17

Networking

Objectives

•To understand Java networking with URIs, sockets and datagrams.

•To implement Java networking applications by using sockets and datagrams.

•To understand how to implement Java clients and servers that communicate with one another.

•To understand how to implement network-based collaborative applications.

•To construct a multithreaded server.

If the presence of electricity can be made visible in any part of a circuit, I see no reason why intelligence may not be transmitted instantaneously by electricity.

Samuel F. B. Morse

Mr. Watson, come here, I want you.

Alexander Graham Bell

What networks of railroads, highways and canals were in another age, the networks of telecommunications, information and computerization … are today.

Bruno Kreisky, Austrian Chancellor

Science may never come up with a better officecommunication system than the coffee break.

Earl Wilson

It’s currently a problem of access to gigabits through punybaud.

J. C. R. Licklider

Outline

17.1Introduction

17.2Manipulating URIs

17.3Reading a File on a Web Server

17.4Establishing a Simple Server Using Stream Sockets

17.5Establishing a Simple Client Using Stream Sockets

17.6Client/Server Interaction with Stream Socket Connections

17.7Connectionless Client/Server Interaction with Datagrams

17.8Client/Server Tic-Tac-Toe Using a Multithreaded Server

17.9Security and the Network

17.10DeitelMessenger Chat Server and Client

17.10.1DeitelMessengerServer and Supporting Classes

17.10.2DeitelMessenger Client and Supporting Classes

17.11(Optional) Discovering Design Patterns: Design Patterns Used in Packages java.io and java.net

17.11.1Creational Design Patterns

17.11.2Structural Design Patterns

17.11.3Architectural Patterns

17.11.4Conclusion

Summary • Terminology • Self-Review Exercises • Answers to Self-Review Exercises • Exercises

17.1 Introduction

There is much excitement over the Internet and the World Wide Web. The Internet ties the “information world” together. The World Wide Web makes the Internet easy to use and gives it the flair and sizzle of multimedia. Organizations see the Internet and the Web as crucial to their information-systems strategies. Java provides a number of built-in networking capabilities that make it easy to develop Internet-based and Web-based applications. Not only can Java specify parallelism through multithreading, but it can enable programs to search the world for information and to collaborate with programs running on other computers internationally, nationally or just within an organization. Java can enable applets and applications to communicate with one another (subject to security constraints).

Networking is a massive and complex topic. Computer science and computer engineering students will typically take a full-semester, upper level course in computer networking and continue with further study at the graduate level. Java provides a rich complement of networking capabilities and will likely be used as an implementation vehicle in computer networking courses. In Java How to Program, Fourth Edition, we introduce a portion of Java’s networking concepts and capabilities. For more advanced networking capabilities, refer to our book Advanced Java 2 Platform How to Program.

Java’s networking capabilities are grouped into several packages. The fundamental networking capabilities are defined by classes and interfaces of package java.net,

through which Java offers socket-based communications that enable applications to view networking as streams of data. The classes and interfaces of package java.net also offer packet-based communications that enable individual packets of information to be trans- mitted—this is commonly used to transmit audio and video over the Internet. In this chapter, we show how to create and manipulate sockets and how to communicate with packets of data.

Our discussion of networking focuses on both sides of a client-server relationship. The client requests that some action be performed, and the server performs the action and responds to the client. A common implementation of the request-response model is between World Wide Web browsers and World Wide Web servers. When a user selects a Web site to browse through a browser (the client application), a request is sent to the appropriate Web server (the server application). The server normally responds to the client by sending an appropriate HTML Web page.

We demonstrate the Swing GUI component JEditorPane and its ability to render an HTML document downloaded from the World Wide Web. We also introduce Java’s a socket-based communications, which enable applications to view networking as if it were file I/O—a program can read from a socket or write to a socket as simply as reading from a file or writing to a file. We show how to create and manipulate sockets.

Java provides stream sockets and datagram sockets. With stream sockets, a process establishes a connection to another process. While the connection is in place, data flows between the processes in continuous streams. Stream sockets are said to provide a connec- tion-oriented service. The protocol used for transmission is the popular TCP (Transmission Control Protocol).

With datagram sockets, individual packets of information are transmitted. This is not the right protocol for everyday users, because, unlike TCP, the protocol used—UDP, the User Datagram Protocol—is a connectionless service, and it does not guarantee that packets arrive in any particular order. In fact, packets can be lost, can be duplicated and can even arrive out of sequence. So, with UDP, significant extra programming is required on the user’s part to deal with these problems (if the user chooses to do so). UDP is most appropriate for network applications that do not require the error checking and reliability of TCP. Stream sockets and the TCP protocol will be the most desirable for the vast majority of Java programmers.

Performance Tip 17.1

Connectionless services generally offer greater performance, but less reliability than con- nection-oriented services.

Portability Tip 17.1

The TCP protocol and its related set of protocols enable a great variety of heterogeneous computer systems (i.e., computer systems with different processors and different operating systems) to intercommunicate.

The chapter ends with a case study in which we implement a client/server chat application similar to the instant-messaging services popular on the Web today. The program incorporates many networking techniques introduced in this chapter. The program also introduces multicasting, in which a server can publish information and clients can subscribe to that information. Each time the server publishes more information, all subscribers receive that information. Throughout the examples of this chapter, we will see that many of the networking details are handled by the Java classes we use.

String title = getParameter( "title0" );

gets the value associated with the "title0" parameter and assigns it to String reference title. If there is not a param tag containing the specified parameter, getParameter returns null.

The applet (Fig. 17.2) obtains from the HTML document (Fig. 17.1) the choices that will be displayed in the applet’s JList. Class SiteSelector uses a Hashtable (package java.util) to store the World Wide Web site names and URIs. A Hashtable stores key/ value pairs. The program uses the key to store and retrieve the associated value in the Hashtable. In this example, the key is the String in the JList that represents the Web site name, and the value is a URL object that stores the URI of the Web site to display in the browser. Class Hashtable provides two methods of importance in this example—put and get. Method put takes two arguments—a key and its associated value—and places the value in the Hashtable at a location determined by the key. Method get takes one argu- ment—a key—and retrieves the value (as an Object reference) associated with the key. Class SiteSelector also contains a Vector (package java.util) in which the site names are placed so they can be used to initialize the JList (one version of the JList constructor receives a Vector object). A Vector is a dynamically resizable array of Objects. Class Vector provides method add to add a new element to the end of the Vector. Classes Hashtable and Vector are discussed in detail in Chapter 20.

Lines 23–24 in method init (lines 20–63) create the Hashtable and Vector objects. Line 27 calls our utility method getSitesFromHTMLParameters (lines 66– 108) to obtain the HTML parameters from the HTML document that invoked the applet.

In method getSitesFromHTMLParameters, line 75 uses Applet method getParameter to obtain a Web site title. If the title is not null, the loop at lines 78– 106 begins executing. Line 81 uses Applet method getParameter to obtain the corresponding location. Line 87 uses the location as the initial value of a new URL object. The URL constructor determines whether the String passed as an argument represents a valid Uniform Resource Identifier. If not, the URL constructor throws a MalformedURLException. Notice that the URL constructor must be called in a try block. If the

URL constructor generates a MalformedURLException, the call to printStackTrace (line 98) causes the program to display a stack trace. Then the program attempts to obtain the next Web site title. The program does not add the site for the invalid URI to the Hashtable, so the title will not be displayed in the JList.

Common Programming Error 17.1

A MalformedURLException is thrown when a String that is not in proper URI format is passed to a URL constructor.

For a proper URL, line 90 places the title and URL into the Hashtable, and line 93 adds the title to the Vector. Line 104 gets the next title from the HTML document. When the call to getParameter at line 104 returns null, the loop terminates.

When method getSitesFromHTMLParameters returns to init, lines 30–61 construct the applet’s GUI. Lines 31–32 add the JLabel “Choose a site to browse” to the NORTH of the content pane’s BorderLayout. Lines 36–58 register an instance of an anonymous inner class that implements ListSelectionListener to handle the siteChooser’s events. Lines 60–61 add siteChooser to the CENTER of the content pane’s BorderLayout.

1// Fig. 17.2: SiteSelector.java

2 // This program uses a button to load a document from a URL.

3

4 // Java core packages

5 import java.net.*;

6 import java.util.*;

7import java.awt.*;

8 import java.applet.AppletContext;

9

10// Java extension packages

11import javax.swing.*;

12import javax.swing.event.*;

19// read HTML parameters and set up GUI

20public void init()

21{

22// create Hashtable and Vector

23sites = new Hashtable();

24siteNames = new Vector();

25

26// obtain parameters from HTML document

27getSitesFromHTMLParameters();

28

29// create GUI components and layout interface

30Container container = getContentPane();

31container.add( new JLabel( "Choose a site to browse" ),

Fig. 17.2 Loading a document from a URL into a browser (part 1 of 3).

65// obtain parameters from HTML document

66private void getSitesFromHTMLParameters()

67{

68// look for applet parameters in the HTML document

69// and add sites to Hashtable

70String title, location;

71URL url;

72int counter = 0;

73

74// obtain first site title

75title = getParameter( "title" + counter );

77// loop until no more parameters in HTML document

78while ( title != null ) {

Fig. 17.2 Loading a document from a URL into a browser (part 2 of 3).

107

108 } // end method getSitesFromHTMLParameters

109

110 } // end class SiteSelector

Fig. 17.2 Loading a document from a URL into a browser (part 3 of 3).

When the user selects one of the Web sites in siteChooser, the program calls method valueChanged (lines 41–54). Line 44 obtains the selected site name from the JList. Line 47 passes the selected site name (the key) to Hashtable method get, which locates and returns an Object reference to the corresponding URL object (the value). The URL cast operator converts the reference to a URL that can be assigned to reference newDocument.

Line 50 uses Applet method getAppletContext to get a reference to an AppletContext object that represents the applet container. Line 53 uses the AppletContext reference browser to invoke AppletContext method showDocument, which receives a URL object as an argument and passes it to the AppletContext (i.e., the browser). The browser displays in the current browser window the World Wide Web resource associated with that URL. In this example, all the resources are HTML documents.

For programmers familiar with HTML frames, there is a second version of AppletContext method showDocument that enables an applet to specify the so-called target frame in which to display the World Wide Web resource. The second version of showDocument takes two arguments—a URL object specifying the resource to display and a String representing the target frame. There are some special target frames that can be used as the second argument. The target frame _blank results in a new Web browser