J2EE Basics

Question: What is J2EE?
Answer: J2EE Stands for Java 2 Enterprise Edition. J2EE is an environment for developing and deploying enterprise applications. J2EE specification is defined by Sun Microsystems Inc. The J2EE platform is one of the best platform for the development and deployment of enterprise applications. The J2EE platform is consists of a set of services, application programming interfaces (APIs), and protocols, which provides the functionality necessary for developing multi-tiered, web-based applications

Question: What do you understand by a J2EE module?
Answer: A J2EE module is a software unit that consists of one or more J2EE components of the same container type along with one deployment descriptor of that type. J2EE specification defines four types of modules:
a) EJB
b) Web
c) application client and
d) resource adapter

In the J2EE applications modules can be deployed as stand-alone units. Modules can also be assembled into J2EE applications.

Question: Tell me something about J2EE component?
Answer: J2EE component is a self-contained functional software unit supported by a container and configurable at deployment time. The J2EE specification defines the following J2EE components:
• Application clients and applets are components that run on the client.
• Java servlet and JavaServer Pages (JSP) technology components are Web components that run on the server.
• Enterprise JavaBeans (EJB) components (enterprise beans) are business components that run on the server. J2EE components are written in the Java programming language and are compiled in the same way as any program in the language. The difference between J2EE components and "standard" Java classes is that J2EE components are assembled into a J2EE application, verified to be well formed and in compliance with the J2EE specification, and deployed to production, where they are run and managed by the J2EE server or client container.
Source: J2EE v1.4 Glossary

Question: What are the contents of web module?
Answer: A web module may contain:
a) JSP files
b) Java classes
c) gif and html files and
d) web component deployment descriptors

Question: Differentiate between .ear, .jar and .war files.
Answer: These files are simply zipped file using java jar tool. These files are created for different purposes. Here is the description of these files:
.jar files: These files are with the .jar extenstion. The .jar files contains the libraries, resources and accessories files like property files.
.war files: These files are with the .war extension. The war file contains the web application that can be deployed on the any servlet/jsp container. The .war file contains jsp, html, javascript and other files for necessary for the development of web applications.
.ear files: The .ear file contains the EJB modules of the application.

Question: What is the difference between Session Bean and Entity Bean?
Answer:
Session Bean: Session is one of the EJBs and it represents a single client inside the Application Server. Stateless session is easy to develop and its efficient. As compare to entity beans session beans require few server resources.

A session bean is similar to an interactive session and is not shared; it can have only one client, in the same way that an interactive session can have only one user. A session bean is not persistent and it is destroyed once the session terminates.

Entity Bean: An entity bean represents persistent global data from the database. Entity beans data are stored into database.

Question: Why J2EE is suitable for the development distributed multi-tiered enterprise applications?
Answer: The J2EE platform consists of multi-tiered distributed application model. J2EE applications allows the developers to design and implement the business logic into components according to business requirement. J2EE architecture allows the development of multi-tired applications and the developed applications can be installed on different machines depending on the tier in the multi-tiered J2EE environment . The J2EE application parts are:

a) Client-tier components run on the client machine.
b) Web-tier components run on the J2EE server.
c) Business-tier components run on the J2EE server and the
d) Enterprise information system (EIS)-tier software runs on the EIS servers

Question: Why do understand by a container?
Answer: Normally, thin-client multi-tiered applications are hard to write because they involve many lines of intricate code to handle transaction and state management, multithreading, resource pooling, and other complex low-level details. The component-based and platform-independent J2EE architecture makes J2EE applications easy to write because business logic is organized into reusable components. In addition, the J2EE server provides underlying services in the form of a container for every component type. Because you do not have to develop these services yourself, you are free to concentrate on solving the business problem at hand (Source: http://java.sun.com/j2ee/1.3/docs/tutorial/doc/Overview4.html ).

In short containers are the interface between a component and the low-level platform specific functionality that supports the component. The application like Web, enterprise bean, or application client component must be assembled and deployed on the J2EE container before executing.

Question: What are the services provided by a container?
Answer: The services provided by container are as follows:
a) Transaction management for the bean
b) Security for the bean
c) Persistence of the bean
d) Remote access to the bean
e) Lifecycle management of the bean
f) Database-connection pooling
g) Instance pooling for the bean

Question: What are types of J2EE clients?
Answer: J2EE clients are the software that access the services components installed on the J2EE container. Following are the J2EE clients:
a) Applets
b) Java-Web Start clients
c) Wireless clients
d) Web applications
Question: What is Deployment Descriptor?
Answer: A deployment descriptor is simply an XML(Extensible Markup Language) file with the extension of .xml. Deployment descriptor describes the component deployment settings. Application servers reads the deployment descriptor to deploy the components contained in the deployment unit. For example ejb-jar.xml file is used to describe the setting of the EJBs.

Question: What do you understand by JTA and JTS?
Answer: JTA stands for Java Transaction API and JTS stands for Java Transaction Service. JTA provides a standard interface which allows the developers to demarcate transactions in a manner that is independent of the transaction manager implementation. The J2EE SDK uses the JTA transaction manager to implement the transaction. The code developed by developers does not calls the JTS methods directly, but only invokes the JTA methods. Then JTA internally invokes the JTS routines. JTA is a high level transaction interface used by the application code to control the transaction.

Question: What is JAXP?
Answer: The Java API for XML Processing (JAXP) enables applications to parse and transform XML documents independent of a particular XML processing implementation. JAXP or Java API for XML Parsing is an optional API provided by Javasoft. It provides basic functionality for reading, manipulating, and generating XML documents through pure Java APIs. It is a thin and lightweight API that provides a standard way to seamlessly integrate any XML-compliant parser with a Java application.
More at http://java.sun.com/xml/

Question: What is J2EE Connector architecture?
Answer: J2EE Connector Architecture (JCA) is a Java-based technology solution for connecting application servers and enterprise information systems (EIS) as part of enterprise application integration (EAI) solutions. While JDBC is specifically used to connect Java EE applications to databases, JCA is a more generic architecture for connection to legacy systems (including databases). JCA was developed under the Java Community Process as JSR 16 (JCA 1.0) and JSR 112 (JCA 1.5). As of 2006, the current version of JCA is version 1.5. The J2EE Connector API is used by J2EE tools developers and system integrators to create resource adapters. Home page for J2EE Connector architecture http://java.sun.com/j2ee/connector/.

Question: What is difference between Java Bean and Enterprise Java Bean?
Answer: Java Bean as is a plain java class with member variables and getter setter methods. Java Beans are defined under JavaBeans specification as Java-Based software component model which includes the features like introspection, customization, events, properties and persistence.
Enterprise JavaBeans or EJBs for short are Java-based software components that comply with Java's EJB specification. EJBs are delpoyed on the EJB container and executes in the EJB container. EJB is not that simple, it is used for building distributed applications. Examples of EJB are Session Bean, Entity Bean and Message Driven Bean. EJB is used for server side programming whereas java bean is a client side. Bean is only development but the EJB is developed and then deploy on EJB Container.

Question: What is the difference between JTS and JTA?
Answer: In any J2EE application transaction management is one of the most crucial requirements of the application. Given the complexity of today's business requirements, transaction processing occupies one of the most complex segments of enterprise level distributed applications to build, deploy and maintain. JTS specifies the implementation of a Java transaction manager. JTS specifies the implementation of a Transaction Manager which supports the Java Transaction API (JTA) 1.0 This transaction manager supports the JTA, using which application servers can be built to support transactional Java applications. Internally the JTS implements the Java mapping of the OMG OTS 1.1 specifications. The Java mapping is specified in two packages: org.omg.CosTransactions and org.omg.CosTSPortability. The JTS thus provides a new architecture for transactional application servers and applications, while complying to the OMG OTS 1.1 interfaces internally. This allows the JTA compliant applications to interoperate with other OTS 1.1 complaint applications through the standard IIOP. Java-based applications and Java-based application servers access transaction management functionality via the JTA interfaces. The JTA interacts with a transaction management implementation via JTS. Similarly, the JTS can access resources via the JTA XA interfaces or can access OTS-enabled non-XA resources. JTS implementations can interoperate via CORBA OTS interfaces.

The JTA specifies an architecture for building transactional application servers and defines a set of interfaces for various components of this architecture. The components are: the application, resource managers, and the application server. The JTA specifies standard interfaces for Java-based applications and application servers to interact with transactions, transaction managers, and resource managers JTA transaction management provides a set of interfaces utilized by an application server to manage the beginning and completion of transactions. Transaction synchronization and propagation services are also provided under the domain of transaction management.

In the Java transaction model, the Java application components can conduct transactional operations on JTA compliant resources via the JTS. The JTS acts as a layer over the OTS. The applications can therefore initiate global transactions to include other OTS transaction managers, or participate in global transactions initiated by other OTS compliant transaction managers.

Question: Can Entity Beans have no create() methods?
Answer: Entity Beans can have no create() methods. Entity Beans have no create() method, when entity bean is not used to store the data in the database. In this case entity bean is used to retrieve the data from database.

Question: What are the call back methods in Session bean?
Answer: Callback methods are called by the container to notify the important events to the beans in its life cycle. The callback methods are defined in the javax.ejb.EntityBean interface.The callback methods example are ejbCreate(), ejbPassivate(), and ejbActivate().

Question: What is bean managed transaction?
Answer: In EJB transactions can be maintained by the container or developer can write own code to maintain the transaction. If a developer doesn’t want a Container to manage transactions, developer can write own code to maintain the database transaction.

Question: What are transaction isolation levels in EJB?
Answer: Thre are four levels of transaction isolation are:
* Uncommitted Read
* Committed Read
* Repeatable Read
* Serializable
The four transaction isolation levels and the corresponding behaviors are described below:
Isolation Level Dirty Read Non-Repeatable Read Phantom Read
Read Uncommitted Possible Possible Possible
Read Committed Not possible Possible Possible
Repeatable Read Not possible Not possible Possible
Serializable Not possible Not possible Not possible

Java J2EE conceptual questions and answers

Q 01: Give a few reasons for using Java?
A 01: Java is a fun language. Let’s look at some of the reasons:
• Built-in support for multi-threading, socket communication, and memory management (automatic garbage
• collection).
• Object Oriented (OO).
• Better portability than other languages across operating systems.
• Supports Web based applications (Applet, Servlet, and JSP), distributed applications (sockets, RMI. EJB etc) and network protocols



Q 02: What is the main difference between the Java platform and the other software platforms?
A 02: Java platform is a software-only platform, which runs on top of other hardware-based platforms like UNIX, NT etc.
The Java platform has 2 components:
• Java Virtual Machine (JVM) – ‘JVM’ is a software that can be ported onto various hardware platforms. Byte
• codes are the machine language of the JVM.
• Java Application Programming Interface (Java API)


Q 03: What is the difference between C++ and Java?
A 03: Both C++ and Java use similar syntax and are Object Oriented, but:
• Java does not support pointers. Pointers are inherently tricky to use and troublesome.
• Java does not support multiple inheritances because it causes more problems than it solves. Instead Java
supports multiple interface inheritance, which allows an object to inherit many method signatures from
different interfaces with the condition that the inheriting object must implement those inherited methods. The
multiple interface inheritance also allows an object to behave polymorphically on those methods.

• Java does not support destructors but rather adds a finalize() method. Finalize methods are invoked by the
garbage collector prior to reclaiming the memory occupied by the object, which has the finalize() method. This
means you do not know when the objects are going to be finalized. Avoid using finalize() method to
release non-memory resources like file handles, sockets, database connections etc because Java has only
a finite number of these resources and you do not know when the garbage collection is going to kick in to
release these resources through the finalize() method.

• Java does not include structures or unions because the traditional data structures are implemented as an
object oriented framework
Java
12
All the code in Java program is encapsulated within classes therefore Java does not have global variables or
functions.
C++ requires explicit memory management, while Java includes automatic garbage collection

Q 04: Explain Java class loaders? Explain dynamic class loading? LF
A 04: Class loaders are hierarchical. Classes are introduced into the JVM as they are referenced by name in a class that
is already running in the JVM. So how is the very first class loaded? The very first class is specially loaded with
the help of static main() method declared in your class. All the subsequently loaded classes are loaded by the
classes, which are already loaded and running. A class loader creates a namespace. All JVMs include at least one
class loader that is embedded within the JVM called the primordial (or bootstrap) class loader. Now let’s look at
non-primordial class loaders. The JVM has hooks in it to allow user defined class loaders to be used in place of
primordial class loader. Let us look at the class loaders created by the JVM.
CLASS LOADER reloadable? Explanation
Bootstrap
(primordial)
No Loads JDK internal classes, java.* packages. (as defined in the sun.boot.class.path
system property, typically loads rt.jar and i18n.jar)
Extensions No Loads jar files from JDK extensions directory (as defined in the java.ext.dirs system
property – usually lib/ext directory of the JRE)
System No Loads classes from system classpath (as defined by the java.class.path property, which
is set by the CLASSPATH environment variable or –classpath or –cp command line
options)

Explain static vs dynamic class loading?
Static class loading Dynamic class loading
Classes are statically loaded with Java’s
“new” operator.
class MyClass {
public static void main(String args[]) {
Car c = new Car();
}
}
Dynamic loading is a technique for programmatically invoking the functions of a
class loader at run time. Let us look at how to load classes dynamically.
Class.forName (String className); //static method which returns a Class
The above static method returns the class object associated with the class
name. The string className can be supplied dynamically at run time. Unlike the
static loading, the dynamic loading will decide whether to load the class Car or
the class Jeep at runtime based on a properties file and/or other runtime
conditions. Once the class is dynamically loaded the following method returns an
instance of the loaded class. It’s just like creating a class object with no
arguments.
class.newInstance (); //A non-static method, which creates an instance of a
class (ie creates an object).
Jeep myJeep = null ;
//myClassName should be read from a properties file or Constants interface.
//stay away from hard coding values in your program.
String myClassName = "au.com.Jeep" ;
Class vehicleClass = Class.forName(myClassName) ;
myJeep = (Jeep) vehicleClass.newInstance();
myJeep.setFuelCapacity(50);
A NoClassDefFoundException is
thrown if a class is referenced with
Java’s “new” operator (i.e. static loading)
but the runtime system cannot find the
referenced class.
A ClassNotFoundException is thrown when an application tries to load in a
class through its string name using the following methods but no definition for the
class with the specified name could be found:
• The forName(..) method in class - Class.
• The findSystemClass(..) method in class - ClassLoader.
• The loadClass(..) method in class - ClassLoader.

Q 05: What are the advantages of Object Oriented Programming Languages (OOPL)?
A 05: The Object Oriented Programming Languages directly represent the real life objects like Car, Jeep, Account,
Customer etc. The features of the OO programming languages like polymorphism, inheritance and
encapsulation make it powerful

Q 06: How does the Object Oriented approach improve software development?
A 06: The key benefits are:
Re-use of previous work: using implementation inheritance and object composition.
Real mapping to the problem domain: Objects map to real world and represent vehicles, customers,
products etc: with encapsulation.
Modular Architecture: Objects, systems, frameworks etc are the building blocks of larger systems.

The increased quality and reduced development time are the by-products of the key benefits discussed above.
If 90% of the new application consists of proven existing components then only the remaining 10% of the code
have to be tested from scratch.

Q 07: How do you express an ‘is a’ relationship and a ‘has a’ relationship or explain inheritance and composition? What
is the difference between composition and aggregation?
A 07: The ‘is a’ relationship is expressed with inheritance and ‘has a’ relationship is expressed with composition. Both
inheritance and composition allow you to place sub-objects inside your new class. Two of the main techniques for
code reuse are class inheritance and object composition.