Java How to Program Fourth Edition - DCC

Chapter 17 Networking 1011
146
147 application.setDefaultCloseOperation(
148 JFrame.EXIT_ON_CLOSE );
149
150 application.waitForPackets();
151 }
152
153 } // end class Client
The Client window after sending a
packet to the server and receiving
the packet back from the server
Fig. Fig. 17.7 17.7 Demonstrating the client side of connectionless client/server computing
with datagrams (part 4 of 4).
Notice that the DatagramSocket constructor call (line 84) in this application does
not specify any arguments. This constructor allows the computer to select the next available
port number for the DatagramSocket. The client does not need a specific port number,
because the server receives the client’s port number as part of each DatagramPacket
sent by the client. Thus, the server can send packets back to the same computer and port
number from which the server receives a packet of information.
Client method waitForPackets (lines 97–125) uses an infinite loop to wait for
packets from the server. Line 111 blocks until a packet arrives. Note that this does not prevent
the user from sending a packet, because the GUI events are handled in the event dispatch
thread. It only prevents the while loop from continuing until a packet arrives at the
Client. When a packet arrives, line 111 stores the packet in receivePacket, and line
114 calls method displayPacket (128–140) to display the packet’s contents in the
JTextArea.
17.8 Client/Server Tic-Tac-Toe Using a Multithreaded Server
In this section, we present the popular game Tic-Tac-Toe implemented by using client/server
techniques with stream sockets. The program consists of a TicTacToeServer application
(Fig. 17.8) that allows two TicTacToeClient applets (Fig. 17.9) to connect to the server
and play Tic-Tac-Toe (outputs shown in Fig. 17.10). As the server receives each client connection,
it creates an instance of inner class Player (lines 158–279 of Fig. 17.8) to process
the client in a separate thread. These threads enable the clients to play the game independently.
The server assigns Xs to the first client to connect (X makes the first move) and assigns