An Example Client Server Application in Java

An Example Client Server Application in Java

In order to to explain socket programming for both TCP and UDP, we use the following simple client-server application:

1. A client reads a line from its standard input (keyboard) and sends the line out its socket to the server.
2. The server reads a line from its connection socket.
3. The server converts the line to uppercase.
4. The server sends the modified line out its connection socket to the client.
5.The client reads the modified line from its socket and prints the line on its standard output (monitor).

Figure 1, demonstrates the main socket-related activity of the client and server. Next we provide the client-server program pair for a TCP implementation of the application. We provide a detailed, line-by-line analysis after each program.

The client server application using connection-oriented transport services

The client program is called, and the server program is called In order to emphasize the key issues, we intentionally provide code that is to the point but not bulletproof. "Good code" would definitely have a few more auxiliary lines.

Once the two programs are compiled on their respective hosts, the server program is first executed at the server host, which creates a server process at the server host. As discussed above, the server process waits to be contacted by a client process. In this example application, when the client program is executed, a process is created at the client, and this process instantly contacts the server and establishes a TCP connection with it. The user at the client may then use the application to send a line and then receive a capitalized version of the line.


Here is the code for the client side of the application:

The program TCPClient creates three streams and one socket, as illustrated in Figure 2. The socket is called clientSocket. The stream inFromUser is an input stream to the program: it is attached to the standard input (that is, the keyboard). When the user types characters on the keyboard, the characters flow into the stream inFromUser. The stream inFromServer is another input stream to the program; it is attached to the socket. Characters that arrive from the network flow into the stream inFromServer. Finally, the stream outToServer is an output stream from the program; it is also attached to the socket. Characters that the client sends to the network flow into the stream outToServer.

TCPclient has three streams through which characters flow

Let's now take a look at the various lines in the code.

Import*; and are Java packages. The package includes classes for input and output streams. Particularly, the package includes the BufferedReader and DataOutputStream classes, classes that the program uses to create the three streams previously demonstrated. The package provides classes for network support. Especially, it includes the Socket and ServerSocket classes. The clientSocket object of this program is derived from the Socket class.

class TCPClient {
    public static void main( String argv[] ) throws Exception
      {. . . . . . }

Up to now, what we've seen is standard stuff that you see at the beginning of most Java code. The third line is the beginning of a class definition block. The keyword class begins the class definition for the class named TCPClient. A class includes variables and methods. The variables and methods of the class are embraced by the curly brackets that begin and end the class definition block. The class TCPClient has no class variables and exactly one method, the main ( ) method. Methods are similar to the functions or procedures in languages such as C; the main ( ) method in the Java language is similar to the main ( ) function in C and C++. When the Java interpreter executes an application (by being invoked upon the application's controlling class), it starts by calling the class's main ( ) method. The main ( ) method then calls all the other methods required to run the application. For this introduction to socket programming in Java, you may ignore the keywords public, static, void, main, and throws Exceptions (although you must include them in the code).

String sentence;
String modifiedSentence;

These above two lines declare objects of type String. The object sentence is the string typed by the user and sent to the server. The object modifiedSentence is the string obtained from the server and sent to the user's standard output.

BufferedReader inFromUser = new BufferedReader ( new InputStreamReader( ) ;

The above line creates the stream object inFromUser of type Buffered Reader. The input stream is initialized with, which attaches the stream to the standard input. The command allows the client to read text from its keyboard.

Socket clientSocket = new Socket ( "hostname", 6789 ) ;

The above line creates the object clientSocket of type Socket. It also starts the TCP connection between client and server. The string "host-name" must be replaced with the host name of the server (for example, ""). Before the TCP connection is actually started, the client performs a DNS lookup on the host name to obtain the hosts IP address. The number 6789 is the port number. You can use a different port number, but you must make sure that you use the same port number at the server side of the application. As discussed earlier, the host's IP address along with the application's port number identifies the server process.

The above two lines create stream objects that are attached to the socket. The outToServer stream provides the process output to the socket. The inFromServer stream provides the process input from the socket (see Figure 2).

sentence = inFromUser.readLine( ) ;

This line places a line typed by the user into the string sentence. The string sentence continues to gather characters until the user ends the line by typing a carriage return. The line passes from standard input through the stream inFromUser into the string sentence.

outToServer.writeBytes ( sentence + '\n' ) ;

The above line sends the string sentence augmented with a carriage return into the outToServer stream. The augmented sentence flows through the client's socket and into the TCP pipe. The client then waits to receive characters from the server.

modifiedSentence = inFromServer.readLine ( ) ;

When characters arrive from the server, they flow through the stream inFrom-Server and get placed into the string modifiedSentence. Characters continue to accumulate in modifiedSentence until the line ends with a carriage return character.

System.out.printIn ( "FROM SERVER" + modifiedSentence ) :

The above line prints to the monitor the string modifiedSentence returned by the server.

clientSocket.close ( ) ;

This last line closes the socket and, hence, closes the TCP connection between the client and the server. It causes TCP in the client to send a TCP message to TCP in the server.

Now let's take a look at the server program.

TCPServer has many similarities with TCPClient. Let's now take a look at the lines in We will not comment on the lines that are identical or similar to commands in

The first line in TCPServer is substantially different from what we saw in TCPClient:

ServerSocket welcomeSocket = new ServerSocket ( 6789 ) ;

This line creates the object welcomeSocket, which is of type ServerSocket. The welcomeSocket is a sort of door that listens for a knock from some client. The welcomeSocket listens on port number 6789. The next line is

Socket connectionSocket  =  welcomeSocket.accept ( ) ;

This line creates a new socket, called connectionSocket, when some client knocks on welcomeSocket. This socket also has port number 6789. (We'll explain why both sockets have the same port number in "Transport Layer") TCP then establishes a direct virtual pipe between clientSocket at the client and connectionSocket at the server. The client and server can then send bytes to each other over the pipe, and all bytes sent arrive at the other side in order.  With connectionSocket established, the server can continue to listen for requests from other clients for the application using welcomeSocket. (This version of the program doesn't actually listen for more connection requests, but it can be modified with threads to do so.) The program then creates various stream objects, analogous to the stream objects created in clientSocket. Now look at

capitalizedSentence  =  clientSentence.toUpperCase () + '\n' ;  

This command is the heart of the application. It takes the line sent by the client, capitalizes it, and adds a carriage return. It uses the method toUpperCase (  ). All the other commands in the program are peripheral; they are used for communication with the client.

To test the program pair, you install and compile in one host and in another host. Be sure to include the proper hostname of the server in You next execute TCPServer.class, the compiled server program, in the server. This creates a process in the server that idles until it is contacted by some client. Then you execute TCPClient.class, the compiled client program, in the client. This creates a process in the client and establishes a TCP connection between the client and server processes. Finally, to use the application, you type a sentence followed by a carriage return.

To develop your own client-server application, you can begin by slightly modifying the programs. For instance, instead of converting all the letters to  uppercase, the server can count the number of times the letter s appears and return this number.


socket programming, standard input, standard output, streams, socket

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