August 2010

Master of Computer Application (MCA) – Semester 4

MC0078 – Java Programming – 4 Credits

(Book ID: B0831 & B0832)

1. Define RMI. Define the architecture of RMI invocation. ANS:-

Introduction to RMIRMI is the distributed object system that is built into the core Java environment. You can think of RMI as a built-in facility for Java that allows you to interact with objects that are actually running in Java virtual machines on remote hosts on the network. With RMI (and other distributed object APIs we discuss in this book), you can get a reference to an object that "lives" in a remote process and invoke methods on it as if it were a local object running within the same virtual machine as your code (hence the name, "Remote Method Invocation API"). RMI was added to the core Java API in Version 1.1 of the JDK (and enhanced for Version 1.2 of the Java 2 platform), in recognition of the critical need for support for distributed objects in distributed-application development. Prior to RMI, writing a distributed application involved basic socket programming, where a "raw" communication channel was used to pass messages and data between two remote processes. Now, with RMI and distributed objects, you can "export" an object as a remote object, so that other remote processes/agents can access it directly as a Java object. So, instead of defining a low-level message protocol and data transmission format between processes in your distributed application, you use Java interfaces as the "protocol" and the exported method arguments become the data transmission format. The distributed object system (RMI in this case) handles all the underlying networking needed to make your remote method calls work.Java RMI is a Java-only distributed object scheme; the objects in an RMI-based distributed application have to be implemented in Java. Some other distributed object schemes, most notably CORBA, are language-independent, which means that the objects can be implemented in any language that has a defined binding. With CORBA, for example, bindings exist for C, C++, Java, Smalltalk, and Ada, among other languages.The advantages of RMI primarily revolve around the fact that it is "Java-native." Since RMI is part of the core Java API and is built to work directly with Java objects within the Java VM, the integration of its remote object facilities into a Java application is almost seamless. You really can use RMI-enabled objects as if they live in the local Java environment. And since Java RMI is built on the assumption that both the client and server are Java objects, RMI can extend the internal garbage-collection mechanisms of the standard Java VM to provide distributed garbage collection of remotely exported objects.If you have a distributed application with heterogeneous components, some of which are written in Java and some that aren't, you have a few choices. You can use RMI, wrapping the

non-Java code with RMI-enabled Java objects using the Java Native Interface (JNI). At the end of this chapter, we discuss this first option in some detail, to give you a feeling for where it could be useful and where it wouldn't. Another option is to use another object distribution scheme, such as CORBA, that supports language-independent object interfaces. Chapter 4, "Java IDL", covers the Java interface to CORBA that is included in the Java 2 SDK. A third option involves the new RMI/IIOP functionality that allows RMI objects to communicate directly with remote CORBA objects over IIOP. We also discuss this option in some detail at the end of this chapter.

RMI in Action

Before we start examining the details of using RMI, let's look at a simple RMI remote object at work. We can create an Account object that represents some kind of bank account and then use RMI to export it as a remote object so that remote clients (e.g., ATMs, personal finance software running on a PC) can access it and carry out transactions.The first step is to define the interface for our remote object. Example 3-1 shows the Account interface. You can tell that it's an RMI object because it extends the java.rmi.Remote interface. Another signal that this is meant for remote access is that each method can throw a java.rmi.RemoteException. The Account interface includes methods to get the account name and balance and to make deposits, withdrawals, and transfers.

Example 3-1. A Remote Account Interface

import java.rmi.Remote;import java.rmi.RemoteException;import java.util.List;

public interface Account extends Remote { public String getName() throws RemoteException; public float getBalance() throws RemoteException; public void withdraw(float amt) throws RemoteException; public void deposit(float amt) throws RemoteException; public void transfer(float amt, Account src) throws RemoteException; public void transfer(List amts, List srcs) throws RemoteException;}

The next step is to create an implementation of this interface, which leads to the AccountImpl class shown in Example 3-2. This class implements all the methods listed in the Account interface and adds a constructor that takes the name of the new account to be created. Notice that the AccountImpl class implements the Account interface, but it also extends the java.rmi.UnicastRemoteObject class. This RMI class provides some of the basic remote functionality for server objects.Example 3-2. Implementation of the Remote Account Interface

import java.rmi.server.UnicastRemoteObject;import java.rmi.RemoteException;import java.util.List;import java.util.ListIterator;

public class AccountImpl extends UnicastRemoteObject implements Account { private float mBalance = 0; private String mName = "";

// Create a new account with the given name public AccountImpl(String name) throws RemoteException { mName = name; }

public String getName() throws RemoteException { return mName; } public float getBalance() throws RemoteException { return mBalance; }

// Withdraw some funds public void withdraw(float amt) throws RemoteException { mBalance -= amt; // Make sure balance never drops below zero mBalance = Math.max(mBalance, 0); }

// Deposit some funds public void deposit(float amt) throws RemoteException { mBalance += amt; }

// Move some funds from another (remote) account into this one public void transfer(float amt, Account src) throws RemoteException { src.withdraw(amt); this.deposit(amt); }

// Make several transfers from other (remote) accounts into this one public void transfer(List amts, List srcs) throws RemoteException { ListIterator amtCurs = amts.listIterator(); ListIterator srcCurs = srcs.listIterator(); // Iterate through the accounts and the amounts to be transferred from

// each (assumes amounts are given as Float objects) while (amtCurs.hasNext() && srcCurs.hasNext()) { Float amt = (Float)amtCurs.next(); Account src = (Account)srcCurs.next(); this.transfer(amt.floatValue(), src); } }}

Once the remote interface and an implementation of it are complete, you need to compile both Java files with your favorite Java compiler. After this is done, you use the RMI stub/skeleton compiler to generate a client stub and a server skeleton for the AccountImpl object. The stub and skeleton handle the communication between the client application and the server object. With Sun's Java SDK, the RMI compiler is called RMIC, and you can invoke it for this example like so:% rmic -d /home/classes AccountImplThe stub and skeleton classes are generated and stored in the directory given by the -d option (/HOME/CLASSES, in this case). This example assumes that the AccountImpl class is already in your CLASSPATH before you run the RMI compiler.There's just one more thing we need to do before we can actually use our remote object: register it with an RMI registry, so that remote clients can find it on the network. The utility class that follows, RegAccount, does this by creating an AccountImpl object and then binding it to a name in the local registry using the java.rmi.Naming interface. After it's done registering the object, the class goes into a wait(), which allows remote clients to connect to the remote object:

import java.rmi.Naming;public class RegAccount { public static void main(String argv[]) { try { // Make an Account with a given name AccountImpl acct = new AccountImpl("JimF");

// Register it with the local naming registry Naming.rebind("JimF", acct); System.out.println("Registered account."); } catch (Exception e) { e.printStackTrace();

} }}

After you compile the RegAccount class, you can run its main() method to register an Account with the local RMI registry. First, however, you need to start the registry. With Sun's Java SDK, the registry can be started using the RMIREGISTRY utility. On a Unix machine, this can be done like so:objhost% rmiregistry &Once the registry is started, you can invoke the main() method on the RegAccount class simply by running it:objhost% java RegAccountRegistered account.Now we have a remote Account object that is ready and waiting for a client to access it and call its methods. The following client code does just this, by first looking up the remote Account object using the java.rmi.Naming interface (and assuming that the Account object was registered on a machine named OBJHOST.ORG), and then calling the deposit method on the

Account object:

import java.rmi.Naming;

public class AccountClient { public static void main(String argv[]) { try { // Lookup account object Account jimAcct = (Account)Naming.lookup("rmi://objhost.org/JimF");

// Make deposit jimAcct.deposit(12000);

// Report results and balance. System.out.println("Deposited 12,000 into account owned by " + jimAcct.getName()); System.out.println("Balance now totals: " + jimAcct.getBalance()); } catch (Exception e) { System.out.println("Error while looking up account:"); e.printStackTrace(); } }}The first time you run this client, here's what you'd do:% java AccountClientDeposited 12,000 into account owned by JimFBalance now totals: 12000.0

For the sake of this example, I've assumed that the client process is running on a machine with all the necessary classes available locally (the Account interface and the stub and skeleton classes generated from the AccountImpl implementation). Later in the chapter, we'll see how to deal with loading these classes remotely when the client doesn't have them locally.

RMI Architecture

Now that we've seen a complete example of an RMI object in action, let's look at what makes remote objects work, starting with an overview of the underlying RMI architecture. There are three layers that comprise the basic remote-object communication facilities in RMI:The STUB/SKELETON layer, which provides the interface that client and server application objects use to interact with each other.The REMOTE REFERENCE layer, which is the middleware between the stub/skeleton layer and the underlying transport protocol. This layer handles the creation and management of remote object references.The TRANSPORT PROTOCOL layer, which is the binary data protocol that sends remote object requests over the wire.These layers interact with each other as shown in Figure 3-1. In this figure, the server is the application that provides remotely accessible objects, while the client is any remote application that communicates with these server objects. In a distributed object system, the distinctions between clients and servers can get pretty blurry at times. Consider the case where one process registers a remote-enabled object with the RMI naming service, and a number of remote processes are accessing it. We might be tempted to call the first process the server and the other processes the clients. But what if one of the clients calls a method on the remote object, passing a reference to an RMI object that's local to the client. Now the server has a reference to and is using an object exported from the client, which turns the tables somewhat. The "server" is really the server for one object and the client of another object, and the "client" is a client and a server, too. For the sake of discussion, I'll refer to a process in a distributed application as a server or client if its role in the overall system is generally limited to one or the other. In peer-to-peer systems, where there is no clear client or server, I'll refer to elements of the system in terms of application-specific roles (e.g., chat participant, chat facilitator).

Figure 3-1. The RMI runtime architecture

As you can see in Figure 3-1, a client makes a request of a remote object using a client-side stub; the server object receives this request from a server-side object skeleton. A client initiates a remote method invocation by calling a method on a stub object. The stub maintains an internal reference to the remote object it represents and forwards the method invocation request through the remote reference layer by MARSHALLING the method arguments into serialized form and asking the remote reference layer to forward the method request and arguments to the appropriate remote object. Marshalling involves converting local objects into portable form so that they can be transmitted to a remote process. Each object is checked as it is marshaled, to determine whether it implements the java.rmi.Remote interface. If it does, its remote reference is used as its marshaled data. If it isn't a Remote object, the argument is serialized into bytes that are sent to the remote host and reconstituted into a copy of the local object. If the argument is neither Remote nor Serializable, the stub throws a java.rmi.MarshalException back to the client. If the marshalling of method arguments succeeds, the client-side remote reference layer receives the remote reference and marshaled arguments from the stub. This layer converts the client request into low-level RMI transport requests according to the type of remote object communication being used. In RMI, remote objects can (potentially) run under several different communication styles, such as point-to-point object references, replicated objects, or multicast

objects. The remote reference layer is responsible for knowing which communication style is in effect for a given remote object and generating the corresponding transport-level requests. In the current version of RMI (Version 1.2 of Java 2), the only communication style provided out of the box is point-to-point object references, so this is the only style we'll discuss in this chapter. For a point-to-point communication, the remote reference layer constructs a single network-level request and sends it over the wire to the sole remote object that corresponds to the remote reference passed along with the request.On the server, the server-side remote reference layer receives the transport-level request and converts it into a request for the server skeleton that matches the referenced object. The skeleton converts the remote request into the appropriate method call on the actual server object, which involves UNMARSHALLING the method arguments into the server environment and passing them to the server object. As you might expect, unmarshalling is the inverse procedure to the marshalling process on the client. Arguments sent as remote references are converted into local stubs on the server, and arguments sent as serialized objects are converted into local copies of the originals. If the method call generates a return value or an exception, the skeleton marshals the object for transport back to the client and forwards it through the server reference layer. This result is sent back using the appropriate transport protocol, where it passes through the client reference layer and stub, is unmarshaled by the stub, and is finally handed back to the client thread that invoked the remote method. RMI Object ServicesOn top of its remote object architecture, RMI provides some basic object services you can use in your distributed application. These include an object naming/registry service, a remote object activation service, and distributed garbage collection.3.1.3.1. Naming/registry serviceWhen a server process wants to export some RMI-based service to clients, it does so by registering one or more RMI-enabled objects with its local RMI registry (represented by the Registry interface). Each object is registered with a name clients can use to reference it. A client can obtain a stub reference to the remote object by asking for the object by name through the Naming interface. The Naming.lookup() method takes the fully qualified name of a remote object and locates the object on the network. The object's fully qualified name is in a URL-like syntax that includes the name of the object's host and the object's registered name.It's important to note that, although the Naming interface is a default naming service provided with RMI, the RMI registry can be tied into other naming services by vendors. Sun has provided a binding to the RMI registry through the Java Naming and Directory Interface ( JNDI), Once the lookup() method locates the object's host, it consults the RMI registry on that host and asks for the object by name. If the registry finds the object, it generates a remote reference to the object and delivers it to the client process, where it is converted into a stub reference that is returned to the caller. Once the client has a remote reference to the server object, communication between the client and the server commences as described earlier. We'll talk in more detail about the Naming and Registry interfaces later in this chapter.

Object activation service

The remote object activation service is new to RMI as of Version 1.2 of the Java 2 platform. It provides a way for server objects to be started on an as-needed basis. Without remote activation, a server object has to be registered with the RMI registry service from within a running Java virtual machine. A remote object registered this way is only available during the lifetime of the Java VM that registered it. If the server VM halts or crashes for some reason, the server object becomes unavailable and any existing client references to the object become invalid. Any further attempts by clients to call methods through these now-invalid references result in RMI exceptions being thrown back to the client. The RMI activation service provides a way for a server object to be activated automatically when a client requests it. This involves creating the server object within a new or existing virtual machine and obtaining a reference to this newly created object for the client that caused the activation. A server object that wants to be activated automatically needs to register an activation method with the RMI activation daemon running on its host. We'll discuss the RMI activation service in more detail later in the chapter.

Distributed garbage collection

The last of the remote object services, distributed garbage collection, is a fairly automatic process that you as an application developer should never have to worry about. Every server that contains RMI-exported objects automatically maintains a list of remote references to the objects it serves. Each client that requests and receives a reference to a remote object, either explicitly through the registry/naming service or implicitly as the result of a remote method call, is issued this remote object reference through the remote reference layer of the object's host process. The reference layer automatically keeps a record of this reference in the form of an expirable "lease" on the object. When the client is done with the reference and allows the remote stub to go out of scope, or when the lease on the object expires, the reference layer on the host automatically deletes the record of the remote reference and notifies the client's reference layer that this remote reference has expired. The concept of expirable leases, as opposed to strict on/off references, is used to deal with situations where a client-side failure or a network failure keeps the client from notifying the server that it is done with its reference to an object. When an object has no further remote references recorded in the remote reference layer, it becomes a candidate for garbage collection. If there are also no further local references to the object (this reference list is kept by the Java VM itself as part of its normal garbage-collection algorithm), the object is marked as garbage and picked up by the next run of the system garbage collector.

Q. 2 Write the steps in creating and running applets in Java along with a sample program to demonstrate the same. Also describe various ways of running Applets.ANS:-

A Java applet is an applet delivered to the users in the form of Java bytecode. Java applets can run in a Web browser using a Java Virtual Machine (JVM), or in Sun's AppletViewer, a stand-alone tool for testing applets. Java applets were introduced in the first version of the Java language in 1995. Java applets are usually written in the Java programming language but they can also be written in other languages that compile to Java bytecode such as Jython,[8] JRuby,[9] or Eiffel (via SmartEiffel).[10]

Applets are used to provide interactive features to web applications that cannot be provided by HTML alone. They can capture mouse input (like rotating 3D object) and also have controls like buttons or check boxes. In response to the user action an applet can change the provided graphic content. This makes applets well suitable for demonstration, visualization and teaching. There are online applet collections for studying various subjects, from physics[11] to heart physiology.[3] Applets are also used to create online game collections that allow players to compete against live opponents in real-time.An applet can also be a text area only, providing, for instance, a cross platform command-line interface to some remote system.[12] If needed, an applet can leave the dedicated area and run as a separate window. However, applets have very little control over web page content outside the applet dedicated area, so they are less useful for improving the site appearance in general (while applets like news tickers [13] or WYSIWYG editors[14] are also known). Applets can also play media in formats that are not natively supported by the browser[15]

Java applets run at a speed that is comparable to (but generally slower than) other compiled languages such as C++, but many times faster than JavaScript.[16] In addition they can use 3D hardware acceleration that is available from Java. This makes applets well suited for non trivial, computation intensive visualizations.HTML pages may embed parameters that are passed to the applet. Hence the same applet may appear differently depending on the parameters that were passed. The first implementations involved downloading an applet class by class. While classes are small files, there are frequently a lot of them, so applets got a reputation as slow loading components. However, since jars were introduced, an applet is usually delivered as a single file that has a size of the bigger image (hundreds of kilobytes to several megabytes).Since Java's bytecode is platform independent, Java applets can be executed by browsers for many platforms, including Microsoft Windows, Unix, Mac OS and Linux. It is also trivial to run a Java applet as an application with very little extra code. This has the advantage of running a Java applet in offline mode without the need for any Internet browser software and also directly from the development IDE.Many Java developers, blogs and magazines are recommending that the Java Web Start technology be used in place of Applets.[17][18]

A Java Servlet is sometimes informally compared to be "like" a server-side applet, but it is different in its language, functions, and in each of the characteristics described here about applets.

ExampleThe following example is made simple enough to illustrate the essential use of Java applets through its java.applet package. It also uses classes from the Java Abstract Window Toolkit (AWT) for producing actual output (in this case, the "Hello, world!" message).

import java.applet.Applet;import java.awt.*; // Applet code for the "Hello, world!" example.// This should be saved in a file named as "HelloWorld.java".public class HelloWorld extends Applet { // This method is mandatory, but can be empty (i.e., have no actual code). public void init() { } // This method is mandatory, but can be empty.(i.e.,have no actual code). public void stop() { } // Print a message on the screen (x=20, y=10). public void paint(Graphics g) { g.drawString("Hello, world!", 20,10); // Draws a circle on the screen (x=40, y=30). g.drawArc(40,30,20,20,0,360); }}

More simple applets are available at Wikiversity.[24]

For compilation, this code is saved on a plain-ASCII file with the same name as the class and .java extension, i.e. HelloWorld.java. The resulting HelloWorld.class applet should be placed on the web server and is invoked within an HTML page by using an <APPLET> or an <OBJECT> tag. For example:<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> <HTML><HEAD><TITLE>HelloWorld_example.html</TITLE></HEAD><BODY><H1>A Java applet example</H1><P>Here it is: <APPLET code="HelloWorld.class" WIDTH="200" HEIGHT="40">This is where HelloWorld.class runs.</APPLET></P></BODY></HTML>

Displaying the HelloWorld_example.html page from a Web server, the result should look as this:

A Java applet exampleHere it is: Hello, world!

Q. 3 Create a User Interface to perform arithmetic operations to demonstrate event handling mechanisms.

ANS:- The JavaFX Script programming language enables you to make a desktop or a mobile application respond in a preprogrammed way to events through a convenient event-handling mechanism. Each JavaFX object that can potentially expose behavior has instance variables that map to event-related functions. You can define these functions to handle events such as the click of a mouse button, tap of a stylus, or the release of a key. For example, you can define a function that will render text when you click a circle with your mouse. For a complete list of events that can be handled by objects, see the JavaFX Script API. The following screen captures show all the possible button states.

Figure 1: States of the Button Create a file with an .fx extension, for example Events.fx. Avoid using file names that match the names of existing classes, instance variables, or reserved words because this type of naming leads to errors during compilation. For more information about existing classes, variables, and reserved words, see JavaFX Script API and Learning the JavaFX Script Programming Language.Adding Graphics

All button states are available in the following PNG images: play_onMouseExited.png, play_onMousePressed.png, stop_onMouseExited.png, and stop_onMousePressed.png. To use the images as scene objects, use a combination of the Image and ImageView classes.

Add import statements for the Image and ImageView classes.Define four Image objects corresponding to different states of the button.

When specifying the image url, you can set the URI of the resource or use the relative codebase. In this example, the image url is set by using the __DIR__ variable that indicates the directory where the image is located. By default it points to the current directory, so ensure that the images are located in the same directory as application-compiled classes. To run the application on the mobile emulator ensure that the images are packed into the application jar file along with the compiled classes.

Define the image variable to store an image of the current button state, and set it to the initial state, playNormal.Define the button variable to the a scene object corresponding to the current state of the button, and bind it to the image variable.Specify the scene of the application and add the button to its content.

import javafx.stage.Stage;import javafx.scene.Scene;import javafx.scene.Group;import javafx.scene.image.Image;import javafx.scene.image.ImageView;

def playNormal = Image { url: "{__DIR__}play_onMouseExited.png"};def playPressed = Image { url: "{__DIR__}play_onMousePressed.png"};def stopNormal = Image { url: "{__DIR__}stop_onMouseExited.png"};def stopPressed = Image { url: "{__DIR__}stop_onMousePressed.png"};

var image = playNormal;

var button = ImageView {image: bind image}

Stage { title: "Play Button" scene: Scene { width: 300 height: 240 content: Group { content: button } }}

 

Note: The button is added to the Group construct for further application enhancements. In your application you can add an ImageView object directly to the scene content..

 Handling the Press Event

This application handles three types of mouse events: mouse is pressed, mouse is released, and mouse is dragged. Each of these events is processed by a specific Node function. Because the ImageView class inherits all Node instance variables and functions, you can apply the onMousePressed, onMouseReleased, and onMouseDragged function to your button.The press event happens when you press the button with a stylus or a mouse without releasing it. In this example, the press event changes the button's appearance.To handle the mouse press event, perform the following steps.Define a Boolean variable named mode to fix whether the button is in the Play or Stop mode. Set the true value so that the button will be in Play mode when the application is started.

Define the X and Y variables to use them for calculating the button's position when processing the drag event.Use the if construction to check the mode of the button. If it is in Play mode, the image is set to playPressed, and stopPressed otherwise.

var mode = true; //true for the Play mode, //false for the Stop mode

var X: Number;var Y: Number;

onMousePressed: function(event) { X = event.sceneX - event.node.translateX; Y = event.sceneY - event.node.translateY; image = if (mode){ playPressed; } else { stopPressed; };}

 As a result, the button changes its appearance when you press it depending on the mode of the button.Handling the Release Event

The release event occurs when you release a mouse pointer from the button. In this example, when you release the pointer from the button, it switches its mode.To handle the mouse release event, use the following code:

onMouseReleased: function(event) { if (mode){ image = stopNormal; mode = false; } else { image = playNormal; mode = true; }}

 After an image is changed, the mode variable changes its value to the opposite. At this point, the application implements switching between the Play and Stop modes and changing the button appearance on the mouse press. The next section concerns dragging the button within the scene.Handling the Drag Event

Unlike the mouse events mentioned in the previous sections, the onMouseDragged event does not change the button's appearance. This event enables you to alter the button position dragging it when the mouse is pressed. In this example, you cannot move the button beyond the bounds of the scene.Use the following code fragment to handle the drag event:

onMouseDragged: function(event) { if (event.sceneX - X <0) { event.node.translateX = 0; } else { if (event.sceneX - X > 300 - image.width){ event.node.translateX = 300 - image.width; } else { event.node.translateX = event.sceneX - X; } } if (event.sceneY - Y <0) { event.node.translateY = 0; } else {if (event.sceneY - Y > 240 - image.height){ event.node.translateY = 240 - image.height; } else{ event.node.translateY = event.sceneY - Y; } }}

 Two if constructions are used to check whether the button has been dragged outside of the scene, and to set values for the translateX and translateY variables of the button. Consider an example when the drag event occurred at the point (320, 100). Suppose that the X was fixed as 5, and the Y was fixed as 10. Then the event.node.translateX - X = 320 - 5 = 315, while 300 - image.width = 300 -63 = 237. The value 315 is greater than 237, so the translateX variable will be set to 300 - image.width = 237, and the button will be placed at the right border of the scene. The translateY variable will be set to event.sceneY - Y = 100 - 10 = 90. Therefore, the button will be moved to the following position: translateX = 237, translateY = 90.The following screen capture shows the application run on the Touch Phone emulator.

Figure 2: Event-Handling Application Run on the Mobile Emulator

 You can find the complete code of this application in the EventsCommon.fx file. The next section discusses how to handle some additional desktop-specific events.Desktop Profile - Adding Tooltips- Handling the Mouse-Enter Event - Handling the Mouse-Exit Event  In this section, you will learn how to enhance the existing example by using additional features specific for desktop applications. Consider handling the mouse-enter and mouse-exit events by changing the button's appearance when the mouse pointer is on it. In addition, the modified application will show two variants of tooltips depending in the mode of the button. Try the following applet to evaluate event handling. Hover, press, release, and drag the button.

Two new images represent the button's state: play_onMouseEntered.png and stop_onMouseEntered.png. So you need to define two more Image variables.

def playHover = Image { url: "{__DIR__}play_onMouseEntered.png"};def stopHover = Image { url: "{__DIR__}stop_onMouseEntered.png"};

 

Adding Tooltips

Perform the following steps to create a textual tooltip and add it to the scene. Add import statements for the Text, Font, Timeline, and Color classes.Declare a Text object specifying the string to render, the color, and the position of the tooltip. Set the content variable depending on the current mode of the button. Set "Play Button" when the button is in Play mode, and "Stop Button" otherwise. Also bind the translateX and translateY variables to the corresponding variables of the button object.Add the tooltip object to the scene.

import javafx.stage.Stage;import javafx.scene.Scene;import javafx.scene.Group;import javafx.animation.Timeline;import javafx.scene.image.Image;import javafx.scene.image.ImageView;import javafx.scene.paint.Color;import javafx.scene.text.Font;import javafx.scene.text.Text;

def tooltip = Text { content: bind if (mode) "Play Button" else "Stop Button" translateX: bind button.translateX translateY: bind button.translateY + 80 opacity: 0.0 font: Font { size: 12 name: "Tahoma" } fill: Color.BLACK};

Stage { title: "Play Button" scene: Scene { fill: Color.WHITE width: 300 height: 240 content: Group { content: [button, tooltip] }}

 Handling the Mouse-Enter Event

This event happens when you position the mouse pointer in the button area. This event is controlled by the onMouseEntered function.To handle the mouse enter event define the onMouseEntered function. Create a Timeline object to alter the opacity of tooltips so that they do not appear instantly as you hover the button, but gradually within five seconds. The following code fragment performs these tasks.

def appear = Timeline { keyFrames: [ at(0s) {tooltip.opacity => 0.0}, at(0.5s) {tooltip.opacity => 1.0} ]}

...

onMouseEntered: function(event) { image = if (mode){ playHover; } else { stopHover }

appear.rate = 1; appear.play();}

 

After you enter the button area with the mouse pointer, the playHover or stopHover image appears and an animation timeline starts adding the tooltip. For more information about the onMouseEntered function, see JavaFX Script API. For more information about the MouseEvent class, see JavaFX Script API. For more information about animation, see Creating Animated Objects.

Note: Because a new state was introduced, you need to apply the following code for the onMouseReleased function. When the mouse is released the button should be in a hovered state, not in the initial state.

onMouseReleased: function(event) { if (mode){ image = stopHover; mode = false; } else { image = playHover; mode = true; }}

 Handling the Mouse-Exit Event

This type of event occurs when the mouse pointer exits the button area. The event is defined by the onMouseExited function.To define the mouse-exit event, use the following code:

onMouseExited: function(event) { image = if (mode){ playNormal; } else { stopNormal } appear.rate = -1; appear.play();}

 

When the mouse pointer exits the area defined by the graphic button, the button's appearance returns to its initial state. The tooltip disappears, because the rate variable of the animation timeline is set to -1 and the tooltip opacity alters from 1.0 to 0.0.For more information about the onMouseExited function, see JavaFX Script API.

Here is the complete Events.fx file.

Q.4. Describe the Web architecture and servlet life cycle.ANS:-

A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servletThe servlet is initialized by calling the init () method.The servlet calls service() method to process a client's request.The servlet is terminated by calling the destroy() method.Finally, servlet is garbage collected by the garbage collector of the JVM.Now let us discuss the life cycle methods in details.

The init() method :The init method is designed to be called only once. It is called when the servlet is first created, and not called again for each user request. So, it is used for one-time initializations, just as with the init method of applets.The servlet is normally created when a user first invokes a URL corresponding to the servlet, but you can also specify that the servlet be loaded when the server is first started.When a user invokes a servlet, a single instance of each servlet gets created, with each user request resulting in a new thread that is handed off to doGet or doPost as appropriate. The init() method simply creates or loads some data that will be used throughout the life of the servlet.The init method definition looks like this:

public void init() throws ServletException { // Initialization code...}

The service() method :The service() method is the main method to perform the actual task. The servlet container (i.e. web server) calls the service() method to handle requests coming from the client( browsers) and to write the formatted response back to the client.Each time the server receives a request for a servlet, the server spawns a new thread and calls service. The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc. methods as appropriate.Here is the signature of this method:

public void service(ServletRequest request, ServletResponse response) throws ServletException, IOException{}

The service () method is called by the container and service method invokes doGe, doPost, doPut, doDelete, etc. methods as appropriate. So you have nothing to do with service() method but you override either doGet() or doPost() depending on what type of request you receive from the client.The doGet() and doPost() are most frequently used methods with in each service request. Here are the signature of these two methods.The doGet() MethodA GET request results from a normal request for a URL or from an HTML form that has no METHOD specified and it should be handled by doGet() method.

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Servlet code}

The doPost() MethodA POST request results from an HTML form that specifically lists POST as the METHOD and it should be handled by doPost() method.

public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Servlet code

}

The destroy() method :The destroy() method is called only once at the end of the life cycle of a servlet. This method gives your servlet a chance to close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities.After the destroy() method is called, the servlet object is marked for garbage collection. The destroy method definition looks like this:

public void destroy() { // Finalization code... }

Architecture Digram:The following figure depicts a typical servlet life-cycle scenario. First the HTTP requests coming to the server are delegated to the servlet container.The servlet container loads the servlet before invoking the service() method.Then the servlet container handles multiple requests by spawning multiple threads, each thread executing the service() method of a single instance of the servlet

Book ID: B0832

Q.5. Explain the importance of CORBA and its applications in running client server programs.ANS:

Exceptions in CORBA IDL: CORBA IDL allows exceptions to be defined in interfaces and thrown by their ethods. To illustrate this point, we have defined our list of shapes inthe server as a sequence of a fixed length (line 4) and have defined FullException (line6), which is thrown by the method newShape (line 7) if the client attempts to add a shape when the sequence is full.Invocation semantics: Remote invocation in CORBA has at-most-once call semantics as the default. However, IDL may specify that the invocation of a particular method has maybe semantics by using the oneway keyword. The client does not block on oneway requests, which can be used only for methods without results. For an example of aoneway request, see the example on callbacks at the end of Section 17.2.1.The CORBA Naming service◊ The CORBA Naming Service is discussed in Section17.3.1. It is a binder that provides operations including rebind for servers to register theremote object references of CORBA objects by name and resolve for clients to look them up by name. The names are structured in a hierarchic fashion, and each name in a path is inside a structure called a NameComponent. This makes access in a simple example seem rather complex.CORBA pseudo objects (Jaa 2 version 1.4) ◊ Implementations of CORBA provide some interfaces to the functionality of the ORB that programmers need to use. They are called pseudo-objects because they cannot be used like CORBA objects; for example, they cannot be passed as arguments in RMIs. They have IDL interfaces and are implemented as libraries. Those relevant to our simple example are: • The ORB interface includes: The method init, which must be called to initialize the ORB; the method resolve_initial_references, which is used to find services such as the Naming Service and the root POA; other methods, which enable conversions between remote object references and strings.• The POA (Portable Object Adaptor – see Figure 17.6 and page 678) interface includes: A method for activating a POAmanager; a method servant_to_reference for registering a CORBA object.

CORBA client and server example (for Java 2 version 1.4)

This section outlines the steps necessary to produce client and server programs that use the IDL Shape and ShapeList interfaces shown in Figure 17.1. This is followed by a discussion of callbacks in CORBA. We use Java as the client and server languages, but the approach is similar for other languages. The interface compiler idlj can be applied to the CORBA interfaces to generate the following items: • The equivalent Java interfaces – two per IDL interface. For example, the interfaces ShapeListOperations and ShapeList are shown in Figure 17.2.• The server skeletons for each idl interface. The names of skeleton classes end in POA, for example ShapeListPOA.• The proxy classes or client stubs, one for each IDL interface. The names of these classes end in Stub, for example _ShapeListStub. CORBA CASE STUDY• A Java class to correspond to each of the structs defined with the IDL interfaces.In our example, classes Rectangle and GraphicalObject are generated. Each ofthese classes contains a declaration of one instance variable for each field in thecorresponding struct and a pair of constructors, but no other methods.• Classes called helpers and holders, one for each of the types defined in the IDLinterface. A helper class contains the narrow method, which is used to cast downfrom a given object reference to the class to which it belongs, which is lower downthe class hierarchy. For example, the narrow method in ShapeHelper casts down toclass Shape. The holder classes deal with out and inout arguments, which cannot bemapped directly onto Java. See Exercise 17.9 for an example of the use of holders.Server program◊ The server program should contain implementations of one or moreIDL interfaces. For a server written in an object-oriented language such as Java or C++,these implementations are implemented as servant classes. CORBA objects areinstances of servant classes.When a server creates an instance of a servant class, it must register it with thePOA, which makes the instance into a CORBA object and gives it a remote objectreference. Unless this is done, it will not be able to receive remote invocations. Readerswho studied Chapter 5 carefully may realize that registering the object with the POAcauses it to be recorded in the CORBA equivalent of the remote object table.In our example, the server contains implementations of the interfaces Shape andShapeList in the form of two servant classes, together with a server class that contains ainitialization section (see Section 5.2.5) in its main method.The servant classes: Each servant class extends the corresponding skeleton class andimplements the methods of an IDL interface using the method signatures defined inthe equivalent Java interface. The servant class that implements the ShapeListinterface is named ShapeListServant, although any other name could have beenchosen. Its outline is shown in Figure 17.3. Consider the method newShape in line 1,which is a factory method because it creates Shape objects. To make a Shape objecta CORBA object, it is registered with the POA by means of its servant_to_referencemethod, as shown in line 2. Complete versions of the IDL interface and the client andserver classes in this example are available at cdk3.net/corba.The server: The main method in the server class ShapeListServer is shown in Figure17.4. It first creates and initializes the ORB (line 1). It gets a reference to the rootFigure 17.2 Java interfaces generated by idlj from CORBA interface ShapeList.public interface ShapeListOperations {Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException;

Shape[] allShapes();int getVersion();}public interface ShapeList extends ShapeListOperations, org.omg.CORBA.Object,org.omg.CORBA.portable.IDLEntity { } // interface ShapeListSECTION 17.2 CORBA RMI 675POA and activates the POAManager (lines 2 & 3). Then it creates an instance ofShapeListServant, which is just a Java object (line 4). It then makes it into a CORBAobject by registering it with the POA (line 5). After this, it registers the server withthe Naming Service. It then waits for incoming client requests (line 10).Servers using the Naming Service first get a root naming context (line 6), then make aNameComponent (line 7), define a path (line 8) and finally use the rebind method (line9) to register the name and remote object reference. Clients carry out the same steps butuse the resolve method as shown in Figure 17.5 line 2.The client program◊ An example client program is shown in Figure 17.5. It creates andinitializes an ORB (line 1), then contacts the Naming Service to get a reference to theremote ShapeList object by using its resolve method (line 2). After that it invokes itsmethod allShapes (line 3) to obtain a sequence of remote object references to all theShapes currently held at the server. It then invokes the getAllState method (line 4),giving as argument the first remote object reference in the sequence returned; the resultis supplied as an instance of the GraphicalObject class.Figure 17.3 ShapeListServant class of the Java server program for CORBA interface ShapeListimport org.omg.CORBA.*;import org.omg.PortableServer.POA;class ShapeListServant extends ShapeListPOA {private POA theRootpoa;private Shape theList[];private int version;private static int n=0;public ShapeListServant(POA rootpoa){theRootpoa = rootpoa;// initialize the other instance variables}public Shape newShape(GraphicalObject g) throws ShapeListPackage.FullException {1version++; Shape s = null;ShapeServant shapeRef = new ShapeServant( g, version);try {org.omg.CORBA.Object ref = theRoopoa.servant_to_reference(shapeRef); 2s = ShapeHelper.narrow(ref);} catch (Exception e) {}if(n >=100) throw new ShapeListPackage.FullException();theList[n++] = s;return s;}public Shape[] allShapes(){ ... }public int getVersion() { ... }}

676 CHAPTER 17 CORBA CASE STUDYThe getAllState method seems to contradict our earlier statement that objectscannot be passed by value in CORBA, because both client and server deal in instancesof the class GraphicalObject. However, there is no contradiction: the CORBA objectreturns a struct, and clients using a different language might see it differently. Forexample, in the C++ language the client would see it as a struct. Even in Java, thegenerated class GraphicalObject is more like a struct because it has no methods.Client programs should always catch CORBA SystemExceptions, which report onerrors due to distribution (see line 5). Client programs should also catch the exceptionsdefined in the IDL interface, such as the FullException thrown by the newShape method.This example illustrates the use of the narrow operation: the resolve operation ofthe Naming Service returns a value of type Object; this type is narrowed to suit theparticular type required – ShapeList.Callbacks◊ Callbacks can be implemented in CORBA in a manner similar to the onedescribed for Java RMI in Section 5.5.1. For example, the WhiteboardCallbackinterface may be defined as follows:interface WhiteboardCallback {oneway void callback(in int version);};Figure 17.4 Java class ShapeListServerimport org.omg.CosNaming.*;import org.omg.CosNaming.NamingContextPackage.*;import org.omg.CORBA.*;import org.omg.PortableServer.*;public class ShapeListServer {public static void main(String args[]) {try{ORB orb = ORB.init(args, null); 1POA rootpoa = POAHelper.narrow(orb.resolve_initial_references("RootPOA"));2rootpoa.the_POAManager().activate(); 3ShapeListServant shapeRef = new ShapeListServant(rootpoa); 4org.omg.CORBA.Object ref = rootpoa.servant_to_reference(SLSRef); 5ShapeList SLRef = ShapeListHelper.narrow(ref);org.omg.CORBA.ObjectobjRef= orb.resolve_initial_references("NameService");NamingContext ncRef = NamingContextHelper.narrow(objRef);NameComponent nc = new NameComponent("ShapeList", ""); 6NameComponent path[] = {nc}; 7ncRef.rebind(path, SLRef); 8orb.run(); 9} catch (Exception e) { ... }}}SECTION 17.2 CORBA RMI 677This interface is implemented as a CORBA object by the client, enabling the server tosend the client a version number whenever new objects are added. But before the servercan do this, the client needs to inform the server of the remote object reference of itsobject. To make this possible, the ShapeList interface requires additional methods such

as register and deregister, as follows:int register(in WhiteboardCallback callback);void deregister(in int callbackId);After a client has obtained a reference to the ShapeList object and created an instance ofWhiteboardCallback, it uses the register method of ShapeList to inform the server thatit is interested in receiving callbacks. The ShapeList object in the server is responsiblefor keeping a list of interested clients and notifying all of them each time its versionnumber increases when a new object is added. The callback method is declared asoneway so that the server may use asynchronous calls to avoid delay as it notifies eachclient.17.2.2 The architecture of CORBAThe architecture is designed to support the role of an object request broker that enablesclients to invoke methods in remote objects, where both clients and servers can beimplemented in a variety of programming languages. The main components of theCORBA architecture are illustrated in Figure 17.6.Figure 17.5 Java client program for CORBA interfaces Shape and ShapeListimport org.omg.CosNaming.*;import org.omg.CosNaming.NamingContextPackage.*;import org.omg.CORBA.*;public class ShapeListClient{public static void main(String args[]) {try{ORB orb = ORB.init(args, null); 1org.omg.CORBA.Object objRef =orb.resolve_initial_references("NameService");NamingContext ncRef = NamingContextHelper.narrow(objRef);NameComponent nc = new NameComponent("ShapeList", "");NameComponent path [] = { nc };ShapeList shapeListRef =ShapeListHelper.narrow(ncRef.resolve(path)); 2Shape[] sList = shapeListRef.allShapes(); 3GraphicalObject g = sList[0].getAllState(); 4} catch(org.omg.CORBA.SystemException e) {...}}

Q.7. Explain the importance, applications and working of Ajax.ANS:- Importance of Ajax in Web Application

 

Introduction

Web applications are fun to build. They are like the funky airplanes of Web sites. Web applications allow the designer and developer to get together and solve a problem for their customers that the customers might not have even know they had. But most Web applications are slow and tedious. Even the fastest of them has lots of free time for your customers to go get a snack (worst of all) head off to a faster Web site. It's that dreaded hourglass! You click a process button and the hourglass appears as the Web application consults the server and the server thinks about what it's going to send back to you.

What is AJAX

The name AJAX is short for Asynchronous JavaScript and XML. It uses the JavaScript XMLHttpRequest function to create a tunnel from the client's browser to the server and transmit information back and forth without having to refresh the page. The data travels in XML format because it transmits complex data types over clear text.

AJAX uses XHTML for the data presentation of the view layer, DOM, short for Document Object Model, which dynamically manipulates the presentation, XML for data exchange, and XMLHttpRequest as the exchange engine that ties everything together.

Because of these requirements, AJAX works on I.E. 5.0+, Mozilla 1.0+, Firefox 1.0+, Netscape 7.0+, and Apple added it to Safari 1.2+.

Traditional HTML sends a request to the server, which processes it and either returns a static HTML page or dispatches the request to some scripting language such as ColdFusion, which creates and returns an HTML page for the browser to render. When this method has to retrieve new data from the server it has to repost and reload another HTML file. In many cases perhaps only a small portion of the returned HTML code varies and the shell itself remains the same resulting in huge overhead because the data has to be downloaded every time.

Some classic examples of applications that would benefit from AJAX are searching for information and displaying it back in a table, related select dropdowns, or checking if a user exists before submitting an entire form.

As we can see, AJAX offers many advantages over traditional HTML applications, but we shouldn't overestimate it. Because the data is JavaScript-driven, one of the main drawbacks is that search engines won't index any of the dynamically generated content. It's definitely not SEO-friendly.

People familiar with MVC will have a better grasp of the concept. Though details of MVC are outside of the scope of this article, the three defined components are Model, View, and Controller. The controller mediates between the data model and the view. It responds to events, which are usually actions by users, and changes the view or model as appropriate. The view consists of the HTML. JavaScript reacts to events triggered by the user and alters the existing rendered content with DOM. ColdFusion will be our model layer and can consist of one or more files.

Building an AJAX platform or engine from scratch can be a difficult and lengthy procedure. There are many AJAX engines available for download and you're welcome to use any of them. The only difference between implementations will be the data encoding, transmission, and decoding methods. The views and models of the MVC will be the same. My examples will be based on CFAJAX, a community-driven Open Source project. One of the problems with CFAJAX is its poor documentation. There is no manual or even a complete FAQ. So I will explain how to set it up step-by-step and work around its downside.

To use AJAX you'll need to really know JavaScript and DOM. But by the end of this article you'll be able to set up AJAX, a ColdFusion model, make basic calls, and exchange simple data. As the applications grow and become more complex, the AJAX engine will remain the same, and the CF model will have more functions, but your JavaScript will have to manipulate more and more objects and that's where it's really at.

Ajax is a way of developing Web applications that combines: XHTML and CSS standards based presentation Interaction with the page through the DOM Data interchange with XML and XSLT Asynchronous data retrieval with XMLHttpRequest JavaScript to tie it all together In the traditional Web application, the interaction between the customer and the server goes like this: In standard Web applications, Customer accesses Web application the interaction between the customer and the server is synchronous. This means that one has to

happen after the other. If a customer clicks a link, the request is sent to the server, which then sends the result back. Server processes request and sends data to the browser while the customer waits Customer clicks on a link or interacts with the application Server processes request and sends data back to the browser while the customer waits etc.... There is a lot of customer waiting.

Why Ajax is widely used in Web Applications

They can use a standard web browser, such as Firefox, Internet Explorer or Safari, as their only user interface. They don't force the user to wait for the web server every time the user clicks a button. This is what "asynchronous" means. For instance, gmail fetches new email messages in the background ("asynchronously") without forcing the user to wait. This makes an AJAX application respond much more like a "real" application on the user's computer, such as Microsoft Outlook. The Ajax engine works within the Web browser (through JavaScript and the DOM) to render the Web application and handle any requests that the customer might have of the Web server. The beauty of it is that because the Ajax engine is handling the requests, it can hold most information in the engine itself, while allowing the interaction with the application and the customer to happen as asynchronously and independently of any interaction with the server. They use standard JavaScript features (including the unofficial XMLHTTPRequest standard, pioneered by Microsoft and adopted by Firefox and other browsers) to fetch data in the background and display different email messages or other data "on the fly" when the user clicks on appropriate parts of the interface. Usually they manipulate data in XML format. This allows AJAX applications to interact easily with server-side code written in a variety of languages, such as PHP, Perl/CGI, Python and ASP.NET. Using XML isn't absolutely necessary, and in fact many "AJAX" applications don't -- they use the XMLHTTPRequest object to send and receive data "on the fly," but they don't actually bother packaging that data as XML. with Ajax, the JavaScript that is loaded when the page loads handles most of the basic tasks such as data validation and manipulation, as well as display rendering the Ajax engine handles without a trip to the server. At the same time that it is making display changes for the customer, it is sending data back and forth to the server. But the data transfer is not dependent upon actions of the customer.