Programming Practices (CS-406) - Unit 5 Short Notes

How unit 5 is examined

This unit is an overview of four technologies (J2ME, J2EE, EJB, XML); none has been asked in the supplied papers, so each is taught as a short answer-ready section, and the likeliest exam forms are "explain", "differentiate" and "write short notes".

J2ME

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. <mark>J2ME (Java 2 Platform, Micro Edition, now Java ME) is the Java platform for small, resource-limited devices such as mobile phones, PDAs, set-top boxes, smart cards and embedded controllers.</mark>

Diagram.

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Key points.

1. J2ME uses a small-footprint virtual machine (the KVM, "kilobyte" VM) and a trimmed set of libraries, because these devices have little memory, a slow processor, a small screen and limited battery.
2. A configuration defines the basic virtual machine and core libraries for a family of devices: CLDC (Connected Limited Device Configuration) is for phones and pagers, and CDC (Connected Device Configuration) is for more capable devices such as set-top boxes.
3. A profile sits on a configuration and adds APIs for the user interface, storage and networking; MIDP (Mobile Information Device Profile) is the profile used with CLDC on mobile phones.
4. Applications are called MIDlets; they extend the class MIDlet, are downloaded over the air to the phone, and are started, paused and destroyed by the device's application manager.
5. A MIDlet has three states, paused, active and destroyed, controlled by the methods startApp(), pauseApp() and destroyApp().
6. J2ME keeps the write-once-run-anywhere idea, but only a subset of the standard J2SE classes is present, and features such as floating point and reflection are limited in CLDC.

public class Hello extends MIDlet {
    public void startApp() { /* build the screen and show it */ }
    public void pauseApp() { }
    public void destroyApp(boolean unconditional) { }
}

Answer frame. Open with the definition; draw the layer figure; develop configuration, profile, MIDlet and its three life-cycle methods in that order; close with one line that J2ME is J2SE cut down for mobile devices.

J2EE

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. <mark>J2EE (Java 2 Platform, Enterprise Edition, now Java EE) is a standard platform of APIs and a runtime environment for developing and deploying large, distributed, multi-tier, server-side enterprise applications.</mark>

Diagram.

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Key points.

1. J2EE builds on J2SE and adds enterprise APIs: Servlets, JSP, EJB, JDBC, JNDI (naming), JMS (messaging), JTA (transactions), JavaMail and RMI.
2. Applications follow a multi-tier model: the client tier is a browser or standalone program, the web tier runs Servlets and JSP, the business tier runs EJB components, and the enterprise information system (EIS) tier holds the database and legacy systems.
3. Each component runs inside a container provided by the application server, and the container supplies services such as transaction management, security, connection pooling, threading and life-cycle management, so the programmer writes only business logic.
4. The web container hosts Servlets and JSP, and the EJB container hosts enterprise beans; both are part of the application server (for example GlassFish, JBoss or WebLogic).
5. Applications are packaged as archives: JAR for EJB modules, WAR for web modules and EAR for the complete application, with deployment descriptors (web.xml, ejb-jar.xml) written in XML.
6. J2EE gives scalable, portable, vendor-independent and secure applications, because the same code runs on any compliant server.

Answer frame. Open with the definition; draw the tier figure; develop the APIs, tiers, containers and packaging in that order; close with portability and scalability.

EJB

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. <mark>Enterprise JavaBeans (EJB) is the J2EE server-side component architecture in which the business logic of an enterprise application is written as reusable components called beans, managed by an EJB container.</mark>

Diagram.

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Key points.

1. A session bean performs business logic for one client; it is stateless if it keeps no client data between calls (and can be shared through a pool) and stateful if it remembers the conversation with one client.
2. An entity bean represents persistent data, usually one row of a database table, and is identified by a primary key; in later versions it was replaced by JPA entities.
3. A message-driven bean is invoked by the arrival of a JMS message, so it handles asynchronous requests and has no client-visible interface.
4. The EJB container provides transaction management, security, persistence, concurrency, remote access and instance pooling, so the bean developer does not code these services.
5. A client reaches a bean through its home interface (to create or find the bean) and its remote interface (the business methods), locating them with a JNDI lookup and calling them over RMI.
6. The bean class, its interfaces and the deployment descriptor ejb-jar.xml are packaged into a JAR and deployed on the application server.

Answer frame. Open with the definition; draw the call-path figure; develop the three bean types, then the container services, then the interfaces; close with the benefit that the container removes plumbing code.

XML

<span style="display:inline-block;padding:.16em .6em;border:1.5px solid currentColor;border-radius:999px;font-size:.68em;font-weight:700;letter-spacing:.06em;text-transform:uppercase;opacity:.75">Not asked since 2022</span>

Definition. <mark>XML (Extensible Markup Language) is a W3C text-based markup language in which the user defines their own tags to describe and carry data in a platform-independent, self-describing form.</mark>

Diagram.

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Key points.

1. XML describes what the data is, not how it looks, unlike HTML, which has fixed tags and describes display; its tags are user-defined and it is case-sensitive.
2. A document must be well-formed: it has exactly one root element, tags are properly nested, every start tag has an end tag, and attribute values are quoted.
3. A DTD or an XML Schema defines the allowed elements and their order, and a well-formed document that follows it is called valid.
4. XML parsing means reading a document into a program: a parser checks well-formedness and gives the program access to the elements.
5. A DOM parser loads the whole document as a tree in memory, so it allows random access and modification but uses more memory.
6. A SAX parser is event-driven and reads the document sequentially, raising events such as start element and end element, so it is fast and uses little memory but cannot go backwards.
7. Java supports both through JAXP (DocumentBuilder for DOM, SAXParser for SAX), and XML is used for data exchange, configuration files such as web.xml, and web services.

<?xml version="1.0"?>
<student>
  <name>Amit</name>
  <roll>101</roll>
  <branch>CSE</branch>
</student>

Answer frame. Open with the definition; write a small XML document and draw its tree; develop well-formed versus valid, then DOM and SAX with the table; close with XML's use in data exchange.

Pitfall: Writing an unclosed tag, two root elements or mismatched case in an example makes the document not well-formed and loses the marks.

Last-minute revision

• J2ME is the Java platform for small devices; its programs are MIDlets running on the KVM.
• J2ME structure: configuration (CLDC or CDC) plus profile (MIDP).
• MIDlet life-cycle methods: startApp(), pauseApp(), destroyApp().
• J2EE is the Java platform for distributed multi-tier enterprise applications.
• J2EE tiers: client, web (Servlet and JSP), business (EJB), EIS (database).
• The J2EE container supplies transactions, security and pooling; packaging is JAR, WAR and EAR.
• EJB is the server-side component model for business logic, managed by the EJB container.
• EJB types: session (stateful or stateless), entity, message-driven.
• XML lets users define their own tags; it is self-describing and platform independent.
• Well-formed means correct syntax; valid means it also follows a DTD or Schema.
• DOM loads a tree in memory (random access); SAX is event-based and light (forward only).

Memory hooks

• Micro means Mobile: J2ME, MIDlet, KVM, CLDC plus MIDP.
• Enterprise means Every tier: J2EE is client, web, business, database.
• Beans come in three flavours: Session, Entity, Message.
• DOM is a Document tree in memory; SAX is a Stream of events.
• Well-formed is grammar, valid is grammar plus the DTD rulebook.

Coverage checklist

• J2ME: definition, configuration and profile, MIDlet life cycle (no past questions).
• J2EE: definition, tiers, containers, packaging, comparison with J2SE (no past questions).
• EJB: definition, bean types, container services, interfaces (no past questions).
• XML: definition, well-formed and valid, DOM and SAX parsing (no past questions).