an API for performing basic CRUD (Create, Read, Update, Delete) operations on objects of persistent classes

• Should have a language or an API for specifying queries that refer to the classesand the properties of classes

• An ability for specifying mapping metadata

• It should have a technique for ORM implementation to interact with transactional objects to perform dirty checking, lazy association fetching, and other optimization functions.

4. What are the different levels of ORM quality?

There are four levels defined for ORM quality.

i. Pure relational
ii. Light object mapping
iii. Medium object mapping
iv. Full object mapping

5. What is a pure relational ORM?

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

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.

9. 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 ofthe 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 arethere, hibernate provides a much easier way to develop a cross platform 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.

<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC
"http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd">
<hibernate-mapping>
<class name="sample.MyPersistanceClass" table="MyPersitaceTable">
<id name="id" column="MyPerId">
<generator class="increment"/>
</id>
<property name="text" column="Persistance_message"/>
<many-to-one name="nxtPer" cascade="all" column="NxtPerId"/>
</class>
</hibernate-mapping>

Everything should be included under <hibernate-mapping> tag. This is the main tag for an xml mapping document.

12. Show Hibernate overview?

13. What the Core interfaces are of hibernate framework?

There are many benefits from these. Out of which the following are the most important one.

i. 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.

ii. 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 the application threads.

iii. 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.

iv. 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.

v. Query and Criteria Interface – This interface allows the user to perform queries and also control the flow of the query execution.

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.

ı ProxyFactory interface - used to create proxies
ı ConnectionProvider interface – used for JDBC connection management
ı TransactionFactory interface – Used for transaction management
ı Transaction interface – Used for transaction management
ı TransactionManagementLookup interface – Used in transaction management.
ı Cahce interface – provides caching techniques and strategies
ı CacheProvider interface – same as Cache interface
ı ClassPersister interface – provides ORM strategies
ı IdentifierGenerator interface – used for primary key generation
ı 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 : filename.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.

19. What 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().

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();

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.

25. What is HQL?

HQL stands for Hibernate Query Language. Hibernate 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.

26. What are the different types of property and class mappings?

• Typical and most common property mapping

<property name="description" column="DESCRIPTION" type="string"/>
Or
<property name="description" type="string">
<column name="DESCRIPTION"/>
</property>

• Derived properties

<property name="averageBidAmount" formula="( select AVG(b.AMOUNT) from BID b
where b.ITEM_ID = ITEM_ID )" type="big_decimal"/>

• Typical and most common property mapping

<property name="description" column="DESCRIPTION" type="string"/>

• Controlling inserts and updates

<property name="name" column="NAME" type="string"
insert="false" update="false"/>

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 the application 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.

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 bi-directional associations. Coming to hibernate associations,
these are unidirectional.

Read more...

Java J2EE Job Interview Questions eBook

This Book features all faq's on Java/J2EE Technolgies. Much useful for Interviews.

Download Java J2EE Job Interview Companion eBook

Read more...